/**
*
* ==============================================================================================================
* ==============================================================================================================
* ==============================================================================================================
* ========================== 拷贝的 AbstractQueuedSynchronizer(AQS) 的源码，进行代码注释 ===========================
* ========================== 拷贝的 AbstractQueuedSynchronizer(AQS) 的源码，进行代码注释 ===========================
* ========================== 拷贝的 AbstractQueuedSynchronizer(AQS) 的源码，进行代码注释 ===========================
* ==============================================================================================================
* ==============================================================================================================
* ==============================================================================================================
*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package com.yang.base.juc.A001_Source.java.util.concurrent.locks;
import java.lang.reflect.Field;
import java.util.concurrent.TimeUnit;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Date;
import java.util.concurrent.locks.ReadWriteLock;
import sun.misc.Unsafe;
/**
* Provides a framework for implementing blocking locks and related
* synchronizers (semaphores, events, etc) that rely on
* first-in-first-out (FIFO) wait queues. This class is designed to
* be a useful basis for most kinds of synchronizers that rely on a
* single atomic {@code int} value to represent state. Subclasses
* must define the protected methods that change this state, and which
* define what that state means in terms of this object being acquired
* or released. Given these, the other methods in this class carry
* out all queuing and blocking mechanics. Subclasses can maintain
* other state fields, but only the atomically updated {@code int}
* value manipulated using methods {@link #getState}, {@link
* #setState} and {@link #compareAndSetState} is tracked with respect
* to synchronization.
*
* <p>Subclasses should be defined as non-public internal helper
* classes that are used to implement the synchronization properties
* of their enclosing class. Class
* {@code AbstractQueuedSynchronizer} does not implement any
* synchronization interface. Instead it defines methods such as
* {@link #acquireInterruptibly} that can be invoked as
* appropriate by concrete locks and related synchronizers to
* implement their public methods.
*
* <p>This class supports either or both a default <em>exclusive</em>
* mode and a <em>shared</em> mode. When acquired in exclusive mode,
* attempted acquires by other threads cannot succeed. Shared mode
* acquires by multiple threads may (but need not) succeed. This class
* does not "understand" these differences except in the
* mechanical sense that when a shared mode acquire succeeds, the next
* waiting thread (if one exists) must also determine whether it can
* acquire as well. Threads waiting in the different modes share the
* same FIFO queue. Usually, implementation subclasses support only
* one of these modes, but both can come into play for example in a
* {@link ReadWriteLock}. Subclasses that support only exclusive or
* only shared modes need not define the methods supporting the unused mode.
*
* <p>This class defines a nested {@link java.util.concurrent.locks.AbstractQueuedSynchronizer.ConditionObject} class that
* can be used as a {@link java.util.concurrent.locks.Condition} implementation by subclasses
* supporting exclusive mode for which method {@link
* #isHeldExclusively} reports whether synchronization is exclusively
* held with respect to the current thread, method {@link #release}
* invoked with the current {@link #getState} value fully releases
* this object, and {@link #acquire}, given this saved state value,
* eventually restores this object to its previous acquired state. No
* {@code AbstractQueuedSynchronizer} method otherwise creates such a
* condition, so if this constraint cannot be met, do not use it. The
* behavior of {@link java.util.concurrent.locks.AbstractQueuedSynchronizer.ConditionObject} depends of course on the
* semantics of its synchronizer implementation.
*
* <p>This class provides inspection, instrumentation, and monitoring
* methods for the internal queue, as well as similar methods for
* condition objects. These can be exported as desired into classes
* using an {@code AbstractQueuedSynchronizer} for their
* synchronization mechanics.
*
* <p>Serialization of this class stores only the underlying atomic
* integer maintaining state, so deserialized objects have empty
* thread queues. Typical subclasses requiring serializability will
* define a {@code readObject} method that restores this to a known
* initial state upon deserialization.
*
* <h3>Usage</h3>
*
* <p>To use this class as the basis of a synchronizer, redefine the
* following methods, as applicable, by inspecting and/or modifying
* the synchronization state using {@link #getState}, {@link
* #setState} and/or {@link #compareAndSetState}:
*
* <ul>
* <li> {@link #tryAcquire}
* <li> {@link #tryRelease}
* <li> {@link #tryAcquireShared}
* <li> {@link #tryReleaseShared}
* <li> {@link #isHeldExclusively}
* </ul>
*
* Each of these methods by default throws {@link
* UnsupportedOperationException}. Implementations of these methods
* must be internally thread-safe, and should in general be short and
* not block. Defining these methods is the <em>only</em> supported
* means of using this class. All other methods are declared
* {@code final} because they cannot be independently varied.
*
* <p>You may also find the inherited methods from {@link
* java.util.concurrent.locks.AbstractOwnableSynchronizer} useful to keep track of the thread
* owning an exclusive synchronizer. You are encouraged to use them
* -- this enables monitoring and diagnostic tools to assist users in
* determining which threads hold locks.
*
* <p>Even though this class is based on an internal FIFO queue, it
* does not automatically enforce FIFO acquisition policies. The core
* of exclusive synchronization takes the form:
*
* <pre>
* Acquire:
* while (!tryAcquire(arg)) {
* <em>enqueue thread if it is not already queued</em>;
* <em>possibly block current thread</em>;
* }
*
* Release:
* if (tryRelease(arg))
* <em>unblock the first queued thread</em>;
* </pre>
*
* (Shared mode is similar but may involve cascading signals.)
*
* <p id="barging">Because checks in acquire are invoked before
* enqueuing, a newly acquiring thread may <em>barge</em> ahead of
* others that are blocked and queued. However, you can, if desired,
* define {@code tryAcquire} and/or {@code tryAcquireShared} to
* disable barging by internally invoking one or more of the inspection
* methods, thereby providing a <em>fair</em> FIFO acquisition order.
* In particular, most fair synchronizers can define {@code tryAcquire}
* to return {@code false} if {@link #hasQueuedPredecessors} (a method
* specifically designed to be used by fair synchronizers) returns
* {@code true}. Other variations are possible.
*
* <p>Throughput and scalability are generally highest for the
* default barging (also known as <em>greedy</em>,
* <em>renouncement</em>, and <em>convoy-avoidance</em>) strategy.
* While this is not guaranteed to be fair or starvation-free, earlier
* queued threads are allowed to recontend before later queued
* threads, and each recontention has an unbiased chance to succeed
* against incoming threads. Also, while acquires do not
* "spin" in the usual sense, they may perform multiple
* invocations of {@code tryAcquire} interspersed with other
* computations before blocking. This gives most of the benefits of
* spins when exclusive synchronization is only briefly held, without
* most of the liabilities when it isn't. If so desired, you can
* augment this by preceding calls to acquire methods with
* "fast-path" checks, possibly prechecking {@link #hasContended}
* and/or {@link #hasQueuedThreads} to only do so if the synchronizer
* is likely not to be contended.
*
* <p>This class provides an efficient and scalable basis for
* synchronization in part by specializing its range of use to
* synchronizers that can rely on {@code int} state, acquire, and
* release parameters, and an internal FIFO wait queue. When this does
* not suffice, you can build synchronizers from a lower level using
* {@link java.util.concurrent.atomic atomic} classes, your own custom
* {@link java.util.Queue} classes, and {@link java.util.concurrent.locks.LockSupport} blocking
* support.
*
* <h3>Usage Examples</h3>
*
* <p>Here is a non-reentrant mutual exclusion lock class that uses
* the value zero to represent the unlocked state, and one to
* represent the locked state. While a non-reentrant lock
* does not strictly require recording of the current owner
* thread, this class does so anyway to make usage easier to monitor.
* It also supports conditions and exposes
* one of the instrumentation methods:
*
* <pre> {@code
* class Mutex implements Lock, java.io.Serializable {
*
* // Our internal helper class
* private static class Sync extends AbstractQueuedSynchronizer {
* // Reports whether in locked state
* protected boolean isHeldExclusively() {
* return getState() == 1;
* }
*
* // Acquires the lock if state is zero
* public boolean tryAcquire(int acquires) {
* assert acquires == 1; // Otherwise unused
* if (compareAndSetState(0, 1)) {
* setExclusiveOwnerThread(Thread.currentThread());
* return true;
* }
* return false;
* }
*
* // Releases the lock by setting state to zero
* protected boolean tryRelease(int releases) {
* assert releases == 1; // Otherwise unused
* if (getState() == 0) throw new IllegalMonitorStateException();
* setExclusiveOwnerThread(null);
* setState(0);
* return true;
* }
*
* // Provides a Condition
* Condition newCondition() { return new ConditionObject(); }
*
* // Deserializes properly
* private void readObject(ObjectInputStream s)
* throws IOException, ClassNotFoundException {
* s.defaultReadObject();
* setState(0); // reset to unlocked state
* }
* }
*
* // The sync object does all the hard work. We just forward to it.
* private final Sync sync = new Sync();
*
* public void lock() { sync.acquire(1); }
* public boolean tryLock() { return sync.tryAcquire(1); }
* public void unlock() { sync.release(1); }
* public Condition newCondition() { return sync.newCondition(); }
* public boolean isLocked() { return sync.isHeldExclusively(); }
* public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); }
* public void lockInterruptibly() throws InterruptedException {
* sync.acquireInterruptibly(1);
* }
* public boolean tryLock(long timeout, TimeUnit unit)
* throws InterruptedException {
* return sync.tryAcquireNanos(1, unit.toNanos(timeout));
* }
* }}</pre>
*
* <p>Here is a latch class that is like a
* {@link java.util.concurrent.CountDownLatch CountDownLatch}
* except that it only requires a single {@code signal} to
* fire. Because a latch is non-exclusive, it uses the {@code shared}
* acquire and release methods.
*
* <pre> {@code
* class BooleanLatch {
*
* private static class Sync extends AbstractQueuedSynchronizer {
* boolean isSignalled() { return getState() != 0; }
*
* protected int tryAcquireShared(int ignore) {
* return isSignalled() ? 1 : -1;
* }
*
* protected boolean tryReleaseShared(int ignore) {
* setState(1);
* return true;
* }
* }
*
* private final Sync sync = new Sync();
* public boolean isSignalled() { return sync.isSignalled(); }
* public void signal() { sync.releaseShared(1); }
* public void await() throws InterruptedException {
* sync.acquireSharedInterruptibly(1);
* }
* }}</pre>
*
* @since 1.5
* @author Doug Lea
*/
public abstract class AbstractQueuedSynchronizer extends AbstractOwnableSynchronizer implements java.io.Serializable {
private static final long serialVersionUID = 7373984972572414691L;
/**
* Creates a new {@code AbstractQueuedSynchronizer} instance
* with initial synchronization state of zero.
*/
protected AbstractQueuedSynchronizer() { }
/**
* Wait queue node class.
*
* <p>The wait queue is a variant of a "CLH" (Craig, Landin, and
* Hagersten) lock queue. CLH locks are normally used for
* spinlocks. We instead use them for blocking synchronizers, but
* use the same basic tactic of holding some of the control
* information about a thread in the predecessor of its node. A
* "status" field in each node keeps track of whether a thread
* should block. A node is signalled when its predecessor
* releases. Each node of the queue otherwise serves as a
* specific-notification-style monitor holding a single waiting
* thread. The status field does NOT control whether threads are
* granted locks etc though. A thread may try to acquire if it is
* first in the queue. But being first does not guarantee success;
* it only gives the right to contend. So the currently released
* contender thread may need to rewait.
*
* <p>To enqueue into a CLH lock, you atomically splice it in as new
* tail. To dequeue, you just set the head field.
* <pre>
* +------+ prev +-----+ +-----+
* head | | <---- | | <---- | | tail
* +------+ +-----+ +-----+
* </pre>
*
* <p>Insertion into a CLH queue requires only a single atomic
* operation on "tail", so there is a simple atomic point of
* demarcation from unqueued to queued. Similarly, dequeuing
* involves only updating the "head". However, it takes a bit
* more work for nodes to determine who their successors are,
* in part to deal with possible cancellation due to timeouts
* and interrupts.
*
* <p>The "prev" links (not used in original CLH locks), are mainly
* needed to handle cancellation. If a node is cancelled, its
* successor is (normally) relinked to a non-cancelled
* predecessor. For explanation of similar mechanics in the case
* of spin locks, see the papers by Scott and Scherer at
* http://www.cs.rochester.edu/u/scott/synchronization/
*
* <p>We also use "next" links to implement blocking mechanics.
* The thread id for each node is kept in its own node, so a
* predecessor signals the next node to wake up by traversing
* next link to determine which thread it is. Determination of
* successor must avoid races with newly queued nodes to set
* the "next" fields of their predecessors. This is solved
* when necessary by checking backwards from the atomically
* updated "tail" when a node's successor appears to be null.
* (Or, said differently, the next-links are an optimization
* so that we don't usually need a backward scan.)
*
* <p>Cancellation introduces some conservatism to the basic
* algorithms. Since we must poll for cancellation of other
* nodes, we can miss noticing whether a cancelled node is
* ahead or behind us. This is dealt with by always unparking
* successors upon cancellation, allowing them to stabilize on
* a new predecessor, unless we can identify an uncancelled
* predecessor who will carry this responsibility.
*
* <p>CLH queues need a dummy header node to get started. But
* we don't create them on construction, because it would be wasted
* effort if there is never contention. Instead, the node
* is constructed and head and tail pointers are set upon first
* contention.
*
* <p>Threads waiting on Conditions use the same nodes, but
* use an additional link. Conditions only need to link nodes
* in simple (non-concurrent) linked queues because they are
* only accessed when exclusively held. Upon await, a node is
* inserted into a condition queue. Upon signal, the node is
* transferred to the main queue. A special value of status
* field is used to mark which queue a node is on.
*
* <p>Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill
* Scherer and Michael Scott, along with members of JSR-166
* expert group, for helpful ideas, discussions, and critiques
* on the design of this class.
*/
static final class Node {
/**
* Marker to indicate a node is waiting in shared mode
*
* 标记：该节点正在共享模式下等待
*/
static final Node SHARED = new Node();
/**
* Marker to indicate a node is waiting in exclusive mode
*
* 标记：该节点正在独占模式下等待
*/
static final Node EXCLUSIVE = null;
// =================== waitStatus 状态值 ===================
/**
* waitStatus value to indicate thread has cancelled
*
* 取消状态：表示该线程已取消
*
* 该节点因超时或中断而被取消。节点永远不会离开这个状态。特别是，取消节点的线程再也不会阻塞。
*/
static final int CANCELLED = 1;
/**
* waitStatus value to indicate successor's thread needs unparking
*
* 通知状态：指示后续线程需要被唤醒
*
* 这个节点的后继节点被(或即将被)阻塞(通过park)，因此当前节点在释放或取消时必须唤醒（通过unpark）其后继节点的。
* 为了避免竞争，获取方法必须首先表明它们需要一个信号，然后重试原子获取，然后在失败时阻塞。
*
* 注意：这个状态一般都是由后继的线程设置的
*/
static final int SIGNAL = -1;
/**
* waitStatus value to indicate thread is waiting on condition
*
* 条件等待状态 ：当前节点在 Condition 节点队列当中，节点在等待 Condition 通知
*
* 该节点当前处于条件队列中。在传输之前，它不会被用作同步队列节点，此时状态将被设置为0。(这里这个值的使用与该字段的其他用途无关，只是简化了机制。)
*/
static final int CONDITION = -2;
/**
* waitStatus value to indicate the next acquireShared should unconditionally propagate
*
* 传播状态：指示下一个 acquireShared 应无条件传播
* 将唤醒后继线程的能力，传递下去（主要用在共享模式下）
*
* releaseShared 应传播到其他节点。这是在 doReleaseShared 中设置的（仅适用于头节点），以确保传播继续，即使其他操作已经介入。
*
*/
static final int PROPAGATE = -3;
/**
* Status field, taking on only the values:
* SIGNAL: The successor of this node is (or will soon be)
* blocked (via park), so the current node must
* unpark its successor when it releases or
* cancels. To avoid races, acquire methods must
* first indicate they need a signal,
* then retry the atomic acquire, and then,
* on failure, block.
* CANCELLED: This node is cancelled due to timeout or interrupt.
* Nodes never leave this state. In particular,
* a thread with cancelled node never again blocks.
* CONDITION: This node is currently on a condition queue.
* It will not be used as a sync queue node
* until transferred, at which time the status
* will be set to 0. (Use of this value here has
* nothing to do with the other uses of the
* field, but simplifies mechanics.)
* PROPAGATE: A releaseShared should be propagated to other
* nodes. This is set (for head node only) in
* doReleaseShared to ensure propagation
* continues, even if other operations have
* since intervened.
* 0: None of the above
*
* The values are arranged numerically to simplify use.
* Non-negative values mean that a node doesn't need to
* signal. So, most code doesn't need to check for particular
* values, just for sign.
*
* The field is initialized to 0 for normal sync nodes, and
* CONDITION for condition nodes. It is modified using CAS
* (or when possible, unconditional volatile writes).
*/
volatile int waitStatus;
/**
* Link to predecessor node that current node/thread relies on
* for checking waitStatus. Assigned during enqueuing, and nulled
* out (for sake of GC) only upon dequeuing. Also, upon
* cancellation of a predecessor, we short-circuit while
* finding a non-cancelled one, which will always exist
* because the head node is never cancelled: A node becomes
* head only as a result of successful acquire. A
* cancelled thread never succeeds in acquiring, and a thread only
* cancels itself, not any other node.
*
* 前驱节点
*/
volatile Node prev;
/**
* Link to the successor node that the current node/thread
* unparks upon release. Assigned during enqueuing, adjusted
* when bypassing cancelled predecessors, and nulled out (for
* sake of GC) when dequeued. The enq operation does not
* assign next field of a predecessor until after attachment,
* so seeing a null next field does not necessarily mean that
* node is at end of queue. However, if a next field appears
* to be null, we can scan prev's from the tail to
* double-check. The next field of cancelled nodes is set to
* point to the node itself instead of null, to make life
* easier for isOnSyncQueue.
*
* 后继节点
*/
volatile Node next;
/**
* The thread that enqueued this node. Initialized on construction and nulled out after use.
* 该节点的线程
*/
volatile Thread thread;
/**
* Link to next node waiting on condition, or the special
* value SHARED. Because condition queues are accessed only
* when holding in exclusive mode, we just need a simple
* linked queue to hold nodes while they are waiting on
* conditions. They are then transferred to the queue to
* re-acquire. And because conditions can only be exclusive,
* we save a field by using special value to indicate shared
* mode.
*
* 取值范围：Node.SHARED ，Node.EXCLUSIVE
* SHARED: 共享模式
* EXCLUSIVE: 独占模式
*
* 1.在条件队列下：表示条件队列当中的后继节点（链接到下一个等待条件的节点）
* 2.在共享模式下：表示特殊值 Shared
*
*/
Node nextWaiter;
/**
* Returns true if node is waiting in shared mode.
* 如果节点在共享模式下等待，则返回true。
*/
final boolean isShared() {
return nextWaiter == SHARED;
}
/**
* Returns previous node, or throws NullPointerException if null.
* Use when predecessor cannot be null. The null check could
* be elided, but is present to help the VM.
*
* @return the predecessor of this node
*/
final Node predecessor() throws NullPointerException {
Node p = prev;
if (p == null)
throw new NullPointerException();
else
return p;
}
Node() { // Used to establish initial head or SHARED marker
}
Node(Thread thread, Node mode) { // Used by addWaiter
this.nextWaiter = mode;
this.thread = thread;
}
Node(Thread thread, int waitStatus) { // Used by Condition
this.waitStatus = waitStatus;
this.thread = thread;
}
}
/**
* Head of the wait queue, lazily initialized. Except for
* initialization, it is modified only via method setHead.
* Note: If head exists, its waitStatus is guaranteed not to be CANCELLED.
*
* 等待队列头部 （该节点不存储线程，只做指向功能，指向队列中第一个等待节点）
*/
private transient volatile Node head;
/**
* Tail of the wait queue, lazily initialized. Modified only via method enq to add new wait node.
*
* 等待队列尾部（该节点不存储线程，只做指向功能，指向队列中最后一个等待节点）
*/
private transient volatile Node tail;
/**
* The synchronization state.
*
* 同步状态（用不同的值，来表示锁是否被持有等等，例如：0 表示该锁未被占用 1表示该锁被占用 2 表示锁重入）
*/
private volatile int state;
/**
* Returns the current value of synchronization state.
* This operation has memory semantics of a {@code volatile} read.
* @return current state value
*
* [返回同步状态的当前值] 此操作具有｛@code volatile｝读取的内存语义
*/
protected final int getState() {
return state;
}
/**
* Sets the value of synchronization state.
* This operation has memory semantics of a {@code volatile} write.
* @param newState the new state value
*
* [设置同步状态的值] 此操作具有｛@code volatile｝写入的内存语义。
*/
protected final void setState(int newState) {
state = newState;
}
/**
* Atomically sets synchronization state to the given updated
* value if the current state value equals the expected value.
* This operation has memory semantics of a {@code volatile} read
* and write.
*
* @param expect the expected value
* @param update the new value
* @return {@code true} if successful. False return indicates that the actual
* value was not equal to the expected value.
*
* [CAS 方式设置 同步状态值]
*/
protected final boolean compareAndSetState(int expect, int update) {
// See below for intrinsics setup to support this
return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
}
//========================== 排队实用程序 ==========================
// Queuing utilities
/**
* The number of nanoseconds for which it is faster to spin
* rather than to use timed park. A rough estimate suffices
* to improve responsiveness with very short timeouts.
*
* 默认自旋的超时时间 （当等待的时间超过该值，则 park 该线程，当等待的时间小于该值，通过自旋尝试获取锁）
*/
static final long spinForTimeoutThreshold = 1000L;
/**
* 将节点插入队列，必要时进行初始化 （返回该节点的前置节点）
*/
private Node enq(final Node node) {
for (;;) {
Node t = tail;
if (t == null) { // Must initialize
if (compareAndSetHead(new Node()))
tail = head;
} else {
node.prev = t;
if (compareAndSetTail(t, node)) {
t.next = node;
return t;
}
}
}
}
/**
* 根据给定的模式，为当前线程创建排队节点
*
* Creates and enqueues node for current thread and given mode.
*
* @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
* @return the new node
*/
private Node addWaiter(Node mode) {
Node node = new Node(Thread.currentThread(), mode);
// Try the fast path of enq; backup to full enq on failure
Node pred = tail;
if (pred != null) {
node.prev = pred;
if (compareAndSetTail(pred, node)) {
pred.next = node;
return node;
}
}
enq(node);
return node;
}
/**
* Sets head of queue to be node, thus dequeuing. Called only by
* acquire methods. Also nulls out unused fields for sake of GC
* and to suppress unnecessary signals and traversals.
*
* @param node the node
* 将队列头设置为节点，从而使其退出队列（设置该节点为虚拟节点）
*
* head ---> node(虚节点，thread=null，pre = null ) ---> 等待节点Node1 ---> 等待节点Node2 ---> tail
*/
private void setHead(Node node) {
head = node;
node.thread = null;
node.prev = null;
}
/**
* 唤醒节点的后继节点（如果存在）。
*/
private void unparkSuccessor(Node node) {
/*
* If status is negative (i.e., possibly needing signal) try
* to clear in anticipation of signalling. It is OK if this
* fails or if status is changed by waiting thread.
*
* 如果状态为负（即，可能需要信号），尝试在信号发出之前清除。
* 如果失败或等待线程更改了状态，则可以。
*/
int ws = node.waitStatus;
if (ws < 0)
compareAndSetWaitStatus(node, ws, 0);
/*
* Thread to unpark is held in successor, which is normally
* just the next node. But if cancelled or apparently null,
* traverse backwards from tail to find the actual
* non-cancelled successor.
*
* 要唤醒(unpark)的线程保存在后继节点中，通常只是下一个节点。
* 但如果被取消或明显为空，则从 tail 向后遍历以找到实际的非取消后继对象。
*/
// 当前节点的后继节点（需要唤醒的节点）
Node s = node.next;
// 当 要唤醒的节点为空
// 或 要唤醒的节点已取消
// 则置空当前要唤醒的线程，从 tail 向后遍历找到实际的非取消的节点（最后一个非取消的节点），赋值给要唤醒的节点
if (s == null || s.waitStatus > 0) {
s = null;
for (Node t = tail; t != null && t != node; t = t.prev)
if (t.waitStatus <= 0)
s = t;
}
// 唤醒后继节点
if (s != null)
LockSupport.unpark(s.thread);
}
/**
* Release action for shared mode -- signals successor and ensures
* propagation. (Note: For exclusive mode, release just amounts
* to calling unparkSuccessor of head if it needs signal.)
*
* 释放共享模式的动作 —— 发出后续信号并确保传播。
* （注意：对于独占模式，如果需要信号，release相当于调用 head 的 unparkSuccessor。）
*/
private void doReleaseShared() {
/*
* Ensure that a release propagates, even if there are other
* in-progress acquires/releases. This proceeds in the usual
* way of trying to unparkSuccessor of head if it needs
* signal. But if it does not, status is set to PROPAGATE to
* ensure that upon release, propagation continues.
* Additionally, we must loop in case a new node is added
* while we are doing this. Also, unlike other uses of
* unparkSuccessor, we need to know if CAS to reset status
* fails, if so rechecking.
*/
for (;;) {
Node h = head;
if (h != null && h != tail) {
int ws = h.waitStatus;
if (ws == Node.SIGNAL) {
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
continue; // loop to recheck cases
unparkSuccessor(h);
}
else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
continue; // loop on failed CAS
}
if (h == head) // loop if head changed
break;
}
}
/**
* Sets head of queue, and checks if successor may be waiting
* in shared mode, if so propagating if either propagate > 0 or
* PROPAGATE status was set.
*
* @param node the node
* @param propagate the return value from a tryAcquireShared
*
* 设置头节点并传播行为 （共享模式下使用）
* 设置队列头，并检查后续程序是否可能在共享模式下等待，如果是，则在设置了 propagate>0 或 PROPAGATE 状态时进行传播。
*/
private void setHeadAndPropagate(Node node, int propagate) {
// 记录旧头部，以便在下面进行检查
Node h = head; // Record old head for check below
// 设置队列头，当前节点出队
setHead(node);
/*
* Try to signal next queued node if:
* Propagation was indicated by caller,
* or was recorded (as h.waitStatus either before
* or after setHead) by a previous operation
* (note: this uses sign-check of waitStatus because
* PROPAGATE status may transition to SIGNAL.)
* and
* The next node is waiting in shared mode,
* or we don't know, because it appears null
*
* The conservatism in both of these checks may cause
* unnecessary wake-ups, but only when there are multiple
* racing acquires/releases, so most need signals now or soon
* anyway.
*
* 当 propagate > 0
* 或 旧的 head 为空 (标识当前队列为空)
* 或 旧的 head.waitStatus < 0 (SIGNAL,CONDITION,PROPAGATE 状态)
* 或 新的 head 为空
* 或 新的 head.waitStatus < 0 (SIGNAL,CONDITION,PROPAGATE 状态)
*
* 判断当前节点的后继节点：为空 或 是共享节点
*
* 执行释放
*
*/
if (propagate > 0 || h == null || h.waitStatus < 0 ||
(h = head) == null || h.waitStatus < 0) {
Node s = node.next;
if (s == null || s.isShared())
doReleaseShared();
}
}
//========================== 用于不同版本的 acquire 的实用程序 ==========================
// Utilities for various versions of acquire
/**
* Cancels an ongoing attempt to acquire.
*
* @param node the node
*
* 取消正在尝试获取锁的节点（一般用于线程被打断，需要立即返回的情况）
*/
private void cancelAcquire(Node node) {
// Ignore if node doesn't exist
// 如果节点不存在，则忽略
if (node == null)
return;
node.thread = null;
/**
* Skip cancelled predecessors
* 跳过已取消节点的前置任务
*/
Node pred = node.prev;
// pred.waitStatus > 0 表示前置节点也被取消了 （剔除前面 所有取消的节点）
// 目的：找到 node 前面第一个未取消的节点
while (pred.waitStatus > 0)
node.prev = pred = pred.prev;
// predNext is the apparent node to unsplice. CASes below will
// fail if not, in which case, we lost race vs another cancel
// or signal, so no further action is necessary.
// 当前取消节点的前置节点的后继节点（不一定是当前节点）
Node predNext = pred.next;
// Can use unconditional write instead of CAS here.
// After this atomic step, other Nodes can skip past us.
// Before, we are free of interference from other threads.
// 当前节点 node 设置为： 取消状态
node.waitStatus = Node.CANCELLED;
// If we are the tail, remove ourselves.
// 如果当前节点为 tail (说明为尾节点)，将当前节点的剔除掉
if (node == tail && compareAndSetTail(node, pred)) {
//设置前置节点的next 为 null （因为现在它为 tail 尾节点）
compareAndSetNext(pred, predNext, null);
} else {
// If successor needs signal, try to set pred's next-link
// so it will get one. Otherwise wake it up to propagate.
// 如果当前节点不是尾节点
int ws;
// 当 前置节点不是 head
// 并且 前置节点的 waitStatus状态是 SIGNAL 或者 （前置节点的waitStatus状态不是取消状态 并且 设置前置状态为 SIGNAL 成功）
// 并且 前置节点的 thread 不为空
if (pred != head &&
((ws = pred.waitStatus) == Node.SIGNAL || (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
pred.thread != null) {
Node next = node.next;
// 将当前节点的next 设置为前置节点next （主要是跳过当前节点）
if (next != null && next.waitStatus <= 0)
compareAndSetNext(pred, predNext, next);
} else {
// 唤醒后继节点
unparkSuccessor(node);
}
//将当前节点的next设置为自己（闭环），便于垃圾回收
node.next = node; // help GC
}
}
/**
* Checks and updates status for a node that failed to acquire.
* Returns true if thread should block. This is the main signal
* control in all acquire loops. Requires that pred == node.prev.
*
* @param pred node's predecessor holding status
* @param node the node
* @return {@code true} if thread should block
*
* 检查并更新未能获取的节点的状态。如果线程应阻塞，则返回true
* 维护节点状态
*/
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
int ws = pred.waitStatus;
if (ws == Node.SIGNAL)
/*
* This node has already set status asking a release to signal it, so it can safely park.
* 当前节点的前驱节点状态是 Single （Single 代表，释放锁时，唤醒后继节点），则后续执行（parkAndCheckInterrupt）挂起等待
*/
return true;
if (ws > 0) {
/*
* Predecessor was cancelled. Skip over predecessors and indicate retry.
* ws > 0 代表前置节点已取消 (剔除该前置节点)
* 从当前前置节点往前找到一个最近的未取消的节点作为当前节点的前置节点
*/
do {
node.prev = pred = pred.prev;
} while (pred.waitStatus > 0);
pred.next = node;
} else {
/*
* waitStatus must be 0 or PROPAGATE. Indicate that we
* need a signal, but don't park yet. Caller will need to
* retry to make sure it cannot acquire before parking.
*
* waitStatus必须为0或PROPAGATE ， 表示我们需要信号，但还没有 park。
* 调用者将需要重试，以确保它不能获取之前 park。
*
* 设置前驱节点的 waitStatus = SIGNAL （Single 代表，释放锁时，唤醒后继节点）
*/
compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
}
return false;
}
/**
* Convenience method to interrupt current thread.
*
* 中断当前线程
*/
static void selfInterrupt() {
Thread.currentThread().interrupt();
}
/**
* Convenience method to park and then check if interrupted
*
* @return {@code true} if interrupted
*
* 唤醒当前线程，并返回中断状态
*/
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);
return Thread.interrupted();
}
/**
* 不同的获取方式，不同的独占/共享和控制模式。每一个都大致相同，但令人讨厌的是不同。
* 由于异常机制(包括确保我们在tryAcquire抛出异常时取消)和其他控制的相互作用，
* 只有一点点因子分解是可能的，至少不会对性能造成太大影响。
*/
/*
* Various flavors of acquire, varying in exclusive/shared and
* control modes. Each is mostly the same, but annoyingly
* different. Only a little bit of factoring is possible due to
* interactions of exception mechanics (including ensuring that we
* cancel if tryAcquire throws exception) and other control, at
* least not without hurting performance too much.
*/
/**
* Acquires in exclusive uninterruptible mode for thread already in
* queue. Used by condition wait methods as well as acquire.
*
* @param node the node
* @param arg the acquire argument
* @return {@code true} if interrupted while waiting
*
* 以独占不可中断模式获取队列中已存在的线程。用于条件等待方法和acquire方法。
*
* 如果被中断，线程执行完成之后，返回中断标识
*/
final boolean acquireQueued(final Node node, int arg) {
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
// 获取前置节点是否为 head ；
// 是：尝试获取锁
// 否：向下执行，继续阻塞（park）
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
// 该节点获取锁成功
// 该节点出队
setHead(node);
// 将之前的虚节点 next 置为空，方便垃圾回收
p.next = null; // help GC
failed = false;
// 返回中断状态
return interrupted;
}
// 保证前置节点的状态是 SIGNAL ，保证前置节点释放之后，唤醒后续节点;
// 阻塞当前线程（等待唤醒后继续执行）
if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt())
//当线程被中断，则返回中断状态
interrupted = true;
}
} finally {
//当线程获取锁成功后，执行失败，则取消该节点
if (failed)
cancelAcquire(node);
}
}
/**
* Acquires in exclusive interruptible mode.
* @param arg the acquire argument
*
* 以独占可中断模式获取。（可响应中断）。
*/
private void doAcquireInterruptibly(int arg) throws InterruptedException {
// 将当前节点加入等待队列
final Node node = addWaiter(Node.EXCLUSIVE);
boolean failed = true;
try {
for (;;) {
// 获取前置节点是否为 head ；
// 是：尝试获取锁
// 否：向下执行，继续阻塞（park）
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
// 该节点获取锁成功
// 该节点出队
setHead(node);
// 将之前的虚节点 next 置为空，方便垃圾回收
p.next = null; // help GC
failed = false;
return;
}
// 保证前置节点的状态是 SIGNAL ，保证前置节点释放之后，唤醒后续节点;
// 阻塞当前线程（等待唤醒后继续执行）
if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt())
//当线程被中断，则抛出异常
throw new InterruptedException();
}
} finally {
//当线程获取锁成功后，执行失败，则取消该节点
if (failed)
cancelAcquire(node);
}
}
/**
* Acquires in exclusive timed mode.
*
* @param arg the acquire argument
* @param nanosTimeout max wait time
* @return {@code true} if acquired
*
* 以独占计时模式获取。
*/
private boolean doAcquireNanos(int arg, long nanosTimeout) throws InterruptedException {
// 判断最大等待时间是否合规
if (nanosTimeout <= 0L)
return false;
// 计算出 截止时间
final long deadline = System.nanoTime() + nanosTimeout;
// 加入等待队列
final Node node = addWaiter(Node.EXCLUSIVE);
boolean failed = true;
try {
for (;;) {
// 获取前置节点是否为 head ；
// 是：尝试获取锁
// 否：向下执行，继续阻塞（park）
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
// 该节点获取锁成功
// 该节点出队
setHead(node);
// 将之前的虚节点 next 置为空，方便垃圾回收
p.next = null; // help GC
failed = false;
return true;
}
//计算距离截止时间还有多少时间
nanosTimeout = deadline - System.nanoTime();
// 判断等待时间是否用完，用完则返回 false（因为这一步的时候，还没有获取锁）
if (nanosTimeout <= 0L)
return false;
// 1.保证前置节点的状态是 SIGNAL ，保证前置节点释放之后，唤醒后续节点;
// 2.判断剩余的时间小于系统定义的自旋时间，
// 小于：线程执行自旋方式获取锁（for循环）
// 否则：阻塞当前线程（阻塞 nanosTimeout 时间，时间到了，自动唤醒）
if (shouldParkAfterFailedAcquire(p, node) && nanosTimeout > spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
//响应线程的中断
if (Thread.interrupted())
throw new InterruptedException();
}
} finally {
//当线程获取锁成功后，执行失败，则取消该节点
if (failed)
cancelAcquire(node);
}
}
/**
* Acquires in shared uninterruptible mode.
* @param arg the acquire argument
*
* 在共享不中断模式下获取。【忽略中断】
*
* 1.为当前线程创建排队节点，加入等待队列
* 2.for循环开始：判断当前节点的前置节点是否为 head？
* 是: 尝试获取锁
* 获取成功，节点出队，
* 获取失败，执行步骤 3
* 否：执行步骤 3
* 3.判断当前节点的前置节点 waitStatus == Node.SIGNAL
* 如果不是，设置为 Node.SIGNAL ，执行步骤 2
* 如果是， 阻塞（park）当前节点的线程，等待唤醒；节点被唤醒之后，执行步骤 2
*
*/
private void doAcquireShared(int arg) {
// 加入到等待队列中
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
boolean interrupted = false; //线程的中断标识
for (;;) {
// 判断当前节点的前置节点是否是 head
final Node p = node.predecessor();
if (p == head) {
int r = tryAcquireShared(arg);
if (r >= 0) {
// 设置 head 并向下传播（唤醒线程）
setHeadAndPropagate(node, r);
p.next = null; // help GC
if (interrupted)
selfInterrupt();
failed = false;
return;
}
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}
/**
* Acquires in shared interruptible mode.
* @param arg the acquire argument
*
* 在共享可中断模式下获取。
*/
private void doAcquireSharedInterruptibly(int arg) throws InterruptedException {
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head) {
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return;
}
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
/**
* Acquires in shared timed mode.
*
* @param arg the acquire argument
* @param nanosTimeout max wait time
* @return {@code true} if acquired
*
* 以共享定时模式获取。
*/
private boolean doAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException {
if (nanosTimeout <= 0L)
return false;
//截止时间
final long deadline = System.nanoTime() + nanosTimeout;
// 加入队列
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head) {
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return true;
}
}
//距离截止时间还有多少时间
nanosTimeout = deadline - System.nanoTime();
if (nanosTimeout <= 0L) //时间用完，直接返回 false
return false;
if (shouldParkAfterFailedAcquire(p, node) &&
// 如果剩余的时间小于系统定义的自旋时间，才会执行自旋（for循环）；
// 否则：阻塞当前 nanosTimeout ，时间到了，自动唤醒；
nanosTimeout > spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
if (Thread.interrupted())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
//========================== 主要暴露的方法 ==========================
// Main exported methods
/**
* Attempts to acquire in exclusive mode. This method should query
* if the state of the object permits it to be acquired in the
* exclusive mode, and if so to acquire it.
*
* <p>This method is always invoked by the thread performing
* acquire. If this method reports failure, the acquire method
* may queue the thread, if it is not already queued, until it is
* signalled by a release from some other thread. This can be used
* to implement method {@link Lock#tryLock()}.
*
* <p>The default
* implementation throws {@link UnsupportedOperationException}.
*
* @param arg the acquire argument. This value is always the one
* passed to an acquire method, or is the value saved on entry
* to a condition wait. The value is otherwise uninterpreted
* and can represent anything you like.
* @return {@code true} if successful. Upon success, this object has
* been acquired.
* @throws IllegalMonitorStateException if acquiring would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if exclusive mode is not supported
*
* 尝试以独占模式获取。该方法应该查询对象的状态是否允许以独占模式获取，如果允许则获取它。
*
* 该方法总是由执行 acquire的线程调用。如果此方法报告失败，acquire方法可能会对线程进行排队(如果它还没有排队)，直到从其他线程释放信号。
*
* 如果获取成功，则返回true
*/
protected boolean tryAcquire(int arg) {
throw new UnsupportedOperationException();
}
/**
* Attempts to set the state to reflect a release in exclusive mode.
*
* <p>This method is always invoked by the thread performing release.
*
* <p>The default implementation throws
* {@link UnsupportedOperationException}.
*
* @param arg the release argument. This value is always the one
* passed to a release method, or the current state value upon
* entry to a condition wait. The value is otherwise
* uninterpreted and can represent anything you like.
* @return {@code true} if this object is now in a fully released
* state, so that any waiting threads may attempt to acquire;
* and {@code false} otherwise.
* @throws IllegalMonitorStateException if releasing would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if exclusive mode is not supported
*
* 试图设置状态以反映独占模式下的释放。(此方法总是由执行释放的线程调用。)
*
*/
protected boolean tryRelease(int arg) {
throw new UnsupportedOperationException();
}
/**
* Attempts to acquire in shared mode. This method should query if
* the state of the object permits it to be acquired in the shared
* mode, and if so to acquire it.
*
* <p>This method is always invoked by the thread performing
* acquire. If this method reports failure, the acquire method
* may queue the thread, if it is not already queued, until it is
* signalled by a release from some other thread.
*
* <p>The default implementation throws {@link
* UnsupportedOperationException}.
*
* @param arg the acquire argument. This value is always the one
* passed to an acquire method, or is the value saved on entry
* to a condition wait. The value is otherwise uninterpreted
* and can represent anything you like.
* @return a negative value on failure; zero if acquisition in shared
* mode succeeded but no subsequent shared-mode acquire can
* succeed; and a positive value if acquisition in shared
* mode succeeded and subsequent shared-mode acquires might
* also succeed, in which case a subsequent waiting thread
* must check availability. (Support for three different
* return values enables this method to be used in contexts
* where acquires only sometimes act exclusively.) Upon
* success, this object has been acquired.
* @throws IllegalMonitorStateException if acquiring would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if shared mode is not supported
*
* 尝试以共享模式获取。该方法应该查询对象的状态是否允许以共享模式获取，如果允许，则获取它。
*
* 返回值 < 0 ：失败
* 返回值 = 0 ：获取成功，但后续的共享模式获取无法成功
* 返回值 > 0 ：获取成功，并且后续的共享模式的获取也可能成功
*
*/
protected int tryAcquireShared(int arg) {
throw new UnsupportedOperationException();
}
/**
* Attempts to set the state to reflect a release in shared mode.
*
* <p>This method is always invoked by the thread performing release.
*
* <p>The default implementation throws
* {@link UnsupportedOperationException}.
*
* @param arg the release argument. This value is always the one
* passed to a release method, or the current state value upon
* entry to a condition wait. The value is otherwise
* uninterpreted and can represent anything you like.
* @return {@code true} if this release of shared mode may permit a
* waiting acquire (shared or exclusive) to succeed; and
* {@code false} otherwise
* @throws IllegalMonitorStateException if releasing would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if shared mode is not supported
*
* 试图设置状态以反映共享模式下的释放。
*
*/
protected boolean tryReleaseShared(int arg) {
throw new UnsupportedOperationException();
}
/**
* Returns {@code true} if synchronization is held exclusively with
* respect to the current (calling) thread. This method is invoked
* upon each call to a non-waiting {@link ConditionObject} method.
* (Waiting methods instead invoke {@link #release}.)
*
* <p>The default implementation throws {@link
* UnsupportedOperationException}. This method is invoked
* internally only within {@link ConditionObject} methods, so need
* not be defined if conditions are not used.
*
* @return {@code true} if synchronization is held exclusively;
* {@code false} otherwise
* @throws UnsupportedOperationException if conditions are not supported
*/
protected boolean isHeldExclusively() {
throw new UnsupportedOperationException();
}
/**
* Acquires in exclusive mode, ignoring interrupts. Implemented
* by invoking at least once {@link #tryAcquire},
* returning on success. Otherwise the thread is queued, possibly
* repeatedly blocking and unblocking, invoking {@link
* #tryAcquire} until success. This method can be used
* to implement method {@link Lock#lock}.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
*
* 以独占模式获取，忽略中断.（如果被中断，线程执行完成之后，acquireQueued 方法返回中断标识）
*
* 实现方式是至少调用一次 tryAcquire()，成功返回。否则线程会排队，可能会反复阻塞和解阻塞，调用 tryAcquire() 直到成功
*/
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
//响应线程的中断
selfInterrupt();
}
/**
* Acquires in exclusive mode, aborting if interrupted.
* Implemented by first checking interrupt status, then invoking
* at least once {@link #tryAcquire}, returning on
* success. Otherwise the thread is queued, possibly repeatedly
* blocking and unblocking, invoking {@link #tryAcquire}
* until success or the thread is interrupted. This method can be
* used to implement method {@link Lock#lockInterruptibly}.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
* @throws InterruptedException if the current thread is interrupted
*
* 以独占模式可中断的获取。 如果中断则终止（如果获取过程中被中断，则会抛出 InterruptedException 异常）
*
* 首先检查中断状态，然后至少调用一次 tryAcquire()，成功后返回。
* 否则，线程会排队，可能会反复阻塞和解阻塞，调用 tryAcquire()，直到成功或线程中断
*/
public final void acquireInterruptibly(int arg) throws InterruptedException {
//判断是否中断，中断则抛出异常
if (Thread.interrupted())
throw new InterruptedException();
if (!tryAcquire(arg))
// 如果唤醒的线程被中断，则响应中断 抛出异常，并取消该节点
doAcquireInterruptibly(arg);
}
/**
* Attempts to acquire in exclusive mode, aborting if interrupted,
* and failing if the given timeout elapses. Implemented by first
* checking interrupt status, then invoking at least once {@link
* #tryAcquire}, returning on success. Otherwise, the thread is
* queued, possibly repeatedly blocking and unblocking, invoking
* {@link #tryAcquire} until success or the thread is interrupted
* or the timeout elapses. This method can be used to implement
* method {@link Lock#tryLock(long, TimeUnit)}.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
* @param nanosTimeout the maximum number of nanoseconds to wait
* @return {@code true} if acquired; {@code false} if timed out
* @throws InterruptedException if the current thread is interrupted
*
* 以独占模式可中断的获取，如果中断将中止，如果超时将获取失败。（如果被中断则终止，抛出 InterruptedException 异常）。
*
* 首先检查中断状态，然后至少调用一次 tryAcquire()，成功后返回。
* 否则，线程会排队，可能会反复阻塞和解阻塞，调用 tryAcquire()，直到成功，或者线程被中断，或者超时过去。
*/
public final boolean tryAcquireNanos(int arg, long nanosTimeout) throws InterruptedException {
// 线程是否被中断，中断则抛出异常
if (Thread.interrupted())
throw new InterruptedException();
// tryAcquire
// 获取锁成功：直接返回 true
// 获取锁失败：在指定时间内以独占模式获取锁。并返回获取锁的情况
return tryAcquire(arg) || doAcquireNanos(arg, nanosTimeout);
}
/**
* Releases in exclusive mode. Implemented by unblocking one or
* more threads if {@link #tryRelease} returns true.
* This method can be used to implement method {@link Lock#unlock}.
*
* @param arg the release argument. This value is conveyed to
* {@link #tryRelease} but is otherwise uninterpreted and
* can represent anything you like.
* @return the value returned from {@link #tryRelease}
*
* 以独占模式释放锁。
*/
public final boolean release(int arg) {
// 调用子类实现，释放锁
if (tryRelease(arg)) {
// 唤醒后继节点
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}
/**
* Acquires in shared mode, ignoring interrupts. Implemented by
* first invoking at least once {@link #tryAcquireShared},
* returning on success. Otherwise the thread is queued, possibly
* repeatedly blocking and unblocking, invoking {@link
* #tryAcquireShared} until success.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquireShared} but is otherwise uninterpreted
* and can represent anything you like.
*
* 以共享模式获取。忽略中断（如果被中断，线程执行完成之后，doAcquireShared 方法返回中断标识）
*
* 首先至少调用一次 tryAcquireShared()，成功后返回。否则线程会排队，可能会反复阻塞和解阻塞，调用 tryAcquireShared()直到成功。
*/
public final void acquireShared(int arg) {
/**
* 返回值 < 0 ：失败
* 返回值 = 0 ：获取成功，但后续的共享模式获取无法成功
* 返回值 > 0 ：获取成功，并且后续的共享模式的获取也可能成功
*/
if (tryAcquireShared(arg) < 0)
doAcquireShared(arg);
}
/**
* Acquires in shared mode, aborting if interrupted. Implemented
* by first checking interrupt status, then invoking at least once
* {@link #tryAcquireShared}, returning on success. Otherwise the
* thread is queued, possibly repeatedly blocking and unblocking,
* invoking {@link #tryAcquireShared} until success or the thread
* is interrupted.
* @param arg the acquire argument.
* This value is conveyed to {@link #tryAcquireShared} but is
* otherwise uninterpreted and can represent anything
* you like.
* @throws InterruptedException if the current thread is interrupted
*
* 以共享模式可中断的获取，如果中断将中止。（如果获取过程中被中断，则会抛出 InterruptedException 异常）
*
* 首先检查中断状态，然后至少调用一次 tryAcquireShared()，成功后返回。
* 否则，线程会排队，可能会反复阻塞和解阻塞，调用 tryacquishared()，直到成功或线程中断。
*/
public final void acquireSharedInterruptibly(int arg) throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (tryAcquireShared(arg) < 0)
doAcquireSharedInterruptibly(arg);
}
/**
* Attempts to acquire in shared mode, aborting if interrupted, and
* failing if the given timeout elapses. Implemented by first
* checking interrupt status, then invoking at least once {@link
* #tryAcquireShared}, returning on success. Otherwise, the
* thread is queued, possibly repeatedly blocking and unblocking,
* invoking {@link #tryAcquireShared} until success or the thread
* is interrupted or the timeout elapses.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquireShared} but is otherwise uninterpreted
* and can represent anything you like.
* @param nanosTimeout the maximum number of nanoseconds to wait
* @return {@code true} if acquired; {@code false} if timed out
* @throws InterruptedException if the current thread is interrupted
*
* 在共享模式下尝试获取，如果中断将中止，如果超时将获取失败。（如果被中断则终止，抛出 InterruptedException 异常）。
*
* 首先检查中断状态，然后至少调用一次 tryAcquireShared()，成功后返回。
* 否则，线程会排队，可能会反复阻塞和解阻塞，调用 tryAcquireShared()，直到成功，或者线程被中断，或者超时过去。
*/
public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
return tryAcquireShared(arg) >= 0 ||
doAcquireSharedNanos(arg, nanosTimeout);
}
/**
* Releases in shared mode. Implemented by unblocking one or more
* threads if {@link #tryReleaseShared} returns true.
*
* @param arg the release argument. This value is conveyed to
* {@link #tryReleaseShared} but is otherwise uninterpreted
* and can represent anything you like.
* @return the value returned from {@link #tryReleaseShared}
*
* 以共享模式释放。
*
* 通过在 tryReleaseShared() 返回true时解除一个或多个线程的阻塞来实现。
*/
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) { //判断释放是否成功
doReleaseShared();
return true;
}
return false;
}
//=================================== 队列检查方法 ==========================================
// Queue inspection methods
/**
* Queries whether any threads are waiting to acquire. Note that
* because cancellations due to interrupts and timeouts may occur
* at any time, a {@code true} return does not guarantee that any
* other thread will ever acquire.
*
* <p>In this implementation, this operation returns in
* constant time.
*
* @return {@code true} if there may be other threads waiting to acquire
*
* 查询是否有线程正在等待获取。
* 注意，由于中断和超时导致的取消可能随时发生，因此返回 true 并不保证任何其他线程都会获取。
*/
public final boolean hasQueuedThreads() {
return head != tail;
}
/**
* Queries whether any threads have ever contended to acquire this
* synchronizer; that is if an acquire method has ever blocked.
*
* <p>In this implementation, this operation returns in
* constant time.
*
* @return {@code true} if there has ever been contention
*
* 查询是否有线程争用过这个同步器；也就是说，如果一个获取方法曾经被阻止。
*/
public final boolean hasContended() {
return head != null;
}
/**
* Returns the first (longest-waiting) thread in the queue, or
* {@code null} if no threads are currently queued.
*
* <p>In this implementation, this operation normally returns in
* constant time, but may iterate upon contention if other threads are
* concurrently modifying the queue.
*
* @return the first (longest-waiting) thread in the queue, or {@code null} if no threads are currently queued
*
* 返回队列中第一个（等待时间最长的）线程，如果当前没有线程排队，则返回 null。
*/
public final Thread getFirstQueuedThread() {
// handle only fast path, else relay
return (head == tail) ? null : fullGetFirstQueuedThread();
}
/**
* Version of getFirstQueuedThread called when fastpath fails
* fastpath失败时调用的getFirstQueuedThread版本
*/
private Thread fullGetFirstQueuedThread() {
/*
* The first node is normally head.next. Try to get its
* thread field, ensuring consistent reads: If thread
* field is nulled out or s.prev is no longer head, then
* some other thread(s) concurrently performed setHead in
* between some of our reads. We try this twice before
* resorting to traversal.
*/
Node h, s;
Thread st;
if (((h = head) != null && (s = h.next) != null &&
s.prev == head && (st = s.thread) != null) ||
((h = head) != null && (s = h.next) != null &&
s.prev == head && (st = s.thread) != null))
return st;
/*
* Head's next field might not have been set yet, or may have
* been unset after setHead. So we must check to see if tail
* is actually first node. If not, we continue on, safely
* traversing from tail back to head to find first,
* guaranteeing termination.
*/
Node t = tail;
Thread firstThread = null;
while (t != null && t != head) {
Thread tt = t.thread;
if (tt != null)
firstThread = tt;
t = t.prev;
}
return firstThread;
}
/**
* Returns true if the given thread is currently queued.
*
* <p>This implementation traverses the queue to determine
* presence of the given thread.
*
* @param thread the thread
* @return {@code true} if the given thread is on the queue
* @throws NullPointerException if the thread is null
*
* 如果给定线程当前已排队，则返回true。
*/
public final boolean isQueued(Thread thread) {
if (thread == null)
throw new NullPointerException();
for (Node p = tail; p != null; p = p.prev)
if (p.thread == thread)
return true;
return false;
}
/**
* Returns {@code true} if the apparent first queued thread, if one
* exists, is waiting in exclusive mode. If this method returns
* {@code true}, and the current thread is attempting to acquire in
* shared mode (that is, this method is invoked from {@link
* #tryAcquireShared}) then it is guaranteed that the current thread
* is not the first queued thread. Used only as a heuristic in
* ReentrantReadWriteLock.
*
* 如果明显的第一个排队线程（如果存在）正在以独占模式等待，则返回 true。
* 如果此方法返回 true，并且当前线程正在尝试以共享模式获取（即，这个方法是从从 tryAcquireShared 调用的），
* 那么可以保证当前线程不是第一个排队线程。仅在 ReentrantReadWriteLock中用作启发式。
*/
final boolean apparentlyFirstQueuedIsExclusive() {
Node h, s;
return (h = head) != null &&
(s = h.next) != null &&
!s.isShared() &&
s.thread != null;
}
/**
* Queries whether any threads have been waiting to acquire longer
* than the current thread.
*
* <p>An invocation of this method is equivalent to (but may be
* more efficient than):
* <pre> {@code
* getFirstQueuedThread() != Thread.currentThread() &&
* hasQueuedThreads()}</pre>
*
* <p>Note that because cancellations due to interrupts and
* timeouts may occur at any time, a {@code true} return does not
* guarantee that some other thread will acquire before the current
* thread. Likewise, it is possible for another thread to win a
* race to enqueue after this method has returned {@code false},
* due to the queue being empty.
*
* <p>This method is designed to be used by a fair synchronizer to
* avoid <a href="AbstractQueuedSynchronizer#barging">barging</a>.
* Such a synchronizer's {@link #tryAcquire} method should return
* {@code false}, and its {@link #tryAcquireShared} method should
* return a negative value, if this method returns {@code true}
* (unless this is a reentrant acquire). For example, the {@code
* tryAcquire} method for a fair, reentrant, exclusive mode
* synchronizer might look like this:
*
* <pre> {@code
* protected boolean tryAcquire(int arg) {
* if (isHeldExclusively()) {
* // A reentrant acquire; increment hold count
* return true;
* } else if (hasQueuedPredecessors()) {
* return false;
* } else {
* // try to acquire normally
* }
* }}</pre>
*
* @return {@code true} if there is a queued thread preceding the
* current thread, and {@code false} if the current thread
* is at the head of the queue or the queue is empty
* @since 1.7
*
* 查询是否有线程等待获取的时间比当前线程长。
*/
public final boolean hasQueuedPredecessors() {
// The correctness of this depends on head being initialized
// before tail and on head.next being accurate if the current
// thread is first in queue.
Node t = tail; // Read fields in reverse initialization order
Node h = head;
Node s;
return h != t &&
((s = h.next) == null || s.thread != Thread.currentThread());
}
//========================================= 仪器仪表和监测方法 ====================================
// Instrumentation and monitoring methods
/**
* Returns an estimate of the number of threads waiting to
* acquire. The value is only an estimate because the number of
* threads may change dynamically while this method traverses
* internal data structures. This method is designed for use in
* monitoring system state, not for synchronization
* control.
*
* @return the estimated number of threads waiting to acquire
*
* 返回等待获取的线程数的估计值。该值只是一个估计值，因为当该方法遍历内部数据结构时，线程数可能会动态变化。该方法设计用于监控系统状态，而非同步控制。
*/
public final int getQueueLength() {
int n = 0;
for (Node p = tail; p != null; p = p.prev) {
if (p.thread != null)
++n;
}
return n;
}
/**
* Returns a collection containing threads that may be waiting to
* acquire. Because the actual set of threads may change
* dynamically while constructing this result, the returned
* collection is only a best-effort estimate. The elements of the
* returned collection are in no particular order. This method is
* designed to facilitate construction of subclasses that provide
* more extensive monitoring facilities.
*
* @return the collection of threads
*
* 返回包含可能正在等待获取的线程的集合。由于在构造此结果时，实际的线程集可能会动态变化，因此返回的集合只是一个尽力而为的估计。
* 返回集合的元素没有特定的顺序。此方法旨在帮助构建提供更广泛监控设施的子类。
*/
public final Collection<Thread> getQueuedThreads() {
ArrayList<Thread> list = new ArrayList<Thread>();
for (Node p = tail; p != null; p = p.prev) {
Thread t = p.thread;
if (t != null)
list.add(t);
}
return list;
}
/**
* Returns a collection containing threads that may be waiting to
* acquire in exclusive mode. This has the same properties
* as {@link #getQueuedThreads} except that it only returns
* those threads waiting due to an exclusive acquire.
*
* @return the collection of threads
*
* 返回包含可能正在等待以独占模式获取的线程的集合。这个方法与 getQueuedThreads() 具有相同的属性，不同之处在于它只返回那些由于独占获取而等待的线程。
*/
public final Collection<Thread> getExclusiveQueuedThreads() {
ArrayList<Thread> list = new ArrayList<Thread>();
for (Node p = tail; p != null; p = p.prev) {
if (!p.isShared()) {
Thread t = p.thread;
if (t != null)
list.add(t);
}
}
return list;
}
/**
* Returns a collection containing threads that may be waiting to
* acquire in shared mode. This has the same properties
* as {@link #getQueuedThreads} except that it only returns
* those threads waiting due to a shared acquire.
*
* @return the collection of threads
*
* 返回一个集合，其中包含可能等待以共享模式获取的线程。这个方法与 getQueuedThreads() 具有相同的属性，不同之处在于它只返回那些因为共享获取而等待的线程。
*/
public final Collection<Thread> getSharedQueuedThreads() {
ArrayList<Thread> list = new ArrayList<Thread>();
for (Node p = tail; p != null; p = p.prev) {
if (p.isShared()) {
Thread t = p.thread;
if (t != null)
list.add(t);
}
}
return list;
}
/**
* Returns a string identifying this synchronizer, as well as its state.
* The state, in brackets, includes the String {@code "State ="}
* followed by the current value of {@link #getState}, and either
* {@code "nonempty"} or {@code "empty"} depending on whether the
* queue is empty.
*
* @return a string identifying this synchronizer, as well as its state
* 返回标识此同步器及其状态的字符串.后跟 getState() 的当前值，以及队列是否空。
*/
public String toString() {
int s = getState();
String q = hasQueuedThreads() ? "non" : "";
return super.toString() +
"[State = " + s + ", " + q + "empty queue]";
}
// =============================== 条件的内部支持方法 ============================
// Internal support methods for Conditions
/**
* Returns true if a node, always one that was initially placed on
* a condition queue, is now waiting to reacquire on sync queue.
* @param node the node
* @return true if is reacquiring
*
* 如果节点(总是最初放置在条件队列中的节点)现在正在同步队列上等待重新获取，则返回true。
*/
final boolean isOnSyncQueue(Node node) {
// 当前节点处于条件等待状态
// 或 当前节点的前置节点为空
// 返回false，当前节点不在同步等待队列中，而是在 条件等待队列中
if (node.waitStatus == Node.CONDITION || node.prev == null)
return false;
// If has successor, it must be on queue
// 前提条件：节点不是条件等待节点、节点的前置节点非空
// 判断：如果节点有后继节点，则一定在队列中（一定不在条件等待队列）
if (node.next != null)
return true;
/*
* node.prev can be non-null, but not yet on queue because
* the CAS to place it on queue can fail. So we have to
* traverse from tail to make sure it actually made it. It
* will always be near the tail in calls to this method, and
* unless the CAS failed (which is unlikely), it will be
* there, so we hardly ever traverse much.
*/
/**
*
* 前置条件： 节点不是条件等待节点、节点的前置节点非空、节点的后置节点为空
*
* node.prev 不为空 并且 不在同步队列中，因为将其放入队列的CAS可能会失败（enq()的时候）。 所以我们必须从尾部开始遍历以确保它确实成功了。
*
* 在对这个方法的调用中，它总是在尾部附近，除非CAS失败(这是不太可能的)，否则它会在那里，所以我们几乎不会遍历太多。
*
*/
return findNodeFromTail(node);
}
/**
* Returns true if node is on sync queue by searching backwards from tail.
* Called only when needed by isOnSyncQueue.
* @return true if present
*
* 通过从尾部向后搜索，如果节点位于同步队列中，则返回true。仅在 isOnSyncQueue需要时调用。
*/
private boolean findNodeFromTail(Node node) {
Node t = tail;
for (;;) {
if (t == node)
return true;
if (t == null)
return false;
t = t.prev;
}
}
/**
* Transfers a node from a condition queue onto sync queue.
* Returns true if successful.
* @param node the node
* @return true if successfully transferred (else the node was
* cancelled before signal)
*
* 将节点从条件队列转移到同步队列。如果成功返回true。
*/
final boolean transferForSignal(Node node) {
/*
* If cannot change waitStatus, the node has been cancelled.
* 将节点的状态值 由 CONDITION 改为 0
*/
if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
return false;
/*
* Splice onto queue and try to set waitStatus of predecessor to
* indicate that thread is (probably) waiting. If cancelled or
* attempt to set waitStatus fails, wake up to resync (in which
* case the waitStatus can be transiently and harmlessly wrong).
*/
//将该节点添加到 同步队列中去
Node p = enq(node);
int ws = p.waitStatus;
// ws > 0 如果该节点已取消 ， 则从 await 方法中唤醒
// ws <= 0 ,尝试修改状态为 SIGNAL， 则从 await 方法中唤醒
if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
LockSupport.unpark(node.thread);
return true;
}
/**
* Transfers node, if necessary, to sync queue after a cancelled wait.
* Returns true if thread was cancelled before being signalled.
*
* @param node the node
* @return true if cancelled before the node was signalled
*
* 必要时，在取消等待后将节点转移到同步队列。如果线程在发出信号之前被取消，则返回true
*/
final boolean transferAfterCancelledWait(Node node) {
// 设置节点状态 从条件等待状态，设置为普通状态
if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
enq(node);
return true;
}
/*
* If we lost out to a signal(), then we can't proceed
* until it finishes its enq(). Cancelling during an
* incomplete transfer is both rare and transient, so just
* spin.
*/
while (!isOnSyncQueue(node))
Thread.yield();
return false;
}
/**
* Invokes release with current state value; returns saved state.
* Cancels node and throws exception on failure.
* @param node the condition node for this wait
* @return previous sync state
*
* 使用当前状态值调用 release; 返回保存的状态。 取消node并在失败时抛出异常。
*/
final int fullyRelease(Node node) {
boolean failed = true;
try {
// 注意：是完全释放锁（传的值是 savedState ）
int savedState = getState();
if (release(savedState)) {
failed = false;
return savedState;
} else {
throw new IllegalMonitorStateException();
}
} finally {
if (failed)
node.waitStatus = Node.CANCELLED;
}
}
//================================================= 条件仪表方法 =========================================
// Instrumentation methods for conditions
/**
* Queries whether the given ConditionObject uses this synchronizer as its lock.
*
* @param condition the condition
* @return {@code true} if owned
* @throws NullPointerException if the condition is null
*
* 查询给定的ConditionObject是否使用此同步器作为其锁。
*/
public final boolean owns(ConditionObject condition) {
return condition.isOwnedBy(this);
}
/**
* Queries whether any threads are waiting on the given condition
* associated with this synchronizer. Note that because timeouts
* and interrupts may occur at any time, a {@code true} return
* does not guarantee that a future {@code signal} will awaken
* any threads. This method is designed primarily for use in
* monitoring of the system state.
*
* @param condition the condition
* @return {@code true} if there are any waiting threads
* @throws IllegalMonitorStateException if exclusive synchronization
* is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this synchronizer
* @throws NullPointerException if the condition is null
*
* 查询是否有任何线程正在等待与此同步器关联的给定条件。
* 注意，由于超时和中断可能随时发生，因此 true返回不能保证将来的信号将唤醒任何线程。
* 该方法主要用于监测系统状态。
*/
public final boolean hasWaiters(ConditionObject condition) {
if (!owns(condition))
throw new IllegalArgumentException("Not owner");
return condition.hasWaiters();
}
/**
* Returns an estimate of the number of threads waiting on the
* given condition associated with this synchronizer. Note that
* because timeouts and interrupts may occur at any time, the
* estimate serves only as an upper bound on the actual number of
* waiters. This method is designed for use in monitoring of the
* system state, not for synchronization control.
*
* @param condition the condition
* @return the estimated number of waiting threads
* @throws IllegalMonitorStateException if exclusive synchronization
* is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this synchronizer
* @throws NullPointerException if the condition is null
*
* 返回与此同步器关联的给定条件下等待的线程数的估计值。
* 请注意，由于超时和中断可能随时发生，因此估计值仅作为实际服务员数量的上限。
* 方法设计用于监控系统状态，而不是用于同步控制。
*/
public final int getWaitQueueLength(ConditionObject condition) {
if (!owns(condition))
throw new IllegalArgumentException("Not owner");
return condition.getWaitQueueLength();
}
/**
* Returns a collection containing those threads that may be
* waiting on the given condition associated with this
* synchronizer. Because the actual set of threads may change
* dynamically while constructing this result, the returned
* collection is only a best-effort estimate. The elements of the
* returned collection are in no particular order.
*
* @param condition the condition
* @return the collection of threads
* @throws IllegalMonitorStateException if exclusive synchronization
* is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this synchronizer
* @throws NullPointerException if the condition is null
*
* 返回一个集合，其中包含可能正在等待与此同步器关联的给定条件的线程。
* 由于在构造此结果时，实际的线程集可能会动态变化，因此返回的集合只是一个尽力而为的估计。
* 返回集合的元素没有特定的顺序。
*/
public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
if (!owns(condition))
throw new IllegalArgumentException("Not owner");
return condition.getWaitingThreads();
}
/**
* Condition implementation for a {@link AbstractQueuedSynchronizer}
* serving as the basis of a {@link Lock} implementation.
*
* <p>Method documentation for this class describes mechanics,
* not behavioral specifications from the point of view of Lock
* and Condition users. Exported versions of this class will in
* general need to be accompanied by documentation describing
* condition semantics that rely on those of the associated
* {@code AbstractQueuedSynchronizer}.
*
* <p>This class is Serializable, but all fields are transient,
* so deserialized conditions have no waiters.
*/
public class ConditionObject implements Condition, java.io.Serializable {
private static final long serialVersionUID = 1173984872572414699L;
/**
* First node of condition queue.
* 条件队列的第一个节点
*/
private transient Node firstWaiter;
/**
* Last node of condition queue.
* 条件队列的最后一个节点
*/
private transient Node lastWaiter;
/**
* Creates a new {@code ConditionObject} instance.
*/
public ConditionObject() { }
// ================== 内部方法 ==================
// Internal methods
/**
* Adds a new waiter to wait queue.
* @return its new wait node
*
* 向等待队列中添加一个新的等待节点（当前线程）
*/
private Node addConditionWaiter() {
Node t = lastWaiter;
// If lastWaiter is cancelled, clean out.
// 如果最后一个等待节点被取消，则清空队列
if (t != null && t.waitStatus != Node.CONDITION) {
unlinkCancelledWaiters();
t = lastWaiter;
}
// 封装该线程
Node node = new Node(Thread.currentThread(), Node.CONDITION);
if (t == null)
firstWaiter = node;
else
t.nextWaiter = node;
lastWaiter = node;
return node;
}
/**
* Removes and transfers nodes until hit non-cancelled one or
* null. Split out from signal in part to encourage compilers
* to inline the case of no waiters.
* @param first (non-null) the first node on condition queue
*
* 唤醒条件队列中等待的节点（unpark），后续添加到同步等待队尾部
*/
private void doSignal(Node first) {
do {
if ( (firstWaiter = first.nextWaiter) == null)
lastWaiter = null;
first.nextWaiter = null;
} while (!transferForSignal(first) &&
(first = firstWaiter) != null);
}
/**
* Removes and transfers all nodes.
* @param first (non-null) the first node on condition queue
*
* 唤醒所有等待线程（unpark）
*/
private void doSignalAll(Node first) {
lastWaiter = firstWaiter = null;
do {
Node next = first.nextWaiter;
first.nextWaiter = null;
transferForSignal(first);
first = next;
} while (first != null);
}
/**
* Unlinks cancelled waiter nodes from condition queue.
* Called only while holding lock. This is called when
* cancellation occurred during condition wait, and upon
* insertion of a new waiter when lastWaiter is seen to have
* been cancelled. This method is needed to avoid garbage
* retention in the absence of signals. So even though it may
* require a full traversal, it comes into play only when
* timeouts or cancellations occur in the absence of
* signals. It traverses all nodes rather than stopping at a
* particular target to unlink all pointers to garbage nodes
* without requiring many re-traversals during cancellation
* storms.
*
* 从条件队列中取消已取消的等待节点，仅在持有锁时调用。
*/
private void unlinkCancelledWaiters() {
Node t = firstWaiter;
Node trail = null;
while (t != null) {
Node next = t.nextWaiter;
if (t.waitStatus != Node.CONDITION) {
t.nextWaiter = null;
if (trail == null)
firstWaiter = next;
else
trail.nextWaiter = next;
if (next == null)
lastWaiter = trail;
}
else
trail = t;
t = next;
}
}
// ================== 公共方法 ==================
// public methods
/**
* Moves the longest-waiting thread, if one exists, from the
* wait queue for this condition to the wait queue for the
* owning lock.
*
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*
* 唤醒最长条件等待的线程（也就是队列头的线程）
*/
public final void signal() {
// 判断是否 独占锁
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
Node first = firstWaiter;
if (first != null)
doSignal(first);
}
/**
* Moves all threads from the wait queue for this condition to
* the wait queue for the owning lock.
*
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*
* 唤醒所有等待线程
*/
public final void signalAll() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
Node first = firstWaiter;
if (first != null)
doSignalAll(first);
}
/**
* Implements uninterruptible condition wait.
* <ol>
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li> Block until signalled.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* </ol>
*
* 导致当前线程等待（park），直到收到信号
*/
public final void awaitUninterruptibly() {
// 添加到条件等待队列
Node node = addConditionWaiter();
// 完全释放独占锁
int savedState = fullyRelease(node);
//标记中断状态
boolean interrupted = false;
//判断节点是否在同步队列中
while (!isOnSyncQueue(node)) {
LockSupport.park(this);
if (Thread.interrupted())
interrupted = true;
}
// 以独占不中断模式获取已经在队列中的线程
// 判断中断状态，重新中断线程
if (acquireQueued(node, savedState) || interrupted)
selfInterrupt();
}
//================== 对于可中断的等待 ==================
/*
* For interruptible waits, we need to track whether to throw
* InterruptedException, if interrupted while blocked on
* condition, versus reinterrupt current thread, if
* interrupted while blocked waiting to re-acquire.
*/
/**
* Mode meaning to reinterrupt on exit from wait
*
* 该模式：在退出等待时重新中断
*/
private static final int REINTERRUPT = 1;
/**
* Mode meaning to throw InterruptedException on exit from wait
*
* 该模式：在退出等待时抛出 InterruptedException
*/
private static final int THROW_IE = -1;
/**
* Checks for interrupt, returning THROW_IE if interrupted
* before signalled, REINTERRUPT if after signalled, or
* 0 if not interrupted.
*
* 检查等待时的中断
*/
private int checkInterruptWhileWaiting(Node node) {
return Thread.interrupted() ?
(transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
0;
}
/**
* Throws InterruptedException, reinterrupts current thread, or
* does nothing, depending on mode.
*
* [根据中断模式，执行具体的操作]
* 1.THROW_IE 抛出异常
* 2.REINTERRUPT 重新中断
* 3.什么也不做
*
*/
private void reportInterruptAfterWait(int interruptMode)
throws InterruptedException {
if (interruptMode == THROW_IE)
throw new InterruptedException();
else if (interruptMode == REINTERRUPT)
selfInterrupt();
}
/**
* Implements interruptible condition wait.
* <ol>
* <li> If current thread is interrupted, throw InterruptedException.
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li> Block until signalled or interrupted.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* <li> If interrupted while blocked in step 4, throw InterruptedException.
* </ol>
*
* 使当前线程等待（park），直到收到信号或被中断
* 1.获取锁之前先检查中断状态，如果被中断，则抛出异常，结束运行
* 2.在等待队列中被中断，唤醒获取锁之后被中断
*/
public final void await() throws InterruptedException {
// 如果当前线程被中断，则抛出InterruptedException。
if (Thread.interrupted())
throw new InterruptedException();
//将该线程封装添加到条件等待队列
Node node = addConditionWaiter();
// 完全释放独占锁
int savedState = fullyRelease(node);
// 中断模式
int interruptMode = 0;
//判断是否在同步队列中，如果不在，继续执行以下代码
while (!isOnSyncQueue(node)) {
LockSupport.park(this);
// 线程被条件队列（transferForSignal 方法）唤醒之后，继续向下运行
// 检查是否被中断
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
}
// ========== 尝试在同步等待队列中被唤醒 ，执行完毕之后，继续向下执行 ==========
// 以独占不中断模式获取已经在队列中的线程
// 判断线程是否需要重新中断
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
// clean up if cancelled 取消时清理
if (node.nextWaiter != null)
unlinkCancelledWaiters();
//响应中断
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
}
/**
* Implements timed condition wait.
* <ol>
* <li> If current thread is interrupted, throw InterruptedException.
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li> Block until signalled, interrupted, or timed out.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* <li> If interrupted while blocked in step 4, throw InterruptedException.
* </ol>
*
* 导致当前线程等待，直到发出信号或中断，或指定的等待时间结束。
*/
public final long awaitNanos(long nanosTimeout)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
final long deadline = System.nanoTime() + nanosTimeout;
int interruptMode = 0;
while (!isOnSyncQueue(node)) {
if (nanosTimeout <= 0L) {
transferAfterCancelledWait(node);
break;
}
if (nanosTimeout >= spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
nanosTimeout = deadline - System.nanoTime();
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null)
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
return deadline - System.nanoTime();
}
/**
* Implements absolute timed condition wait.
* <ol>
* <li> If current thread is interrupted, throw InterruptedException.
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li> Block until signalled, interrupted, or timed out.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* <li> If interrupted while blocked in step 4, throw InterruptedException.
* <li> If timed out while blocked in step 4, return false, else true.
* </ol>
*
* 使当前线程等待，直到收到信号或中断，或超过指定的截止日期。
*/
public final boolean awaitUntil(Date deadline)
throws InterruptedException {
long abstime = deadline.getTime();
if (Thread.interrupted())
throw new InterruptedException();
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
boolean timedout = false;
int interruptMode = 0;
while (!isOnSyncQueue(node)) {
if (System.currentTimeMillis() > abstime) {
timedout = transferAfterCancelledWait(node);
break;
}
LockSupport.parkUntil(this, abstime);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null)
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
return !timedout;
}
/**
* Implements timed condition wait.
* <ol>
* <li> If current thread is interrupted, throw InterruptedException.
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li> Block until signalled, interrupted, or timed out.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* <li> If interrupted while blocked in step 4, throw InterruptedException.
* <li> If timed out while blocked in step 4, return false, else true.
* </ol>
*
* 使当前线程等待，直到发出信号或中断，或经过指定的等待时间
*/
public final boolean await(long time, TimeUnit unit)
throws InterruptedException {
long nanosTimeout = unit.toNanos(time);
if (Thread.interrupted())
throw new InterruptedException();
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
final long deadline = System.nanoTime() + nanosTimeout;
boolean timedout = false;
int interruptMode = 0;
while (!isOnSyncQueue(node)) {
if (nanosTimeout <= 0L) {
timedout = transferAfterCancelledWait(node);
break;
}
if (nanosTimeout >= spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
nanosTimeout = deadline - System.nanoTime();
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null)
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
return !timedout;
}
// ================== support for instrumentation ==================
// support for instrumentation
/**
* Returns true if this condition was created by the given
* synchronization object.
*
* @return {@code true} if owned
*
* 如果此条件是由给定的同步对象创建的，则返回true。
*/
final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
return sync == AbstractQueuedSynchronizer.this;
}
/**
* Queries whether any threads are waiting on this condition.
* Implements {@link AbstractQueuedSynchronizer#hasWaiters(ConditionObject)}.
*
* @return {@code true} if there are any waiting threads
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*
* 查询是否有线程在此条件下等待。
*/
protected final boolean hasWaiters() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
if (w.waitStatus == Node.CONDITION)
return true;
}
return false;
}
/**
* Returns an estimate of the number of threads waiting on
* this condition.
* Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength(ConditionObject)}.
*
* @return the estimated number of waiting threads
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*
* 返回在此条件下等待的线程数
*/
protected final int getWaitQueueLength() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
int n = 0;
for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
if (w.waitStatus == Node.CONDITION)
++n;
}
return n;
}
/**
* Returns a collection containing those threads that may be
* waiting on this Condition.
* Implements {@link AbstractQueuedSynchronizer#getWaitingThreads(ConditionObject)}.
*
* @return the collection of threads
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*
* 返回正在等待该条件的线程的集合
*/
protected final Collection<Thread> getWaitingThreads() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
ArrayList<Thread> list = new ArrayList<Thread>();
for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
if (w.waitStatus == Node.CONDITION) {
Thread t = w.thread;
if (t != null)
list.add(t);
}
}
return list;
}
}
/**
* Setup to support compareAndSet. We need to natively implement
* this here: For the sake of permitting future enhancements, we
* cannot explicitly subclass AtomicInteger, which would be
* efficient and useful otherwise. So, as the lesser of evils, we
* natively implement using hotspot intrinsics API. And while we
* are at it, we do the same for other CASable fields (which could
* otherwise be done with atomic field updaters).
*/
// TODO：注释源码
// private static final Unsafe unsafe = Unsafe.getUnsafe();
// TODO：新增
private static final Unsafe unsafe;
private static final long stateOffset;
private static final long headOffset;
private static final long tailOffset;
private static final long waitStatusOffset;
private static final long nextOffset;
static {
try {
//TODO：新增 初始化 unsafe 基于反射获取Unsafe实例
Field f = Unsafe.class.getDeclaredField("theUnsafe");
f.setAccessible(true);
unsafe = (Unsafe) f.get(null);
stateOffset = unsafe.objectFieldOffset
(AbstractQueuedSynchronizer.class.getDeclaredField("state"));
headOffset = unsafe.objectFieldOffset
(AbstractQueuedSynchronizer.class.getDeclaredField("head"));
tailOffset = unsafe.objectFieldOffset
(AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
waitStatusOffset = unsafe.objectFieldOffset
(Node.class.getDeclaredField("waitStatus"));
nextOffset = unsafe.objectFieldOffset
(Node.class.getDeclaredField("next"));
} catch (Exception ex) { throw new Error(ex); }
}
/**
* CAS head field. Used only by enq.
*/
private final boolean compareAndSetHead(Node update) {
return unsafe.compareAndSwapObject(this, headOffset, null, update);
}
/**
* CAS tail field. Used only by enq.
*/
private final boolean compareAndSetTail(Node expect, Node update) {
return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
}
/**
* CAS waitStatus field of a node.
*/
private static final boolean compareAndSetWaitStatus(Node node, int expect, int update) {
return unsafe.compareAndSwapInt(node, waitStatusOffset, expect, update);
}
/**
* CAS next field of a node.
*/
private static final boolean compareAndSetNext(Node node, Node expect, Node update) {
return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
}
}
