异步任务二：FutureTask源码分析
- 参考文献
Unsafe方法详解
1.FutureTask简介
FutureTask实现的RunnableFuture接口继承了Runnable, Future接口。
FutureTask是可取消的异步计算，是Future的基本实现，提供了开始、取消任务以及查询其计算结果的方法——get()方法一直阻塞获取结果(接受中断信号)，get(time)超时则会抛异常。
除非任务使用runAndReset，否则一旦计算完成，任务就不能重置或者取消了。
FutureTask可以使用Callable或者(Runnable+result)作为构造参数——后者也会转换成Callable，因为他实现了Runnable接口，因此可以提交给线程池执行——会执行构造参数中的任务。
2.状态变迁
异步任务共有7中状态，分别是：new、completing、normal、cancelled、exceptional、interrupting、interrupted。
可能的状态变迁包括以下四种情况：
- 正常完成:
new->completing->normal; - 异常：
new->completing->exceptinal; - 取消:
new->cancelled; - 中断:
new->interrupting->interrupted。
run()方法执行过程中set方法执行完成—正常完成则NORMAL，抛异常则EXCEPTIONAL;
run()方法中set执行之前如果调用cancel(boolean)方法并修改了任务状态，则最终状态根据参数变成CANCELLED或者INTERRUPTED。详见下节代码实现：
[注] 等待线程状态变迁
线程可能被挂起也可能被释放(停止挂起):- 调用
get(timed)方法获取任务结果时，线程被挂起； - 调用
cancel()或者run()方法执行结束之后调用finishCompletion()时，所有等待结果的线程被释放。
3.主要方法的实现
FutureTask主要有三个特点:1.只执行一次；2.取消；3.异步获取结果。
run()方法
- 为了保证任务只执行一次，设置准入条件
只有new状态并且成功将runner变量设置为当前线程才可执行任务； - 可能的状态改变，通过准入条件后：
- 如果正常结束，则通过setXX方法设置返回值-可能为异常，最终状态为COMPLETED或EXCEPTIONAL；
- 如果在setXX执行之前某线程执行了
cancel(boolean)，则setXX方法内容不能成功执行，并且handlePossibleCancellationInterrupt中负责将状态修改为INTERRUPTED。
综上：如果正常结束则setXX中将状态修改为NORMAL或者INTERRUPTED;如果在setXX方法修改状态之前调用cancel方法修改了状态，则最终状态根据cancel参数相应的变成CANCELLED或者INTERRUPTED。
public void run() {
/**
* FutureTask保证任务只执行一次，因为：
* 1. 如果任务不是新建状态(已经开始执行)
* 或者2. 是新建状态但是设置执行callable的工作线程为当前线程失败，则返回
* ——fixme:在多个线程获取任务并执行时，可能两个任务并发执行到这里，则此代码保证只有一个线程成功进行下去
*/
if (state != NEW || !UNSAFE.compareAndSwapObject(this, runnerOffset, null, Thread.currentThread()))
return;
//以上，state是NEW并且成功将runner设置为当前线程才能继续执行
try {
Callable<V> c = callable;
//state可能在其他地方被修改，比如取消、中断
//setXX方法改变状态：completing->completed 或者 interrupting -> interrupted
if (c != null && state == NEW) {
V result;
boolean ran;
//fixme 如果任务执行过程中抛异常，则调用setException(ex)方法，否则调用set(res)方法设置结果
try {
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result);
}
} finally {
// runner must be non-null until state is settled to
// prevent concurrent calls to run()
//在确定状态之前，runnable必须为null，防止并发调用run()
runner = null;
// state must be re-read after nulling runner to prevent
// leaked interrupts
int s = state;
if (s >= INTERRUPTING)//如果状态是INTERRUPTING和INTERRUPTED。如果是CANCELLED则不用管
//处理可能的取消中断
// fixme run方法中在set结果之前并没有修改状态，如果此时被cancel，则set方法不能成功执行，此处需要处理cancel方法的影响：等待程序执行到INTERRUPTED处
//详见cancel(boolean)源码
handlePossibleCancellationInterrupt(s);
}
}
cancel(boolean mayInterruptIfRunning)方法：执行此方法最终状态可能是CANCELLED或者INTERRUPTED(如果当前状态是NEW且此通过CAS成功的更新了状态，则继续执行返回true)- 如果是不可中断的取消，则状态置为
CANCELLED； - 如果是可中断的取消，则状态置为
INTERRUPTING,并在执行t.interrupt()后，将状态更新为INTERRUPTED。
- 如果是不可中断的取消，则状态置为
public boolean cancel(boolean mayInterruptIfRunning) {
/**
* 如果当前状态是新建、而且使用CAS操作改变了新建状态(因为其他线程也可能在修改):
* 如果可中断则修改为中断状态，否则修改为CANCELLED状态
*/
if (!(state == NEW &&
UNSAFE.compareAndSwapInt(this, stateOffset, NEW,
mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
return false;
/**
* 如果可以中断，则先中断线程，在修改当前任务对象状态值。最后 移除并signal所有等待的线程，调用done()，置空callable
*/
try { // in case call to interrupt throws exception
if (mayInterruptIfRunning) {
try {
//执行具体业务逻辑（构造参数值）所在的线程。fixme：通过执行线程对象的interrupt()方法中断线程
Thread t = runner;
if (t != null)
t.interrupt();
} finally { // final state
//避免从排序的更新值
//putOrderedObject，Unsafe. putOrderedInt，Unsafe. putOrderedLong这三个方法，JDK会在执行这三个方法时插入StoreStore内存屏障，避免发生写操作重排序。
UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
}
}
} finally {
finishCompletion();
}
return true;
}
get()方法
/**
* 阻塞一定的时间获取结果，超时则抛 TimeoutException
* @throws CancellationException {@inheritDoc}
*/
public V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
if (unit == null)
throw new NullPointerException();
int s = state;
if (s <= COMPLETING &&
(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
throw new TimeoutException();
return report(s);
}
/**
* fixme: 被get方法调用，因此方法中的线程都是等待获取结果的线程
*
* 等待结束或者中断\超时时抛异常
*/
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
//如果调用函数get设置了等待时间，则获取需要目标时间，否则目标时间是0L
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
boolean queued = false;
for (;;) {
//get()获取结果的线程被中断_get()方法调用方法
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
int s = state;
//如果当前状态不是新建和开始状态,返回当前状态
if (s > COMPLETING) {//NORMAL、EXCEPTIONAL、CANCELLED、INTERRUPTING、INTERRUPTED
if (q != null)
q.thread = null;
return s;
}
//如果当前状态是正完成COMPLETING，则让出cpu资源给同等甚至更高优先级的线程
else if (s == COMPLETING) // cannot time out yet
Thread.yield();
//fixme: 如果状态是NEW而且q为null，则将当前线程包装复制给q
else if (q == null)
q = new WaitNode();//fixme 线程节点等于当前线程
// 如果任务状态为NEW而且q不为null，则：1.将waiters节点赋值给q.next，然后跟waiters节点比较是否相同(todo 肯定不同啊)，相同则将waiters复制给q
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this,waitersOffset,q.next = waiters,q);
//如果get方法设置了超时时间；//如果已经超时，则移除当前等待线程;否则关起当前线程；
else if (timed) {
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
LockSupport.parkNanos(this, nanos);
}
else//如果没有设置超时时间，则直接挂起当前线程
LockSupport.park(this);
}
}
4. Unsafe的基本操作
FutureTask和基本类型对应的原子类，比如AtomicInteger等都是通过sum.misc.Unsafe进行CAS操作，使用模式相同：
- 通过
sun.misc.Unsafe.getUnsafe()方法获取Unsafe对象； - 获取类中指定字段的与域对象,然后用其作为参数获取此字段在Java对象中相对对象起始位置的相对偏移量；
- 使用
Unsafe对象方法执行CAS或者防止指令重拍的一系列操作。示例如下：
public class UnsafeDemo {
private int status;
private static final Unsafe UNSAFE;
private static final long statusOffset;
static {
try {
UNSAFE = Unsafe.getUnsafe();
Class<?> clz = UnsafeDemo.class;
statusOffset = UNSAFE.objectFieldOffset(clz.getField("status"));
} catch (Exception e) {
throw new Error(e);
}
}
/**
* 使用CAS累加，并返回旧值
*/
public int casFunc() {
int old;
{
old = UNSAFE.getIntVolatile(this, statusOffset);
}
while (!UNSAFE.compareAndSwapInt(this, statusOffset, old, old + 1)) ;
return old;
}
/**
* getAndAddInt使用comparaAndSwapInt实现，三个参数含义分别是：
* 要改变的对象、对象偏移量、增加的值。操作是线程安全的
*/
public int addFunc() {
return UNSAFE.getAndAddInt(this, statusOffset, 1);
}
}
源码注释
package com.sankuai.meituan.waimai.algorithm.thrift;
/**
* <p>可取消的异步计算</p>
* A cancellable asynchronous computation.
* <p>这个类是Future的基本实现，提供了开始、取消、查询其计算结果的方法。</p>
* This class provides a base
* implementation of {@link Future}, with methods to start and cancel
* a computation, query to see if the computation is complete, and
* retrieve the result of the computation.
* <p>结果只有在执行结束后才能够获取：get方法将一直阻塞知道计算完成，get(time)则会超时后抛异常</p>
* The result can only be
* retrieved when the computation has completed; the {@code get}
* methods will block if the computation has not yet completed.
* <p>一旦计算完成，任务就不能重制或者取消了，除非任务使用runAndReset方法调用</p>
* Once
* the computation has completed, the computation cannot be restarted
* or cancelled (unless the computation is invoked using
* {@link #runAndReset}).
*
* <p>FutureTask可以用来包装Callable或者Runnable，因为他实现了Runnable接口，也可以提交给线程池执行</p>
* <p>A {@code FutureTask} can be used to wrap a {@link Callable} or
* {@link Runnable} object. Because {@code FutureTask} implements
* {@code Runnable}, a {@code FutureTask} can be submitted to an
* {@link Executor} for execution.
*
*
* <p>In addition to serving as a standalone class, this class provides
* {@code protected} functionality that may be useful when creating
* customized(自定义) task classes.
*
* @since 1.5
* @author Doug Lea
* @param <V> The result type returned by this FutureTask's {@code get} methods
*/
public class FutureTask<V> implements RunnableFuture<V> {
/*
* Revision notes: This differs from previous versions of this
* class that relied on AbstractQueuedSynchronizer, mainly to
* avoid surprising users about retaining interrupt status during
* cancellation races. Sync control in the current design relies
* on a "state" field updated via CAS to track completion, along
* with a simple Treiber stack to hold waiting threads.
*
* Style note: As usual, we bypass overhead of using
* AtomicXFieldUpdaters and instead directly use Unsafe intrinsics.
*/
/**
* The run state of this task, initially NEW. The run state
* transitions to a terminal state only in methods set,
* setException, and cancel. During completion(完成), state may take on
* transient短暂的 values of COMPLETING (while outcome is being set) or
* INTERRUPTING (only while interrupting the runner to satisfy a
* cancel(true)). Transitions from these intermediate(中间状态) to final
* states use cheaper ordered/lazy writes because values are unique
* and cannot be further modified.
*
* Possible state transitions(转变):
* NEW -> COMPLETING -> NORMAL
* NEW -> COMPLETING -> EXCEPTIONAL
* NEW -> CANCELLED
* NEW -> INTERRUPTING -> INTERRUPTED
*/
/**
* 以下变量记录此类对象任务的运行状态，初始化为NEW 0：
* 1. 执行以下方法时状态转变到最终态：set-COMPLETING,setException-EXCEPTIONAL和cancle-CANCELLED;
* 2. 在计算过程中，状态可能短暂的变成COMPLETING()、INTERRUPTING();
* 3. 状态转变到最终状态时使用的是低消耗的 ordered/lazy写,因为最终态不会在被写；
* 4. 可能的转变：
* 1）新建-》完成中-》正常结束 NORMAL；
* 2）新建-》完成中-》异常；
* 3）新建-》取消；
* 3）新建-》中断中-》已中断。
*/
private volatile int state;
private static final int NEW = 0;
private static final int COMPLETING = 1;
private static final int NORMAL = 2;//正常结束
private static final int EXCEPTIONAL = 3;//异常
private static final int CANCELLED = 4;
private static final int INTERRUPTING = 5;
private static final int INTERRUPTED = 6;
/** The underlying callable; nulled out after running */
//具体执行的任务，运行之后会置空
private Callable<V> callable;
/** The thread running the callable; CASed during run() */
/**运行构造参数中callable的线程：具体业务逻辑所在。在run()方法中通过CAS操作写入**/
private volatile Thread runner;
/** The result to return or exception to throw from get() */
/**get()方法返回的结果或者抛出的异常**/
private Object outcome; // non-volatile, protected by state reads/writes
/** Treiber stack of waiting threads */
private volatile WaitNode waiters;
/**
* 使用Unsafe操作 status、runner和waiters（private volatile WaitNode waiters）；
*/
private static final sun.misc.Unsafe UNSAFE;
private static final long stateOffset;
private static final long runnerOffset;
private static final long waitersOffset;
static {
try {
UNSAFE = sun.misc.Unsafe.getUnsafe();
//直接使用 Class 效果基本一致，但是这样写更加规范，在某些类型转换时可以避免不必要的 unchecked 错误
Class<?> k = FutureTask.class;
/**
* 1. getDeclaredField(filedName)：获取类中指定字段的域对象；
* 2. objectFieldOffset()方法用于获取某个字段相对Java对象的“起始地址”的偏移量。
*/
stateOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("state"));
runnerOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("runner"));
waitersOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("waiters"));
} catch (Exception e) {
throw new Error(e);
}
}
/**
* Simple linked list nodes to record waiting threads in a Treiber（齿轮）stack.
* <p>链表节点是 "等待线程"</p>
*/
static final class WaitNode {
volatile Thread thread;
volatile WaitNode next;
WaitNode() { thread = Thread.currentThread(); }
}
/**
* Creates a {@code FutureTask} that will, upon running, execute the
* given {@code Callable}.
*
* @param callable the callable task
* @throws NullPointerException if the callable is null
*/
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
/**
* Creates a {@code FutureTask} that will, upon running, execute the
* given {@code Runnable}, and arrange that {@code get} will return the
* given result on successful completion.
*
* @param runnable the runnable task
* @param result the result to return on successful completion. If
* you don't need a particular result, consider using
* constructions of the form:
* {@code Future<?> f = new FutureTask<Void>(runnable, null)}
* @throws NullPointerException if the runnable is null
*/
public FutureTask(Runnable runnable, V result) {
//Executors.callable(Runnable,Object)：构造一个执行runnable任务并且返回值是指定值的任务；
this.callable = Executors.callable(runnable, result);
this.state = NEW; // ensure visibility of callable
}
public void run() {
/**
* FutureTask保证任务只执行一次，因为：
* 1. 如果任务不是新建状态(已经开始执行)
* 或者2. 是新建状态但是设置执行callable的工作线程为当前线程失败，则返回
* ——fixme:在多个线程获取任务并执行时，可能两个任务并发执行到这里，则此代码保证只有一个线程成功进行下去
*/
if (state != NEW || !UNSAFE.compareAndSwapObject(this, runnerOffset, null, Thread.currentThread()))
return;
//以上，state是NEW并且成功将runner设置为当前线程才能继续执行
try {
Callable<V> c = callable;
//state可能在其他地方被修改，比如取消、中断
//setXX方法改变状态：completing->completed 或者 interrupting -> interrupted
if (c != null && state == NEW) {
V result;
boolean ran;
//fixme 如果任务执行过程中抛异常，则调用setException(ex)方法，否则调用set(res)方法设置结果
try {
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result);
}
} finally {
// runner must be non-null until state is settled to
// prevent concurrent calls to run()
//在确定状态之前，runnable必须为null，防止并发调用run()
runner = null;
// state must be re-read after nulling runner to prevent
// leaked interrupts
int s = state;
if (s >= INTERRUPTING)//如果状态是INTERRUPTING和INTERRUPTED。如果是CANCELLED则不用管
//处理可能的取消中断
// fixme run方法中在set结果之前并没有修改状态，如果此时被cancel，则set方法不能成功执行，此处需要处理cancel方法的影响：等待程序执行到INTERRUPTED处
//详见cancel(boolean)源码
handlePossibleCancellationInterrupt(s);
}
}
/**
* Executes the computation without setting its result, and then
* resets this future to initial state, failing to do so if the
* computation encounters an exception or is cancelled. This is
* designed for use with tasks that intrinsically execute more
* than once.
*
* <p>在ScheduledThreadPoolExecutor中被调用</p>
* @return {@code true} if successfully run and reset
*/
protected boolean runAndReset() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return false;
boolean ran = false;
int s = state;
try {
Callable<V> c = callable;
if (c != null && s == NEW) {
try {
c.call(); // don't set result
ran = true;
} catch (Throwable ex) {
setException(ex);
}
}
} finally {
// runner must be non-null until state is settled to
// prevent concurrent calls to run()
runner = null;
// state must be re-read after nulling runner to prevent
// leaked interrupts
s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
return ran && s == NEW;
}
/**
* Sets the result of this future to the given value unless
* this future has already been set or has been cancelled.
*
* <p>This method is invoked internally by the {@link #run} method
* upon successful completion of the computation.
*
* <p>更新状态和结果变量，然后进行完成计算处理</p>
* @param v the value
*/
protected void set(V v) {
/**
* 四个参数分别是：被修改的对象、要更新的属性在内存中的位置、期待的值、要更新的值(如果是期待的值、则更新为要更新的值)
* 更新成功则返回true，失败则返回false；
*/
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}
/**
* Causes this future to report an {@link ExecutionException}
* with the given throwable as its cause, unless this future has
* already been set or has been cancelled.
*
* <p>This method is invoked internally by the {@link #run} method
* upon failure of the computation.
*
* <p>更新状态和结果变量，然后进行完成计算处理</p>
*
* @param t the cause of failure
*/
protected void setException(Throwable t) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = t;
UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
finishCompletion();
}
}
/**
* 执行此方法最终状态可能是CANCELLED或者INTERRUPTED(如果当前状态是NEW且此通过CAS成功的更新了状态，则继续执行返回true)
* 1.如果是不可中断的取消，则状态置为CANCELLED；
* 2.如果是可中断的取消，则状态置为INTERRUPTING,并在执行t.interrupt()后，将状态更新为INTERRUPTED。
*/
public boolean cancel(boolean mayInterruptIfRunning) {
/**
* 如果当前状态是新建、而且使用CAS操作改变了新建状态(因为其他线程也可能在修改):
* 如果可中断则修改为中断状态，否则修改为CANCELLED状态
*/
if (!(state == NEW &&
UNSAFE.compareAndSwapInt(this, stateOffset, NEW,
mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
return false;
/**
* 如果可以中断，则先中断线程，在修改当前任务对象状态值。最后 移除并signal所有等待的线程，调用done()，置空callable
*/
try { // in case call to interrupt throws exception
if (mayInterruptIfRunning) {
try {
//执行具体业务逻辑（构造参数值）所在的线程。fixme：通过执行线程对象的interrupt()方法中断线程
Thread t = runner;
if (t != null)
t.interrupt();
} finally { // final state
//避免从排序的更新值
//putOrderedObject，Unsafe. putOrderedInt，Unsafe. putOrderedLong这三个方法，JDK会在执行这三个方法时插入StoreStore内存屏障，避免发生写操作重排序。
UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
}
}
} finally {
finishCompletion();
}
return true;
}
/**
* 阻塞获取结果
*
* 如果状态为1，即没有以任何形式结束或发起中断流程，则将当前线程放入 等待链表；
*/
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}
/**
* 阻塞一定的时间获取结果，超时则抛 TimeoutException
* @throws CancellationException {@inheritDoc}
*/
public V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
if (unit == null)
throw new NullPointerException();
int s = state;
if (s <= COMPLETING &&
(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
throw new TimeoutException();
return report(s);
}
/**
* 任务借宿后返回结果或者抛异常；
* Returns result or throws exception for completed task.
*
* @param s completed state value
*/
@SuppressWarnings("unchecked")
private V report(int s) throws ExecutionException {
Object x = outcome;
if (s == NORMAL)
return (V)x;
if (s >= CANCELLED)
throw new CancellationException();
throw new ExecutionException((Throwable)x);
}
/**
* Protected method invoked when this task transitions to state
* {@code isDone} (whether normally or via cancellation). The
* default implementation does nothing. Subclasses may override
* this method to invoke completion callbacks or perform
* bookkeeping. Note that you can query status inside the
* implementation of this method to determine whether this task
* has been cancelled.
*/
protected void done() { }
/**
* CANCELLED、INTERRUPTING和INTERRUPTED都将会返回true
*/
public boolean isCancelled() {
return state >= CANCELLED;
}
/**
* 只要不是NEW都返回true，包括COMPLETING、NORMAL、EXCEPTIONAL、CANCELLED、INTERRUPTING、INTERRUPTED
*/
public boolean isDone() {
return state != NEW;
}
/**
* Ensures that any interrupt from a possible cancel(true) is only
* delivered to a task while in run or runAndReset.
*/
private void handlePossibleCancellationInterrupt(int s) {
// It is possible for our interrupter to stall before getting a
// chance to interrupt us. Let's spin-wait patiently.
if (s == INTERRUPTING)
while (state == INTERRUPTING)
//Thread.yield()是指当前线程已经完成重要操作，建议线程调度器将cpu资源从转移给同等或更高优先级的处于就需状态的线程
//yield()不会阻塞该线程，它只是将该线程从运行状态转入就绪状态。
Thread.yield(); // wait out pending interrupt
//sleep()方法会将线程转入time waiting状态，直到阻塞时间结束，才会转入就绪状态
// assert state == INTERRUPTED;
// We want to clear any interrupt we may have received from
// cancel(true). However, it is permissible to use interrupts
// as an independent mechanism for a task to communicate with
// its caller, and there is no way to clear only the
// cancellation interrupt.
//
// Thread.interrupted();
}
/**
* Removes and signals all waiting threads, invokes done(), and nulls out callable.
* <p>移除和signal所有等待的线程，调用done()，置空callable</p>
*/
private void finishCompletion() {
// assert state > COMPLETING;
for (WaitNode q; (q = waiters) != null;) {
//移除所有等待的线程，如果移除成功， todo 线程等待什么
if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
for (;;) {
Thread t = q.thread;
if (t != null) {
q.thread = null;
LockSupport.unpark(t);
}
WaitNode next = q.next;
if (next == null)
break;
q.next = null; // unlink to help gc fixme：引用存活的对象的对象也不会进行垃圾回收
q = next;
}
break;
}
}
done();
callable = null; // to reduce footprint
}
/**
* fixme: 被get方法调用，因此方法中的线程都是等待获取结果的线程
*
* 等待结束或者中断\超时时抛异常
*/
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
//如果调用函数get设置了等待时间，则获取需要目标时间，否则目标时间是0L
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
boolean queued = false;
for (;;) {
//get()获取结果的线程被中断_get()方法调用方法
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
int s = state;
//如果当前状态不是新建和开始状态,返回当前状态
if (s > COMPLETING) {//NORMAL、EXCEPTIONAL、CANCELLED、INTERRUPTING、INTERRUPTED
if (q != null)
q.thread = null;
return s;
}
//如果当前状态是正完成COMPLETING，则让出cpu资源给同等甚至更高优先级的线程
else if (s == COMPLETING) // cannot time out yet
Thread.yield();
//fixme: 如果状态是NEW而且q为null，则将当前线程包装复制给q
else if (q == null)
q = new WaitNode();//fixme 线程节点等于当前线程
// 如果任务状态为NEW而且q不为null，则：1.将waiters节点赋值给q.next，然后跟waiters节点比较是否相同(todo 肯定不同啊)，相同则将waiters复制给q
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this,waitersOffset,q.next = waiters,q);
//如果get方法设置了超时时间；//如果已经超时，则移除当前等待线程;否则关起当前线程；
else if (timed) {
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
LockSupport.parkNanos(this, nanos);
}
else//如果没有设置超时时间，则直接挂起当前线程
LockSupport.park(this);
}
}
/**
* Tries to unlink a timed-out or interrupted wait node to avoid
* accumulating garbage. Internal nodes are simply unspliced
* without CAS since it is harmless if they are traversed anyway
* by releasers. To avoid effects of unsplicing(拼接) from already
* removed nodes, the list is retraversed in case of an apparent
* race. This is slow when there are a lot of nodes, but we don't
* expect lists to be long enough to outweigh higher-overhead
* schemes.
*
* <p>尝试 unlink 一个超时或者中断的wait node 来避免内存垃圾的累加。</p>
*/
private void removeWaiter(WaitNode node) {
if (node != null) {
node.thread = null;
retry:
for (;;) { // restart on removeWaiter race
for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
s = q.next;
if (q.thread != null)
pred = q;
else if (pred != null) {
pred.next = s;
if (pred.thread == null) // check for race
continue retry;
}
else if (!UNSAFE.compareAndSwapObject(this, waitersOffset, q, s))
continue retry;
}
break;
}
}
}
}
