JUC同步锁原理源码解析五----Phaser
JUC同步锁原理源码解析五----Phaser
Phaser
Phaser的来源
A reusable synchronization barrier, similar in functionality to {@link java.util.concurrent.CyclicBarrier CyclicBarrier} and {@link java.util.concurrent.CountDownLatch CountDownLatch} but supporting more flexible usage.
JDK中对Phaser的定义时，一个可重用的同步栅栏。其作用相当于CyclicBarrier和CountDownLatch的结合体，但是支持更加灵活的使用
Phaser的底层实现
Phaser的底层实现依旧依赖于CAS的自旋锁操作，通过cas保证原子性的操作
2.Phaser
基本使用
import java.util.List;
import java.util.concurrent.Phaser;
public class PhaserDemo {
void runTasks(List<Runnable> tasks) {
final Phaser phaser = new Phaser(1); // "1" to register self
// create and start threads
for (final Runnable task : tasks) {
phaser.register();
new Thread() {
public void run() {
phaser.arriveAndAwaitAdvance(); // await all creation
task.run();
}
}.start();
}
}
void startTasks(List<Runnable> tasks, int iterations) {
Phaser phaser = new Phaser() {
protected boolean onAdvance(int phase, int registeredParties) {
return phase >= iterations - 1 || registeredParties == 0;
}
};
phaser.register();
for (Runnable task : tasks) {
phaser.register();
new Thread(() -> {
do {
task.run();
phaser.arriveAndAwaitAdvance();
} while (!phaser.isTerminated());
}).start();
}
// allow threads to proceed; don't wait for them
phaser.arriveAndDeregister();
}
}
Phaser类
public class Phaser {
private volatile long state;//采用long 64 位表示state变量。使用位操作来表示，cas单原子性变量保证多变量的原子性
private static final int MAX_PARTIES = 0xffff;
private static final int MAX_PHASE = Integer.MAX_VALUE;
private static final int PARTIES_SHIFT = 16;
private static final int PHASE_SHIFT = 32;
private static final int UNARRIVED_MASK = 0xffff; // to mask ints
private static final long PARTIES_MASK = 0xffff0000L; // to mask longs
private static final long COUNTS_MASK = 0xffffffffL;
private static final long TERMINATION_BIT = 1L << 63;
// some special values
private static final int ONE_ARRIVAL = 1;
private static final int ONE_PARTY = 1 << PARTIES_SHIFT;
private static final int ONE_DEREGISTER = ONE_ARRIVAL|ONE_PARTY;
private static final int EMPTY = 1;
// The following unpacking methods are usually manually inlined
private static int unarrivedOf(long s) {
int counts = (int)s;
return (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK);
}
private static int partiesOf(long s) {
return (int)s >>> PARTIES_SHIFT;
}
private static int phaseOf(long s) {
return (int)(s >>> PHASE_SHIFT);
}
private static int arrivedOf(long s) {
int counts = (int)s;
return (counts == EMPTY) ? 0 :
(counts >>> PARTIES_SHIFT) - (counts & UNARRIVED_MASK);
}
/**
* The parent of this phaser, or null if none
*/
private final Phaser parent;
/**
* The root of phaser tree. Equals this if not in a tree.
*/
private final Phaser root;
/**
* Heads of Treiber stacks for waiting threads. To eliminate
* contention when releasing some threads while adding others, we
* use two of them, alternating across even and odd phases.
* Subphasers share queues with root to speed up releases.
*/
private final AtomicReference<QNode> evenQ;
private final AtomicReference<QNode> oddQ;
QNode类
static final class QNode implements ForkJoinPool.ManagedBlocker {
final Phaser phaser;
final int phase;
final boolean interruptible;
final boolean timed;
boolean wasInterrupted;
long nanos;
final long deadline;
volatile Thread thread; // nulled to cancel wait
QNode next;
QNode(Phaser phaser, int phase, boolean interruptible,
boolean timed, long nanos) {
this.phaser = phaser;
this.phase = phase;
this.interruptible = interruptible;
this.nanos = nanos;
this.timed = timed;
this.deadline = timed ? System.nanoTime() + nanos : 0L;
thread = Thread.currentThread();
}
Phaser的构造器
public Phaser(int parties) {
this(null, parties);
}
public Phaser(Phaser parent) {
this(parent, 0);
}
//最终都是走这个构造器方法
public Phaser(Phaser parent, int parties) {
if (parties >>> PARTIES_SHIFT != 0)//
throw new IllegalArgumentException("Illegal number of parties");
int phase = 0;
this.parent = parent;
if (parent != null) {//判断父阶段是否为空。如果有父阶段，子阶段的行为由父阶段控制，调用父阶段去处理
final Phaser root = parent.root;//root为父阶段
this.root = root;
this.evenQ = root.evenQ;//使用父阶段的偶队列
this.oddQ = root.oddQ;//使用父阶段的奇队列
if (parties != 0)//如果父阶段不为空
phase = parent.doRegister(1);//将当前阶段注册到父阶段中
}
else {//表示没有父阶段
this.root = this;
this.evenQ = new AtomicReference<QNode>();
this.oddQ = new AtomicReference<QNode>();
}
this.state = (parties == 0) ? (long)EMPTY :
((long)phase << PHASE_SHIFT) | //64位中高32位表示阶段数，也即phase的数量
((long)parties << PARTIES_SHIFT) | //64位中低32位的高16位表示参与者的数量
((long)parties);//64位中低32位的低16位表示未完成的数量
}
register方法
public int register() {
return doRegister(1);
}
private int doRegister(int registrations) {
// adjustment to state
long adjust = ((long)registrations << PARTIES_SHIFT) | registrations;//对当前state变量的参与数量和未完成数量都加 1
final Phaser parent = this.parent;//如果有父阶段，获取父阶段
int phase;
for (;;) {
long s = (parent == null) ? state : reconcileState();//拿到state的值
int counts = (int)s;//将64位取低32位的值
int parties = counts >>> PARTIES_SHIFT;//右移16位，取高16位的值，也即parties的数量
int unarrived = counts & UNARRIVED_MASK;//获取低16位的数值，也即未到达的数量
if (registrations > MAX_PARTIES - parties)//越界判断
throw new IllegalStateException(badRegister(s));
phase = (int)(s >>> PHASE_SHIFT);//获取阶段数
if (phase < 0)//阶段数为0,表示已经超过阶段数了，不需要继续处理了
break;
if (counts != EMPTY) { // not 1st registration
if (parent == null || reconcileState() == s) {
if (unarrived == 0) // wait out advance 如果未完成数量等于0
root.internalAwaitAdvance(phase, null);//阻塞等待或者等到下一阶段推进
else if (UNSAFE.compareAndSwapLong(this, stateOffset,//将当前需要参与的数量放到state变量中
s, s + adjust))
break;//退出循环
}
}
else if (parent == null) {//没有父阶段或自己就是父阶段
long next = ((long)phase << PHASE_SHIFT) | adjust; //阶段数量增加
if (UNSAFE.compareAndSwapLong(this, stateOffset, s, next))//cas尝试将阶段数量增加，成功就腿很粗
break;
}
else {
synchronized (this) { //走到这里表示，自身属于子阶段，需要接受父阶段的调度
if (state == s) { //重新检测state变量是否改变
phase = parent.doRegister(1);//向父阶段注册
if (phase < 0)//阶段数已经超过了最大阶段数
break;
//while循环，设置state的中phase阶段数直至成功
while (!UNSAFE.compareAndSwapLong
(this, stateOffset, s,
((long)phase << PHASE_SHIFT) | adjust)) {
s = state;
phase = (int)(root.state >>> PHASE_SHIFT);
// assert (int)s == EMPTY;
}
break;
}
}
}
}
return phase;
}
reconcileState方法:
//只要使用在有父子阶段的存在的情况下
private long reconcileState() {
final Phaser root = this.root;//获取到当前阶段
long s = state;//取得当前state
if (root != this) {//如果root不是当前阶段
int phase, p;
// CAS to root phase with current parties, tripping unarrived
while ((phase = (int)(root.state >>> PHASE_SHIFT)) != //phase等于root的阶段数
(int)(s >>> PHASE_SHIFT) &&//root的阶段数不等于当前阶段state变量的阶段数
!UNSAFE.compareAndSwapLong//
(this, stateOffset, s,
s = (((long)phase << PHASE_SHIFT) | //root的阶段数
((phase < 0) ? (s & COUNTS_MASK) : //如果阶段数已经超了，直接取低32位
(((p = (int)s >>> PARTIES_SHIFT) == 0) ? EMPTY : //获取到s对应的parties数量,复制给p
((s & PARTIES_MASK) | p))))))
s = state;
}
return s;
}
arriveAndAwaitAdvance方法
public int arriveAndAwaitAdvance() {
// Specialization of doArrive+awaitAdvance eliminating some reads/paths
final Phaser root = this.root;//获取当前阶段
for (;;) {
long s = (root == this) ? state : reconcileState();//获取到state的状态，如果有父阶段调用reconcileState
int phase = (int)(s >>> PHASE_SHIFT);//等到当前阶段数
if (phase < 0)//阶段数超了，直接返回
return phase;
int counts = (int)s;//获取state的低32位
int unarrived = (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK);//获取到未到达的参与者数量
if (unarrived <= 0)//未到达的参与者数量越界检查
throw new IllegalStateException(badArrive(s));
if (UNSAFE.compareAndSwapLong(this, stateOffset, s,
s -= ONE_ARRIVAL)) {//cas将未到达的数量减1
if (unarrived > 1)//如果未到达的数量大于1
return root.internalAwaitAdvance(phase, null);//调用父阶段控制其去睡眠等待
if (root != this)//如果this不是父阶段
return parent.arriveAndAwaitAdvance();//由父类处理，将到达线程数减1或滚动到下一阶段
long n = s & PARTIES_MASK; // base of next state//获得参与者parties数量
int nextUnarrived = (int)n >>> PARTIES_SHIFT;//获得下一个阶段参与者的数量
if (onAdvance(phase, nextUnarrived))//调用onAdvance会掉方法
n |= TERMINATION_BIT;//TERMINATION_BIT:1<<63，标识阶段数结束
else if (nextUnarrived == 0)//如果下一个阶段参与者为0
n |= EMPTY;//异或上EMPTY
else
n |= nextUnarrived;//否则将低32位的低16位置为下一阶段参与者的数量，表示未完成的数量等于下一个阶段参与者的数量
int nextPhase = (phase + 1) & MAX_PHASE;//获得下一个阶段的phase的数量
n |= (long)nextPhase << PHASE_SHIFT;//n异或上下一阶段phase的数量组合成state比那辆
if (!UNSAFE.compareAndSwapLong(this, stateOffset, s, n))//cas设置state变量。当前线程如果设置state变量失败，是否可以允许爆炸唤醒，不直接退出？
return (int)(state >>> PHASE_SHIFT); // cas失败，返回state中的阶段数
releaseWaiters(phase);//释放等待线程
return nextPhase;
}
}
}
internalAwaitAdvance方法
private int internalAwaitAdvance(int phase, QNode node) {
// assert root == this;
releaseWaiters(phase-1); // ensure old queue clean 将上一个阶段等待线程唤醒，将队列清空
boolean queued = false; // true when node is enqueued
int lastUnarrived = 0; // to increase spins upon change
int spins = SPINS_PER_ARRIVAL;//SPINS_PER_ARRIVAL = (NCPU < 2) ? 1 : 1 << 8,单核CPU没有自旋的必要，浪费时间
long s;
int p;
while ((p = (int)((s = state) >>> PHASE_SHIFT)) == phase) {//判断当前阶段数是否等于phase
if (node == null) { // spinning in noninterruptible mode
int unarrived = (int)s & UNARRIVED_MASK;//获取未到达的参与者数量
if (unarrived != lastUnarrived &&//未到达的参与者数量不等于lastUnarrived
(lastUnarrived = unarrived) < NCPU)//lastUnarrived 赋值lastUnarrived。小于CPU的核心数，证明任务很快可以调度，值得等待。但是考虑业务线程，实际中如果CPU的核心数没有大于2，其实没有自旋的必要。
spins += SPINS_PER_ARRIVAL;//增加自旋次数
boolean interrupted = Thread.interrupted();//判断中断标志位
if (interrupted || --spins < 0) { // need node to record intr //如果中断了，或者自旋次数小于0
node = new QNode(this, phase, false, false, 0L);
node.wasInterrupted = interrupted;//将中断标识赋值
}
}
else if (node.isReleasable()) // done or aborted 判断是否已经完成，或者说中断等方式释放
break;
else if (!queued) { // push onto queue 不在队列中
AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ; //根据phase的奇偶性，选择队列
QNode q = node.next = head.get();//头插法
if ((q == null || q.phase == phase) &&
(int)(state >>> PHASE_SHIFT) == phase) // avoid stale enq
queued = head.compareAndSet(q, node);
}
else {
try {
ForkJoinPool.managedBlock(node);//由于兼容forkJoin线程池，所以这里提供模板。这里进行阻塞等待
} catch (InterruptedException ie) {
node.wasInterrupted = true;
}
}
}
if (node != null) {//进入这里表示，当前阶段不一致
if (node.thread != null)
node.thread = null; // avoid need for unpark() //将thread置为空
if (node.wasInterrupted && !node.interruptible) //节点被中断，并且节点不可中断
Thread.currentThread().interrupt();//重置中断标志位
if (p == phase && (p = (int)(state >>> PHASE_SHIFT)) == phase)//当前阶段数量一致，也即属于同一阶段
return abortWait(phase); // possibly clean up on abort
}
releaseWaiters(phase);//唤醒等待的线程
return p;
}
releaseWaiters方法
private void releaseWaiters(int phase) {
QNode q; // first element of queue
Thread t; // its thread
AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ;//根据阶段数判断是奇数队列还是偶数队列
while ((q = head.get()) != null &&//获取到头结点，如果头结点补位空
q.phase != (int)(root.state >>> PHASE_SHIFT)) {//并且当前阶段数已经滚动到下一个阶段
if (head.compareAndSet(q, q.next) &&//cas替换头结点
(t = q.thread) != null) {//如果旧的头结点不为空
q.thread = null;//将节点q的线程置为空
LockSupport.unpark(t);//唤醒节点q的线程
}
}
}
arriveAndDeregister方法
public int arriveAndDeregister() {
return doArrive(ONE_DEREGISTER);//ONE_DEREGISTER = ONE_ARRIVAL|ONE_PARTY;
}
doArrive方法
private int doArrive(int adjust) {
final Phaser root = this.root;//获取当前阶段
for (;;) {
long s = (root == this) ? state : reconcileState();//如果有父阶段获取父阶段的state,没有取当前阶段的state
int phase = (int)(s >>> PHASE_SHIFT);//获取阶段数
if (phase < 0)//如果阶段数已经小于0，表示已经结束，直接返回
return phase;
int counts = (int)s;//获取state的低32位，业绩参与者和未完成的参与者
int unarrived = (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK);//获取未到达的参与者数量EMPTY是特殊值，表示没有未到达的参与者
if (unarrived <= 0)//如果未到达的参与者小于0，非法直接抛出异常
throw new IllegalStateException(badArrive(s));
if (UNSAFE.compareAndSwapLong(this, stateOffset, s, s-=adjust)) {//直接cas自旋，更新state变量
if (unarrived == 1) {//如果当前线程是最后一个未完成的参与者，需要做收尾工作
long n = s & PARTIES_MASK; // base of next state 获取参与者的数量
int nextUnarrived = (int)n >>> PARTIES_SHIFT;//设置未到达的参与者数量为下一个阶段的参与者数量
if (root == this) {//如果root是当前阶段
if (onAdvance(phase, nextUnarrived))//回调钩子函数
n |= TERMINATION_BIT;//置为TERMINATION状态，TERMINATION_BIT = 1L << 63;最高位符号位表示终止标志位
else if (nextUnarrived == 0)//如果下一个阶段没有参与者
n |= EMPTY;//直接或上一个EMPTY
else
n |= nextUnarrived;//否则直接或上下一个阶段的未达到的参与者数量
int nextPhase = (phase + 1) & MAX_PHASE;//阶段数加1
n |= (long)nextPhase << PHASE_SHIFT;//将阶段数组合到变量n中，
UNSAFE.compareAndSwapLong(this, stateOffset, s, n);//cas自旋将state置为n,表示滚动到下一个阶段
releaseWaiters(phase);//释放所有等待的节点
}
else if (nextUnarrived == 0) { // propagate deregistration 这里表示root有父阶段且自己已经完成
phase = parent.doArrive(ONE_DEREGISTER);//父阶段中标识自己已经完成并且将参与者数量减1，未到达的参与者也减1
UNSAFE.compareAndSwapLong(this, stateOffset,//将当前的state,case自旋，置为EMPTY
s, s | EMPTY);
}
else
phase = parent.doArrive(ONE_ARRIVAL);//当前线程不是最后一个完成的线程，将未到达的参与者数量减1即可
}
return phase;
}
}
}
4.留言
到了这里，其实AQS的源码基本已经覆盖了，对于AQS的源码也应该有了清楚的认知。总结就是：一个volatile 的state变量，两个等待队列(竞争队列，条件队列)，通过cas的方式保证单变量的原子性。后续将会对Exchanger以及Phaser进行源码解析，到此基本AQS已经到了一个段落了。后续观看源码时，请注意多考虑一下多线程并发时可能出现的情况，去理解doug lea写代码的思路。
