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JUC同步锁原理源码解析五----Phaser 今日热搜

来源：博客园


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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 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 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 evenQ;    private final AtomicReference 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();        this.oddQ = new AtomicReference();    }    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 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 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。

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