java多线程6：ReentrantLock

下面看下JUC包下的一大并发神器ReentrantLock，是一个可重入的互斥锁，具有比synchronized更为强大的功能。

ReentrantLock基本用法

先来看一下ReentrantLock的简单用法

public class MyDomain1 {

    private Lock lock = new ReentrantLock();

    public void method1() {
        System.out.println("进入method1方法");
        try {
            lock.lock();
            for (int i = 0; i < 5; i++) {
                System.out.println(Thread.currentThread().getName() + " i=" + i);
                Thread.sleep(1000);
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }
}

public class Mythread1_1 extends Thread {

    private MyDomain1 myDomain1;

    public Mythread1_1(MyDomain1 myDomain1) {
        this.myDomain1 = myDomain1;
    }

    @Override
    public void run() {
        myDomain1.method1();
    }
}

　　开启三个线程同时执行测试方法

@Test
    public void test1() throws InterruptedException {
        MyDomain1 myDomain1 = new MyDomain1();
        Mythread1_1 a = new Mythread1_1(myDomain1);
        Mythread1_1 c = new Mythread1_1(myDomain1);
        Mythread1_1 d = new Mythread1_1(myDomain1);
        a.start();
	c.start();
	d.start();

        a.join();
	c.join();
	d.join();
    }

　　执行结果：

进入method1方法
Thread-0 i=0
进入method1方法
进入method1方法
Thread-0 i=1
Thread-0 i=2
Thread-0 i=3
Thread-0 i=4
Thread-1 i=0
Thread-1 i=1
Thread-1 i=2
Thread-1 i=3
Thread-1 i=4
Thread-2 i=0
Thread-2 i=1
Thread-2 i=2
Thread-2 i=3
Thread-2 i=4

　　可以看到，代码流程进入到lock.lock()以后没有任何的交替打印，都是一个线程执行完后一个线程才开始执行，说明ReentrantLock具有加锁的功能。

看下ReentrantLock源码的构造方法：

/**
     * Creates an instance of {@code ReentrantLock}.
     * This is equivalent to using {@code ReentrantLock(false)}.
     */
    public ReentrantLock() {
        sync = new NonfairSync();
    }

    /**
     * Creates an instance of {@code ReentrantLock} with the
     * given fairness policy.
     *
     * @param fair {@code true} if this lock should use a fair ordering policy
     */
    public ReentrantLock(boolean fair) {
        sync = fair ? new FairSync() : new NonfairSync();
    }

　　可以看到ReentrantLock支持两种加锁模式：公平锁和非公平锁。它是如何实现的呢？继续往下看

我们测试用例中，默认使用的是非公平锁的加锁方法，看下 NonfairSync 的lock() 方法

/**
     * Sync object for non-fair locks
     */
    static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;

        /**
         * Performs lock.  Try immediate barge, backing up to normal
         * acquire on failure.
         */
        final void lock() {
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1);
        }

        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
        }
    }

　　　第12行的 compareAndSetState方法，当第一个线程执行次方法时，会将 state 设置为1，执行成功后，exclusiveOwnerThread=线程1。

此时线程1正常执行业务，当线程2走到lock方法时，此时线程12执行compareAndSetState方法将返回false，执行 acquire(1)

/**
     * 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.
     */
    public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

　　非公平锁实现的tryAcquire

/**
         * Performs non-fair tryLock.  tryAcquire is implemented in
         * subclasses, but both need nonfair try for trylock method.
         */
        final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }

　　此时线程2得到的state应该是1，并且 current != getExclusiveOwnerThread()，所以线程2会继续执行 acquireQueued(addWaiter(Node.EXCLUSIVE), arg)。

注意第8行到第13行，如果此时线程1已经释放了锁，那么线程2得到的state就是0了，它将走获取锁的逻辑，

第14行到第20行，这块就是ReentrantLock支持可重入的实现，也就是如果当前执行的线程是持有锁的线程，那么就可以获取锁，并将state+1。

如果线程1此时还没有释放锁，那么线程2将走到等待队列里

*
     * @param node the node
     * @param arg the acquire argument
     * @return {@code true} if interrupted while waiting
     */
    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

　　这个for循环对于线程2来说，首先再次尝试去获取锁，因为此时线程1可能已经释放锁了，如果依旧获取锁失败，则执行

/**
     * 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
     */
    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.
             */
            return true;
        if (ws > 0) {
            /*
             * Predecessor was cancelled. Skip over predecessors and
             * indicate retry.
             */
            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.
             */
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

　　这块代码，最好打个断点一步一步去执行，更容易看出每一步执行的逻辑以及值。

这个ws是节点predecessor的waitStatus，很明显是0，所以此时把pred的waitStatus设置为Noed.SIGNAL即-1并返回false。

既然返回了false，上面的if自然不成立，再走一次for循环，还是先尝试获取锁，不成功，继续走shouldParkAfterFailedAcquire，此时waitStatus为-1，小于0，走第三行的判断，返回true。

/**
     * Convenience method to park and then check if interrupted
     *
     * @return {@code true} if interrupted
     */
    private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);
        return Thread.interrupted();
    }

　　最后一步，线程2调用LockSupport的park方法。

接下来就到线程1执行完任务后，将执行unlock方法 释放锁

public void unlock() {
        sync.release(1);
    }

/**
     * 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}
     */
    public final boolean release(int arg) {
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }

　　首先tryRelease(1) ,代码逻辑比较简单，就是将state设置0 （注意这是同一个锁只lock一次的情况下），并将 exclusiveOwnerThread设置为null

protected final boolean tryRelease(int releases) {
            int c = getState() - releases;
            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            boolean free = false;
            if (c == 0) {
                free = true;
                setExclusiveOwnerThread(null);
            }
            setState(c);
            return free;
        }

　　当锁释放完成后，继续执行release方法的 unparkSuccessor(h),

/**
     * Wakes up node's successor, if one exists.
     *
     * @param node the node
     */
    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.
         */
        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.
         */
        Node s = node.next;
        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;
        }
        if (s != null)
            LockSupport.unpark(s.thread);
    }

　　h的下一个Node，这个Node里面的线程就是线程2，由于这个Node不等于null，线程2最终被unpark了，线程2可以继续运行。

有一个很重要的问题是：锁被解了怎样保证整个FIFO队列减少一个Node呢？

还记得线程2被park在 acquireQueued方法

final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

　　被阻塞的线程2是被阻塞在第14行，注意这里并没有return语句，阻塞完成线程2继续进行for循环。线程2所在的Node的前驱Node是p，线程2尝试tryAcquire，成功，

然后线程2就成为了head节点了，把p的next设置为null，这样原头Node里面的所有对象都不指向任何块内存空间，h属于栈内存的内容，方法结束被自动回收，

这样随着方法的调用完毕，原头Node也没有任何的引用指向它了，这样它就被GC自动回收了。此时，遇到一个return语句，acquireQueued方法结束，后面的Node也是一样的原理。

至此线程2 lock方法执行完成，并成功获取到锁。

至此ReentrantLock的非公平锁的加锁与锁释放逻辑已经大致清楚了，那么公平锁的加锁过程又是如何呢？

/**
         * Fair version of tryAcquire.  Don't grant access unless
         * recursive call or no waiters or is first.
         */
        protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (!hasQueuedPredecessors() &&
                    compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }
    }

　　1:tryAcquire(1),因为是第一个线程，所以当前status=0，尝试获取锁，hasQueuedPredecessors方法也是和非公平锁一个代码上的区别

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());
    }

　  公平锁获取锁之前首先判断当前队列是否存在（head==tail）[不存在]，设置staus=1，获取锁成功。

　  如果等待队列中存在等待线程，则取出第一个等待的线程(head.next)，并返回第一个等待的线程是否是当前线程，

　  只有当等到队列的第一个等待的线程是当前线程尝试获取锁的线程，才会获取锁成功。

假如此时线程t2，也来获取锁，调用tryAcquire(1)时，因为status!=0，返回fasle，调用addWaiter(Node.EXCLUSIVE)，

此时会生成一个队列，队列的head为 new Node(), tail为t2的Node，调用acquireQueued(t2的Node)，因为此时t2所在Node的prev为head，所以会尝试直接获取一次锁，

如果获取成功，将t2的Node设置为head，如果没有获取锁，shouldParkAfterFailedAcquire()，t2 Park()。

ReentrantLock持有的锁

定义一个对象，分别有两个测试方法，一个用ReentrantLock加锁，一个用synchronized加锁

public class MyDomain1 {

    private Lock lock = new ReentrantLock();

    public void method1() {
        System.out.println("进入method1方法");
        try {
            lock.lock();
            for (int i = 0; i < 5; i++) {
                System.out.println(Thread.currentThread().getName() + " i=" + i);
                Thread.sleep(1000);
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    // 为了测试 lock 和 synchronized同步方法不是同一把锁
    public synchronized void method2() {
        System.out.println("进入method2方法");
        for (int j = 0; j < 5; j++) {
            System.out.println(Thread.currentThread().getName() + " j=" + j);
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}　

　　定义两个线程类，分别调用一个方法

public class Mythread1_1 extends Thread {

    private MyDomain1 myDomain1;

    public Mythread1_1(MyDomain1 myDomain1) {
        this.myDomain1 = myDomain1;
    }

    @Override
    public void run() {
        myDomain1.method1();
    }
}

public class Mythread1_2 extends Thread {

    private MyDomain1 myDomain1;

    public Mythread1_2(MyDomain1 myDomain1) {
        this.myDomain1 = myDomain1;
    }

    @Override
    public void run() {
        myDomain1.method2();
    }
}

　　

@Test
    public void test1() throws InterruptedException {
        MyDomain1 myDomain1 = new MyDomain1();
        Mythread1_1 a = new Mythread1_1(myDomain1);
        Mythread1_2 b = new Mythread1_2(myDomain1);
        a.start();
        b.start();

        a.join();
        b.join();
    }

　　执行结果：

进入method1方法
Thread-0 i=0
进入method2方法
Thread-1 j=0
Thread-0 i=1
Thread-1 j=1
Thread-1 j=2
Thread-0 i=2
Thread-0 i=3
Thread-1 j=3
Thread-0 i=4
Thread-1 j=4

　　可以看到两个线路交替打印，说明 ReentrantLock 和 synchronized同步方法不是同一把锁

Condition

ReentrantLock实现等待/通知模型，这也是比synchronized更为强大的功能点之一。

1、一个ReentrantLock里面可以创建多个Condition实例，实现多路通知

2、notify()方法进行通知时，被通知的线程时Java虚拟机随机选择的，但是ReentrantLock结合Condition可以实现有选择性地通知

3、await()和signal()之前，必须要先lock()获得锁，使用完毕在finally中unlock()释放锁，这和wait()、notify()/notifyAll()使用前必须先获得对象锁是一样的

先看个示例

定义一个对象并new了两个condition，然后分别执行await方法，再定义一个signal方法，只唤醒其中一个condition

public class MyDomain2 {

    private Lock lock = new ReentrantLock();
    private Condition conditionA = lock.newCondition();
    private Condition conditionB = lock.newCondition();

    public void await() {
        System.out.println("进入await方法");
        try {
            lock.lock();
            System.out.println(Thread.currentThread().getName() + " conditionA await " + System.currentTimeMillis());
            conditionA.await();
            System.out.println(Thread.currentThread().getName() + " conditionA await out " + System.currentTimeMillis());
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public void await2() {
        System.out.println("进入await2方法");
        try {
            lock.lock();
            System.out.println(Thread.currentThread().getName() + " conditionB await " + System.currentTimeMillis());
            conditionB.await();
            System.out.println(Thread.currentThread().getName() + " conditionB await out " + System.currentTimeMillis());
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public void signal() {
        System.out.println("进入signal方法");
        try {
            lock.lock();
            System.out.println(Thread.currentThread().getName() + " conditionA signal " + System.currentTimeMillis());
            conditionA.signal();
            Thread.sleep(3000);
            System.out.println(Thread.currentThread().getName() + " conditionA signal " + System.currentTimeMillis());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }
}

　　一个线程执行await方法，一个线程负责执行signal

public class Mythread2_1 extends Thread {

    private MyDomain2 myDomain2;

    public Mythread2_1(MyDomain2 myDomain2) {
        this.myDomain2 = myDomain2;
    }

    @Override
    public void run() {
        myDomain2.await();
    }
}

public class Mythread2_2 extends Thread {

    private MyDomain2 myDomain2;

    public Mythread2_2(MyDomain2 myDomain2) {
        this.myDomain2 = myDomain2;
    }

    @Override
    public void run() {
        myDomain2.signal();
    }
}

　　测试方法

@Test
    public void test2() throws InterruptedException {
        MyDomain2 myDomain2 = new MyDomain2();
        Mythread2_1 a = new Mythread2_1(myDomain2);
        Mythread2_2 b = new Mythread2_2(myDomain2);
        a.start();
        Thread.sleep(5000);
        b.start();

        a.join();
        b.join();

    }

　　执行结果：

进入await方法
Thread-0 conditionA await 1639549418811
进入signal方法
Thread-1 conditionA signal 1639549423817
Thread-1 conditionA signal 1639549426820
Thread-0 conditionA await out 1639549426820

　　可以看到进入await方法后，线程1 park住了，5秒钟后，待signal执行完成后，线程1才开始继续执行。

同时condition还有signalAll方法，可以唤醒同一个condition所有在等待的线程。

　　

看过 ReentrantLock源码的应该注意到  AbstractQueuedSynchronizer, 它也是JUC包实现的核心抽象同步器，

也是CountDownLatch、Semphore等并发类的核心组件，这个我们后续再继续研究。

参考文献

1：《Java并发编程的艺术》

2：《Java多线程编程核心技术》