【Java】 之 ReentraintLock 浅析

关系图:

一、简介

ReentrantLock 可重入锁

ReentrantLock 对于 Lock 接口的实现主要依赖了 Sync

使用步骤:

二、使用

比如同时累加

public class ReentrantLockTest extends Thread {

    private static ReentrantLock lock = new ReentrantLock();
    private String name;
    private static int i = 0;

    private ReentrantLockTest(String name) {
        this.name = name;
    }

    @Override
    public void run() {

        System.out.println(name + " 开始增加");

        for (int j = 0; j < 100; j++) {

            lock.lock();

            try {

                ++i;
            } finally {
                lock.unlock();
            }
        }

        System.out.println(name + " 结束增加");
    }

    public static void main(String[] args) throws InterruptedException {
        ReentrantLockTest test1 = new ReentrantLockTest("thread1");
        ReentrantLockTest test2 = new ReentrantLockTest("thread2");

        test1.start();
        test2.start();
        test1.join();
        test2.join();
        System.out.println(i);
    }
}

三、源码分析

方法图:

(0) ReentrantLock()

默认创建不公平锁, 即当上一个线程释放锁的时候, 随机下一个线程获得锁

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

(1) lock()

// ReentrantLock.java
public void lock() {
    sync.lock();
}

// ReentrantLock.java  NonfairSync.java
final void lock() {
    if (compareAndSetState(0, 1)) // CAS, 尝试设置 state = 1
        setExclusiveOwnerThread(Thread.currentThread()); // 设置拥有这个锁的线程
    else
        acquire(1); // 尝试获取
}

// AQS
public final void acquire(int arg) {
    // 获取失败, 且加入了队列
    if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
        selfInterrupt(); // 自我中断, 阻塞
}

// ReentrantLock.java  tryAcquire
final boolean nonfairTryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    if (c == 0) {
        if (compareAndSetState(0, acquires)) { // 尝试设置 state = 1
            setExclusiveOwnerThread(current);  // 获取锁成功, 则设置自己拥有锁的线程为当前线程
            return true;
        }
    }
    else if (current == getExclusiveOwnerThread()) {  // 如果拥有锁的线程为当前线程
        int nextc = c + acquires;
        if (nextc < 0) // overflow
            throw new Error("Maximum lock count exceeded");
        setState(nextc);   // 记录重入次数
        return true;
    }
    return false;
}


// AQS
final boolean acquireQueued(final Node node, int arg) {
    boolean failed = true;
    try {
        boolean interrupted = false;
        for (;;) {
            final Node p = node.predecessor();  // 获取前一个节点
            if (p == head && tryAcquire(arg)) { // 如果为头节点 且 尝试获取锁成功
                setHead(node);
                p.next = null; // help GC
                failed = false;
                return interrupted;
            }
            // 获取失败后挂起, 挂起后并检查中断情况
            if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                interrupted = true;
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
}

(2) lockInterruptibly()

// ReentrantLock.java
public void lockInterruptibly() throws InterruptedException {
    sync.acquireInterruptibly(1);
}

// AQS
public final void acquireInterruptibly(int arg)
        throws InterruptedException {
    if (Thread.interrupted())             // 是否可中断
        throw new InterruptedException();
    if (!tryAcquire(arg))                 // 尝试获取
        doAcquireInterruptibly(arg);
}

// AQS
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);
    }
}

(3) tryLock()

// ReentrantLock.java
public boolean tryLock() {
    return sync.nonfairTryAcquire(1);
}

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

(4) unlock()

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

// ReentrantLock.java
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;
}

四、应用场景

多线程并发同步

五、参考资料

1. <<码出高效>>