简单了解ReentrantLock

什么是ReentrantLock？

ReentrantLock是Java的一个锁，用于协作多线程，同时也是一个独占锁

ReentrantLock应用场景

保证并发安全

代替synchronized实现同步

线程通信

如线程交替打印1~100

public class JUCTest {

    private static volatile int count = 0;
    private static final int MAX = 100;
    
    private static final ReentrantLock lock = new ReentrantLock();
    private static final Condition condition = lock.newCondition();

    public static void main(String[] args) {
        Thread t1 = new Thread(new Print(), "thread-1");
        Thread t2 = new Thread(new Print(), "thread-2");
        t1.start();
        t2.start();
    }

    static class Print implements Runnable {

        @Override
        public void run() {
            while (count <= MAX) {
                lock.lock();
                try {
                    if (count <= MAX) {
                        System.out.println(Thread.currentThread().getName() + ": " + count);
                        count++;
                    }
                    condition.signalAll();
                    condition.await();
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }
            }

        }
    }
}
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ReentrantLock源码解析

构造器

默认是非公平锁

public ReentrantLock() {
    sync = new NonfairSync();
}

public ReentrantLock(boolean fair) {
    sync = fair ? new FairSync() : new NonfairSync();
}
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lock()

public void lock() {
    sync.lock();
}
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公平锁

调用了AQS的acquire()

final void lock() {
    acquire(1);
}

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

protected final boolean tryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    // 当前资源没人占用才能尝试获取锁
    if (c == 0) {
        // 先看看有没有前面的节点，如果没有其他节点再CAS尝试获取锁
        if (!hasQueuedPredecessors() &&
            compareAndSetState(0, acquires)) {
            setExclusiveOwnerThread(current);
            return true;
        }
    }
    // 可重入锁原理
    else if (current == getExclusiveOwnerThread()) {
        int nextc = c + acquires;
        if (nextc < 0)
            throw new Error("Maximum lock count exceeded");
        setState(nextc);
        return true;
    }
    return false;
}
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非公平锁

调用了AQS的acquire()，只不过多一个先CAS抢锁的判断

final void lock() {
    if (compareAndSetState(0, 1))
        setExclusiveOwnerThread(Thread.currentThread());
    else
        acquire(1);
}
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非公平锁tryAcquire()

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

final boolean nonfairTryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    // 当前资源没人占用才能尝试获取锁
    if (c == 0) {
        // CAS尝试获取锁
        if (compareAndSetState(0, acquires)) {
            setExclusiveOwnerThread(current);
            return true;
        }
    }
    // 可重入锁原理
    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;
}
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unlock()

调用了AQS的release()

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

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

调用AQS的await()

public final void await() throws InterruptedException {
    if (Thread.interrupted())
        throw new InterruptedException();
    Node node = addConditionWaiter();
    int savedState = fullyRelease(node);
    int interruptMode = 0;
    while (!isOnSyncQueue(node)) {
        LockSupport.park(this);
        if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
            break;
    }
    if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
        interruptMode = REINTERRUPT;
    if (node.nextWaiter != null) // clean up if cancelled
        unlinkCancelledWaiters();
    if (interruptMode != 0)
        reportInterruptAfterWait(interruptMode);
}
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signal()

调用AQS的signal()

public final void signal() {
    if (!isHeldExclusively())
        throw new IllegalMonitorStateException();
    Node first = firstWaiter;
    if (first != null)
        doSignal(first);
}
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signalAll()

调用AQS的signalAll()

public final void signalAll() {
    if (!isHeldExclusively())
        throw new IllegalMonitorStateException();
    Node first = firstWaiter;
    if (first != null)
        doSignalAll(first);
}
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