ReentrantLock源码简析

Posted 2021-03-22 expiator

概念

ReentrantLock，可重入锁。在多线程中，可以通过加锁保证线程安全。

ReentrantLock，是基于AQS实现的。

加锁和解锁

• 加锁：

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

• 解锁

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

内部类Sync继承AQS，因此可以维护状态变量state，通过acquire()获取state、release()释放state。后文会涉及。

构造方法

• 无参构造方法

默认使用非公平锁。

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

• 有参构造方法

根据参数判断是否公平锁。

/**
 * 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();
}

Sync类

Sync类是一个继承AQS的内部静态抽象类。

Sync继承AQS，因此也可以维护状态变量state，通过acquire()获取state、release()释放state。

公平锁FairSync和非公平锁NonfairSync，都继承Sync类，并且重写了Sync类中的抽象方法Lock()。

其中的nonfairTryAcquire()方法，是ReentrantLock最核心的实现。

**如果状态变量state为0，说明目前还没有其他线程加锁，当前线程可以进行加锁，通过CAS将状态变量state加1。
并且当前线程会获得锁。

如果状态变量state不为0，且获取锁的是当前线程，则将state加1。**

    /**
     * Base of synchronization control for this lock. Subclassed
     * into fair and nonfair versions below. Uses AQS state to
     * represent the number of holds on the lock.
     */
    abstract static class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = -5179523762034025860L;

        /**
         * Performs {@link Lock#lock}. The main reason for subclassing
         * is to allow fast path for nonfair version.
         */
        abstract void lock();

        /**
         * 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();
            //如果状态变量state为0，说明目前还没有其他线程加锁，当前线程可以进行加锁，通过CAS将状态变量state加1。
            //并且当前线程会获得锁。
            if (c == 0) {
                if (compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            //如果状态变量state不为0，且获取锁的是当前线程，则将state加1。
            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;
        }

        //释放state
        protected final boolean tryRelease(int releases) {
            int c = getState() - releases;
            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            boolean free = false;
            //如果state为0，则说明已经释放锁，当前没有线程获得锁。
            if (c == 0) {
                free = true;
                setExclusiveOwnerThread(null);
            }
            setState(c);
            return free;
        }

        protected final boolean isHeldExclusively() {
            // While we must in general read state before owner,
            // we don't need to do so to check if current thread is owner
            return getExclusiveOwnerThread() == Thread.currentThread();
        }

        final ConditionObject newCondition() {
            return new ConditionObject();
        }

        // Methods relayed from outer class

        final Thread getOwner() {
            return getState() == 0 ? null : getExclusiveOwnerThread();
        }

        final int getHoldCount() {
            return isHeldExclusively() ? getState() : 0;
        }

        final boolean isLocked() {
            return getState() != 0;
        }

        /**
         * Reconstitutes the instance from a stream (that is, deserializes it).
         */
        private void readObject(java.io.ObjectInputStream s)
            throws java.io.IOException, ClassNotFoundException {
            s.defaultReadObject();
            setState(0); // reset to unlocked state
        }
    }

非公平锁和公平锁

非公平锁和公平锁，都重写了tryAcquire()。而acquire()是AQS中的方法。

• 非公平锁：

非公平锁的调用过程： lock()---> acquire(1) ---> tryAcquire(1) ---> nonfairTryAcquire(1)

/**
 * 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);
    }
}

• 公平锁：

主要流程：lock()--->acquire(1)--->tryAcquire(1)---> hasQueuedPredecessors()

公平锁和非公平锁的主要区别是：

公平锁，会去判断“当前线程”是不是AQS的线程等待队列(CLH队列)中的第一个线程，并对state进行CAS操作。

这个区别在hasQueuedPredecessors()方法中体现。

    /**
     * Sync object for fair locks
     */
    static final class FairSync extends Sync {
        private static final long serialVersionUID = -3000897897090466540L;

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

        /**
         * 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();
            //如果当前状态变量为0(也就是没有线程获取到锁)
            if (c == 0) {
                //判断“当前线程”是不是AQS的线程等待队列(CLH队列)中的第一个线程，并对state进行CAS操作。
                if (!hasQueuedPredecessors() &&
                    compareAndSetState(0, acquires)) {
                    //设置当前线程获取锁
                    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;
        }
    }

其他资料

AQS详情见： https://www.cnblogs.com/expiator/p/12052125.html