可重入锁ReentrantLock源码解析

ReentrantLock：一个可重入互斥Lock具有与使用synchronized方法和语句访问的隐式监视锁相同的基本行为和语义，但具有扩展功能。比如实现公平锁、超时处理、锁中断。

A ReentrantLock由最后一个成功锁定且尚未解锁的线程拥有 。当锁不是由另一个线程拥有时，调用lock的线程将成功获取锁。 如果当前线程已经拥有该锁，该方法将立即返回。 当前线程是否持有该锁可以使用方法isHeldByCurrentThread()和getHoldCount()进行检查。
该类的构造函数接受可选的公平参数，当设置true ，即可以使用公平锁。在竞争时，锁有利于授予访问最长等待的线程。 否则，该锁不保证任何特定的访问顺序。 使用许多线程访问的公平锁的程序可能会比使用默认设置的整体吞吐量小（通常要慢很多），但是具有更小的时间差异来获得锁定并保证更少的饥饿死锁。 但是请注意，锁的公平性不能保证线程调度的公平性。 因此，使用公平锁的线程可以连续获得多次，而其他活动线程不进行而且不是当前锁拥有者。
建议的做法是始终立即跟随lock与try块的通话，最常见的是在之前/之后的建设，如：

class X {
    private final ReentrantLock lock = new ReentrantLock();
    // ...
    public void m() {
      lock.lock();  // block until condition holds（阻塞,直到条件成立）
      try {
        // ... method body
      } finally {
        lock.unlock()
      }
    }
  }} 

除了实现Lock接口，这个类定义了许多public种protected方法用于检查锁的状态。 其中一些方法仅适用于仪器和监控。
此类的序列化与内置锁的操作方式相同：反序列化锁处于未锁定状态，无论其序列化时的状态如何。
此锁最多支持同一个线程的2147483647递归锁。 尝试超过此限制会导致Error从锁定方法中抛出。

我们看下Lock源码：

/**
 * Lock实现提供比使用synchronized方法和语句可以获得的更广泛的锁定操作。
 * 它们允许更灵活的结构化，可能具有完全不同的属性，并且可以支持多个相关联的对象Condition 。
 */
public interface Lock {
    void lock();
    void lockInterruptibly() throws InterruptedException;
    boolean tryLock();
    boolean tryLock(long time, TimeUnit unit) throws InterruptedException;
    void unlock();
    Condition newCondition();
}

ReentrantLock使用了NonfairSync、FairSync这两个Sync的子类来实现公平锁与非公平锁，默认是非公平锁。所以我们先看看Sync、NonfairSync、FairSync的实现。

一、Sync源码

    /**
     * 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;

        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();
            if (c == 0) {
                //这个线程第一次加锁，使用CAS设置state=1
                if (compareAndSetState(0, acquires)) {
                    //设置当前锁的当前持有线程，然后退出并返回成功
                    setExclusiveOwnerThread(current);
                    return true;
                }
            } else if (current == getExclusiveOwnerThread()) {
                //如果不是第一次加锁，则state向上加acquires（这里默认是1）
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded"); //最大支持2147483647次重入
                setState(nextc);
                return true;
            }
            return false;
        }

        protected final boolean tryRelease(int releases) {
            int c = getState() - releases;
            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            boolean free = false;
            if (c == 0) {
                //只有释放锁的次数与加锁次数一致，才能完全释放锁
                free = true;
                setExclusiveOwnerThread(null);
            }
            setState(c);
            return free;
        }

        protected final boolean isHeldExclusively() {
            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).
         * 反序列化，重置为未加锁状态
         */
        private void readObject(java.io.ObjectInputStream s)
            throws java.io.IOException, ClassNotFoundException {
            s.defaultReadObject();
            setState(0); // reset to unlocked state
        }
    }

二、NonfairSync源码

    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() {
            //如果是第一次加锁，那么直接设置持有线程即可，不必调用acquire()方法去阻塞等待。
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1);
        }

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

三、FairSync源码

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();
            if (c == 0) {
                //为保证公平锁，除非是第一个，否则不能加锁
                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;
        }
    }

四、ReentrantLock源码实现

public class ReentrantLock implements Lock, java.io.Serializable {
    private static final long serialVersionUID = 7373984872572414699L;

    private final Sync sync;

    abstract static class Sync extends AbstractQueuedSynchronizer {
        // ... 省略 ...
    }

    static final class NonfairSync extends Sync {
        // ... 省略 ...
    }

    static final class FairSync extends Sync {
        // ... 省略 ...
    }

    /**
     * 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();
    }

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

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

    //tryLock()方法默认调用nonfairTryAcquire()，只要锁可用，即使当前有其他线程正在等待，它也会成功
    public boolean tryLock() {
        return sync.nonfairTryAcquire(1);
    }

    public boolean tryLock(long timeout, TimeUnit unit)
            throws InterruptedException {
        return sync.tryAcquireNanos(1, unit.toNanos(timeout));
    }

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

    public Condition newCondition() {
        return sync.newCondition();
    }

    public int getHoldCount() {
        return sync.getHoldCount();
    }

    public boolean isHeldByCurrentThread() {
        return sync.isHeldExclusively();
    }

    public boolean isLocked() {
        return sync.isLocked();
    }

    public final boolean isFair() {
        return sync instanceof FairSync;
    }

    protected Thread getOwner() {
        return sync.getOwner();
    }

    public final boolean hasQueuedThreads() {
        return sync.hasQueuedThreads();
    }

    public final boolean hasQueuedThread(Thread thread) {
        return sync.isQueued(thread);
    }

    public final int getQueueLength() {
        return sync.getQueueLength();
    }

    protected Collection<Thread> getQueuedThreads() {
        return sync.getQueuedThreads();
    }

    public boolean hasWaiters(Condition condition) {
        if (condition == null)
            throw new NullPointerException();
        if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
            throw new IllegalArgumentException("not owner");
        return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
    }

    public int getWaitQueueLength(Condition condition) {
        if (condition == null)
            throw new NullPointerException();
        if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
            throw new IllegalArgumentException("not owner");
        return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition);
    }

    protected Collection<Thread> getWaitingThreads(Condition condition) {
        if (condition == null)
            throw new NullPointerException();
        if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
            throw new IllegalArgumentException("not owner");
        return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
    }

    public String toString() {
        Thread o = sync.getOwner();
        return super.toString() + ((o == null) ?
                                   "[Unlocked]" :
                                   "[Locked by thread " + o.getName() + "]");
    }
}