ReentrantLock源码解析

妖狐艹你老母 2022-05-08 09:12 212阅读 0赞

ReentrantLock支持公平锁和非公平锁，默认构造使用非公平锁实现。

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

首先查看获取锁 `lock` 方法

// ReentrantLock.NonfairSync final void lock() { // CAS设置同步状态state为1，成功表明获取锁成功，设置当前线程为独占锁的持有线程 if (compareAndSetState(0, 1)) setExclusiveOwnerThread(Thread.currentThread()); else acquire(1); // CAS失败，重新尝试获取锁 }

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

// ReentrantLock.NonfairSync protected final boolean tryAcquire(int acquires) { return nonfairTryAcquire(acquires); // 尝试获取非公共独占锁 }

// ReentrantLock.Sync final boolean nonfairTryAcquire(int acquires) { final Thread current = Thread.currentThread(); int c = getState(); // 获取同步状态state值 if (c == 0) { // 0代表没有被线程持有，CAS获取锁，获取成功则将当前线程设置为独占锁的持有线程 if (compareAndSetState(0, acquires)) { setExclusiveOwnerThread(current); return true; } } // 当前线程与独占锁持有者线程相同 else if (current == getExclusiveOwnerThread()) { int nextc = c + acquires; if (nextc < 0) // overflow throw new Error("Maximum lock count exceeded"); setState(nextc); // 设置锁重入次数 return true; } return false; }

对于公平锁实现，仅仅在 `tryAcquire` 中添加判断是否有等待获取锁的线程等待时间比当前线程久。

// ReentrantLock.FairSync protected final boolean tryAcquire(int acquires) { final Thread current = Thread.currentThread(); int c = getState(); if (c == 0) { // 相比非公平锁，添加判断是否有线程等待获取锁的时间比当前线程还久 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; }

释放锁 `unlock` 方法

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

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

// ReentrantLock.Sync protected final boolean tryRelease(int releases) { int c = getState() - releases; // 递减锁重入次数 // 判断当前线程是否为独占锁持有线程，否则抛出异常 if (Thread.currentThread() != getExclusiveOwnerThread()) throw new IllegalMonitorStateException(); boolean free = false; // 锁释放标识 // c等于0，说明锁已经释放，设置锁释放标识为true if (c == 0) { free = true; setExclusiveOwnerThread(null); // 置空独占锁持有线程 } setState(c); // 设置同步状态state return free; // 返回锁释放标识 }