ReentrantLock 非公平锁实现原理详解

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

 // ReentrantLock.java
 public void lock() {
     sync.lock();
 }
 ​
 public boolean tryLock() {
     return sync.nonfairTryAcquire(1);
 }
 ​
 public void unlock() {
     sync.release(1);
 }

 // NonfairSync.java
 final void lock() {
     if (compareAndSetState(0, 1))
         setExclusiveOwnerThread(Thread.currentThread());
     else
         acquire(1);
 }

 // AbstractQueuedSynchronizer.java
 private volatile int state;
 protected final boolean compareAndSetState(int expect, int update) {
     // See below for intrinsics setup to support this
     return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
 }

 // AbstractOwnableSynchronizer.java
 private transient Thread exclusiveOwnerThread;
 ​
 protected final void setExclusiveOwnerThread(Thread thread) {
     exclusiveOwnerThread = thread;
 }

 // NonfairSync.java
 final void lock() {
     if (compareAndSetState(0, 1))
         setExclusiveOwnerThread(Thread.currentThread());
     else
         acquire(1);
 }
 ​
 // AbstractQueuedSynchronizer.java
 public final void acquire(int arg) {
     if (!tryAcquire(arg) &&
         acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
         selfInterrupt();
 }

 // NonfairSync.java
 protected final boolean tryAcquire(int acquires) {
     return nonfairTryAcquire(acquires);
 }

 // Sync.java
 final boolean nonfairTryAcquire(int acquires) {
     final Thread current = Thread.currentThread();
     // 获取当前同步资源状态
     int c = getState();
     // 同步资源空闲
     if (c == 0) {
         // 占用同步资源更新资源状态
         if (compareAndSetState(0, acquires)) {
             // 设置当前线程为资源对应的独占锁持有者
             setExclusiveOwnerThread(current);
             return true;
         }
     }
     // 如果当前线程就是持有锁线程，更新锁状态，直接持有锁
     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;
 }

// AbstractQueuedSynchronizer.java
private Node enq(final Node node) {
    for (;;) {
        Node t = tail;
        if (t == null) { // Must initialize
            if (compareAndSetHead(new Node()))
                tail = head;
        } else {
            node.prev = t;
            if (compareAndSetTail(t, node)) {
                t.next = node;
                return t;
            }
        }
    }
}

// AbstractQueuedSynchronizer.java
private Node addWaiter(Node mode) {
    // 创建新的Node对象
    Node node = new Node(Thread.currentThread(), mode);
    // Try the fast path of enq; backup to full enq on failure
    Node pred = tail;
    if (pred != null) {
        node.prev = pred;
        if (compareAndSetTail(pred, node)) {
            pred.next = node;
            return node;
        }
    }
    enq(node);
    return node;
}

// Node实体类
static final class Node {
    ...
    // 前一个元素
    volatile Node prev;
    // 下一个元素
    volatile Node next;
    // 线程对象
    volatile Thread thread;
    Node() {    // Used to establish initial head or SHARED marker
    }
    Node(Thread thread, Node mode) {     // Used by addWaiter
        this.nextWaiter = mode;
        this.thread = thread;
    }
    Node(Thread thread, int waitStatus) { // Used by Condition
        this.waitStatus = waitStatus;
        this.thread = thread;
    }
    ...
}
// AbstractQueuedSynchronizer.java中声明Node对象
// 链表头
private transient volatile Node head;
// 链表尾部
private transient volatile Node tail;

// AbstractQueuedSynchronizer.java
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);
    }
}

// AbstractQueuedSynchronizer.java
private final boolean parkAndCheckInterrupt() {
    LockSupport.park(this);
    return Thread.interrupted();
}

// ReentrantLock.java
public void unlock() {
    sync.release(1);
}
// AQS.java
public final boolean release(int arg) {
    if (tryRelease(arg)) {
        Node h = head;
        if (h != null && h.waitStatus != 0)
            unparkSuccessor(h);
        return true;
    }
    return false;
}

static final class Node {
    ...
    // waitStatus的四种可能取值，表示当前节点及其后续节点对应线程的运行状态
    static final int CANCELLED =  1;
    static final int SIGNAL    = -1;
    static final int CONDITION = -2;
    static final int PROPAGATE = -3;
    // 声明waitStatus
    volatile int waitStatus;
    ...
}

protected final boolean tryRelease(int releases) {
    // 获取AQS的锁状态标记，计算剩余锁状态标记
    int c = getState() - releases;
    if (Thread.currentThread() != getExclusiveOwnerThread())
        throw new IllegalMonitorStateException();
    boolean free = false;
    // 锁状态标记为0，当前没有其他线程持有锁，锁处于空闲状态，设置锁持有者为null
    if (c == 0) {
        free = true;
        setExclusiveOwnerThread(null);
    }
    // 更新锁状态
    setState(c);
    return free;
}

private void unparkSuccessor(Node node) {
    // 获取头节点的waitStatus状态，将头节点的waitStatus状态设置为0，head waitStatus恢复初始状态
    int ws = node.waitStatus;
    if (ws < 0)
        compareAndSetWaitStatus(node, ws, 0);
    // 获取头节点的下一个节点，如果下一个节点为null或者状态为取消状态(waitStatus大于0只有取消状态)，则从尾节点开始遍历，查找未被取消的后续节点对象
    Node s = node.next;
    if (s == null || s.waitStatus > 0) {
        s = null;
        for (Node t = tail; t != null && t != node; t = t.prev)
            if (t.waitStatus <= 0)
                s = t;
    }
    // 对上一步获取到的节点对应的线程执行unpark唤醒，去抢占锁
    if (s != null)
        LockSupport.unpark(s.thread);
}