1. ReentrantLock是什么

Lock提供了比synchronized方法和语句更广泛的锁定操作。更灵活的结构化，并且支持多个相关联的对象Condition。它实现了Lock、Serializable序列化接口。

图1 ReentrantLock实现接口图

1.1 Lock

1.1.1 lock

// 获取锁
void lock();

1.1.2 lockInterruptibly

// 跟lock一样的功能--获取锁，但是lock中途不能被终端，lockInterruptibly允许中途被中断
void lockInterruptibly();

1.1.3 tryLock

// 尝试获取锁，成功true，失败false
boolean tryLock();

1.1.4 unlock

// 解锁，在finally中使用，不然容易死锁
void unlock();

1.1.5 newCondition

// 获取Condition对象（await、signal）
Condition newCondition();

1.2 ReentrantLock源码剖析

图2 ReentrantLock总方法

1.2.1 非公平锁

// 无参构造方法
public ReentrantLock() {
    // 创建非公平锁
    sync = new NonfairSync();
}

1.2.1.1 NonfairSync

// 非公平锁继承Sync
static final class NonfairSync extends Sync {

    // 序列化ID
    private static final long serialVersionUID = 7316153563782823691L;
    
    // 获取锁
    final void lock() {
        // 通过Unsafe接口的CAS函数设置值，将对象的设置为使用状态
        if (compareAndSetState(0, 1))
            // 使用对象为当前操作线程
            setExclusiveOwnerThread(Thread.currentThread());
        else
            // 获取不到，则加入等待队列
            acquire(1);
    }
    
    // 尝试获取锁
    protected final boolean tryAcquire(int acquires) {
        return nonfairTryAcquire(acquires);
    }
}

1.2.1.2 acquire

// 加入等待队列
public final void acquire(int arg) {
    if (!tryAcquire(arg) &&
        acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
        selfInterrupt();
}

1.2.1.3 tryAcquire

// 尝试是否为可重入锁
protected final boolean tryAcquire(int acquires) {
    return nonfairTryAcquire(acquires);
}

// 
final boolean nonfairTryAcquire(int acquires) {
    // 获取当前线程
    final Thread current = Thread.currentThread();
    // 查看对象锁状态
    int c = getState();
    // 空闲状态
    if (c == 0) {
        // CAS设置占有状态
        if (compareAndSetState(0, acquires)) {
            // 设置当前线程持有
            setExclusiveOwnerThread(current);
            return true;
        }
    }
    // 非空闲，但占有对象的线程为当前线程，则按可重入锁解决
    else if (current == getExclusiveOwnerThread()) {
        // 统计可重入锁次数
        int nextc = c + acquires;
        // 非法次数
        if (nextc < 0) // overflow
            throw new Error("Maximum lock count exceeded");
        // 设置对象可重入锁次数
        setState(nextc);
        return true;
    }
    return false;
}

1.2.1.4 Sync

// 核心！抽象队列同步器(AQS)
abstract static class Sync extends AbstractQueuedSynchronizer {
    // 序列ID
    private static final long serialVersionUID = -5179523762034025860L;

    // 获取锁
    abstract void lock();

    // 非公平锁实现方法，跟上面一样，不重复解释
    final boolean nonfairTryAcquire(int acquires) {
        final Thread current = Thread.currentThread();
        int c = getState();
        if (c == 0) {
            if (compareAndSetState(0, acquires)) {
                setExclusiveOwnerThread(current);
                return true;
            }
        }
        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;
    }
    
    // 尝试释放锁
    protected final boolean tryRelease(int releases) {
        // 统计对象可重入锁次数
        int c = getState() - releases;
        // 当前线程不是对象持有锁对象，非法操作
        if (Thread.currentThread() != getExclusiveOwnerThread())
            throw new IllegalMonitorStateException();
        // 对象操作中，有点"内存屏障"的意思
        boolean free = false;
        // 如果状态为0，不用操作了，没有线程占有该对象
        if (c == 0) {
            // 对象可操作状态
            free = true;
            // 对象无线程占有
            setExclusiveOwnerThread(null);
        }
        // 设置对象可重入锁次数
        setState(c);
        return free;
    }
    
    // 判断当前对象占有线程是否为此时操作线程，可重入锁的必要检查
    protected final boolean isHeldExclusively() {
        return getExclusiveOwnerThread() == Thread.currentThread();
    }
    
    // 获取Condition对象，可对对象进行阻塞（await）和就绪（signal）状态操作
    final ConditionObject newCondition() {
        return new ConditionObject();
    }

    
    // 获取对象占用线程
    final Thread getOwner() {
        return getState() == 0 ? null : getExclusiveOwnerThread();
    }
    
    // 获取对象锁可重入次数
    final int getHoldCount() {
        return isHeldExclusively() ? getState() : 0;
    }
    
    // 对象是否被占用
    final boolean isLocked() {
        return getState() != 0;
    }
}

1.2.2 公平锁

// 传入true，则为创建公平锁
public ReentrantLock(boolean fair) {
    sync = fair ? new FairSync() : new NonfairSync();
}

1.2.2.1 FairSync

// 公平锁
static final class FairSync extends Sync {
    // 序列ID
    private static final long serialVersionUID = -3000897897090466540L;
    
    // 获取锁
    final void lock() {
        acquire(1);
    }

    // 获取可重入锁
    protected final boolean tryAcquire(int acquires) {
        // 当前线程
        final Thread current = Thread.currentThread();
        // 统计对象可重入次数
        int c = getState();
        // 对象无锁状态
        if (c == 0) {
            // 队列为空（这里与非公平不一样，队列没有等待线程他才可以持有）
            if (!hasQueuedPredecessors() &&
                // 设置对象占用状态
                compareAndSetState(0, acquires)) {
                // 设置值对象占用线程
                setExclusiveOwnerThread(current);
                return true;
            }
        }
        // 与非公平锁解释相同
        else if (current == getExclusiveOwnerThread()) {
            int nextc = c + acquires;
            if (nextc < 0)
                throw new Error("Maximum lock count exceeded");
            setState(nextc);
            return true;
        }
        return false;
    }
}

1.2.2.2 acquire

// 获取锁
public final void acquire(int arg) {
    // 获取失败
    if (!tryAcquire(arg) &&
        // 加入队列成功
        acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
        // 设置线程中断状态
        selfInterrupt();
}

1.3 AbstractQueuedSynchronizer

图3 AQS祖宗图

关键其实就是等待队列，也叫"CLH"（Craig, Landin, and Hagersten）锁队列。

图4 CLH结构图（双端队列）

1.3.1 Node节点

static final class Node {

    // 共享节点
    static final Node SHARED = new Node();
    // 排他节点
    static final Node EXCLUSIVE = null;

    // 取消状态：其实就是废物了，等待被GC
    static final int CANCELLED =  1;
    // 
    static final int SIGNAL    = -1;
    /** waitStatus value to indicate thread is waiting on condition */
    static final int CONDITION = -2;
    /**
     * waitStatus value to indicate the next acquireShared should
     * unconditionally propagate
     */
    static final int PROPAGATE = -3;

    /**
     * Status field, taking on only the values:
     *   SIGNAL:     The successor of this node is (or will soon be)
     *               blocked (via park), so the current node must
     *               unpark its successor when it releases or
     *               cancels. To avoid races, acquire methods must
     *               first indicate they need a signal,
     *               then retry the atomic acquire, and then,
     *               on failure, block.
     *   CANCELLED:  This node is cancelled due to timeout or interrupt.
     *               Nodes never leave this state. In particular,
     *               a thread with cancelled node never again blocks.
     *   CONDITION:  This node is currently on a condition queue.
     *               It will not be used as a sync queue node
     *               until transferred, at which time the status
     *               will be set to 0. (Use of this value here has
     *               nothing to do with the other uses of the
     *               field, but simplifies mechanics.)
     *   PROPAGATE:  A releaseShared should be propagated to other
     *               nodes. This is set (for head node only) in
     *               doReleaseShared to ensure propagation
     *               continues, even if other operations have
     *               since intervened.
     *   0:          None of the above
     *
     * The values are arranged numerically to simplify use.
     * Non-negative values mean that a node doesn't need to
     * signal. So, most code doesn't need to check for particular
     * values, just for sign.
     *
     * The field is initialized to 0 for normal sync nodes, and
     * CONDITION for condition nodes.  It is modified using CAS
     * (or when possible, unconditional volatile writes).
     */
    volatile int waitStatus;

    /**
     * Link to predecessor node that current node/thread relies on
     * for checking waitStatus. Assigned during enqueuing, and nulled
     * out (for sake of GC) only upon dequeuing.  Also, upon
     * cancellation of a predecessor, we short-circuit while
     * finding a non-cancelled one, which will always exist
     * because the head node is never cancelled: A node becomes
     * head only as a result of successful acquire. A
     * cancelled thread never succeeds in acquiring, and a thread only
     * cancels itself, not any other node.
     */
    volatile Node prev;

    /**
     * Link to the successor node that the current node/thread
     * unparks upon release. Assigned during enqueuing, adjusted
     * when bypassing cancelled predecessors, and nulled out (for
     * sake of GC) when dequeued.  The enq operation does not
     * assign next field of a predecessor until after attachment,
     * so seeing a null next field does not necessarily mean that
     * node is at end of queue. However, if a next field appears
     * to be null, we can scan prev's from the tail to
     * double-check.  The next field of cancelled nodes is set to
     * point to the node itself instead of null, to make life
     * easier for isOnSyncQueue.
     */
    volatile Node next;

    /**
     * The thread that enqueued this node.  Initialized on
     * construction and nulled out after use.
     */
    volatile Thread thread;

    /**
     * Link to next node waiting on condition, or the special
     * value SHARED.  Because condition queues are accessed only
     * when holding in exclusive mode, we just need a simple
     * linked queue to hold nodes while they are waiting on
     * conditions. They are then transferred to the queue to
     * re-acquire. And because conditions can only be exclusive,
     * we save a field by using special value to indicate shared
     * mode.
     */
    Node nextWaiter;

    /**
     * Returns true if node is waiting in shared mode.
     */
    final boolean isShared() {
        return nextWaiter == SHARED;
    }

    /**
     * Returns previous node, or throws NullPointerException if null.
     * Use when predecessor cannot be null.  The null check could
     * be elided, but is present to help the VM.
     *
     * @return the predecessor of this node
     */
    final Node predecessor() throws NullPointerException {
        Node p = prev;
        if (p == null)
            throw new NullPointerException();
        else
            return p;
    }

    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;
    }
}