ReentrantLock

Posted 2021-02-28 zerodslearnjava

ReentrantLock

1 数据结构

从上图可以看出，ReentrantLock的功能都是通过sync这个对象提供的。

public class ReentrantLock implements Lock, java.io.Serializable {
    private final Sync sync;
}

2 获取锁是否要走公平非公平逻辑

• 区分公平非公平锁，公平锁老实排队
  • lock
  • lockInterruptibly
  • tryLock(long timeout, TimeUnit unit)
• 不区分是否公平，如果锁可获取就去竞争
  • tryLock()

3 源码解析

/**
 * 锁的同步控制的基础，抽象类提供了基本处理框架，具体逻辑由子类实现
 * 使用AQS的状态作为持有锁的数目
 */
abstract static class Sync extends AbstractQueuedSynchronizer {
    private static final long serialVersionUID = -5179523762034025860L;

    abstract void lock();

    // 执行非公平trylock
    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;
    }

    // 释放
    protected final boolean tryRelease(int releases) {
        int c = getState() - releases;
        if (Thread.currentThread() != getExclusiveOwnerThread())
            throw new IllegalMonitorStateException();
        boolean free = false;
        if (c == 0) {
            free = true;
            setExclusiveOwnerThread(null);
        }
        setState(c);
        return free;
    }

    // 检查当前线程是不是持有锁
    protected final boolean isHeldExclusively() {
        return getExclusiveOwnerThread() == Thread.currentThread();
    }

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

    // 获取持有锁的线程
    final Thread getOwner() {
        return getState() == 0 ? null : getExclusiveOwnerThread();
    }

    // 获取持有锁的次数
    final int getHoldCount() {
        return isHeldExclusively() ? getState() : 0;
    }

    // 是否加锁了
    final boolean isLocked() {
        return getState() != 0;
    }
}