Lock简介

文章转自于技术人成长之路:https://mp.weixin.qq.com/s/Xl5X30dbIYKE-1lB9NuXAw

Concurrent包的结构层次

在针对并发编程中,Doug Lea大师为我们提供了大量实用,高性能的工具类,针对这些代码进行研究会让我们对并发编程的掌握更加透彻也会大大提升我们对并发编程技术的热爱。这些代码在java.util.concurrent包下。如下图,即为concurrent包的目录结构图。

Lock简介

其中包含了两个子包:atomic以及lock,另外在concurrent下的阻塞队列以及executors,这些就是concurrent包中的精华,之后会一一进行学习。而这些类的实现主要是依赖于volatile以及CAS,从整体上来看concurrent包的整体实现图如下图所示:

Lock简介

Lock简介

我们下来看concurent包下的lock子包。锁是用来控制多个线程访问共享资源的方式,一般来说,一个锁能够防止多个线程同时访问共享资源。在Lock接口出现之前,java程序主要是靠synchronized关键字实现锁功能的,而java SE5之后,并发包中增加了lock接口,它提供了与synchronized一样的锁功能。虽然它失去了像synchronize关键字隐式加锁解锁的便捷性,但是却拥有了锁获取和释放的可操作性,可中断的获取锁以及超时获取锁等多种synchronized关键字所不具备的同步特性。通常使用显示使用lock的形式如下:

Lock lock = new ReentrantLock();
lock.lock();
try {
.......
} finally {
   lock.unlock();
}

需要注意的是synchronized同步块执行完成或者遇到异常是锁会自动释放,而lock必须调用unlock()方法释放锁,因此在finally块中释放锁。

Lock接口API

我们现在就来看看lock接口定义了哪些方法:

 //获取锁
void lock();
//获取锁的过程能够响应中断
void lockInterruptibly() throws InterruptedException;
//非阻塞式响应中断能立即返回,获取锁放回true反之返回fasle
boolean tryLock();
//超时获取锁,在未超时内或者未中断的情况下能够获取锁
boolean tryLock(long time, TimeUnit unit) throws InterruptedException;
//释放锁
void unlock();

//获取与lock绑定的等待通知组件,当前线程必须获得了锁才能进行等待,进行等待时会先释放锁,当再次获取锁时才能从等待中返回
Condition newCondition();

上面是lock接口下的六个方法,也只是从源码中英译中翻译了一遍,感兴趣的可以自己的去看看。那么在locks包下有哪些类实现了该接口了?先从最熟悉的ReentrantLock说起。

public class ReentrantLock implements Lock, java.io.Serializable

很显然ReentrantLock实现了lock接口,接下来我们来仔细研究一下它是怎样实现的。当你查看源码时你会惊讶的发现ReentrantLock并没有多少代码,另外有一个很明显的特点是:基本上所有的方法的实现实际上都是调用了其静态内存类**Sync**中的方法,而**Sync**类继承了**AbstractQueuedSynchronizer(AQS)**。可以看出要想理解ReentrantLock关键核心在于对队列同步器AbstractQueuedSynchronizer(简称同步器)的理解。

private final Sync sync;

    /**
     * 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;

        /**
         * Performs {@link Lock#lock}. The main reason for subclassing
         * is to allow fast path for nonfair version.
         */
        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) {
                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;
        }

        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;
        }

        protected final boolean isHeldExclusively() {
            // While we must in general read state before owner,
            // we don‘t need to do so to check if current thread is owner
            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).
         */
        private void readObject(java.io.ObjectInputStream s)
            throws java.io.IOException, ClassNotFoundException {
            s.defaultReadObject();
            setState(0); // reset to unlocked state
        }
    }

Lock简介

Lock简介

Lock简介

Lock简介

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