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对AQS的简单理解及自定义锁的实现

AQS

AQS,即AbstractQueuedSynchronizer,意为队列同步器，是用来构建锁或者其它同步组件的基础框架。

AQS使用一个int类型的state表示同步状态。

AQS使用了模版方法的设计模式，子类继承AQS后，通过实现抽象方法来管理同步状态。

AQS在ReentrantLock、ReentrantReadWriteLock、Semaphore、CountDownLatch等类中都有广泛使用。

ReentrantLock中的AQS

下面以ReentrantLock为例，来看下AQS的具体使用

ReentrantLock中有一个Sync类型的成员变量sync。

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private final Sync sync;

Sync的定义

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abstract static class Sync extends AbstractQueuedSynchronizer {
        //...
    }

Sync是一个继承自AQS的抽象类，只实现了一部分的抽象方法。它有两个子类:

NonfairSync:用于实现非公平锁

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/**
* Sync object for non-fair locks
*/
static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;

        /**
         * Performs lock.  Try immediate barge, backing up to normal
         * acquire on failure.
         */
        final void lock() {
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1);
        }

        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
        }
    }

FairSync:用于实现公平锁

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/**
* Sync object for fair locks
*/
static final class FairSync extends Sync {
        private static final long serialVersionUID = -3000897897090466540L;

        final void lock() {
            acquire(1);
        }

        /**
         * Fair version of tryAcquire.  Don't grant access unless
         * recursive call or no waiters or is first.
         */
        protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                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;
        }
    }

在Sync的子类实现中，主要用到了getState()、setState(int newState)、compareAndSetState(int expect,int update)方法。

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protected final int getState() {
        return state;
    }
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protected final void setState(int newState) {
        state = newState;
    }
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protected final boolean compareAndSetState(int expect, int update) {
        // See below for intrinsics setup to support this
        return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
    }

通过CAS来实现同步操作。

在使用ReentrantLock时，默认是非公平锁

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public ReentrantLock() {
        sync = new NonfairSync();
    }

我们也可以指定使用公平锁或非公平锁

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

CLH队列锁

CLH队列锁即Craig, Landin, and Hagersten (CLH) locks。

CLH队列锁也是一种基于链表的可扩展、高性能、公平的自旋锁，申请线程仅仅在本地变量上自旋，它不断轮询前驱的状态，假设发现前驱释放了锁就结束自旋。

自定义锁

自定义不可重入锁

下面基于AQS实现一个自定义的锁

  1. 定义一个类实现Lock接口
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public class CustomLock implements Lock {
    
    @Override
    public void lock() {

    }

    @Override
    public void lockInterruptibly() throws InterruptedException {

    }

    @Override
    public boolean tryLock() {
        return false;
    }

    @Override
    public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
        return false;
    }

    @Override
    public void unlock() {

    }

    @Override
    public Condition newCondition() {
        return null;
    }
}

可以看到，实现Lock后，生成的默认方法如上。

  1. 定义一个静态内部类继承自AQS,并实现tryAcquire、tryRelease、isHeldExclusively方法
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private static class Sync extends AbstractQueuedSynchronizer {
        @Override
        protected boolean tryAcquire(int arg) {
            if (compareAndSetState(0, 1)) {
                setExclusiveOwnerThread(Thread.currentThread());
                return true;
            }
            return false;
        }

        @Override
        protected boolean tryRelease(int arg) {
            if (getState() == 0) {
                throw new IllegalMonitorStateException();
            }
            setExclusiveOwnerThread(null);
            setState(0);
            return true;
        }

        @Override
        protected boolean isHeldExclusively() {
            return getState() == 1;
        }

        Condition newCondition() {
            return new ConditionObject();
        }
    }
  1. 定义Sync类型的变量，并具体实现Lock接口定义的方法
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private Sync sync = new Sync();

@Override
    public void lock() {
        System.out.println(Thread.currentThread().getName() + " ready get lock");
        sync.acquire(1);
        System.out.println(Thread.currentThread().getName() + " already got lock");
    }

    @Override
    public void lockInterruptibly() throws InterruptedException {
        sync.acquireInterruptibly(1);
    }

    @Override
    public boolean tryLock() {
        return sync.tryAcquire(1);
    }

    @Override
    public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
        return sync.tryAcquireNanos(1, unit.toNanos(time));
    }

    @Override
    public void unlock() {
        System.out.println(Thread.currentThread().getName() + " ready release lock");
        sync.release(1);
        System.out.println(Thread.currentThread().getName() + " already released lock");
    }

    @Override
    public Condition newCondition() {
        return sync.newCondition();
    }
  1. 使用
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Lock lock = new CustomLock();
try{
    lock.lock();
    //执行具体的业务
}
catch(Exception e){
    e.printStackTrace();
}
finally{
    lock.unlock();
}

以上4步，即可以实现简单的自定义锁。

  1. 完整代码
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public class CustomLock implements Lock {

    private Sync sync = new Sync();

    @Override
    public void lock() {
        System.out.println(Thread.currentThread().getName() + " ready get lock");
        sync.acquire(1);
        System.out.println(Thread.currentThread().getName() + " already got lock");
    }

    @Override
    public void lockInterruptibly() throws InterruptedException {
        sync.acquireInterruptibly(1);
    }

    @Override
    public boolean tryLock() {
        return sync.tryAcquire(1);
    }

    @Override
    public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
        return sync.tryAcquireNanos(1, unit.toNanos(time));
    }

    @Override
    public void unlock() {
        System.out.println(Thread.currentThread().getName() + " ready release lock");
        sync.release(1);
        System.out.println(Thread.currentThread().getName() + " already released lock");
    }

    @Override
    public Condition newCondition() {
        return sync.newCondition();
    }

    private static class Sync extends AbstractQueuedSynchronizer {
        @Override
        protected boolean tryAcquire(int arg) {
            if (compareAndSetState(0, 1)) {
                setExclusiveOwnerThread(Thread.currentThread());
                return true;
            }
            return false;
        }

        @Override
        protected boolean tryRelease(int arg) {
            if (getState() == 0) {
                throw new IllegalMonitorStateException();
            }
            setExclusiveOwnerThread(null);
            setState(0);
            return true;
        }

        @Override
        protected boolean isHeldExclusively() {
            return getState() == 1;
        }

        Condition newCondition() {
            return new ConditionObject();
        }
    }
}

自定义可重入锁

和上面的步骤差不多，只需要在继承AQS的类的实现方法中修改即可

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private static class Sync extends AbstractQueuedSynchronizer {
        @Override
        protected boolean tryAcquire(int arg) {
            if (compareAndSetState(0, 1)) {
                setExclusiveOwnerThread(Thread.currentThread());
                return true;
            } else if (getExclusiveOwnerThread() == Thread.currentThread()) {
                setState(getState() + 1);
                return true;
            }
            return false;
        }

        @Override
        protected boolean tryRelease(int arg) {
            if (getExclusiveOwnerThread() != Thread.currentThread()) {
                throw new IllegalMonitorStateException();
            }
            if (getState() == 0) {
                throw new IllegalMonitorStateException();
            }
            setState(getState() - 1);
            if (getState() == 0) {
                setExclusiveOwnerThread(null);
            }
            return true;
        }

        @Override
        protected boolean isHeldExclusively() {
            return getState() > 0;
        }

        Condition newCondition() {
            return new ConditionObject();
        }
    }

完整代码

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public class CustomReentrantLock implements Lock {

    private Sync sync = new Sync();

    @Override
    public void lock() {
        System.out.println(Thread.currentThread().getName() + " ready get lock");
        sync.acquire(1);
        System.out.println(Thread.currentThread().getName() + " already got lock");
    }

    @Override
    public void lockInterruptibly() throws InterruptedException {
        sync.acquireInterruptibly(1);
    }

    @Override
    public boolean tryLock() {
        return sync.tryAcquire(1);
    }

    @Override
    public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
        return sync.tryAcquireNanos(1, unit.toNanos(time));
    }

    @Override
    public void unlock() {
        System.out.println(Thread.currentThread().getName() + " ready release lock");
        sync.release(1);
        System.out.println(Thread.currentThread().getName() + " already released lock");
    }

    @Override
    public Condition newCondition() {
        return sync.newCondition();
    }

    private static class Sync extends AbstractQueuedSynchronizer {
        @Override
        protected boolean tryAcquire(int arg) {
            if (compareAndSetState(0, 1)) {
                setExclusiveOwnerThread(Thread.currentThread());
                return true;
            } else if (getExclusiveOwnerThread() == Thread.currentThread()) {
                setState(getState() + 1);
                return true;
            }
            return false;
        }

        @Override
        protected boolean tryRelease(int arg) {
            if (getExclusiveOwnerThread() != Thread.currentThread()) {
                throw new IllegalMonitorStateException();
            }
            if (getState() == 0) {
                throw new IllegalMonitorStateException();
            }
            setState(getState() - 1);
            if (getState() == 0) {
                setExclusiveOwnerThread(null);
            }
            return true;
        }

        @Override
        protected boolean isHeldExclusively() {
            return getState() > 0;
        }

        Condition newCondition() {
            return new ConditionObject();
        }
    }
}