我们已经了解了AQS的的基本原理了，不如自己仿照其它锁实现一个排他锁吧。

要求如下：同一时间只允许一个线程持有锁，不要求可重入(反复加锁直接忽视即可)。

(1)实现Lock接口

复制
public class Demo23 {
    public static void main(String[] args) {
 
    }
 
    private static class myLock 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;
        }
    }
}

(2) 创建内部类sync继承AQS，内部调用sync完成逻辑

复制
public class Demo23 {
    public static void main(String[] args) {
			 myLock lock = new myLock();
        lock.lock();
    }
 
    private static class myLock implements Lock{
 
        private class sync extends AbstractQueuedSynchronizer {
 
            @Override 
            protected boolean tryAcquire(int arg) {
                return super.tryAcquire(arg);
            }
 
            @Override 
            protected boolean isHeldExclusively() {
                return getExclusiveOwnerThread() == Thread.currentThread();
            }
 
            protected  Condition newCondition() {
                return new ConditionObject();
            }
 
        }
 
        sync sync = new sync();
 
        @Override 
        public void lock() {
            sync.acquire(1);
        }
 
        @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() {
            sync.release(1);
        }
 
        @Override 
        public Condition newCondition() {
            return sync.newCondition();
        }
    }
}

上面代码的运行结果如下。

复制
/home/wangzhou/IdeaProjects/jucdemo/src/com/wangzhou/Demo21.java:11:24
java: 未报告的异常错误java.lang.InterruptedException; 必须对其进行捕获或声明以便抛出

这是因为其父类的tryAcquire方法实现如下。

复制
    protected boolean tryAcquire(int arg) {
        throw new UnsupportedOperationException();
    }

(3)实现tryAcquire 先看看其它锁怎么做的。比如公平锁。

复制
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;
}

上面代码包含可重入逻辑，我们这里如果重入直接忽略，返回false，因此核心逻辑就只需要进行一个CAS操作了。

复制
  @Override
protected boolean tryAcquire(int acquires) {
    if(isHeldExclusively()) {
        return true;
    }
    if (compareAndSetState(0, acquires)) {
        setExclusiveOwnerThread(Thread.currentThread());
        return true;
    }
    return false;
}

加锁过程已经完成，您可以自行测试，接下来我们来实现解锁功能。

同样先看看其它锁怎么实现。

复制
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;
}

我们要实现的逻辑也很简单，没加锁的情况下不允许解锁。加锁的情况下解锁。

复制
 @Override
protected boolean tryRelease(int args) {
    if(getState() == 0) {
        throw new UnsupportedOperationException();
    }
    if(isHeldExclusively()) {
        setExclusiveOwnerThread(null);
        setState(0);
        return true;
    }
    return false;
}

测试.

复制
public static void main(String[] args) throws InterruptedException {
    myLock lock = new myLock();
    lock.lock();
    new Thread(() ->{
        lock.lock();
        System.out.println("thread2 get lock");
        lock.unlock();
    }).start();
    TimeUnit.SECONDS.sleep(1);
    lock.unlock();
    System.out.println("thread 1 unlock");
}

condition请读者自测。