AQS详解

一、什么是AQS

        Java并发编程核心在于java.concurrent.util包而juc当中的大多数同步器实现都是围绕着共同的基础行为，比如等待队列、条件队列、独占获取、共享获取等，而这个行为的抽象就是基于AbstractQueuedSynchronizer简称AQS，AQS定义了一套多线程访问共享资源 的同步器框架，是一个依赖状态(state)的同步器。

一、AQS具备的特性

二、AQS内部维护属性volatile int state

    /**
     * The synchronization state.
     */
    private volatile int state;

三、State三种访问方式

    /**
     * Returns the current value of synchronization state.
     * This operation has memory semantics of a {@code volatile} read.
     * @return current state value
     */
    protected final int getState() {
        return state;
    }

    /**
     * Sets the value of synchronization state.
     * This operation has memory semantics of a {@code volatile} write.
     * @param newState the new state value
     */
    protected final void setState(int newState) {
        state = newState;
    }

    /**
     * Atomically sets synchronization state to the given updated
     * value if the current state value equals the expected value.
     * This operation has memory semantics of a {@code volatile} read
     * and write.
     *
     * @param expect the expected value
     * @param update the new value
     * @return {@code true} if successful. False return indicates that the actual
     *         value was not equal to the expected value.
     */
    protected final boolean compareAndSetState(int expect, int update) {
        // See below for intrinsics setup to support this
        return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
    }

四、AQS定义两种资源共享方式

五、AQS的两种定义队列

 六、AQS 定义了5个队列中节点状态

        /** waitStatus value to indicate thread has cancelled */
        static final int CANCELLED =  1;
        /** waitStatus value to indicate successor's thread needs unparking */
        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;

  七、不同的自定义同步器竞争共享资源的方式也不同。自定义同步器在实现时只需要实现共享资源state的获取与释放方式即可，至于具体线程等待队列的维护（如获取资源失败入队/唤醒出队等），AQS已经在顶层实现好了。自定义同步器实现时主要实现以下几种方法

    //以下方法根据不同的子类同步器重写方法，使用了设计模式的模板模式
    protected boolean tryAcquire(int arg) {
        throw new UnsupportedOperationException();
    }
    protected boolean tryRelease(int arg) {
        throw new UnsupportedOperationException();
    }
    protected int tryAcquireShared(int arg) {
        throw new UnsupportedOperationException();
    }
    protected boolean tryReleaseShared(int arg) {
        throw new UnsupportedOperationException();
    }
    protected boolean isHeldExclusively() {
        throw new UnsupportedOperationException();
    }

八、同步等待队列

九、条件等待队列 

AQS中条件队列是使用单向列表保存的，用nextWaiter来连接:

Condition接口

@Slf4j
public class ConditionTest {

    public static void main(String[] args) {

        Lock lock = new ReentrantLock();
        Condition condition = lock.newCondition();

        new Thread(() -> {
            lock.lock();
            try {
                log.debug(Thread.currentThread().getName() + " 开始处理任务");
                condition.await();
                log.debug(Thread.currentThread().getName() + " 结束处理任务");
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                lock.unlock();
            }
        }).start();

        new Thread(() -> {
            lock.lock();
            try {
                log.debug(Thread.currentThread().getName() + " 开始处理任务");

                Thread.sleep(2000);
                condition.signal();
                log.debug(Thread.currentThread().getName() + " 结束处理任务");
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                lock.unlock();
            }
        }).start();
    }