聊聊canal的BooleanMutex

序

本文主要研究一下canal的BooleanMutex

BooleanMutex

canal-1.1.4/common/src/main/java/com/alibaba/otter/canal/common/utils/BooleanMutex.java

public class BooleanMutex {

    private Sync sync;

    public BooleanMutex(){
        sync = new Sync();
        set(false);
    }

    public BooleanMutex(Boolean mutex){
        sync = new Sync();
        set(mutex);
    }

    /**
     * 阻塞等待Boolean为true
     * 
     * @throws InterruptedException
     */
    public void get() throws InterruptedException {
        sync.innerGet();
    }

    /**
     * 阻塞等待Boolean为true,允许设置超时时间
     * 
     * @param timeout
     * @param unit
     * @throws InterruptedException
     * @throws TimeoutException
     */
    public void get(long timeout, TimeUnit unit) throws InterruptedException, TimeoutException {
        sync.innerGet(unit.toNanos(timeout));
    }

    /**
     * 重新设置对应的Boolean mutex
     * 
     * @param mutex
     */
    public void set(Boolean mutex) {
        if (mutex) {
            sync.innerSetTrue();
        } else {
            sync.innerSetFalse();
        }
    }

    public boolean state() {
        return sync.innerState();
    }

    //......
}

• BooleanMutex定义了sync属性，其get方法执行的是sync.innerGet()，其set方法执行的是sync.innerSetTrue()或者sync.innerSetFalse()，其state方法返回的是sync.innerState()

Sync

canal-1.1.4/common/src/main/java/com/alibaba/otter/canal/common/utils/BooleanMutex.java

    private final class Sync extends AbstractQueuedSynchronizer {

        private static final long serialVersionUID = 2559471934544126329L;
        /** State value representing that TRUE */
        private static final int  TRUE             = 1;
        /** State value representing that FALSE */
        private static final int  FALSE            = 2;

        private boolean isTrue(int state) {
            return (state & TRUE) != 0;
        }

        /**
         * 实现AQS的接口，获取共享锁的判断
         */
        protected int tryAcquireShared(int state) {
            // 如果为true，直接允许获取锁对象
            // 如果为false，进入阻塞队列，等待被唤醒
            return isTrue(getState()) ? 1 : -1;
        }

        /**
         * 实现AQS的接口，释放共享锁的判断
         */
        protected boolean tryReleaseShared(int ignore) {
            // 始终返回true，代表可以release
            return true;
        }

        boolean innerState() {
            return isTrue(getState());
        }

        void innerGet() throws InterruptedException {
            acquireSharedInterruptibly(0);
        }

        void innerGet(long nanosTimeout) throws InterruptedException, TimeoutException {
            if (!tryAcquireSharedNanos(0, nanosTimeout)) throw new TimeoutException();
        }

        void innerSetTrue() {
            for (;;) {
                int s = getState();
                if (s == TRUE) {
                    return; // 直接退出
                }
                if (compareAndSetState(s, TRUE)) {// cas更新状态，避免并发更新true操作
                    releaseShared(0);// 释放一下锁对象，唤醒一下阻塞的Thread
                    return;
                }
            }
        }

        void innerSetFalse() {
            for (;;) {
                int s = getState();
                if (s == FALSE) {
                    return; // 直接退出
                }
                if (compareAndSetState(s, FALSE)) {// cas更新状态，避免并发更新false操作
                    return;
                }
            }
        }

    }

• Sync继承了AbstractQueuedSynchronizer，其tryReleaseShared方法始终返回true，其innerGet方法执行的是acquireSharedInterruptibly方法，其innerSetTrue方法执行compareAndSetState(s, TRUE)，其innerSetFalse执行compareAndSetState(s, FALSE)

小结

BooleanMutex定义了sync属性，其get方法执行的是sync.innerGet()，其set方法执行的是sync.innerSetTrue()或者sync.innerSetFalse()，其state方法返回的是sync.innerState()

doc

• BooleanMutex

来源：oschina

链接：https://my.oschina.net/go4it/blog/3221517