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為true
     * 
     * @throws InterruptedException
     */
    public void get() throws InterruptedException {
        sync.innerGet();
    }

    /**
     * 阻塞等待Boolean為true,允許設定超時時間
     * 
     * @param timeout
     * @param unit
     * @throws InterruptedException
     * @throws TimeoutException
     */
    public void get(long timeout, TimeUnit unit) throws InterruptedException, TimeoutException {
        sync.innerGet(unit.toNanos(timeout));
    }

    /**
     * 重新設定對應的Boolean mutex
     * 
     * @param mutex
     */
    public void set(Boolean mutex) {
        if (mutex) {
            sync.innerSetTrue();
        } else {
            sync.innerSetFalse();
        }
    }

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

    //......
}
複製程式碼

• BooleanMutex定義了sync屬性，其get方法執行的是sync.innerGet()，其set方法執行的是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;
        }

        /**
         * 實現AQS的介面，獲取共享鎖的判斷
         */
        protected int tryAcquireShared(int state) {
            // 如果為true，直接允許獲取鎖物件
            // 如果為false，進入阻塞佇列，等待被喚醒
            return isTrue(getState()) ? 1 : -1;
        }

        /**
         * 實現AQS的介面，釋放共享鎖的判斷
         */
        protected boolean tryReleaseShared(int ignore) {
            // 始終返回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更新狀態，避免併發更新true操作
                    releaseShared(0);// 釋放一下鎖物件，喚醒一下阻塞的Thread
                    return;
                }
            }
        }

        void innerSetFalse() {
            for (;;) {
                int s = getState();
                if (s == FALSE) {
                    return; // 直接退出
                }
                if (compareAndSetState(s, FALSE)) {// cas更新狀態，避免併發更新false操作
                    return;
                }
            }
        }

    }
複製程式碼

• Sync繼承了AbstractQueuedSynchronizer，其tryReleaseShared方法始終返回true，其innerGet方法執行的是acquireSharedInterruptibly方法，其innerSetTrue方法執行compareAndSetState(s, TRUE)，其innerSetFalse執行compareAndSetState(s, FALSE)

小結

BooleanMutex定義了sync屬性，其get方法執行的是sync.innerGet()，其set方法執行的是sync.innerSetTrue()或者sync.innerSetFalse()，其state方法返回的是sync.innerState()

doc

• BooleanMutex