jdk7 AbstractQueuedSynchronizer（AQS） 應用分析

//先拿ReentrantLock分析看看
public class ReentrantLock implements Lock, java.io.Serializable {
    private static final long serialVersionUID = 7373984872572414699L;
    /** Synchronizer providing all implementation mechanics */
    private final Sync sync;//獲取鎖，和釋放鎖，都是變量操做

    /**
     * Base of synchronization control for this lock. Subclassed
     * into fair and nonfair versions below. Uses AQS state to
     * represent the number of holds on the lock. Sync 繼承自AbstractQueuedSynchronizer（所謂AQS）
     * 這個Sync是個抽象類，下面公平鎖FairSync和非公平鎖NonfairSync會實現它
     */
    abstract static class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = -5179523762034025860L;

        /**
         * Performs {@link Lock#lock}. The main reason for subclassing
         * is to allow fast path for nonfair version.
         */
        abstract void lock();//等着公平鎖和非公平鎖的實現

        /**
         * Performs non-fair tryLock.  tryAcquire is
         * implemented in subclasses, but both need nonfair
         * try for trylock method.嘗試獲取鎖
         */
        final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();//是它，就是它，來獲取【依賴狀態變量】，鎖的根基變量
            if (c == 0) {
                if (compareAndSetState(0, acquires)) {//是它，就是它，經過cas原則更新【依賴狀態變量】，鎖的根基方法，更新成功了，可理解是獲取鎖成功，更新失敗了，就算獲取鎖失敗（jdk裏本沒有鎖）
                    setExclusiveOwnerThread(current);//設置當前線程爲 排他的全部者
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {//若是當前線程已是全部者了
                int nextc = c + acquires;//增長 狀態變量值
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);//是它，就是它，更新 【依賴狀態變量】
                return true;
            }
            return false;
        }
       //釋放鎖
        protected final boolean tryRelease(int releases) {
            int c = getState() - releases;//減小 狀態變量值
            if (Thread.currentThread() != getExclusiveOwnerThread())//當前線程是鎖的擁有者線程嗎？
                throw new IllegalMonitorStateException();
            boolean free = false;
            if (c == 0) {//若是沒有線程擁有鎖，就設置null
                free = true;
                setExclusiveOwnerThread(null);
            }
            setState(c);
            return free;
        }
        //是否被當前線程擁有
        protected final boolean isHeldExclusively() {
            // While we must in general read state before owner,
            // we don't need to do so to check if current thread is owner
            return getExclusiveOwnerThread() == Thread.currentThread();
        }
        //題外話：顯示鎖，多等待隊列實現（其實就是new 個新對象，歸根結底，仍是一個對象，一個條件隊列）
        final ConditionObject newCondition() {
            return new ConditionObject();
        }

        // Methods relayed from outer class

        final Thread getOwner() {
            return getState() == 0 ? null : getExclusiveOwnerThread();
        }

        final int getHoldCount() {
            return isHeldExclusively() ? getState() : 0;
        }

        final boolean isLocked() {
            return getState() != 0;
        }

        /**
	 *  從一個輸入流重構鎖。這個怎麼玩的炫？
         * Reconstitutes this lock instance from a stream.
         * @param s the stream
         */
        private void readObject(java.io.ObjectInputStream s)
            throws java.io.IOException, ClassNotFoundException {
            s.defaultReadObject();
            setState(0); // reset to unlocked state
        }
    }

    /**
     * Sync object for non-fair locks
     * 非公平鎖的實現
     */
    static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;

        /**
         * Performs lock.  Try immediate barge, backing up to normal
         * acquire on failure. 嘗試獲取鎖
         */
        final void lock() {//調的仍是AQS裏的方法，因此說AQS是根基類
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1);//這個方法是AQS裏的方法，但它又回調子類tryAcquire(int arg)方法，也就是下面的tryAcquire方法，也就是模板方法模式，AQS負責流程調度，具體每一個鎖的獲取，失敗語義，本身定義。
        }

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

    /**
     * Sync object for fair locks
     * 公平鎖，（弄個隊列FIFO）
     */
    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() &&//沒有在前面等待的線程,(體現公平性)，而且compareAndSetState 成功，就獲取鎖成功了。
                    compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {//若果當前線程，已經獲取鎖了，就更新，【狀態變量值】
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }
    }

    /**
     * Creates an instance of {@code ReentrantLock}.
     * This is equivalent to using {@code ReentrantLock(false)}.默認是非公平鎖
     */
    public ReentrantLock() {
        sync = new NonfairSync();
    }

    /**
     * Creates an instance of {@code ReentrantLock} with the
     * given fairness policy.
     * 能夠構造非公平和公平鎖
     * @param fair {@code true} if this lock should use a fair ordering policy
     */
    public ReentrantLock(boolean fair) {
        sync = fair ? new FairSync() : new NonfairSync();
    }

    /**
     * Acquires the lock.
     *
     * <p>Acquires the lock if it is not held by another thread and returns
     * immediately, setting the lock hold count to one.
     *
     * <p>If the current thread already holds the lock then the hold
     * count is incremented by one and the method returns immediately.
     *
     * <p>If the lock is held by another thread then the
     * current thread becomes disabled for thread scheduling
     * purposes and lies dormant until the lock has been acquired,
     * at which time the lock hold count is set to one.
     */
    public void lock() {
        sync.lock();
    }

    /**
     * Acquires the lock unless the current thread is
     * {@linkplain Thread#interrupt interrupted}.
     *
     * <p>Acquires the lock if it is not held by another thread and returns
     * immediately, setting the lock hold count to one.
     *
     * <p>If the current thread already holds this lock then the hold count
     * is incremented by one and the method returns immediately.
     *
     * <p>If the lock is held by another thread then the
     * current thread becomes disabled for thread scheduling
     * purposes and lies dormant until one of two things happens:
     *
     * <ul>
     *
     * <li>The lock is acquired by the current thread; or
     *
     * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
     * current thread.
     *
     * </ul>
     *
     * <p>If the lock is acquired by the current thread then the lock hold
     * count is set to one.
     *
     * <p>If the current thread:
     *
     * <ul>
     *
     * <li>has its interrupted status set on entry to this method; or
     *
     * <li>is {@linkplain Thread#interrupt interrupted} while acquiring
     * the lock,
     *
     * </ul>
     *
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * <p>In this implementation, as this method is an explicit
     * interruption point, preference is given to responding to the
     * interrupt over normal or reentrant acquisition of the lock.
     *
     * @throws InterruptedException if the current thread is interrupted
     */
    public void lockInterruptibly() throws InterruptedException {
        sync.acquireInterruptibly(1);
    }

    /**
     * Acquires the lock only if it is not held by another thread at the time
     * of invocation.
     *
     * <p>Acquires the lock if it is not held by another thread and
     * returns immediately with the value {@code true}, setting the
     * lock hold count to one. Even when this lock has been set to use a
     * fair ordering policy, a call to {@code tryLock()} <em>will</em>
     * immediately acquire the lock if it is available, whether or not
     * other threads are currently waiting for the lock.
     * This &quot;barging&quot; behavior can be useful in certain
     * circumstances, even though it breaks fairness. If you want to honor
     * the fairness setting for this lock, then use
     * {@link #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) }
     * which is almost equivalent (it also detects interruption).
     *
     * <p> If the current thread already holds this lock then the hold
     * count is incremented by one and the method returns {@code true}.
     *
     * <p>If the lock is held by another thread then this method will return
     * immediately with the value {@code false}.
     *
     * @return {@code true} if the lock was free and was acquired by the
     *         current thread, or the lock was already held by the current
     *         thread; and {@code false} otherwise
     *  經過sync獲取鎖，默認是非公平的。
     */
    public boolean tryLock() {
        return sync.nonfairTryAcquire(1);
    }

    /**
     * Acquires the lock if it is not held by another thread within the given
     * waiting time and the current thread has not been
     * {@linkplain Thread#interrupt interrupted}.
     *
     * <p>Acquires the lock if it is not held by another thread and returns
     * immediately with the value {@code true}, setting the lock hold count
     * to one. If this lock has been set to use a fair ordering policy then
     * an available lock <em>will not</em> be acquired if any other threads
     * are waiting for the lock. This is in contrast to the {@link #tryLock()}
     * method. If you want a timed {@code tryLock} that does permit barging on
     * a fair lock then combine the timed and un-timed forms together:
     *
     * <pre>if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... }
     * </pre>
     *
     * <p>If the current thread
     * already holds this lock then the hold count is incremented by one and
     * the method returns {@code true}.
     *
     * <p>If the lock is held by another thread then the
     * current thread becomes disabled for thread scheduling
     * purposes and lies dormant until one of three things happens:
     *
     * <ul>
     *
     * <li>The lock is acquired by the current thread; or
     *
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     *
     * <li>The specified waiting time elapses
     *
     * </ul>
     *
     * <p>If the lock is acquired then the value {@code true} is returned and
     * the lock hold count is set to one.
     *
     * <p>If the current thread:
     *
     * <ul>
     *
     * <li>has its interrupted status set on entry to this method; or
     *
     * <li>is {@linkplain Thread#interrupt interrupted} while
     * acquiring the lock,
     *
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * <p>If the specified waiting time elapses then the value {@code false}
     * is returned.  If the time is less than or equal to zero, the method
     * will not wait at all.
     *
     * <p>In this implementation, as this method is an explicit
     * interruption point, preference is given to responding to the
     * interrupt over normal or reentrant acquisition of the lock, and
     * over reporting the elapse of the waiting time.
     *
     * @param timeout the time to wait for the lock
     * @param unit the time unit of the timeout argument
     * @return {@code true} if the lock was free and was acquired by the
     *         current thread, or the lock was already held by the current
     *         thread; and {@code false} if the waiting time elapsed before
     *         the lock could be acquired
     * @throws InterruptedException if the current thread is interrupted
     * @throws NullPointerException if the time unit is null
     *  經過sync，獲取鎖，可中斷，可超時
     */
    public boolean tryLock(long timeout, TimeUnit unit)
            throws InterruptedException {
        return sync.tryAcquireNanos(1, unit.toNanos(timeout));
    }

    /**
     * Attempts to release this lock.
     *
     * <p>If the current thread is the holder of this lock then the hold
     * count is decremented.  If the hold count is now zero then the lock
     * is released.  If the current thread is not the holder of this
     * lock then {@link IllegalMonitorStateException} is thrown.
     *
     * @throws IllegalMonitorStateException if the current thread does not
     *         hold this lock
     */
    public void unlock() {//這裏的release方法其實也是AQS的一個模板方法，會回調子類的tryRelease(int arg)方法。
        sync.release(1);
    }
     // 其餘代碼省略幾百行。。。。
    }

    而後翻翻jdk源碼 會發現 ReentrantReadWriteLock ,CountDownLatch,Semaphore 也是AQS實現的

    下面看看https://my.oschina.net/u/146130/blog/881848