一、synchronized原理詳解

　　1. 設計同步器的意義

　　　　多執行緒可能會同時訪問一個共享、可變的資源，這個資源稱之為臨界資源，需要同步機制來協同物件可變狀態的訪問，同步器的本質就是加鎖，即同一時刻只能有同一個執行緒訪問臨界資源，也稱為同步互斥訪問

　　　　sychronized內建鎖是一種物件鎖(鎖的是物件,而非引用),可以實現臨界資源的互斥訪問,是可重入的

　　2. 加鎖的方式

1. 1. 同步例項方法,鎖是當前例項物件
   2. 同步類方法,鎖是當前類物件
   3. 同步程式碼塊,鎖是括號裡面的物件

　　3. synchronized原理詳解

　　　　JVM在1.5之後版本做了重大的優化,如鎖粗化,鎖消除,偏向鎖,輕量級鎖,鎖自旋等操作來減少鎖的開銷

　　　　① 偏向鎖 : 在大多數情況下,鎖不僅存在多執行緒競爭,而且總是由同一個執行緒多次獲得,故而引入偏向鎖,核心思想是 : 如果一個執行緒獲取了鎖,那麼進入偏向模式,此時Mark Word 的結構也就變成了偏向鎖結構,當執行緒再次請求鎖時,無需做任何同步操作,即可獲取鎖的過程,省去了大量的有關鎖申請的操作,從而提高了程式的效能

　　　　② 輕量級鎖 : 輕量級鎖所適應的場景是,執行緒交替執行同步程式碼塊的場合,如果存在同一時間內訪問同一鎖的場合,就會導致輕量級鎖膨脹為重量級鎖

　　　　③ 自旋鎖 : 應用場景是,大多數情況下,執行緒持有鎖的時間都不會太長,如果直接掛起作業系統層面的執行緒就會得不償失,畢竟執行緒之間的切換需要從使用者狀態轉換到核心態,這個狀態之間的轉換需要較長的時間,虛擬機器假設在不久的將來,當前執行緒可以獲得鎖,虛擬機器會讓當前獲取鎖的執行緒做幾個空迴圈,經過幾次空迴圈後,如果得到鎖,就順利進入臨界區,如果得不到鎖,就係統層面掛起

　　　　④ 鎖消除 : java虛擬機器在JIT編譯時,通過上下文的掃描,去除不存在共享資源競爭的鎖,通過這種方式消除沒有必要的鎖,可以節省毫無意義的請求鎖的時間

　　　　⑤ 重量級鎖 : synchronized是基於JVM內建鎖的實現,通過內部物件Monitor(監視器鎖)實現,基於進入與退出monitor物件實現方法與程式碼塊同步,監視器鎖的實現依賴底層作業系統Mutex lock的實現,他是一個重量級鎖,效能較低.

　　4.ReentrantLock

　　　　是一種基於AQS框架的實現,是JDK中的執行緒併發訪問的同步手段,他的功能類似於synchronized,是一種互斥鎖,可以保證執行緒安全.而且比synchronized具有更多的特性,比如它支援手動加鎖與解鎖,支援鎖的公平性.
　　　　除了Lock外, java.current.util當中同步器的實現如Latch, Barrier, BlockingQueue等,都是基於AQS框架實現.

　　　　一般是通過定義內部類Sync整合AQS, 將同步器所有都對映到Sync對應的方法

　　5. AQS

　　　　特性 : 阻塞等待佇列, 共享/獨佔 ,公平/非公平, 可重入, 允許中斷

　　　　內部維護屬性 : volatile int state ,state表示資源的可用狀態

　　　　State三種訪問方式 : getState(), setState(), compareAndSetState()

　　　　兩種資源的共享方式 :
　　　　　　Exclusive-獨佔 : 只有一個執行緒能執行, 如ReentrantLock

　　　　　　share-共享 : 多個執行緒可以同時執行, 如Semphore/CountdownLatch

　　　　定義兩種佇列:

　　　　　　同步等待佇列

　　　　　　條件等待佇列

　　　　自定義同步器主要實現以下幾種方法 :

　　　　　　isHeldExclusively() : 該執行緒是否正在獨佔資源. 只有用到condition才需要去實現它.

　　　　　　tryAcquire(int) : 獨佔方式. 嘗試獲取資源,成功返回true, 失敗返回false.

　　　　　　tryRelease(int) : 獨佔方式, 嘗試釋放資源,成功返回true,失敗返回false.

　　　　　　tryAcquireShared(int) : 共享方式. 嘗試獲取資源,負數表示失敗; 0表示成功,但沒有剩餘剩餘可用資源; 正數表示成功,且有剩餘資源.

　　　　　　tryReleaseShared(int) : 共享方式. 嘗試釋放資源, 如果釋放後允許喚醒後續等待節點返回true,否則返回false.

　　　　CLH同步等待佇列

　　　　　　CLH是一種FIFO先入先出執行緒等待佇列,Java中的CLH是原CLH的一個變種,執行緒由原自旋改為阻塞機制

　　　　Condition條件等待佇列

　　　　　　是一個多執行緒協調通訊的工具類,使得某個或者某些執行緒一起等待某個條件,只有當條件具備時,這些執行緒才會被喚醒,從而重新爭奪鎖

　　AQS原始碼分析

public abstract class AbstractQueuedSynchronizer
        extends AbstractOwnableSynchronizer
        implements java.io.Serializable {
    private static final long serialVersionUID = 7373984972572414691L;

    /**
     * Creates a new {@code AbstractQueuedSynchronizer} instance
     * with initial synchronization state of zero.
     */
    protected AbstractQueuedSynchronizer() { }

    /**
     * Wait queue node class.
     *
     * 不管是條件佇列，還是CLH等待佇列
     * 都是基於Node類
     * 
     * AQS當中的同步等待佇列也稱CLH佇列，CLH佇列是Craig、Landin、Hagersten三人
     * 發明的一種基於雙向連結串列資料結構的佇列，是FIFO先入先出執行緒等待佇列，Java中的
     * CLH佇列是原CLH佇列的一個變種,執行緒由原自旋機制改為阻塞機制。
     */
    static final class Node {
        /**
         * 標記節點未共享模式
         * */
        static final Node SHARED = new Node();
        /**
         *  標記節點為獨佔模式
         */
        static final Node EXCLUSIVE = null;

        /**
         * 在同步佇列中等待的執行緒等待超時或者被中斷，需要從同步佇列中取消等待
         * */
        static final int CANCELLED =  1;
        /**
         *  後繼節點的執行緒處於等待狀態，而當前的節點如果釋放了同步狀態或者被取消，
         *  將會通知後繼節點，使後繼節點的執行緒得以執行。
         */
        static final int SIGNAL    = -1;
        /**
         *  節點在等待佇列中，節點的執行緒等待在Condition上，當其他執行緒對Condition呼叫了signal()方法後，
         *  該節點會從等待佇列中轉移到同步佇列中，加入到同步狀態的獲取中
         */
        static final int CONDITION = -2;
        /**
         * 表示下一次共享式同步狀態獲取將會被無條件地傳播下去
         */
        static final int PROPAGATE = -3;

        /**
         * 標記當前節點的訊號量狀態 (1,0,-1,-2,-3)5種狀態
         * 使用CAS更改狀態，volatile保證執行緒可見性，高併發場景下，
         * 即被一個執行緒修改後，狀態會立馬讓其他執行緒可見。
         */
        volatile int waitStatus;

        /**
         * 前驅節點，當前節點加入到同步佇列中被設定
         */
        volatile Node prev;

        /**
         * 後繼節點
         */
        volatile Node next;

        /**
         * 節點同步狀態的執行緒
         */
        volatile Thread thread;

        /**
         * 等待佇列中的後繼節點，如果當前節點是共享的，那麼這個欄位是一個SHARED常量，
         * 也就是說節點型別(獨佔和共享)和等待佇列中的後繼節點共用同一個欄位。
         */
        Node nextWaiter;

        /**
         * Returns true if node is waiting in shared mode.
         */
        final boolean isShared() {
            return nextWaiter == SHARED;
        }

        /**
         * 返回前驅節點
         */
        final Node predecessor() throws NullPointerException {
            Node p = prev;
            if (p == null)
                throw new NullPointerException();
            else
                return p;
        }
        //空節點，用於標記共享模式
        Node() {    // Used to establish initial head or SHARED marker
        }
        //用於同步佇列CLH
        Node(Thread thread, Node mode) {     // Used by addWaiter
            this.nextWaiter = mode;
            this.thread = thread;
        }
        //用於條件佇列
        Node(Thread thread, int waitStatus) { // Used by Condition
            this.waitStatus = waitStatus;
            this.thread = thread;
        }
    }
    
    /**
     * 指向同步等待佇列的頭節點
     */
    private transient volatile Node head;

    /**
     * 指向同步等待佇列的尾節點
     */
    private transient volatile Node tail;

    /**
     * 同步資源狀態
     */
    private volatile int state;

    /**
     * 
     * @return current state value
     */
    protected final int getState() {
        return state;
    }

    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);
    }

    // Queuing utilities

    /**
     * The number of nanoseconds for which it is faster to spin
     * rather than to use timed park. A rough estimate suffices
     * to improve responsiveness with very short timeouts.
     */
    static final long spinForTimeoutThreshold = 1000L;

    /**
     * 節點加入CLH同步佇列
     */
    private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
            if (t == null) { // Must initialize
                //佇列為空需要初始化，建立空的頭節點
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                node.prev = t;
                //set尾部節點
                if (compareAndSetTail(t, node)) {//當前節點置為尾部
                    t.next = node; //前驅節點的next指標指向當前節點
                    return t;
                }
            }
        }
    }

    /**
     * Creates and enqueues node for current thread and given mode.
     *
     * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
     * @return the new node
     */
    private Node addWaiter(Node mode) {
        // 1. 將當前執行緒構建成Node型別
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
        Node pred = tail;
        // 2. 1當前尾節點是否為null？
        if (pred != null) {
            // 2.2 將當前節點尾插入的方式
            node.prev = pred;
            // 2.3 CAS將節點插入同步佇列的尾部
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        enq(node);
        return node;
    }

    /**
     * Sets head of queue to be node, thus dequeuing. Called only by
     * acquire methods.  Also nulls out unused fields for sake of GC
     * and to suppress unnecessary signals and traversals.
     *
     * @param node the node
     */
    private void setHead(Node node) {
        head = node;
        node.thread = null;
        node.prev = null;
    }

    /**
     *
     */
    private void unparkSuccessor(Node node) {
        //獲取wait狀態
        int ws = node.waitStatus;
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0);// 將等待狀態waitStatus設定為初始值0

        /**
         * 若後繼結點為空，或狀態為CANCEL（已失效），則從後尾部往前遍歷找到最前的一個處於正常阻塞狀態的結點
         * 進行喚醒
         */
        Node s = node.next; //head.next = Node1 ,thread = T3
        if (s == null || s.waitStatus > 0) {
            s = null;
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        if (s != null)
            LockSupport.unpark(s.thread);//喚醒執行緒,T3喚醒
    }

    /**
     * 把當前結點設定為SIGNAL或者PROPAGATE
     * 喚醒head.next(B節點)，B節點喚醒後可以競爭鎖，成功後head->B，然後又會喚醒B.next，一直重複直到共享節點都喚醒
     * head節點狀態為SIGNAL，重置head.waitStatus->0，喚醒head節點執行緒，喚醒後執行緒去競爭共享鎖
     * head節點狀態為0，將head.waitStatus->Node.PROPAGATE傳播狀態，表示需要將狀態向後繼節點傳播
     */
    private void doReleaseShared() {
        for (;;) {
            Node h = head;
            if (h != null && h != tail) {
                int ws = h.waitStatus;
                if (ws == Node.SIGNAL) {//head是SIGNAL狀態
                    /* head狀態是SIGNAL，重置head節點waitStatus為0，E這裡不直接設為Node.PROPAGAT,
                     * 是因為unparkSuccessor(h)中，如果ws < 0會設定為0，所以ws先設定為0，再設定為PROPAGATE
                     * 這裡需要控制併發，因為入口有setHeadAndPropagate跟release兩個，避免兩次unpark
                     */
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                        continue; //設定失敗，重新迴圈
                    /* head狀態為SIGNAL，且成功設定為0之後,喚醒head.next節點執行緒
                     * 此時head、head.next的執行緒都喚醒了，head.next會去競爭鎖，成功後head會指向獲取鎖的節點，
                     * 也就是head發生了變化。看最底下一行程式碼可知，head發生變化後會重新迴圈，繼續喚醒head的下一個節點
                     */
                    unparkSuccessor(h);
                    /*
                     * 如果本身頭節點的waitStatus是出於重置狀態（waitStatus==0）的，將其設定為“傳播”狀態。
                     * 意味著需要將狀態向後一個節點傳播
                     */
                }
                else if (ws == 0 &&
                        !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                    continue;                // loop on failed CAS
            }
            if (h == head) //如果head變了，重新迴圈
                break;
        }
    }

    /**
     * 把node節點設定成head節點，且Node.waitStatus->Node.PROPAGATE
     */
    private void setHeadAndPropagate(Node node, int propagate) {
        Node h = head; //h用來儲存舊的head節點
        setHead(node);//head引用指向node節點
        /* 這裡意思有兩種情況是需要執行喚醒操作
         * 1.propagate > 0 表示呼叫方指明瞭後繼節點需要被喚醒
         * 2.頭節點後面的節點需要被喚醒（waitStatus<0），不論是老的頭結點還是新的頭結點
         */
        if (propagate > 0 || h == null || h.waitStatus < 0 ||
                (h = head) == null || h.waitStatus < 0) {
            Node s = node.next;
            if (s == null || s.isShared())//node是最後一個節點或者 node的後繼節點是共享節點
                /* 如果head節點狀態為SIGNAL，喚醒head節點執行緒，重置head.waitStatus->0
                 * head節點狀態為0(第一次新增時是0)，設定head.waitStatus->Node.PROPAGATE表示狀態需要向後繼節點傳播
                 */
                doReleaseShared();
        }
    }

    // Utilities for various versions of acquire

    /**
     * 終結掉正在嘗試去獲取鎖的節點
     * @param node the node
     */
    private void cancelAcquire(Node node) {
        // Ignore if node doesn't exist
        if (node == null)
            return;

        node.thread = null;

        // 剔除掉一件被cancel掉的節點
        Node pred = node.prev;
        while (pred.waitStatus > 0)
            node.prev = pred = pred.prev;

        // predNext is the apparent node to unsplice. CASes below will
        // fail if not, in which case, we lost race vs another cancel
        // or signal, so no further action is necessary.
        Node predNext = pred.next;

        // Can use unconditional write instead of CAS here.
        // After this atomic step, other Nodes can skip past us.
        // Before, we are free of interference from other threads.
        node.waitStatus = Node.CANCELLED;

        // If we are the tail, remove ourselves.
        if (node == tail && compareAndSetTail(node, pred)) {
            compareAndSetNext(pred, predNext, null);
        } else {
            // If successor needs signal, try to set pred's next-link
            // so it will get one. Otherwise wake it up to propagate.
            int ws;
            if (pred != head &&
                    ((ws = pred.waitStatus) == Node.SIGNAL ||
                            (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
                    pred.thread != null) {
                Node next = node.next;
                if (next != null && next.waitStatus <= 0)
                    compareAndSetNext(pred, predNext, next);
            } else {
                unparkSuccessor(node);
            }

            node.next = node; // help GC
        }
    }

    /**
     * 
     */
    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus;
        if (ws == Node.SIGNAL)
            /*
             * 若前驅結點的狀態是SIGNAL，意味著當前結點可以被安全地park
             */
            return true;
        if (ws > 0) {
            /*
             * 前驅節點狀態如果被取消狀態，將被移除出佇列
             */
            do {
                node.prev = pred = pred.prev;
            } while (pred.waitStatus > 0);
            pred.next = node;
        } else {
            /*
             * 當前驅節點waitStatus為 0 or PROPAGATE狀態時
             * 將其設定為SIGNAL狀態，然後當前結點才可以可以被安全地park
             */
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

    /**
     * 中斷當前執行緒
     */
    static void selfInterrupt() {
        Thread.currentThread().interrupt();
    }

    /**
     * 阻塞當前節點，返回當前Thread的中斷狀態
     * LockSupport.park 底層實現邏輯呼叫系統核心功能 pthread_mutex_lock 阻塞執行緒
     */
    private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);//阻塞
        return Thread.interrupted();
    }

    /**
     * 已經在隊列當中的Thread節點，準備阻塞等待獲取鎖
     */
    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {//死迴圈
                final Node p = node.predecessor();//找到當前結點的前驅結點
                if (p == head && tryAcquire(arg)) {//如果前驅結點是頭結點，才tryAcquire，其他結點是沒有機會tryAcquire的。
                    setHead(node);//獲取同步狀態成功，將當前結點設定為頭結點。
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                /**
                 * 如果前驅節點不是Head，通過shouldParkAfterFailedAcquire判斷是否應該阻塞
                 * 前驅節點訊號量為-1，當前執行緒可以安全被parkAndCheckInterrupt用來阻塞執行緒
                 */
                if (shouldParkAfterFailedAcquire(p, node) &&
                        parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     * 與acquireQueued邏輯相似，唯一區別節點還不在隊列當中需要先進行入隊操作
     */
    private void doAcquireInterruptibly(int arg)
            throws InterruptedException {
        final Node node = addWaiter(Node.EXCLUSIVE);//以獨佔模式放入佇列尾部
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return;
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                        parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     * 獨佔模式定時獲取
     */
    private boolean doAcquireNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (nanosTimeout <= 0L)
            return false;
        final long deadline = System.nanoTime() + nanosTimeout;
        final Node node = addWaiter(Node.EXCLUSIVE);//加入佇列
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return true;
                }
                nanosTimeout = deadline - System.nanoTime();
                if (nanosTimeout <= 0L)
                    return false;//超時直接返回獲取失敗
                if (shouldParkAfterFailedAcquire(p, node) &&
                        nanosTimeout > spinForTimeoutThreshold)
                    //阻塞指定時長，超時則執行緒自動被喚醒
                    LockSupport.parkNanos(this, nanosTimeout);
                if (Thread.interrupted())//當前執行緒中斷狀態
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     * 嘗試獲取共享鎖
     */
    private void doAcquireShared(int arg) {
        final Node node = addWaiter(Node.SHARED);//入隊
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();//前驅節點
                if (p == head) {
                    int r = tryAcquireShared(arg); //非公平鎖實現，再嘗試獲取鎖
                    //state==0時tryAcquireShared會返回>=0(CountDownLatch中返回的是1)。
                    // state為0說明共享次數已經到了，可以獲取鎖了
                    if (r >= 0) {//r>0表示state==0,前繼節點已經釋放鎖，鎖的狀態為可被獲取
                        //這一步設定node為head節點設定node.waitStatus->Node.PROPAGATE，然後喚醒node.thread
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        if (interrupted)
                            selfInterrupt();
                        failed = false;
                        return;
                    }
                }
                //前繼節點非head節點，將前繼節點狀態設定為SIGNAL，通過park掛起node節點的執行緒
                if (shouldParkAfterFailedAcquire(p, node) &&
                        parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     * Acquires in shared interruptible mode.
     * @param arg the acquire argument
     */
    private void doAcquireSharedInterruptibly(int arg)
            throws InterruptedException {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                        parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     * Acquires in shared timed mode.
     *
     * @param arg the acquire argument
     * @param nanosTimeout max wait time
     * @return {@code true} if acquired
     */
    private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (nanosTimeout <= 0L)
            return false;
        final long deadline = System.nanoTime() + nanosTimeout;
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return true;
                    }
                }
                nanosTimeout = deadline - System.nanoTime();
                if (nanosTimeout <= 0L)
                    return false;
                if (shouldParkAfterFailedAcquire(p, node) &&
                        nanosTimeout > spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                if (Thread.interrupted())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    // Main exported methods

    /**
     * 嘗試獲取獨佔鎖，可指定鎖的獲取數量
     */
    protected boolean tryAcquire(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * 嘗試釋放獨佔鎖，在子類當中實現
     */
    protected boolean tryRelease(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * 共享式：共享式地獲取同步狀態。對於獨佔式同步元件來講，同一時刻只有一個執行緒能獲取到同步狀態，
     * 其他執行緒都得去排隊等待，其待重寫的嘗試獲取同步狀態的方法tryAcquire返回值為boolean，這很容易理解；
     * 對於共享式同步元件來講，同一時刻可以有多個執行緒同時獲取到同步狀態，這也是“共享”的意義所在。
     * 本方法待被之類覆蓋實現具體邏輯
     *  1.當返回值大於0時，表示獲取同步狀態成功，同時還有剩餘同步狀態可供其他執行緒獲取；
     *
     *　2.當返回值等於0時，表示獲取同步狀態成功，但沒有可用同步狀態了；

     *　3.當返回值小於0時，表示獲取同步狀態失敗。
     */
    protected int tryAcquireShared(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * 釋放共享鎖，具體實現在子類當中實現
     */
    protected boolean tryReleaseShared(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
     * 當前執行緒是否持有獨佔鎖
     */
    protected boolean isHeldExclusively() {
        throw new UnsupportedOperationException();
    }

    /**
     * 獲取獨佔鎖
     */
    public final void acquire(int arg) {
        //嘗試獲取鎖
        if (!tryAcquire(arg) &&
                acquireQueued(addWaiter(Node.EXCLUSIVE), arg))//獨佔模式
            selfInterrupt();
    }

    /**
     * 
     */
    public final void acquireInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (!tryAcquire(arg))
            doAcquireInterruptibly(arg);
    }

    /**
     * 獲取獨佔鎖，設定最大等待時間
     */
    public final boolean tryAcquireNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        return tryAcquire(arg) ||
                doAcquireNanos(arg, nanosTimeout);
    }

    /**
     * 釋放獨佔模式持有的鎖
     */
    public final boolean release(int arg) {
        if (tryRelease(arg)) {//釋放一次鎖
            Node h = head;
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);//喚醒後繼結點
            return true;
        }
        return false;
    }

    /**
     * 請求獲取共享鎖
     */
    public final void acquireShared(int arg) {
        if (tryAcquireShared(arg) < 0)//返回值小於0，獲取同步狀態失敗，排隊去；獲取同步狀態成功，直接返回去幹自己的事兒。
            doAcquireShared(arg);
    }


    /**
     * Releases in shared mode.  Implemented by unblocking one or more
     * threads if {@link #tryReleaseShared} returns true.
     *
     * @param arg the release argument.  This value is conveyed to
     *        {@link #tryReleaseShared} but is otherwise uninterpreted
     *        and can represent anything you like.
     * @return the value returned from {@link #tryReleaseShared}
     */
    public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }

    // Queue inspection methods

    public final boolean hasQueuedThreads() {
        return head != tail;
    }

    public final boolean hasContended() {
        return head != null;
    }

    public final Thread getFirstQueuedThread() {
        // handle only fast path, else relay
        return (head == tail) ? null : fullGetFirstQueuedThread();
    }

    /**
     * Version of getFirstQueuedThread called when fastpath fails
     */
    private Thread fullGetFirstQueuedThread() {
        Node h, s;
        Thread st;
        if (((h = head) != null && (s = h.next) != null &&
                s.prev == head && (st = s.thread) != null) ||
                ((h = head) != null && (s = h.next) != null &&
                        s.prev == head && (st = s.thread) != null))
            return st;

        Node t = tail;
        Thread firstThread = null;
        while (t != null && t != head) {
            Thread tt = t.thread;
            if (tt != null)
                firstThread = tt;
            t = t.prev;
        }
        return firstThread;
    }

    /**
     * 判斷當前執行緒是否在隊列當中
     */
    public final boolean isQueued(Thread thread) {
        if (thread == null)
            throw new NullPointerException();
        for (Node p = tail; p != null; p = p.prev)
            if (p.thread == thread)
                return true;
        return false;
    }

    final boolean apparentlyFirstQueuedIsExclusive() {
        Node h, s;
        return (h = head) != null &&
                (s = h.next)  != null &&
                !s.isShared()         &&
                s.thread != null;
    }

    /**
     * 判斷當前節點是否有前驅節點
     */
    public final boolean hasQueuedPredecessors() {
        Node t = tail; // Read fields in reverse initialization order
        Node h = head;
        Node s;
        return h != t &&
                ((s = h.next) == null || s.thread != Thread.currentThread());
    }


    // Instrumentation and monitoring methods

    /**
     * 同步佇列長度
     */
    public final int getQueueLength() {
        int n = 0;
        for (Node p = tail; p != null; p = p.prev) {
            if (p.thread != null)
                ++n;
        }
        return n;
    }

    /**
     * 獲取佇列等待thread集合
     */
    public final Collection<Thread> getQueuedThreads() {
        ArrayList<Thread> list = new ArrayList<Thread>();
        for (Node p = tail; p != null; p = p.prev) {
            Thread t = p.thread;
            if (t != null)
                list.add(t);
        }
        return list;
    }

    /**
     * 獲取獨佔模式等待thread執行緒集合
     */
    public final Collection<Thread> getExclusiveQueuedThreads() {
        ArrayList<Thread> list = new ArrayList<Thread>();
        for (Node p = tail; p != null; p = p.prev) {
            if (!p.isShared()) {
                Thread t = p.thread;
                if (t != null)
                    list.add(t);
            }
        }
        return list;
    }

    /**
     * 獲取共享模式等待thread集合
     */
    public final Collection<Thread> getSharedQueuedThreads() {
        ArrayList<Thread> list = new ArrayList<Thread>();
        for (Node p = tail; p != null; p = p.prev) {
            if (p.isShared()) {
                Thread t = p.thread;
                if (t != null)
                    list.add(t);
            }
        }
        return list;
    }


    // Internal support methods for Conditions

    /**
     * 判斷節點是否在同步佇列中
     */
    final boolean isOnSyncQueue(Node node) {
        //快速判斷1：節點狀態或者節點沒有前置節點
        //注：同步佇列是有頭節點的，而條件佇列沒有
        if (node.waitStatus == Node.CONDITION || node.prev == null)
            return false;
        //快速判斷2：next欄位只有同步佇列才會使用，條件佇列中使用的是nextWaiter欄位
        if (node.next != null) // If has successor, it must be on queue
            return true;
        //上面如果無法判斷則進入複雜判斷
        return findNodeFromTail(node);
    }

    private boolean findNodeFromTail(Node node) {
        Node t = tail;
        for (;;) {
            if (t == node)
                return true;
            if (t == null)
                return false;
            t = t.prev;
        }
    }

    /**
     * 將節點從條件隊列當中移動到同步隊列當中，等待獲取鎖
     */
    final boolean transferForSignal(Node node) {
        /*
         * 修改節點訊號量狀態為0，失敗直接返回false
         */
        if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
            return false;

        /*
         * 加入同步佇列尾部當中，返回前驅節點
         */
        Node p = enq(node);
        int ws = p.waitStatus;
        //前驅節點不可用 或者 修改訊號量狀態失敗
        if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
            LockSupport.unpark(node.thread); //喚醒當前節點
        return true;
    }

    final boolean transferAfterCancelledWait(Node node) {
        if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
            enq(node);
            return true;
        }
        /*
         * If we lost out to a signal(), then we can't proceed
         * until it finishes its enq().  Cancelling during an
         * incomplete transfer is both rare and transient, so just
         * spin.
         */
        while (!isOnSyncQueue(node))
            Thread.yield();
        return false;
    }

    /**
     * 入參就是新建立的節點，即當前節點
     */
    final int fullyRelease(Node node) {
        boolean failed = true;
        try {
            //這裡這個取值要注意，獲取當前的state並釋放，這從另一個角度說明必須是獨佔鎖
            //可以考慮下這個邏輯放在共享鎖下面會發生什麼？
            int savedState = getState();
            if (release(savedState)) {
                failed = false;
                return savedState;
            } else {
                //如果這裡釋放失敗，則丟擲異常
                throw new IllegalMonitorStateException();
            }
        } finally {
            /**
             * 如果釋放鎖失敗，則把節點取消，由這裡就能看出來上面新增節點的邏輯中
             * 只需要判斷最後一個節點是否被取消就可以了
             */
            if (failed)
                node.waitStatus = Node.CANCELLED;
        }
    }

    // Instrumentation methods for conditions

    public final boolean hasWaiters(ConditionObject condition) {
        if (!owns(condition))
            throw new IllegalArgumentException("Not owner");
        return condition.hasWaiters();
    }

    /**
     * 獲取條件佇列長度
     */
    public final int getWaitQueueLength(ConditionObject condition) {
        if (!owns(condition))
            throw new IllegalArgumentException("Not owner");
        return condition.getWaitQueueLength();
    }

    /**
     * 獲取條件隊列當中所有等待的thread集合
     */
    public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
        if (!owns(condition))
            throw new IllegalArgumentException("Not owner");
        return condition.getWaitingThreads();
    }

    /**
     * 條件物件，實現基於條件的具體行為
     */
    public class ConditionObject implements Condition, java.io.Serializable {
        private static final long serialVersionUID = 1173984872572414699L;
        /** First node of condition queue. */
        private transient Node firstWaiter;
        /** Last node of condition queue. */
        private transient Node lastWaiter;

        /**
         * Creates a new {@code ConditionObject} instance.
         */
        public ConditionObject() { }

        // Internal methods

        /**
         * 1.與同步佇列不同，條件佇列頭尾指標是firstWaiter跟lastWaiter
         * 2.條件佇列是在獲取鎖之後，也就是臨界區進行操作，因此很多地方不用考慮併發
         */
        private Node addConditionWaiter() {
            Node t = lastWaiter;
            //如果最後一個節點被取消，則刪除佇列中被取消的節點
            //至於為啥是最後一個節點後面會分析
            if (t != null && t.waitStatus != Node.CONDITION) {
                //刪除所有被取消的節點
                unlinkCancelledWaiters();
                t = lastWaiter;
            }
            //建立一個型別為CONDITION的節點並加入佇列，由於在臨界區，所以這裡不用併發控制
            Node node = new Node(Thread.currentThread(), Node.CONDITION);
            if (t == null)
                firstWaiter = node;
            else
                t.nextWaiter = node;
            lastWaiter = node;
            return node;
        }

        /**
         * 發訊號，通知遍歷條件隊列當中的節點轉移到同步隊列當中，準備排隊獲取鎖
         */
        private void doSignal(Node first) {
            do {
                if ( (firstWaiter = first.nextWaiter) == null)
                    lastWaiter = null;
                first.nextWaiter = null;
            } while (!transferForSignal(first) && //轉移節點
                    (first = firstWaiter) != null);
        }

        /**
         * 通知所有節點移動到同步隊列當中，並將節點從條件佇列刪除
         */
        private void doSignalAll(Node first) {
            lastWaiter = firstWaiter = null;
            do {
                Node next = first.nextWaiter;
                first.nextWaiter = null;
                transferForSignal(first);
                first = next;
            } while (first != null);
        }

        /**
         * 刪除條件隊列當中被取消的節點
         */
        private void unlinkCancelledWaiters() {
            Node t = firstWaiter;
            Node trail = null;
            while (t != null) {
                Node next = t.nextWaiter;
                if (t.waitStatus != Node.CONDITION) {
                    t.nextWaiter = null;
                    if (trail == null)
                        firstWaiter = next;
                    else
                        trail.nextWaiter = next;
                    if (next == null)
                        lastWaiter = trail;
                }
                else
                    trail = t;
                t = next;
            }
        }

        // public methods

        /**
         * 發新號，通知條件隊列當中節點到同步隊列當中去排隊
         */
        public final void signal() {
            if (!isHeldExclusively())//節點不能已經持有獨佔鎖
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            if (first != null)
                /**
                 * 發訊號通知條件佇列的節點準備到同步隊列當中去排隊
                 */
                doSignal(first);
        }

        /**
         * 喚醒所有條件佇列的節點轉移到同步隊列當中
         */
            public final void signalAll() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            if (first != null)
                doSignalAll(first);
        }

        /**
         * Implements uninterruptible condition wait.
         * <ol>
         * <li> Save lock state returned by {@link #getState}.
         * <li> Invoke {@link #release} with saved state as argument,
         *      throwing IllegalMonitorStateException if it fails.
         * <li> Block until signalled.
         * <li> Reacquire by invoking specialized version of
         *      {@link #acquire} with saved state as argument.
         * </ol>
         */
        public final void awaitUninterruptibly() {
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            boolean interrupted = false;
            while (!isOnSyncQueue(node)) {
                LockSupport.park(this);
                if (Thread.interrupted())
                    interrupted = true;
            }
            if (acquireQueued(node, savedState) || interrupted)
                selfInterrupt();
        }

        /** 該模式表示在退出等待時重新中斷 */
        private static final int REINTERRUPT =  1;
        /** 異常中斷 */
        private static final int THROW_IE    = -1;

        /**
         * 這裡的判斷邏輯是：
         * 1.如果現在不是中斷的，即正常被signal喚醒則返回0
         * 2.如果節點由中斷加入同步佇列則返回THROW_IE，由signal加入同步佇列則返回REINTERRUPT
         */
        private int checkInterruptWhileWaiting(Node node) {
            return Thread.interrupted() ?
                    (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
                    0;
        }

        /**
         * 根據中斷時機選擇丟擲異常或者設定執行緒中斷狀態
         */
        private void reportInterruptAfterWait(int interruptMode)
                throws InterruptedException {
            if (interruptMode == THROW_IE)
                throw new InterruptedException();
            else if (interruptMode == REINTERRUPT)
                selfInterrupt();
        }

        /**
         * 加入條件佇列等待，條件佇列入口
         */
        public final void await() throws InterruptedException {

            //T2進來
            //如果當前執行緒被中斷則直接丟擲異常
            if (Thread.interrupted())
                throw new InterruptedException();
            //把當前節點加入條件佇列
            Node node = addConditionWaiter();
            //釋放掉已經獲取的獨佔鎖資源
            int savedState = fullyRelease(node);//T2釋放鎖
            int interruptMode = 0;
            //如果不在同步佇列中則不斷掛起
            while (!isOnSyncQueue(node)) {
                LockSupport.park(this);//T1被阻塞
                //這裡被喚醒可能是正常的signal操作也可能是中斷
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
            /**
             * 走到這裡說明節點已經條件滿足被加入到了同步佇列中或者中斷了
             * 這個方法很熟悉吧？就跟獨佔鎖呼叫同樣的獲取鎖方法，從這裡可以看出條件佇列只能用於獨佔鎖
             * 在處理中斷之前首先要做的是從同步佇列中成功獲取鎖資源
             */
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            //走到這裡說明已經成功獲取到了獨佔鎖，接下來就做些收尾工作
            //刪除條件佇列中被取消的節點
            if (node.nextWaiter != null) // clean up if cancelled
                unlinkCancelledWaiters();
            //根據不同模式處理中斷
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
        }


        /**
         * Implements timed condition wait.
         * <ol>
         * <li> If current thread is interrupted, throw InterruptedException.
         * <li> Save lock state returned by {@link #getState}.
         * <li> Invoke {@link #release} with saved state as argument,
         *      throwing IllegalMonitorStateException if it fails.
         * <li> Block until signalled, interrupted, or timed out.
         * <li> Reacquire by invoking specialized version of
         *      {@link #acquire} with saved state as argument.
         * <li> If interrupted while blocked in step 4, throw InterruptedException.
         * <li> If timed out while blocked in step 4, return false, else true.
         * </ol>
         */
        public final boolean await(long time, TimeUnit unit)
                throws InterruptedException {
            long nanosTimeout = unit.toNanos(time);
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            final long deadline = System.nanoTime() + nanosTimeout;
            boolean timedout = false;
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                if (nanosTimeout <= 0L) {
                    timedout = transferAfterCancelledWait(node);
                    break;
                }
                if (nanosTimeout >= spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
                nanosTimeout = deadline - System.nanoTime();
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null)
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
            return !timedout;
        }


        final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
            return sync == AbstractQueuedSynchronizer.this;
        }

        /**
         * Queries whether any threads are waiting on this condition.
         * Implements {@link AbstractQueuedSynchronizer#hasWaiters(ConditionObject)}.
         *
         * @return {@code true} if there are any waiting threads
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *         returns {@code false}
         */
        protected final boolean hasWaiters() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION)
                    return true;
            }
            return false;
        }

        /**
         * Returns an estimate of the number of threads waiting on
         * this condition.
         * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength(ConditionObject)}.
         *
         * @return the estimated number of waiting threads
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *         returns {@code false}
         */
        protected final int getWaitQueueLength() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            int n = 0;
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION)
                    ++n;
            }
            return n;
        }

        /**
         * 得到同步隊列當中所有在等待的Thread集合
         */
        protected final Collection<Thread> getWaitingThreads() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            ArrayList<Thread> list = new ArrayList<Thread>();
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION) {
                    Thread t = w.thread;
                    if (t != null)
                        list.add(t);
                }
            }
            return list;
        }
    }

    /**
     * Setup to support compareAndSet. We need to natively implement
     * this here: For the sake of permitting future enhancements, we
     * cannot explicitly subclass AtomicInteger, which would be
     * efficient and useful otherwise. So, as the lesser of evils, we
     * natively implement using hotspot intrinsics API. And while we
     * are at it, we do the same for other CASable fields (which could
     * otherwise be done with atomic field updaters).
     * unsafe魔法類，直接繞過虛擬機器記憶體管理機制，修改記憶體
     */
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    //偏移量
    private static final long stateOffset;
    private static final long headOffset;
    private static final long tailOffset;
    private static final long waitStatusOffset;
    private static final long nextOffset;

    static {
        try {
            //狀態偏移量
            stateOffset = unsafe.objectFieldOffset
                    (AbstractQueuedSynchronizer.class.getDeclaredField("state"));
            //head指標偏移量，head指向CLH佇列的頭部
            headOffset = unsafe.objectFieldOffset
                    (AbstractQueuedSynchronizer.class.getDeclaredField("head"));
            tailOffset = unsafe.objectFieldOffset
                    (AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
            waitStatusOffset = unsafe.objectFieldOffset
                    (Node.class.getDeclaredField("waitStatus"));
            nextOffset = unsafe.objectFieldOffset
                    (Node.class.getDeclaredField("next"));

        } catch (Exception ex) { throw new Error(ex); }
    }

    /**
     * CAS 修改頭部節點指向. 併發入隊時使用.
     */
    private final boolean compareAndSetHead(Node update) {
        return unsafe.compareAndSwapObject(this, headOffset, null, update);
    }

    /**
     * CAS 修改尾部節點指向. 併發入隊時使用.
     */
    private final boolean compareAndSetTail(Node expect, Node update) {
        return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
    }

    /**
     * CAS 修改訊號量狀態.
     */
    private static final boolean compareAndSetWaitStatus(Node node,
                                                         int expect,
                                                         int update) {
        return unsafe.compareAndSwapInt(node, waitStatusOffset,
                expect, update);
    }

    /**
     * 修改節點的後繼指標.
     */
    private static final boolean compareAndSetNext(Node node,
                                                   Node expect,
                                                   Node update) {
        return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
    }
}


AQS框架具體實現-獨佔鎖實現ReentrantLock

public class ReentrantLock implements Lock, java.io.Serializable {
    private static final long serialVersionUID = 7373984872572414699L;
    /**
     * 內部呼叫AQS的動作，都基於該成員屬性實現
     */
    private final Sync sync;

    /**
     * ReentrantLock鎖同步操作的基礎類,繼承自AQS框架.
     * 該類有兩個繼承類，1、NonfairSync 非公平鎖，2、FairSync公平鎖
     */
        abstract static class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = -5179523762034025860L;

        /**
         * 加鎖的具體行為由子類實現
         */
        abstract void lock();

        /**
         * 嘗試獲取非公平鎖
         */
        final boolean nonfairTryAcquire(int acquires) {
            //acquires = 1
            final Thread current = Thread.currentThread();
            int c = getState();
            /**
             * 不需要判斷同步佇列（CLH）中是否有排隊等待執行緒
             * 判斷state狀態是否為0，不為0可以加鎖
             */
            if (c == 0) {
                //unsafe操作，cas修改state狀態
                if (compareAndSetState(0, acquires)) {
                    //獨佔狀態鎖持有者指向當前執行緒
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            /**
             * state狀態不為0，判斷鎖持有者是否是當前執行緒，
             * 如果是當前執行緒持有 則state+1
             */
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            //加鎖失敗
            return false;
        }

        /**
         * 釋放鎖
         */
        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;
        }

        /**
         * 判斷持有獨佔鎖的執行緒是否是當前執行緒
         */
        protected final boolean isHeldExclusively() {
            return getExclusiveOwnerThread() == Thread.currentThread();
        }

        //返回條件物件
        final ConditionObject newCondition() {
            return new ConditionObject();
        }


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

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

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

        /**
         * Reconstitutes the instance from a stream (that is, deserializes it).
         */
        private void readObject(java.io.ObjectInputStream s)
                throws java.io.IOException, ClassNotFoundException {
            s.defaultReadObject();
            setState(0); // reset to unlocked state
        }
    }

    /**
     * 非公平鎖
     */
    static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;
        /**
         * 加鎖行為
         */
        final void lock() {
            /**
             * 第一步：直接嘗試加鎖
             * 與公平鎖實現的加鎖行為一個最大的區別在於，此處不會去判斷同步佇列(CLH佇列)中
             * 是否有排隊等待加鎖的節點，上來直接加鎖（判斷state是否為0,CAS修改state為1）
             * ，並將獨佔鎖持有者 exclusiveOwnerThread 屬性指向當前執行緒
             * 如果當前有人佔用鎖，再嘗試去加一次鎖
             */
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                //AQS定義的方法,加鎖
                acquire(1);
        }

        /**
         * 父類AbstractQueuedSynchronizer.acquire()中呼叫本方法
         */
        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
        }
    }

    /**
     * 公平鎖
     */
    static final class FairSync extends Sync {
        private static final long serialVersionUID = -3000897897090466540L;
        final void lock() {
            acquire(1);
        }
        /**
         * 重寫aqs中的方法邏輯
         * 嘗試加鎖，被AQS的acquire()方法呼叫
         */
        protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                /**
                 * 與非公平鎖中的區別，需要先判斷隊列當中是否有等待的節點
                 * 如果沒有則可以嘗試CAS獲取鎖
                 */
                if (!hasQueuedPredecessors() &&
                        compareAndSetState(0, acquires)) {
                    //獨佔執行緒指向當前執行緒
                    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;
        }
    }

    /**
     * 預設建構函式，建立非公平鎖物件
     */
    public ReentrantLock() {
        sync = new NonfairSync();
    }

    /**
     * 根據要求建立公平鎖或非公平鎖
     */
    public ReentrantLock(boolean fair) {
        sync = fair ? new FairSync() : new NonfairSync();
    }

    /**
     * 加鎖
     */
    public void lock() {
        sync.lock();
    }

    /**
     * 嘗試獲去取鎖，獲取失敗被阻塞，執行緒被中斷直接丟擲異常
     */
    public void lockInterruptibly() throws InterruptedException {
        sync.acquireInterruptibly(1);
    }

    /**
     * 嘗試加鎖
     */
    public boolean tryLock() {
        return sync.nonfairTryAcquire(1);
    }

    /**
     * 指定等待時間內嘗試加鎖
     */
    public boolean tryLock(long timeout, TimeUnit unit)
            throws InterruptedException {
        return sync.tryAcquireNanos(1, unit.toNanos(timeout));
    }

    /**
     * 嘗試去釋放鎖
     */
    public void unlock() {
        sync.release(1);
    }

    /**
     * 返回條件物件
     */
    public Condition newCondition() {
        return sync.newCondition();
    }

    /**
     * 返回當前執行緒持有的state狀態數量
     */
    public int getHoldCount() {
        return sync.getHoldCount();
    }

    /**
     * 查詢當前執行緒是否持有鎖
     */
    public boolean isHeldByCurrentThread() {
        return sync.isHeldExclusively();
    }

    /**
     * 狀態表示是否被Thread加鎖持有
     */
    public boolean isLocked() {
        return sync.isLocked();
    }

    /**
     * 是否公平鎖？是返回true 否則返回 false
     */
    public final boolean isFair() {
        return sync instanceof FairSync;
    }

    /**
     * 獲取持有鎖的當前執行緒
     */
    protected Thread getOwner() {
        return sync.getOwner();
    }

    /**
     * 判斷隊列當中是否有在等待獲取鎖的Thread節點
     */
    public final boolean hasQueuedThreads() {
        return sync.hasQueuedThreads();
    }

    /**
     * 當前執行緒是否在同步佇列中等待
     */
    public final boolean hasQueuedThread(Thread thread) {
        return sync.isQueued(thread);
    }

    /**
     * 獲取同步佇列長度
     */
    public final int getQueueLength() {
        return sync.getQueueLength();
    }

    /**
     * 返回Thread集合，排隊中的所有節點Thread會被返回
     */
    protected Collection<Thread> getQueuedThreads() {
        return sync.getQueuedThreads();
    }

    /**
     * 條件隊列當中是否有正在等待的節點
     */
    public boolean hasWaiters(Condition condition) {
        if (condition == null)
            throw new NullPointerException();
        if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
            throw new IllegalArgumentException("not owner");
        return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
    }

}