并发编程学习(4)Lock

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void lock() // 如果锁可用就获得锁，如果锁不可用就阻塞直到锁释放
void lockInterruptibly() // 和lock()方法相似, 但阻塞的线程可中断，抛 出java.lang.InterruptedException 异常
boolean tryLock() // 非阻塞获取锁;尝试获取锁，如果成功返回 true
boolean tryLock(longtimeout, TimeUnit timeUnit)//带有超时时间的获取锁方法
void unlock() // 释放锁

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public class ReentrantDemo{
  public synchronized void demo(){// main获得对象锁
      System.out.println("begin:demo");
      demo2();
 }
 public void demo2(){
         System.out.println("begin:demo1");
         // 在次获得对象锁
         synchronized (this){
         }
 }
 public static void main(String[] args) {
       ReentrantDemo rd=new ReentrantDemo();
       new Thread(rd::demo).start();
       }
}

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public class RWLock {
    static ReentrantReadWriteLock wrl=new ReentrantReadWriteLock();
    static Map<String,Object> cacheMap=new HashMap<>();
    static Lock read=wrl.readLock();
    static Lock write=wrl.writeLock();

    //线程B/C/D
    public static final Object get(String key){
        System.out.println("begin read data:"+key);
        read.lock(); //获得读锁-> 阻塞
        try {
            return cacheMap.get(key);
        }finally {
            read.unlock();
        }
    }
    //线程A
    public static final Object put(String key,Object val){
        write.lock();//获得了写锁
        try{
            return cacheMap.put(key,val);
        }finally {
            write.unlock();
        }
    }

    public static void main(String[] args) {
        wrl.readLock();//B线程 ->阻塞
        wrl.writeLock(); //A线程
        //读->读是可以共享
        //读->写 互斥
        //写->写 互斥
        //读多写少的场景
    }
}

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public void lock(){
    sync.lock();
}

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final void lock() {
 if (compareAndSetState(0, 1))  //通过cas操作来修改state状态，表示争抢锁的操作
 setExclusiveOwnerThread(Thread.currentThread());//设置当前获得锁状态的线程
 else
 acquire(1);//尝试去获取锁
}

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    protected final boolean compareAndSetState(int expect, int update) {
        // See below for intrinsics setup to support this
        return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
    }

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    public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

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

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        final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread(); //获取当前执行的线程
            int c = getState(); //获得 state 的值
            if (c == 0) { //表示无锁状态
                if (compareAndSetState(0, acquires)) { //cas 替换 state 的值，cas 成功表示获取锁成功
                    setExclusiveOwnerThread(current); //保存当前获得锁的线程,下次再来的时候不要再尝试竞争锁
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) { //如果同一个线程来获得锁，直接增加重入次数
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }

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    private Node addWaiter(Node mode) {
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
        Node pred = tail; //tail 是 AQS 中表示同比队列队尾的属性，默认是 null
        if (pred != null) { //tail 不为空的情况下，说明队列中存在节点
            node.prev = pred; //把当前线程的 Node 的 prev 指向 tail
            if (compareAndSetTail(pred, node)) { //通过 cas 把 node加入到 AQS 队列，也就是设置为 tail
                pred.next = node; ;//设置成功以后，把原 tail 节点的 next指向当前 node
                return node;
            }
        }
        enq(node); ;//tail=null,把 node 添加到同步队列
        return node;
    }

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    private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
            if (t == null) { // Must initialize
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

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    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor(); ;//获取当前节点的 prev 节点
                if (p == head && tryAcquire(arg)) { //如果是 head 节点，说明有资格去争抢锁
                    setHead(node); //获取锁成功，也就是ThreadA 已经释放了锁，然后设置 head 为 ThreadB 获得执行权限
                    p.next = null; // help GC //把原 head 节点从链表中移除
                    failed = false;
                    return interrupted;
                }
                //ThreadA 可能还没释放锁，使得 ThreadB 在执行 tryAcquire 时会返回 false
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;//并且返回当前线程在等待过程中有没有中断过。
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

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private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus; //前置节点的waitStatus
        if (ws == Node.SIGNAL)//如果前置节点为 SIGNAL，意味着只需要等待其他前置节点的线程被释放
            /*
             * This node has already set status asking a release
             * to signal it, so it can safely park.
             */
            return true; //返回 true，意味着可以直接放心的挂起了
        if (ws > 0) { //ws 大于 0，意味着 prev 节点取消了排队，直接移除这个节点就行
            /*
             * Predecessor was cancelled. Skip over predecessors and
             * indicate retry.
             */
            do {
                node.prev = pred = pred.prev; //相当于: pred=pred.prev;node.prev=pred;
            } while (pred.waitStatus > 0); //这里采用循环，从双向列表中移除 CANCELLED 的节点
            pred.next = node;
        } else { //利用 cas 设置 prev 节点的状态为 SIGNAL(-1)

            /*
             * waitStatus must be 0 or PROPAGATE.  Indicate that we
             * need a signal, but don't park yet.  Caller will need to
             * retry to make sure it cannot acquire before parking.
             */
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

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    private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);
        return Thread.interrupted();
    }

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    public native void unpark(Object var1);
    public native void park(boolean var1, long var2);

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public final boolean release(int arg) {
   if (tryRelease(arg)) { //释放锁成功
      Node h = head; //得到 aqs 中 head 节点
      if (h != null && h.waitStatus != 0) //如果 head 节点不为空并且状态！=0.调用 unparkSuccessor(h)唤醒后续节点
         unparkSuccessor(h);
        return true;
   }
  return false;
}

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        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;
        }

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    private void unparkSuccessor(Node node) {
        /*
         * If status is negative (i.e., possibly needing signal) try
         * to clear in anticipation of signalling.  It is OK if this
         * fails or if status is changed by waiting thread.
         */
        int ws = node.waitStatus; //获得 head 节点的状态
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0); ;// 设置 head 节点状态为 0

        /*
         * Thread to unpark is held in successor, which is normally
         * just the next node.  But if cancelled or apparently null,
         * traverse backwards from tail to find the actual
         * non-cancelled successor.
         */
        Node s = node.next; //得到 head 节点的下一个节点
        if (s == null || s.waitStatus > 0) {
        //如果下一个节点为 null 或者 status>0 表示 cancelled 状态.
            //通过从尾部节点开始扫描，找到距离 head 最近的一个waitStatus<=0 的节点
            s = null;
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        if (s != null) //next 节点不为空，直接唤醒这个线程即可
            LockSupport.unpark(s.thread);
    }

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    private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
            if (t == null) { // Must initialize
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

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final void lock() {
 acquire(1);
}
非公平锁在获取锁的时候，会先通过 CAS 进行抢占，而公平锁则不会
final void lock() {
    if (compareAndSetState(0, 1))
        setExclusiveOwnerThread(Thread.currentThread());
    else
        acquire(1);
}

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公平锁
 protected final boolean tryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    if (c == 0) {
        if (!hasQueuedPredecessors() &&
            compareAndSetState(0, acquires)) {
            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;
}

非公平锁
final boolean nonfairTryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    if (c == 0) {
        if (compareAndSetState(0, acquires)) {
            setExclusiveOwnerThread(current);
            return true;
        }
    }
    else if (current == getExclusiveOwnerThread()) {
        int nextc = c + acquires;
        if (nextc < 0) // overflow
            throw new Error("Maximum lock count exceeded");
        setState(nextc);
        return true;
    }
    return false;
}