多线程之ReentrantLock底层原理与分析

public class ReentrantDemo {

    public synchronized void fun1() {
        System.out.println("fun1");
        fun2();
    }

    public void fun2() {
        synchronized (this) {
            System.out.println("fun2");
        }
    }

    public static void main(String[] args) {
        ReentrantDemo demo = new ReentrantDemo();
        new Thread(demo::fun1).start();
    }
}

public class AtomicDemo {

    private static int count = 0;
    private static ReentrantLock lock = new ReentrantLock();

    public static void inc() {
        lock.lock();
        try {
            Thread.sleep(2);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        count++;
        lock.unlock();
    }

    public static void main(String[] args) throws InterruptedException {
        for (int i = 0; i < 1000; i++) {
            new Thread(()-> AtomicDemo.inc()).start();
        }
        Thread.sleep(3000);
        System.out.println("count = " + count);
    }

}

public class LockDemo {

    static Map<String,Object> cache = new HashMap<>();

    static ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();

    static Lock readLock = rwl.readLock();

    static Lock writeLock = rwl.writeLock();

    public static Object get(String key) {
        readLock.lock();
        try {
            return cache.get(key);
        } finally {
            readLock.unlock();
        }
    }
    
    public static void put(String key, Object obj){
        writeLock.lock();
        try {
            cache.put(key,obj);
        } finally {
            writeLock.unlock();
        }
    } 
}

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

    final void lock() {
        if (compareAndSetState(0, 1))
            setExclusiveOwnerThread(Thread.currentThread());
        else
            acquire(1);
    }

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

UNSAFE_ENTRY(jboolean, Unsafe_CompareAndSwapInt(JNIEnv *env, jobject unsafe, jobject obj, jlong offset, jint e, jint x))
  UnsafeWrapper("Unsafe_CompareAndSwapInt");
  oop p = JNIHandles::resolve(obj); //将 Java 对象解析成 JVM 的 oop（普通对象指针）
  jint* addr = (jint *) index_oop_from_field_offset_long(p, offset);  //根据对象 p 和地址偏移量找到地址
  return (jint)(Atomic::cmpxchg(x, addr, e)) == e;//基于 cas 比较并替换， x 表示需要更新的值，addr 表示 state在内存中的地址，e 表示预期值
UNSAFE_END

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

    protected boolean tryAcquire(int arg) {
        throw new UnsupportedOperationException();
    }

		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;
        }

    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;
        if (pred != null) {
            node.prev = pred;
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        enq(node);
        return node;
    }

    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;
                }
            }
        }
    }

    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

   private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus;
        if (ws == Node.SIGNAL)
            /*
             * This node has already set status asking a release
             * to signal it, so it can safely park.
             */
            return true;
        if (ws > 0) {
            /*
             * Predecessor was cancelled. Skip over predecessors and
             * indicate retry.
             */
            do {
                node.prev = pred = pred.prev;
            } while (pred.waitStatus > 0);
            pred.next = node;
        } else {
            /*
             * 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;
    }

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

    static void selfInterrupt() {
        Thread.currentThread().interrupt();
    }

    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;
    }

        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;
        }

    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;
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 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;
        if (s == null || s.waitStatus > 0) {
            s = null;
            //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
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        if (s != null)
            LockSupport.unpark(s.thread);
    }

        final void lock() {
            acquire(1);
        }

        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;
        }
    }