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Java AQS中ReentrantLock条件锁的使用

作者:飞奔的小付

ReentrantLock继承了Lock接口, lock方法实际上是调用了Sync的子类NonfairSync(非公平锁)的lock方法。ReentrantLock的真正实现在他的两个内部类NonfairSync和FairSync中,默认实现是非公平锁

一.什么是AQS

1.定义

java.util.concurrent包中的大多数同步器实现都是围绕着共同的基础行为,比如等待队列、条件队列、独占获取、共享获取等,而这些行为的抽象就是基于AbstractQueuedSynchronizer(简称AQS)实现的,AQS是一个抽象同步框架,可以用来实现一个依赖状态的同步器。

JDK中提供的大多数的同步器如Lock, Latch, Barrier等,都是基于AQS框架来实现的。

2.特性

3.属性

内部维护属性volatile int state,表示资源的可用状态

4.资源共享方式

5.两种队列

6.队列节点状态

7.实现方法

自定义同步器实现时主要实现以下几种方法:

二.等待队列

1.同步等待队列

AQS当中的同步等待队列也称CLH队列,CLH队列是Craig、Landin、Hagersten三人发明的一种基于双向链表数据结构的队列,是FIFO先进先出线程等待队列,Java中的CLH队列是原CLH队列的一个变种,线程由原自旋机制改为阻塞机制。

AQS 依赖CLH同步队列来完成同步状态的管理:

2.条件等待队列

AQS中条件队列是使用单向列表保存的,用nextWaiter来连接:

三.condition接口

    public static void main(String[] args) {
        Lock lock = new ReentrantLock();
        Condition condition = lock.newCondition();
        new Thread(() -> {
            lock.lock();
            try {
                log.debug(Thread.currentThread().getName() + " 开始处理任务");
                //会释放当前持有的锁,然后阻塞当前线程
                condition.await();
                log.debug(Thread.currentThread().getName() + " 结束处理任务");
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                lock.unlock();
            }
        }).start();
        new Thread(() -> {
            lock.lock();
            try {
                log.debug(Thread.currentThread().getName() + " 开始处理任务");
                Thread.sleep(2000);
                //唤醒因调用Condition#await方法而阻塞的线程
                condition.signal();
                log.debug(Thread.currentThread().getName() + " 结束处理任务");
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                lock.unlock();
            }
        }).start();
    }

Thread-0 开始处理任务

Thread-1 开始处理任务

Thread-1 结束处理任务

Thread-0 结束处理任务

四.ReentrantLock

1.ReentrantLock是什么

ReentrantLock是一种基于AQS框架的应用实现,是JDK中的一种线程并发访问的同步手段,它的功能类似于synchronized是一种互斥锁,可以保证线程安全。

2.特点

3. ReentrantLock和synchronized的区别

4. ReentrantLock的使用

伪代码:

ReentrantLock lock = new ReentrantLock(); //参数默认false,不公平锁  
ReentrantLock lock = new ReentrantLock(true); //公平锁  
//加锁    
lock.lock(); 
try {  
    //临界区 
} finally { 
    // 解锁 
    lock.unlock();  

例子:基本使用

	private static int sum = 0;
    private static Lock lock = new ReentrantLock();
    public static void main(String[] args) throws InterruptedException {
        for (int i = 0; i < 3; i++) {
            Thread thread = new Thread(()->{
                //加锁 一般写在try前面
                lock.lock();
                try {
                    // 临界区代码 业务逻辑
                    for (int j = 0; j < 10000; j++) {
                        sum++;
                    }
                } finally {
                    // 解锁
                    lock.unlock();
                }
            });
            thread.start();
        }
        Thread.sleep(2000);
        System.out.println(sum);
    }

30000

可重入

public static ReentrantLock lock = new ReentrantLock();
    public static void main(String[] args) {
        method1();
    }
    public static void method1() {
        lock.lock();
        try {
            log.debug("execute method1");
            method2();
        } finally {
            lock.unlock();
        }
    }
    public static void method2() {
        lock.lock();
        try {
            log.debug("execute method2");
            method3();
        } finally {
            lock.unlock();
        }
    }
    public static void method3() {
        lock.lock();
        try {
            log.debug("execute method3");
        } finally {
            lock.unlock();
        }
    }

execute method1

execute method2

execute method3

可中断

    public static void main(String[] args) throws InterruptedException {
        ReentrantLock lock = new ReentrantLock();
        Thread t1 = new Thread(() -> {
            log.debug("t1启动...");
            try {
                lock.lockInterruptibly();
                try {
                    log.debug("t1获得了锁");
                } finally {
                    lock.unlock();
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
                log.debug("t1等锁的过程中被中断");
            }
        }, "t1");
        lock.lock();
        try {
            log.debug("main线程获得了锁");
            t1.start();
            //先让线程t1执行
            Thread.sleep(1000);
            t1.interrupt();
            log.debug("线程t1执行中断");
        } finally {
            lock.unlock();
        }
    }

main线程获得了锁

t1启动…

线程t1执行中断

t1等锁的过程中被中断

锁超时

    public static void main(String[] args) throws InterruptedException {
        ReentrantLock lock = new ReentrantLock();
        Thread t1 = new Thread(() -> {
            log.debug("t1启动...");
            try {
                //if (!lock.tryLock()) {
                //  log.debug("t1获取锁失败,立即返回false");
                //  return;
                //}
 				if (!lock.tryLock(1, TimeUnit.SECONDS)) {
                    log.debug("等待 1s 后获取锁失败,返回");
                    return;
                }
            } catch (Exception e) {
                e.printStackTrace();
                return;
            }
            try {
                log.debug("t1获得了锁");
            } finally {
                lock.unlock();
            }
        }, "t1");
        lock.lock();
        try {
            log.debug("main线程获得了锁");
            t1.start();
            //先让线程t1执行
            Thread.sleep(2000);
        } finally {
            lock.unlock();
        }
    }

main线程获得了锁

t1启动…

等待 1s 后获取锁失败,返回

公平锁和非公平锁

public static void main(String[] args) throws InterruptedException {
//        ReentrantLock lock = new ReentrantLock(true); //公平锁
        ReentrantLock lock = new ReentrantLock(); //非公平锁
        for (int i = 0; i < 500; i++) {
            new Thread(() -> {
                lock.lock();
                try {
                    try {
                        Thread.sleep(10);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    log.debug(Thread.currentThread().getName() + " running...");
                } finally {
                    lock.unlock();
                }
            }, "t" + i).start();
        }
        // 1s 之后去争抢锁
        Thread.sleep(1000);
        for (int i = 0; i < 500; i++) {
            new Thread(() -> {
                lock.lock();
                try {
                    log.debug(Thread.currentThread().getName() + " running...");
                } finally {
                    lock.unlock();
                }
            }, "强行插入" + i).start();
        }
    }

条件变量

private static ReentrantLock lock = new ReentrantLock();
    private static Condition cigCon = lock.newCondition();
    private static Condition takeCon = lock.newCondition();
    private static boolean hashcig = false;
    private static boolean hastakeout = false;
    //送烟
    public void cigratee(){
        lock.lock();
        try {
            while(!hashcig){
                try {
                    log.debug("没有烟,歇一会");
                    cigCon.await();
                }catch (Exception e){
                    e.printStackTrace();
                }
            }
            log.debug("有烟了,干活");
        }finally {
            lock.unlock();
        }
    }
    //送外卖
    public void takeout(){
        lock.lock();
        try {
            while(!hastakeout){
                try {
                    log.debug("没有饭,歇一会");
                    takeCon.await();

                }catch (Exception e){
                    e.printStackTrace();
                }
            }
            log.debug("有饭了,干活");
        }finally {
            lock.unlock();
        }
    }
    public static void main(String[] args) {
        ReentrantLockDemo6 test = new ReentrantLockDemo6();
        new Thread(() ->{
            test.cigratee();
        }).start();
        new Thread(() -> {
            test.takeout();
        }).start();
        new Thread(() ->{
            lock.lock();
            try {
                hashcig = true;
                log.debug("唤醒送烟的等待线程");
                cigCon.signal();
            }finally {
                lock.unlock();
            }
        },"t1").start();
        new Thread(() ->{
            lock.lock();
            try {
                hastakeout = true;
                log.debug("唤醒送饭的等待线程");
                takeCon.signal();
            }finally {
                lock.unlock();
            }
        },"t2").start();
    }

没有烟,歇一会
没有饭,歇一会
唤醒送烟的等待线程
唤醒送饭的等待线程
有烟了,干活
有饭了,干活

五.源码解析

首先会调用lock方法

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

lock会调用公平方法或者非公平的方法,默认是非公平锁方法,非公平锁则会cas尝试加锁,state是不是0,是0的话就把它改为1,并设置当前线程为独占线程,加锁成功,待下个线程进来时已经变成1,则失败阻塞。

加锁

	final void lock() {
		// 看状态是不是0,如果是0 则改为1,加锁成功
        if (compareAndSetState(0, 1))
         // 并设置当前线程为独占线程
           setExclusiveOwnerThread(Thread.currentThread());
        else
        	//不是0则失败阻塞
           acquire(1);
    }
protected final void setExclusiveOwnerThread(Thread thread) {
        exclusiveOwnerThread = thread;
    }

加锁失败(入队 阻塞)

 public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            //恢复中断标识位
            selfInterrupt();
    }

首先tryAcquire 又进行了一次判断,看是否能获取锁,

final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            //其他线程进来,状态值是1
            if (c == 0) {
                if (compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
            	// 重入,将状态值+1
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }

添加进队列

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;
        //第一次tail为空
        if (pred != null) {
        	//尾插法
            node.prev = pred;
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        //tail为空则在这里创建队列
        enq(node);
        return node;
    }

创建队列并且入队

private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
            if (t == null) { // Must initialize
            	//创建队列
                if (compareAndSetHead(new Node()))
                	// 将头节点指向前一节点的尾节点,这时候tail不为空了
                    tail = head;
            } else {
            	//双向接口,前一节点的尾节点也指向当前节点的头节点
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

阻塞

final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) { //保证一定获取锁
            	//获取head节点
                final Node p = node.predecessor();
                //是头节点则尝试获取锁
                if (p == head && tryAcquire(arg)) {
                	//设置头节点
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                //获取锁失败的情况,阻塞,在for循环里,第一次shouldParkAfterFailedAcquire为false,会将其设置为-1,第二次就可以阻塞
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

是否需要阻塞,把状态设置为SIGNAL,可以被唤醒了

private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus;
        //是-1了就可以去阻塞
        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 { //把节点去掉
                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.
             */
             //把状态设置为SIGNAL,可以被唤醒了
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

真正的阻塞方法

 private final boolean parkAndCheckInterrupt() {
 		//阻塞
        LockSupport.park(this);
        //清除中断标识位,在加锁失败方法的后面恢复中断标识位,可能其他地方还用到这个锁标识位
        return Thread.interrupted();
    }

唤醒 unlock()

 public void unlock() {
        sync.release(1);
    }
  public final boolean release(int arg) {
  	// 尝试唤醒
     if (tryRelease(arg)) {
         Node h = head;
         if (h != null && h.waitStatus != 0)
         	//唤醒阻塞的线程
             unparkSuccessor(h);
         return true;
     }
     return false;
 }
protected final boolean tryRelease(int releases) {
			//当前状态-1
            int c = getState() - releases;
            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            boolean free = false;
            if (c == 0) {
                free = true;
                setExclusiveOwnerThread(null);
            }
            //设置状态
            setState(c);
            return free;
        }

在这里唤醒

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;
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        //后面一个节点不为空 则直接唤醒当前线程
        if (s != null)
            LockSupport.unpark(s.thread);
    }

线程取消获取锁

private void cancelAcquire(Node node) {
        // Ignore if node doesn't exist
        if (node == null)
            return;
        node.thread = null;
        // Skip cancelled predecessors
        Node pred = node.prev;
        while (pred.waitStatus > 0)
        	//将前一个节点干掉
            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
        }
    }

至此加锁、解锁、阻塞、唤醒的底层源码都梳理完了。

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