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Java线程等待唤醒

2022-02-05 分类：未分类 阅读(265)

Java线程等待唤醒插图
image-20210510110445093
Java线程等待唤醒插图1
image-20210510114027612
  • synchronized-wait-notify
    static Object objectLock = new Object();
    public static void syncWaitNotify()
    {
        new Thread(() -> {
            //暂停几秒钟线程
//            try { TimeUnit.SECONDS.sleep(3); } catch (InterruptedException e) { e.printStackTrace(); }
            synchronized (objectLock){
                System.out.println(Thread.currentThread().getName()+"\t"+"---come in");
                try {
                    objectLock.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName()+"\t"+"---被唤醒");
            }
        },"t1").start();

        //暂停几秒钟线程
//        try { TimeUnit.SECONDS.sleep(1); } catch (InterruptedException e) { e.printStackTrace(); }

        new Thread(() -> {
            synchronized (objectLock){
                objectLock.notify();
                System.out.println(Thread.currentThread().getName()+"\t"+"---发出通知");
            }
        },"t2").start();
    }

RUN>

t1    ---come in
t2    ---发出通知
t1    ---被唤醒
  • wait-notify 不能脱离synchronized代码块单独运行
static Object objectLock = new Object();
    public static void syncWaitNotify()
    {
        new Thread(() -> {
            //暂停几秒钟线程
//            try { TimeUnit.SECONDS.sleep(3); } catch (InterruptedException e) { e.printStackTrace(); }
//            synchronized (objectLock){
                System.out.println(Thread.currentThread().getName()+"\t"+"---come in");
                try {
                    objectLock.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName()+"\t"+"---被唤醒");
//            }
        },"t1").start();

        //暂停几秒钟线程
//        try { TimeUnit.SECONDS.sleep(1); } catch (InterruptedException e) { e.printStackTrace(); }

        new Thread(() -> {
//            synchronized (objectLock){
                objectLock.notify();
                System.out.println(Thread.currentThread().getName()+"\t"+"---发出通知");
//            }
        },"t2").start();
    }

RUN>

t1    ---come in
Exception in thread "t1" Exception in thread "t2" java.lang.IllegalMonitorStateException
  at java.lang.Object.wait(Native Method)
  at java.lang.Object.wait(Object.java:502)
  at com.atguigu.juc.interrupt.LockSupportDemo.lambda$syncWaitNotify$2(LockSupportDemo.java:83)
  at java.lang.Thread.run(Thread.java:748)
java.lang.IllegalMonitorStateException
  at java.lang.Object.notify(Native Method)
  at com.atguigu.juc.interrupt.LockSupportDemo.lambda$syncWaitNotify$3(LockSupportDemo.java:96)
  at java.lang.Thread.run(Thread.java:748)

Process finished with exit code 0
Java线程等待唤醒插图2
image-20210510115127500

synchronized-wait-notify ,先notify后wait，线程运行会是什么情况

    static Object objectLock = new Object();
    public static void syncWaitNotify()
    {
        new Thread(() -> {
            //暂停几秒钟线程
            try { TimeUnit.SECONDS.sleep(3); } catch (InterruptedException e) { e.printStackTrace(); }
            synchronized (objectLock){
                System.out.println(Thread.currentThread().getName()+"\t"+"---come in");
                try {
                    objectLock.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName()+"\t"+"---被唤醒");
            }
        },"t1").start();

        //暂停几秒钟线程
//        try { TimeUnit.SECONDS.sleep(1); } catch (InterruptedException e) { e.printStackTrace(); }

        new Thread(() -> {
            synchronized (objectLock){
                objectLock.notify();
                System.out.println(Thread.currentThread().getName()+"\t"+"---发出通知");
            }
        },"t2").start();
    }

RUN>?????

t2    ---发出通知
t1    ---come in
Java线程等待唤醒插图3
image-20210510114935437
Java线程等待唤醒插图4
image-20210510115207113

Condition接口中的await后signal方法实现线程的等待和唤醒

  • Condition-await-signal
static Lock lock = new ReentrantLock();
    static Condition condition = lock.newCondition();
    public static void lockAwaitSignal()
    {
        new Thread(() -> {
            //暂停几秒钟线程
//            try { TimeUnit.SECONDS.sleep(3); } catch (InterruptedException e) { e.printStackTrace(); }
            lock.lock();
            try
            {
                System.out.println(Thread.currentThread().getName()+"\t"+"---come in");
                condition.await();
                System.out.println(Thread.currentThread().getName()+"\t"+"---被唤醒");
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                lock.unlock();
            }
        },"t1").start();

        new Thread(() -> {
            lock.lock();
            try
            {
                condition.signal();
                System.out.println(Thread.currentThread().getName()+"\t"+"---发出通知");
            }finally {
                lock.unlock();
            }
        },"t2").start();
    }

RUN>??????

t1    ---come in
t2    ---发出通知
t1    ---被唤醒
  • await-signal 不能脱离lock-unlock 单独使用
    static Lock lock = new ReentrantLock();
    static Condition condition = lock.newCondition();
    public static void lockAwaitSignal()
    {
        new Thread(() -> {
            //暂停几秒钟线程
//            try { TimeUnit.SECONDS.sleep(3); } catch (InterruptedException e) { e.printStackTrace(); }
//            lock.lock();
            try
            {
                System.out.println(Thread.currentThread().getName()+"\t"+"---come in");
                condition.await();
                System.out.println(Thread.currentThread().getName()+"\t"+"---被唤醒");
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
//                lock.unlock();
            }
        },"t1").start();

        new Thread(() -> {
//            lock.lock();
            try
            {
                condition.signal();
                System.out.println(Thread.currentThread().getName()+"\t"+"---发出通知");
            }finally {
//                lock.unlock();
            }
        },"t2").start();
    }

RUN>??????

t1    ---come in
Exception in thread "t1" Exception in thread "t2" java.lang.IllegalMonitorStateException
  at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.signal(AbstractQueuedSynchronizer.java:1939)
  at com.atguigu.juc.interrupt.LockSupportDemo.lambda$lockAwaitSignal$1(LockSupportDemo.java:67)
  at java.lang.Thread.run(Thread.java:748)
java.lang.IllegalMonitorStateException
  at java.util.concurrent.locks.ReentrantLock$Sync.tryRelease(ReentrantLock.java:151)
  at java.util.concurrent.locks.AbstractQueuedSynchronizer.release(AbstractQueuedSynchronizer.java:1261)
  at java.util.concurrent.locks.AbstractQueuedSynchronizer.fullyRelease(AbstractQueuedSynchronizer.java:1723)
  at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2036)
  at com.atguigu.juc.interrupt.LockSupportDemo.lambda$lockAwaitSignal$0(LockSupportDemo.java:54)
  at java.lang.Thread.run(Thread.java:748)
  • Condition-await-signal 先signal后await
static Lock lock = new ReentrantLock();
static Condition condition = lock.newCondition();
public static void lockAwaitSignal()
{
    new Thread(() -> {
        //暂停几秒钟线程
        try { TimeUnit.SECONDS.sleep(3); } catch (InterruptedException e) { e.printStackTrace(); }
        lock.lock();
        try
        {
            System.out.println(Thread.currentThread().getName()+"\t"+"---come in");
            condition.await();
            System.out.println(Thread.currentThread().getName()+"\t"+"---被唤醒");
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    },"t1").start();

    new Thread(() -> {
        lock.lock();
        try
        {
            condition.signal();
            System.out.println(Thread.currentThread().getName()+"\t"+"---发出通知");
        }finally {
            lock.unlock();
        }
    },"t2").start();
}

RUN????????

t2    ---发出通知
t1    ---come in
Java线程等待唤醒插图5
image-20210510120144143

Class LockSupport

  • java.lang.Object
    • java.util.concurrent.locks.LockSupport
public class LockSupport
extends Object

Basic thread blocking primitives for creating locks and other synchronization classes.

This class associates, with each thread that uses it, a permit (in the sense of the [Semaphore](dfile:///Users/jiangbin/Library/Application Support/Dash/DocSets/Java_SE8/Java.docset/Contents/Resources/Documents/java/util/concurrent/Semaphore.html) class). A call to park will return immediately if the permit is available, consuming it in the process; otherwise it may block. A call to unpark makes the permit available, if it was not already available. (Unlike with Semaphores though, permits do not accumulate. There is at most one.)

Java线程等待唤醒插图6
image-20210510113312062
  1. LockSupport是用来创建锁和其他同步类的基本线程阻塞原语。
  2. LockSupport类使用了一种名为Permit (许可)的概念来做到阻塞和唤醒线程的功能，每个线程都有一个许可(permit),
  3. permit只有两个值1和零，默认是零。
  4. 可以把许可看成是一种(0,1)信号量(Semaphore)，但与Semaphore不同的是，许可的累加上限是1。
  • LockSupport不需要同步代码块，直接可以使用park unpark 挂起恢复线程运行(无锁块要求)
public static void main(String[] args)
    {
        Thread t1 = new Thread(() -> {
            System.out.println(Thread.currentThread().getName() + "\t" + "---come in");
            LockSupport.park();
            System.out.println(Thread.currentThread().getName() + "\t" + "---被唤醒");
        }, "t1");
        t1.start();

        try { TimeUnit.SECONDS.sleep(3); } catch (InterruptedException e) { e.printStackTrace(); }
        new Thread(() -> {
            LockSupport.unpark(t1);
            System.out.println(Thread.currentThread().getName()+"\t"+"---发出通知");
        },"t2").start();
    }

RUN>??????

t1    ---come in
t2    ---发出通知
t1    ---被唤醒
  • LockSupport - 先unpark后park 线程正常运行(支持先唤醒，后等待)
  public static void main(String[] args)
    {
        Thread t1 = new Thread(() -> {
            try { TimeUnit.SECONDS.sleep(3); } catch (InterruptedException e) { e.printStackTrace(); }
            System.out.println(Thread.currentThread().getName() + "\t" + "---come in");
            LockSupport.park();
            System.out.println(Thread.currentThread().getName() + "\t" + "---被唤醒");
        }, "t1");
        t1.start();

        new Thread(() -> {
            LockSupport.unpark(t1);
            System.out.println(Thread.currentThread().getName()+"\t"+"---发出通知");
        },"t2").start();
    }

RUN>??????

t1    ---come in
t2    ---发出通知
t1    ---被唤醒

重点说明(重要)

LockSupport是用来创建锁和其他同步类的基本线程阻塞原语。

LockSupport是一个线程阻寒工具类，所有的方法都是静态方法，可以让线程在任意位置阻塞，阻寒之后也有对应的唤醒方法。归根结底，LockSupport调用的Unsafe中的native代码。

==LockSupport提供park()和unpark()方法实现阻塞线程和解除线程阻塞的过程==

LockSupport和每个使用它的线程都有- -个许可(permit)关联。 permit相当于1， 0的开关，默认是0,

调用一次unpark就加1变成1,调用一次park会消费permit,也就是将1变成0，同时park 立即返回。

如再次调用park会变成阻塞(因为permit为零了会阻塞在这里，一- 直到permit变为1),这时调用unpark会把permit置为1。

每个线程都有一个相关的permit, permit最多只有一个，重复调用unpark也不会积累凭证。

形象的理解

线程阻塞需要消耗凭证(permit)， 这个凭证最多只有1个。
当调用park方法时

  • 如果有凭证，则会直接消耗掉这个凭证然后正常退出;.

  • 如果无凭证，就必须阻塞等待凭证可用;

而unpark则相反，它会增加一个凭证，但凭证最多只能有1个，累加无效。

为什么可以先唤醒线程后阻塞线程?

因为unpark获得了一个凭证，之后再调用park方法，就可以名正言顺的凭证消费，故不会阻塞。

为什么唤醒两次后阻塞两次，但最终结果还会阻塞线程?

因为凭证的数量最多为1，连续调用两次unpark和调用一次unpark效果一样，只会增加一个凭证;
而调用两次park却需要消费两个凭证，证不够，不能放行。

 public static void main(String[] args)
    {
        Thread t1 = new Thread(() -> {
            try { TimeUnit.SECONDS.sleep(3); } catch (InterruptedException e) { e.printStackTrace(); }
            System.out.println(Thread.currentThread().getName() + "\t" + "---come in");
            LockSupport.park();
            LockSupport.park();
            System.out.println(Thread.currentThread().getName() + "\t" + "---被唤醒");
        }, "t1");
        t1.start();

        new Thread(() -> {
            LockSupport.unpark(t1);
            LockSupport.unpark(t1);
            System.out.println(Thread.currentThread().getName()+"\t"+"---发出通知");
        },"t2").start();
    }
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