java多线程（7）wait()、notify()和notityALL()

时间  2019-12-10
标签 java 多线程 wait notify notityall 栏目 Java 繁體版
原文   原文链接

wait

wait方法是Object中的方法，这个方法的功能特性：
1).执行wait方法的前提是当前线程已经获取到对象的锁，也就是wait方法必须在synchronized修饰的代码块或者方法中使用。
2).执行wait以后，会失去锁的全部权
3).wait方法执行后会一直等待，直到被调用notify()、notifyAll()或者所在线程被中断。
4).被调用notify()或者notifyAll()后，线程仍是会等待，直到拥有锁的全部权，才会继续往下执行。
下面举个例子：java

public static void main(String[] args) throws InterruptedException {
        Object lock1 = new Object();
        Thread t1 = new Thread(new Test().new Tt1(lock1));
        Thread t2 = new Thread(new Test().new Tt2(lock1));
        t1.start();
        Thread.sleep(1000);
        t2.start();
    }
    
    
    class Tt1 implements Runnable{

        private Object lock1;
        
        public Tt1(Object lock1) {
            this.lock1 = lock1;
        }

        @Override
        public void run() {
            try {
                System.out.println(this.getClass()+"-------1");
                synchronized (lock1) {
                    Thread.sleep(2000);
                    System.out.println("waiting start");
                    lock1.wait();
                }
                System.out.println("waiting end");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
    
    class Tt2 implements Runnable{

        private Object lock1;
        
        public Tt2(Object lock1) {
            this.lock1 = lock1;
        }

        @Override
        public void run() {
            System.out.println(this.getClass()+"-------1");
            synchronized (lock1) {
                try {
                    System.out.println(this.getClass()+"-------2");
                    lock1.notify();
                    Thread.sleep(1000);
                    System.out.println(this.getClass()+"-------3");
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
            
        }
    }

执行结果：多线程

class Test$Tt1-------1
class Test$Tt2-------1
waiting start
class Test$Tt2-------2
class Test$Tt2-------3
waiting end

分析一下：
第一、2行：t1和t2启动，t1先获取到锁因此t2一直被阻塞住
第三、4行：t1中执行了wait，锁被释放，因此t2继续执行下去。
第五、6行：t2中调用了notify()可是t1没有立刻执行，由于锁如今是被t2拥有，等t2执行完成释放锁后，t1继续执行。ide

notify、notifyAll

notify已经在上面有提到过，notify和notifyAll 的做用是唤醒正在wait的线程，notify是随机唤醒wait线程中的一个，notifyAll 则是唤醒所有。this

1).执行notify、notifyAll 方法的前提是当前线程已经获取到对象的锁，也就是必须在synchronized修饰的代码块或者方法中使用。这个和wait是同样的。
2).被调用notify()或者notifyAll()后，线程仍是会等待，直到拥有锁的全部权，才会继续往下执行。
3)notify、notifyAll不会释放锁，这个与wait不一样。线程

释放和不释放锁

在多线程的操做中，锁的释放与否是必需要清楚的，wait是会释放锁，而notify(notifyAll)则不会。先举个wait的例子：code

public static void main(String[] args) throws InterruptedException {
        Object lock1 = new Object();
        Thread t1 = new Thread(new Test().new Tt1(lock1));
        Thread t2 = new Thread(new Test().new Tt2(lock1));
        t1.start();
        Thread.sleep(100);
        t2.start();
    }
    
    class Tt1 implements Runnable{
        private Object lock1;
        
        public Tt1(Object lock1) {
            this.lock1 = lock1;
        }

        @Override
        public void run() {
            try {
                synchronized (lock1) {
                    System.out.println(Thread.currentThread().getName()+"---start");
                    lock1.wait();
                    Thread.sleep(3000);
                    System.out.println(Thread.currentThread().getName()+"---end");
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
    
    class Tt2 implements Runnable{

        private Object lock1;
        
        public Tt2(Object lock1) {
            this.lock1 = lock1;
        }

        @Override
        public void run() {
            synchronized (lock1) {
                try {
                    System.out.println(Thread.currentThread().getName()+"---start");
                    System.out.println(Thread.currentThread().getName()+"---end");
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
            
        }
    }

执行结果：对象

Thread-0---start
Thread-1---start
Thread-1---end

Thread-0执行wait后立刻释放了锁，因此Thread-1很快接着就执行。get

再来notify的例子，其实就是把上一个例子wait()改为nofity()：it

public static void main(String[] args) throws InterruptedException {
        Object lock1 = new Object();
        Thread t1 = new Thread(new Test().new Tt1(lock1));
        Thread t2 = new Thread(new Test().new Tt2(lock1));
        t1.start();
        Thread.sleep(100);
        t2.start();
    }
    
    class Tt1 implements Runnable{
        private Object lock1;
        
        public Tt1(Object lock1) {
            this.lock1 = lock1;
        }

        @Override
        public void run() {
            try {
                synchronized (lock1) {
                    System.out.println(Thread.currentThread().getName()+"---start");
                    lock1.notify();
                    Thread.sleep(3000);
                    System.out.println(Thread.currentThread().getName()+"---end");
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
    
    class Tt2 implements Runnable{

        private Object lock1;
        
        public Tt2(Object lock1) {
            this.lock1 = lock1;
        }

        @Override
        public void run() {
            synchronized (lock1) {
                try {
                    System.out.println(Thread.currentThread().getName()+"---start");
                    System.out.println(Thread.currentThread().getName()+"---end");
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
            
        }
    }

执行结果：io

Thread-0---start
Thread-0---end
Thread-1---start
Thread-1---end

可见Thread-0在执行nofity后并无释放锁，而是等待代码块执行完以后才释放锁，Thread-1才能继续执行。

必须先获取锁

不管是notify仍是wait都是要先获取锁，既必须在synchronized内使用，举个反例：

public static void main(String[] args) throws InterruptedException {
            Object lock1 = new Object();
            Thread t1 = new Thread(new Test().new Tt1(lock1));
            t1.run();
        }
        
        class Tt1 implements Runnable{
            private Object lock1;
            
            public Tt1(Object lock1) {
                this.lock1 = lock1;
            }
    
            @Override
            public void run() {
                try {
                    lock1.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }

执行结果：

Exception in thread "main" java.lang.IllegalMonitorStateException
    at java.lang.Object.wait(Native Method)
    at java.lang.Object.wait(Object.java:502)
    at Test$Tt1.run(Test.java:24)
    at java.lang.Thread.run(Thread.java:748)
    at Test.main(Test.java:11)

wait()遇到interrupt()

以前说道interrupt()，并不会直接中断线程，而是会给线程一个中断标志，并且包括sleep、wait、join会抛出InterruptedException。

public static void main(String[] args) throws InterruptedException {
            Object lock1 = new Object();
            Thread t1 = new Thread(new Test().new Tt1(lock1));
            t1.start();
            Thread.sleep(100);
            t1.interrupt();
        }
        
        class Tt1 implements Runnable{
            private Object lock1;
            
            public Tt1(Object lock1) {
                this.lock1 = lock1;
            }
    
            @Override
            public void run() {
                synchronized (lock1) {
                    try {
                        System.out.println(Thread.currentThread().getName()+"---start");
                        lock1.wait();
                        System.out.println(Thread.currentThread().getName()+"---end");
                    }catch (InterruptedException e) {
                        System.out.println("线程被中断了");;
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
            }
        }

执行结果：

Thread-0---start
线程被中断了

notifyAll和nofity

notifyAll和nofity惟一的不一样就是，能够唤醒所有和唤醒一个， 先举个nofity的例子

public static void main(String[] args) throws InterruptedException {
            Object lock1 = new Object();
            Thread t1 = new Thread(new Test().new Tt1(lock1));
            Thread t2 = new Thread(new Test().new Tt1(lock1));
            Thread t3 = new Thread(new Test().new Tt2(lock1));
            t1.start();
            Thread.sleep(100);
            t2.start();
            Thread.sleep(100);
            t3.start();
        }
        
        class Tt1 implements Runnable{
            private Object lock1;
            
            public Tt1(Object lock1) {
                this.lock1 = lock1;
            }
    
            @Override
            public void run() {
                synchronized (lock1) {
                    try {
                        System.out.println(Thread.currentThread().getName()+"---start");
                        System.out.println(Thread.currentThread().getName()+"---wait");
                        lock1.wait();
                        System.out.println(Thread.currentThread().getName()+"---end");
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
            }
        }
        
        class Tt2 implements Runnable{
    
            private Object lock1;
            
            public Tt2(Object lock1) {
                this.lock1 = lock1;
            }
    
            @Override
            public void run() {
                synchronized (lock1) {
                    try {
                        System.out.println(Thread.currentThread().getName()+"---start");
                        System.out.println(Thread.currentThread().getName()+"---notify");
                        lock1.notify();
                        System.out.println(Thread.currentThread().getName()+"---end");
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
                
            }
        }

执行结果：

Thread-0---start
Thread-0---wait
Thread-1---start
Thread-1---wait
Thread-2---start
Thread-2---notify
Thread-2---end
Thread-0---end

Thread-0和Thread-1在wait，Thread-2执行了notify，但只有Thread-0被唤醒，从新开始执行，Thread-1还在wait。

再来看看nofityAll：

public static void main(String[] args) throws InterruptedException {
            Object lock1 = new Object();
            Thread t1 = new Thread(new Test().new Tt1(lock1));
            Thread t2 = new Thread(new Test().new Tt1(lock1));
            Thread t3 = new Thread(new Test().new Tt2(lock1));
            t1.start();
            Thread.sleep(100);
            t2.start();
            Thread.sleep(100);
            t3.start();
        }
        
        class Tt1 implements Runnable{
            private Object lock1;
            
            public Tt1(Object lock1) {
                this.lock1 = lock1;
            }
    
            @Override
            public void run() {
                synchronized (lock1) {
                    try {
                        System.out.println(Thread.currentThread().getName()+"---start");
                        System.out.println(Thread.currentThread().getName()+"---wait");
                        lock1.wait();
                        System.out.println(Thread.currentThread().getName()+"---end");
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
            }
        }
        
        class Tt2 implements Runnable{
    
            private Object lock1;
            
            public Tt2(Object lock1) {
                this.lock1 = lock1;
            }
    
            @Override
            public void run() {
                synchronized (lock1) {
                    try {
                        System.out.println(Thread.currentThread().getName()+"---start");
                        System.out.println(Thread.currentThread().getName()+"---notifyAll");
                        lock1.notifyAll();
                        System.out.println(Thread.currentThread().getName()+"---end");
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
                
            }
        }

执行结果：

Thread-0---start
Thread-0---wait
Thread-1---start
Thread-1---wait
Thread-2---start
Thread-2---notifyAll
Thread-2---end
Thread-1---end
Thread-0---end

Thread-2执行了notifyAll后，Thread-1和Thread-0都被唤醒。

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