Java线程的wait(), notify()和notifyAll()

Posted 2021-12-01 milton

wait(), notify()和notifyAll()方法用于在线程间建立关联. 在对象上调用wait()将使线程进入WAITTING状态, 直到其他线程对同一个对象调用notify()或notifyAll(). 在任何线程上, 对一个对象调用wait(), notify()和notifyAll(), 都需要先获得这个对象的锁, 就是说, 这些方法必须在synchronized方法或代码块中调用.

notify()

调用notify()时, 在所有WAITING状态的线程中只会有一个线程被通知, 这个选择是随机的, 被通知的线程并不会立即得到对象的锁, 而是一直等到调用notify()的线程释放锁, 在这之前线程都是BLOCKED状态. 当获得锁后, 就会从BLOCKED状态变为RUNNING状态. 例子

class Shared {
    synchronized void waitMethod() {
        Thread t = Thread.currentThread();
        System.out.println(t.getName() + " is releasing the lock and going to wait");
        try {
            wait();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(t.getName() + " has been notified and acquired the lock back");
    }

    synchronized void notifyOneThread() {
        Thread t = Thread.currentThread();
        notify();
        System.out.println(t.getName() + " has notified one thread waiting for this object lock");
    }
}

public class MainClass {
    public static void main(String[] args) {
        final Shared s = new Shared();
        //Thread t1 will be waiting for lock of object ‘s‘
        Thread t1 = new Thread() {
            @Override
            public void run() {
                s.waitMethod();
            }
        };
        t1.start();

        //Thread t2 will be waiting for lock of object ‘s‘
        Thread t2 = new Thread() {
            @Override
            public void run() {
                s.waitMethod();
            }
        };
        t2.start();

        //Thread t3 will be waiting for lock of object ‘s‘
        Thread t3 = new Thread() {
            @Override
            public void run() {
                s.waitMethod();
            }
        };
        t3.start();

        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        //Thread t4 will notify only one thread which is waiting for lock of object ‘s‘
        Thread t4 = new Thread() {
            @Override
            public void run() {
                s.notifyOneThread();
            }
        };
        t4.start();
    }
}

.

notifyAll()

当线程在对象上调用notifyAll()时, 所有WAITING状态的线程都会被通知, 所有的线程都会从WAITING状态变成BLOCKED状态, 然后争抢对象的锁. 得到对象锁的线程, 将变成RUNNING状态, 而其他线程则继续保持BLOCKED状态继续等待获取对象锁. 例子

class Shared {
    synchronized void waitMethod() {
        Thread t = Thread.currentThread();
        System.out.println(t.getName() + " is releasing the lock and going to wait");
        try {
            wait();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(t.getName() + " has been notified and acquired the lock back");
    }

    synchronized void notifyAllThread() {
        Thread t = Thread.currentThread();
        notifyAll();
        System.out.println(t.getName() + " has notified all threads waiting for this object lock");
    }
}

public class MainClass {
    public static void main(String[] args) {
        final Shared s = new Shared();
        //Thread t1 will be waiting for lock of object ‘s‘
        Thread t1 = new Thread() {
            @Override
            public void run() {
                s.waitMethod();
            }
        };
        t1.start();

        //Thread t2 will be waiting for lock of object ‘s‘
        Thread t2 = new Thread() {
            @Override
            public void run() {
                s.waitMethod();
            }
        };
        t2.start();

        //Thread t3 will be waiting for lock of object ‘s‘
        Thread t3 = new Thread() {
            @Override
            public void run() {
                s.waitMethod();
            }
        };
        t3.start();

        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        //Thread t4 will notify all threads which are waiting for lock of object ‘s‘
        Thread t4 = new Thread() {
            @Override
            public void run() {
                s.notifyAllThread();
            }
        };
        t4.start();
    }
}

.

一个生产者和消费者的例子

注意, 在1个生产1个消费的情况下, 是能确保生产和消费的互相通知的, 但是在2个生产1个消费的情况下, 有可能要多次notify后消费线程才能拿到queue的锁.

public class DemoThreadWait1 {
    Queue<Integer> queue = new LinkedList<>();

    public void consume() {
        synchronized (queue) {
            while (queue.isEmpty()) {
                try {
                    System.out.println("consume wait");
                    queue.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("remove all");
                queue.clear();
                queue.notify();
            }
        }
    }

    public void produce(int i) {
        synchronized (queue) {
            if (queue.size() < 5) {
                System.out.println("add " + i);
                queue.add(i);
            }
            if (queue.size() >= 5) {
                queue.notify();
                try {
                    System.out.println("produce wait");
                    queue.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    public static void main(String[] args) {
        DemoThreadWait1 demo = new DemoThreadWait1();
        new Thread(()->{
            while(true) {
                demo.consume();
            }
        }).start();

        new Thread(()->{
            while(true) {
                demo.produce((int) (Math.random() * 1000));
            }
        }).start();

        new Thread(()->{
            while(true) {
                demo.produce((int) (Math.random() * 1000));
            }
        }).start();
    }
}