False wake up problem in Java threads

First define the resource class: contains the resource quantity, consumption method, production method

When resource is 0, producer produces resource

When the resource is 1, the consumer consumes the resource

Public class Resource {// The number of current resources int num=0; Int size=1; Public synchronized void remove(){if(num==0){try {system.out.println (" synchronized "); wait(); } catch (InterruptedException e) { e.printStackTrace(); } } num--; System.out.println(" consumer Thread: "+ thread.currentThread ().getName()+ num); notifyAll(); } public synchronized void put(){if(num==size){try {system.out.println (" synchronized "); wait(); } catch (InterruptedException e) { e.printStackTrace(); } } num++; System.out.println(" producer Thread: "+ thread.currentThread ().getName()+ num); notifyAll(); }}Copy the code

Creating a consumer thread

Public class Consumer implements Runnable {private Resource implements Runnable; public Consumer(Resource resource) { this.resource = resource; } @Override public void run() { for(int i=0; i<10; i++){ resource.remove(); }}}Copy the code

Creating a producer thread

Public class Producer implements Runnable {private Resource implements Runnable; public Producer(Resource resource) { this.resource = resource; } @Override public void run() { for(int i=0; i<10; i++){ resource.put(); }}}Copy the code

The thread of execution

public static void main(String[] args) {
    Resource resource = new Resource();
    Consumer consumer = new Consumer(resource);
    Producer producer = new Producer(resource);
    new Thread(consumer).start();
    new Thread(producer).start();
}
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It can be found that the thread is executing normally

But when we have two consumers and two producers, thread execution becomes abnormal

public static void main(String[] args) { Resource resource = new Resource(); Consumer consumer = new Consumer(resource); Producer producer = new Producer(resource); New Thread(producer," producer 1").start(); New Thread(producer," producer 2").start(); New Thread(consumer," consumer 1").start(); New Thread(consumer," consumer 2").start(); }Copy the code