wait_notify_example.java

package com.skazemy;

import java.util.ArrayList;
import java.util.List;

/**
 * This example shows that after calling wait() on an object, it will release that object's monitor lock,
 * so that other threads can enter the synchronized block. It also shows once notifyAll()
 * is called, first that thread has to exit the synchronized block, then each waiting thread starts
 * doing their work one by one, each has to exit their synchronized block so the other can wake up
 */
public class Main {

    public static void main(String[] args) {

        final Object object = new Object();

        class Waiter implements Runnable {
            @Override
            public void run() {
                String thread = Thread.currentThread().getName();
                System.out.println(thread + " - Before sync");
                synchronized (object) {
                    try {
                        System.out.println(thread + " - Before Wait");
                        object.wait();
                        System.out.println(thread + " Wait is done");
                        Thread.sleep(2000);
                        System.out.println(thread + " Sleep is Done");
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                System.out.println(thread + " Wait is done After sync");
            }
        }

        List<Thread> threads = new ArrayList<>(10);
        for (int i = 0 ; i < 10 ; i++) {
            threads.add(new Thread(new Waiter(), "thread " + i));
        }

        Thread thread3 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(5000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("Notifier - Before sync");
                synchronized (object) {
                    System.out.println("Notifier - Before notify");
                    object.notifyAll();
                    System.out.println("Notify is done");
                    System.out.println("Notifier sleeps again");
                    try {
                        Thread.sleep(5000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println("Notifier wakes up");
                }
            }
        });

        for (Thread thread: threads) {
            thread.start();
        }

        thread3.start();

    }
}