Java Object wait() Method

Description

The Java Object wait() method causes current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object. In other words, this method behaves exactly as if it simply performs the call wait(0).

The current thread must own this object's monitor. The thread releases ownership of this monitor and waits until another thread notifies threads waiting on this object's monitor to wake up either through a call to the notify method or the notifyAll method. The thread then waits until it can re-obtain ownership of the monitor and resumes execution.

This method should only be called by a thread that is the owner of this object's monitor. See the notify method for a description of the ways in which a thread can become the owner of a monitor.

Declaration

Following is the declaration for java.lang.Object.wait() method

public final void wait()

Parameters

NA

Return Value

This method does not return a value.

Exception

• IllegalMonitorStateException − if the current thread is not the owner of the object's monitor.

• InterruptedException − if another thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.

Making a Thread to Wait Example

The following example shows the usage of java.lang.Object.wait() method. In this program, we've created ObjectDemo class which is having addElement and removeElement methods. These methods are synchronized in nature and while adding an elment, we're using notifyAll() method to wake up all the thread waiting to access the object's monitor. In removeElement() method, we're checking if list is empty and then using wait() method, thread is put in waiting state. In main method, we've two runnable instances, one to remove element and another to add element. Once threads are created and started, addElement() and removeElement() methods are called.

package com.tutorialspoint;

import java.util.Collections;
import java.util.LinkedList;
import java.util.List;

public class ObjectDemo extends Object {

   private List synchedList;

   public ObjectDemo() {
      // create a new synchronized list to be used
      synchedList = Collections.synchronizedList(new LinkedList());
   }

   // method used to remove an element from the list
   public String removeElement() throws InterruptedException {
      synchronized (synchedList) {

         // while the list is empty, wait
         while (synchedList.isEmpty()) {
            System.out.println("List is empty...");
            synchedList.wait();
            System.out.println("Waiting...");
         }
         String element = (String) synchedList.remove(0);

         return element;
      }
   }

   // method to add an element in the list
   public void addElement(String element) {
      System.out.println("Opening...");
      synchronized (synchedList) {

         // add an element and notify all that an element exists
         synchedList.add(element);
         System.out.println("New Element:'" + element + "'");

         synchedList.notifyAll();
         System.out.println("notifyAll called!");
      }
      System.out.println("Closing...");
   }

   public static void main(String[] args) {
      final ObjectDemo demo = new ObjectDemo();

      Runnable runA = new Runnable() {

         public void run() {
            try {
               String item = demo.removeElement();
               System.out.println("" + item);
            } catch (InterruptedException ix) {
               System.out.println("Interrupted Exception!");
            } catch (Exception x) {
               System.out.println("Exception thrown.");
            }
         }
      };

      Runnable runB = new Runnable() {

         // run adds an element in the list and starts the loop
         public void run() {
            demo.addElement("Hello!");
         }
      };

      try {
         Thread threadA1 = new Thread(runA, "A");
         threadA1.start();

         Thread.sleep(500);

         Thread threadA2 = new Thread(runA, "B");
         threadA2.start();

         Thread.sleep(500);

         Thread threadB = new Thread(runB, "C");
         threadB.start();

         Thread.sleep(1000);

         threadA1.interrupt();
         threadA2.interrupt();
      } catch (InterruptedException x) {
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result −

List is empty...
List is empty...
Opening...
New Element:'Hello!'
notifyAll called!
Closing...
Waiting...
Hello!
Waiting...
List is empty...
Interrupted Exception!