wait( ) method behind the scene ...

[ankur rathi]

public class ThreadTest {

public static void main(String[] args) {

HasRun h  = new HasRun();
Thread t1 = new Thread(h,"Tom");
Thread t2 = new Thread(h,"Jerry");
t1.start();
try {
t1.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
t2.start();

}
 
}
 
class HasRun implements Runnable {

synchronized public void run() {
for(int i=0; i<=1000; i++) {
System.out.println(Thread.currentThread().getName()+" "+i);

}
}

}

This program is giving IlligalMonitorStateException , because I am not calling wait() method in synchronized context . But what actually the reason ?

can any body explain .

thanks a lot .

[Subhash Bhushan C]

Every object, i.e., every object that extends Object, get associated with a "wait set", which is a set of threads. This wait set is manipulated by the wait(), notify() and notifyAll() methods of the Object class. The JLS specifies that, as per Java Thread scheduling mechanism, a thread can be added to a wait set of an object, only when the current thread holds a lock on the object. Meaning that, wait should be called for an object when the currently executing thread holds the lock on the object.

As to the reason of "Why" this is so, the programmers of Java might have experience problems with Thread scheduling mechanism, if the threads that call wait() on objects, did not own the locks on them.

Source: JLS 17.14

[ankur rathi]

public class ThreadTest {

public static void main(String[] args) {

HasRun h  = new HasRun();
Thread t1 = new Thread(h,"A");
Thread t2 = new Thread(h,"B");
t1.start();
synchronized(h) {

try {
t1.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
t2.start();

}
 
}
 
class HasRun implements Runnable {

synchronized public void run() {
for(int i=0; i<=1000; i++) {
System.out.println(Thread.currentThread().getName()+" "+i);
}
}

}

Now , why this code is giving Exception , I am calling wait() method from synchronized block .

thanks a lot .

[Jim Yingst]

You have synchronized using the object h as monitor (a HasRun instance). You are calling wiat() using a different instance, t1 (a Thread). You can only call wait if you've synchronized on the same instance you're using to call wait. Thus, you need to do either

synchronized(h) {
    try {
        h.wait();

or

synchronized(t1) {
    try {
        t1.wait();

I'm not really sure what you're trying to do, so I don't know which is more appropriate.