IllegalMonitorStateException

[Asanka Vithanage]

can any body explain the reason to genarate
IllegalMonitorStateException

[Ankit Garg]

This exception occurs when you call wait or notify on an object whose lock is not held by the thread who is executing the code. Eg

class Try {
   public static void main(String[] args)throws Exception {
      Try t = new Try();
      t.wait();  //it will throw IllegalMonitorStateException
   }
}

To solve the problem, the code must be

class Try {
   public static void main(String[] args)throws Exception {
      Try t = new Try();
      synchronized(t) {
         t.wait();  //no IllegalMonitorStateException this time.
      }
   }
}

[Punit Singh]

First Case:
If you wait or monitor on an object, without taking lock on that object means without synchronizing on that object, then IllegalMonitorStateException is thrown.

  
public class IllegalMonitorThreadExceptionImpl extends Thread{
static String str="";
@Override
public void run() {

try {
                System.out.println("waiting...");
str.wait();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}

}

public static void main(String[] args) {
new IllegalMonitorThreadExceptionImpl().start();
}
}
  

Second Case: If you take lock on an object, here synchronized(str),
but you change the object here str="String object changed here..";
and call wait() on that object here str.wait();
here str object has been changed. It is different Object than you have taken lock on. Now you donot have lock on new str object. so here if str.wait() called it will throw IllegalMonitorStateException, means your are monitoring on such an object that is not locked by you.

 public class IllegalMonitorThreadExceptionImpl extends Thread{
static String str="";
@Override
public void run() {
synchronized (str) {
str="String object changed here..";
System.out.println("waiting...");
try {
str.wait();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}

public static void main(String[] args) {
new IllegalMonitorThreadExceptionImpl().start();
}
}
   

[Asanka Vithanage]

thnak you for you'r explanation...............

but i have problem.In the following code i used wait method without using synchronization .but it compiles and run fine
gives CDA as out put.but it not prints B in out put.
can You explain this

class Try extends Thread{  
public void run(){
try{
System.out.println("A");
this.wait();
"Asa".wait();
System.out.println("B");
}
catch(Exception e){} 
} 
public static void main(String[] args)throws Exception {      
 
Try t = new Try(); 
t.start();
System.out.println("C");
System.out.println("D");
 
  }}

[James Tharakan]

write a print statement in the catch block.

[Asanka Vithanage]

:roll: :roll: :roll:
i got it