Thread -IllegalMonitorStateException and IllegalThreadStateException

[esunilk]

Hi all,

I am newbie to this forum and preparing for scjp exam.

below code is sample program.

 class EnumOne extends Thread {
public static void main(String[] args) {

EnumOne e = new EnumOne();
     Thread t = new Thread(e);
    for(int i=0;i<3;i++){
   t.start();
  
   }
}
 
public void run(){
System.out.println("hi");
}

}   

can anyone explain

1. what is IllegalThreadStateException and when we get this exception with any good example.

2. and what is IllegalMonitorStateException with good example.

Many thanks in advance

[Punit Singh]

Calling 2 times start on a single thread is IllegalThreadStateException.

class ThreadI extends Thread{}
public class IllegalThreadExceptionImpl {
 public static void main(String[] args) {
ThreadI t1=new ThreadI();
t1.start();
t1.start();
}
}    

Calling wait() or notify() on any object without having lock (mean synchronized) on that object is IllegalMonitorStateException.

public class IllegalMonitorStateExceptionImpl extends Thread{
static String str="";
@Override
public void run() {
synchronized (str)//taking lock on str=""
 
  {
str="String object changed here..";//but I am changing the str reference to point to new String="String object changed here.."

                     System.out.println("waiting...");
try {
str.wait();//no no we donot have lock on new String object...
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}

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

[satishkumar janakiraman]

You have called start method of the thread in a for loop. When a thread calls a start method it will invoke run and ends to complete (assume there is timeslicing). After completion of the run method the thread is not in runnable state. But your for loop is iterating and the thread has already completed its run method and not in runnable state.
This the reason for throwing IllegalThreadStateException

For your second question, the api says
Thrown to indicate that a thread has attempted to wait on an object's monitor or to notify other threads waiting on an object's monitor without owning the specified monitor.

class EnumOne extends Thread {
public static void main(String[] args) {

EnumOne e = new EnumOne();
     Thread t = new Thread(e);
   
   t.start();
  
   
}

private void test() {

try {
System.out.print("I am not owning the lock of this object");

wait();
} catch(Exception e) {
e.printStackTrace();
}

}
 
 
public void run(){
System.out.println("hi");
test();
}

}   

sat