Thread Locks

[Tim Holmes]

I am trying to figure out what object has the lock on the thread when "notify()" is called in this code. Ideas?

 
class Test extends Thread {
   publid void run() {
     synchronized(this) {
       try{wait();}catch(InterruptedException ie){}
     }
    }
    public static void main(String args[]){
 
      Test a1 = new Test();
      long startTime = System.currentTimeMillis();
      a1.start();
      a1.notify();
      System.out.println("done"); 
    }
}
 

[Vishwanath Krishnamurthi]

Hi

I am not very sure, but I think wait() and notify() should be called from a synchronized context only... In this code, it isn't the case...

Thanks

[Gosia Wittemann]

Exactly, there's no lock in the main method and you get the java.lang.IllegalMonitorStateException.

[Amit K. Sharma]

Hi Tim,

For the code that you posted :

class Test extends Thread {
   publid void run() {
     synchronized(this) {
       try{wait();}catch(InterruptedException ie){}
     }
    }
    public static void main(String args[]){
 
      Test a1 = new Test();
      long startTime = System.currentTimeMillis();
      a1.start();
      a1.notify(); // line 12
      System.out.println("done"); 
    }
}

The answer is : No thread holds the lock! since at line 12 the code attempts to execute notify() on object a1 without acquiring its lock. Result will be IllegalMonitorStateException

Now, if we fix this code to make it work, it chages to :

class Test extends Thread {
 
public void run() {
synchronized(this) {
try{
wait(); // line 6
}catch(InterruptedException ie){
ie.printStackTrace();
}
}
}

public static void main(String args[]){
 
Test a1 = new Test();
long startTime = System.currentTimeMillis();
a1.start();

try {
Thread.sleep(100); // line 20 
} catch (Exception x) {}

synchronized (a1) {
a1.notify(); // line 24 
}
System.out.println("done"); 
}
}

In this case the code works and the answer to the same question for this code is :
At when notify is called on object `a1` ( line 24 ), "The main() thread has acquired lock on monitor of object a1".

Note : In case you remove the delay incurred by Thread.sleep(100) from line 24, the thread executed by run() method will never come out of wait because notify is called even before wait is executed in line 6

Cheers,
Amit

[Thirugnanam Saravanan]

In case you remove the delay incurred by Thread.sleep(100) from line 24, the thread executed by run() method will never come out of wait because notify is called even before wait is executed in line 6

I do not understand this statement. How are you saying that notify is called even before wait is executed? What is the order of execution of start(), wait() and notify()?

[Sid Robin]

quote: In case you remove the delay incurred by Thread.sleep(100) from line 24, the thread executed by run() method will never come out of wait because notify is called even before wait is executed in line 6

The Quote posted meant to say that ,once a1.start() is invoked it doesnt start executing run method . a1.start() gets scheduled for executing at a later stage . so a1.notify() is executed even before wait() of run method is called
which will make the program wait forever

[Sid Robin]

quote: In case you remove the delay incurred by Thread.sleep(100) from line 24, the thread executed by run() method will never come out of wait because notify is called even before wait is executed in line 6

The Quote posted meant to say that ,once a1.start() is invoked it doesnt start executing run method . a1.start() gets scheduled for executing at a later stage . so a1.notify() is executed even before wait() of run method is called
which is of no use

[Tim Holmes]

Thanks everyone...that helps me to understand a lot better!