synchronization, wait and notify

[susana miranda]

Hello, I have a question. In the following code:

class ThreadC extends Thread{
public static void main(String [] args) {
new ThreadC().metodo1();
}

public synchronized void metodo1()
{
ThreadC b = new ThreadC();
b.start();
// synchronized(b) {
try {
System.out.println("Waiting for b to complete...");
b.wait();
} catch (InterruptedException e) {System.out.println("Error al hacer b.wait()");}
System.out.println("metodo1 terminado");
// }
}

public synchronized void run( ) {
// synchronized(this) {
System.out.println("run into ThreadB");
notify( );
System.out.println("Ya se hizo notify()");
try{
Thread.sleep(2000);
}
catch(InterruptedException e){}
System.out.println("ya se libera el lock de ThreadB");
}
// }
}

At runtime: metodo1 generates IllegalMonitorstateException, as far as I now this is because wait() must be called in a synchronized context.
1. Why isn't enough to make metodo1() synchronized?
2. Why notify() works well in run() method just by making it synchronized?

[Ankit Garg]

Hi Susana. Welcome to javaranch

1. method1 generated IllegalMonitorStateException because you are calling wait on object b

b.wait();

and method1 is synchronized on this.

public synchronized void metodo1()
{
   ThreadC b = new ThreadC();
   b.wait();
}
 
implicitly works as
 
public void metodo1()
{
   synchronized(this)
   {
      ThreadC b = new ThreadC();
      b.wait();
   }
}

2. it works in run method because you are calling notify on this reference. Since the method run is synchronized, and you call notify without using any reference variable, so it will be called on this. So the code will work as

public void run()
{
   synchronized(this)
   {
      this.notify();
   }
}

[susana miranda]

Hi Ankit, I saw my mistake. Thanks!