Beginner Question on threads

[Eugene Armistead]

Hi,
I have an Object that gets locked by various threads via a synchronized block. Something Like:
class MyThread extends Thread{
public void run(){
synchronized(someObject){
// do stuff to some Object
}
}
}
Is there an easy way to have an another instance of MyThread examine "someObject" and see if it is locked by another thread? This way I could tell the user something like "waiting for locked resource..."
Thanks

[Angela Narain]

Hi,
I tried out the following code and started two threads
which share the same object "MyThread" as shown below :

class MyThread extends Thread {
  boolean wait = false ;
  public void run(){
          callMe();
  }
  public synchronized void callMe() {
        while( wait == true )
        {
            System.out.println(" Waiting for releasing the lock");
            try{
       wait();
            }
            catch( InterruptedException e ) { } 
        }  
System.out.println(" Locked ");
wait = true;
notifyAll();
     }
 
  public static void main( String[] args )
  {
     MyThread t = new MyThread();
     new Thread(t).start();
     new Thread(t).start();
  }
}

[This message has been edited by Angela Narain (edited October 04, 2001).]

[Eugene Armistead]

Hi Angela,
This works perfect for what I need. Thanks for your help!
Regards,

[Mr. C Lamont Gilbert]

Originally posted by Angela Narain:
[B]Hi,
I tried out the following code and started two threads
which share the same object "MyThread" as shown below :

class MyThread extends Thread {
  boolean wait = false ;
  public void run(){
          callMe();
  }
  public synchronized void callMe() {
        while( wait == true )
        {
            System.out.println(" Waiting for releasing the lock");
            try{
       wait();
            }
            catch( InterruptedException e ) { } 
        }  
System.out.println(" Locked ");
wait = true;
notifyAll();
     }
 
  public static void main( String[] args )
  {
     MyThread t = new MyThread();
     new Thread(t).start();
     new Thread(t).start();
  }
}

[This message has been edited by Angela Narain (edited October 04, 2001).][/B]

This can not work. Once you are into the callMe function, you will never see wait == false. This is because the lock will not be released by the FIRST thread to acquire it, untill the thread is done and leaving the method. The next thread to grab the lock will always be faced with wait == true. Did you ever see the "waiting" message?
You need something slightly different

class Accessor
{
private boolean waitObject = false;
public void callMe()
{
    boolean sentMessage = false;
    synchronized(waitObject)
    {
       while(waitObject == true)
       {
          if(sentMessage == false)/*send wait message once */
          {
             System.out.println(" Waiting for the lock");
             sentMessage = true;
          }
          try
          {
             waitObject.wait();
          }
          catch(InteruptedException ie)
          {}
       }
       System.out.println(" Locked ");
       waitObject = true;             
    }
    /* do stuff */
    synchronized(waitObject)
    {
        waitObject = false;
        waitObject.notify();
    }
}
}
class Thrd1 extends Thread
{
  Accessor theA;
  public Thrd1(Accessor a)
  {
    theA = a;
  } 
  public void run()
  {
    while(1) 
    {
      theA.callMe();
    }
  }
}
public static void main(String [] args)
{
    Accessor A = new Accessor();
     Thrd1 t1= new Thrd1(A).start();
     Thrd1 t2= new Thrd1(A).start();
     t2.join();//program will run till u break it
}

That should do what you seek. Do you understand the difference here?