thread synch and inner classes

[tom walters]

i have a class with 2 inner classes that are threads

class Parent
{
   SharedObject o;
   String monitor = new String("");
   public static void main(String[] args)
   {
      new Inner1().start();
      new Inner2().start();
   }   
   class Inner1 extends Thread
   {
      public void run()
      {
         Checker c = new Checker();
         while(true){ 
            o = c.check();
           //use o
         }
      }
   }

   class Inner2 extends Thread
   {
      public void run()
      {
         Checker c = new Checker();
         while(true){ 
            o = c.check();
           //use o
         }
      }
   }
}

how do i make this thread safe? make the check() method in Checker synchronized? put the o = c.check(); in a synchronized block using the monitor String object?
[ April 26, 2002: Message edited by: tom walters ]
Formatted and added code tags - George
[ May 28, 2002: Message edited by: George Brown ]

[Thomas Smets]

Hi Tom,
I simplified your code as the stub you gave would never be able to compile :
Please note that I don't like the idea that the []inotification[/i] has to be in a synchronized block, but it's rather logical to have it so... So !
Please also note that this example s inspired from "JAVA threads" by Scott Oaks & Henry Wong at O'Reilly, ISBN 1-56592-216-6
Make sure you get the second edition at least, while these references dates to the first edition (ISBN number)

 
package test.lang;
import org.apache.log4j.Category;
import org.apache.log4j.BasicConfigurator;
import java.util.Random;
/**
  * The purpose of this class is to :
  * 
  * Show how we can use threaded inner classes safely accessing the
  * "carrying/supporting" class attributes !
  *     
  * @author  <a href="mailto:[e-mail]">Thomas SMETS, Brussels</a>
  * @version 0.1.15 pre-final
  */
class Parent
{
  Category log = Category.getInstance (Parent.class);
  Random random = new Random (System.currentTimeMillis ());
  
  Object o;
  String SharedObject = null;
  boolean isBusy = false;
  public static final long MAX_ITER = 200;  
  
  Inner i1, i2;
  Parent()
  {
    log.info ("Parent created");
    i1 = new Inner(1);
    i2 = new Inner(2);
    o = new Object ();
  }
  
  public void  go ()
  {  
    log.info ("Parent::go( )");
    i1.start ();
    i2.start ();
  }
  public void getBusyFlag(int callerNbr)
  {
    log.info ("Requesting the busy-flag (Thread is " + callerNbr + ")");
    synchronized (o)
    {
      while (isBusy)
      {
        try 
        {
          log.info ("Thread " + callerNbr + " is waitng");
          o.wait ();
        }
        catch (Exception ex) 
        {
          log.info ("Thread " + callerNbr + " wait has been disturbed");
        }
      }
      
      if (!isBusy)
      {
        isBusy = true;
        log.info ("Busy flag has been given to Thread : " + callerNbr);
        return;
      }  
    }
  }
  
  public boolean isFlagBusy ()  {    return isBusy;  }  
  
  public void releaseFlag(int callerNbr)
  {
    synchronized (o)
    {
      log.info ("Releasing the flag : " + callerNbr);
      isBusy = false;
      o.notifyAll();    
    }
  }
  public static void main(String[] args)
  {
    BasicConfigurator.resetConfiguration ();
    BasicConfigurator.configure ();
    new Parent().go();
  }
  
  
  class Inner
    extends Thread
  {
    int Nbr;
    Category ilog = null;
    
    Inner ()         
    {      
      Nbr = 0;    
      ilog = Category.getInstance (Inner.class);
    }    
    Inner (int i)    
    {
      this();
      Nbr = i;    
    }  
    
    public void run()
    {
      ilog.info (Nbr + " Inner::run()");
      while(true)
      { 
        getBusyFlag(Nbr);        
        try 
        {          
          if (SharedObject != null)
            ilog.fatal ("SharedObject is not null (Thread : "+ Nbr +"). First part is " 
                        + SharedObject.substring ( 0, 
                                                   (SharedObject.length ()<20)?
                                                    SharedObject.length ():
                                                    20) );                     
          for (int i = 0; i<MAX_ITER; i++)   
          {          
            SharedObject += new Integer (Nbr).toString ();            
            try 
            {
              Thread.sleep (20);
            }
            catch (Exception ex) 
            {
            }            
          }
          
          SharedObject = null;
          releaseFlag(Nbr);
        }
        catch (Exception ex) 
        {
          ilog.warn ("I have been woken up (Thread : "+ Nbr +"). Exception is " 
                    + ex.getClass ().getName () );
        }
        
      }
    }
  }    // End of inner class
  
}
 

Rgds,
Thomas,

[Mr. C Lamont Gilbert]

Tom, either of your proposed solutions would make that a thread safe operation.

[Ilja Preuss]

Originally posted by CL Gilbert:
Tom, either of your proposed solutions would make that a thread safe operation.

Mhhh, you probably meant to write neither?
It's not the check() that is critical, but parallel usage of o. This usage has to be synchronized.

[Mr. C Lamont Gilbert]

Originally posted by Ilja Preuss:

Mhhh, you probably meant to write neither?
It's not the check() that is critical, but parallel usage of o. This usage has to be synchronized.

Since the threads are not sharing anything, their are no synchronization issues.