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JAVA PROGRAMMING

Suspending, Resuming and Stopping Threads

Understanding Deadlock

Because threads can become blocked and because objects can have synchronized methods that prevent threads from accessing that object until the synchronization lock is released, it's possible for one thread to get stuck waiting for another thread, which in turn waits for another thread, etc., until the chain leads back to a thread waiting on the first one. Thus, there's a continuous loop of threads waiting on each other and no one can move. This is called deadlock. The claim is that it doesn't happen that often, but when it happens to you it's frustrating to debug.

There is no language support to help prevent deadlock; it's up to you to avoid it by careful design. These are not comforting words to the person who's trying to debug a deadlocking program.

One change that has been made in Java 1.2 to reduce the possibility of deadlock is the deprecation of Thread's stop( ), suspend( ), and resume( ) methods.

Here is an example code using Thread's stop( ), suspend( ), resume( ) methods.

class newthread implements Runnable
{
String name;
Thread t;
newthread(String threadname)
{
name=threadname;
t=new Thread(this, name);
System.out.println("New Thread: "+t);
t.start();
}
public void run()
{
 try
 {
 for(int i=15;i>0;i--)
 {
 System.out.println(name+":"+i);
 Thread.sleep(200);
 }
 }catch(InterruptedException e)
 {
 System.out.println(name+" interrupted");
 }
 System.out.println(name+"  exiting ");
 }
 }
 public class suspres
 {
  public static void main(String args[])
  {
  newthread ob1=new newthread("one");
  newthread ob2=new newthread("two");
  try
  {
  Thread.sleep(1000);
  ob1.t.suspend();
  System.out.println("Suspending thread one");
  Thread.sleep(1000);
  ob1.t.resume();
  System.out.println("Resuming thread one");
  ob2.t.suspend();
  System.out.println("Suspending thread two");
  Thread.sleep(1000);
  ob2.t.resume();
  System.out.println("Resuming thread two");
  }catch(InterruptedException e)
  {
  System.out.println("Main thread Interrupted");
  }
  try
  {
   System.out.println("Wating for threads to finish");
   ob1.t.join();
   ob2.t.join();
   }catch (InterruptedException e)
   {
   System.out.println("Main thread Interrupted");
   }
   System.out.println("Main thread exiting");
   }
   }


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