Basic thread doubt

[Deep Chand]

a) When a thread calls a wait(), does that release all the locks it holds or just the object on which it has called wait()? i.e. if a thread calls a synchronized method on objA (class A) and then from there it calls a synchronized method on objB (class B) and then calls wait()? At that time whether it releases the locks for objA and objB both or just objB?
b) When a thread is in wait() state and is interrupted and it throws InterruptedException, does it relases the lock? My answer is No.
c) When a thread throws an uncaught exception inside a synchronized method of a class, and so before the controls leave the current method and the exception is propagated to the next level, does the thread releases all the locks ? My answer is Yes.
I know many of you may feel these questions to be very silly but sometimes such silly questions in the exam look lot more difficult.
Thanks,
Deep

[Marlene Miller]

class Test {
   public static void main(String[] args) {
      Runnable r = new R();
      Thread t1 = new Thread(r);
      Thread t2 = new Thread(r);
      t1.start();
      t2.start();
   }
}
class R implements Runnable {
   static A a = new A();
   static B b = new B();
   public void run () {
      Thread t = Thread.currentThread();
      System.out.println(t.getName() + " enter run()");
      a.ma(b);
   }
}
class A {
   synchronized void ma(B b) {
      Thread t = Thread.currentThread();
      System.out.println(t.getName() + " enter ma()");
      b.mb(this);
   }
}
class B {
   synchronized void mb(A a) {
      try {
         Thread t = Thread.currentThread();
         System.out.println(t.getName() + " enter mb()");
         System.out.println(t.getName()
            + " holds lock of A object= " + t.holdsLock(a)
            + ", holds lock of B object= " + t.holdsLock(this));
         System.out.println(t.getName() + " call wait()");
         wait();
      } catch (InterruptedException e) {
      }
   }
}
Thread-1 enter run()
Thread-1 enter ma()
Thread-1 enter mb()
Thread-1 holds lock of A object= true, holds lock of B object= true
Thread-1 call wait()
Thread-2 enter run()

[ September 18, 2003: Message edited by: Marlene Miller ]

[Marlene Miller]

class Test {
   public static void main(String[] args) {
      Runnable r = new R();
      Thread t = new Thread(r);
      t.start();
      Thread.yield();
      t.interrupt();
   }
}
class R implements Runnable {
   public void run () {
      m();
   }
   synchronized void m() {
      Thread t = Thread.currentThread();
      System.out.println(
        t.getName() + " holds lock= " + t.holdsLock(this));
      try {
         wait();
      } catch (InterruptedException e) {
         System.out.println(t.getName() + " catch" );
         System.out.println(
           t.getName() + " holds lock= " + t.holdsLock(this));
      }
   }
}
Thread-1 holds lock= true
Thread-1 catch
Thread-1 holds lock= true

[Marlene Miller]

class Test {
   public static void main(String[] args) {
      new Thread(new R()).start();
   }
}
class R implements Runnable {
   public void run () {
      m();
   }
   synchronized void m() {
      Thread t = Thread.currentThread();
      System.out.println(t.getName() + " holds lock= " + t.holdsLock(this));
      try {
         throw new NullPointerException();
      } catch (RuntimeException e) {
         System.out.println(t.getName() + " catch" );
         System.out.println(t.getName() + " " + e);
         System.out.println(t.getName() + " holds lock= " + t.holdsLock(this));
      }
   }
}
Thread-1 holds lock= true
Thread-1 catch
Thread-1 java.lang.NullPointerException
Thread-1 holds lock= true

[Barkat Mardhani]

posted by Deep:

a) When a thread calls a wait(), does that release all the locks it holds or just the object on which it has called wait()? i.e. if a thread calls a synchronized method on objA (class A) and then from there it calls a synchronized method on objB (class B) and then calls wait()? At that time whether it releases the locks for objA and objB both or just objB?

The answer is "just the object on which it has called wait()"
Here is an example:

 /** This example demonstrates that if a thread (t1 in this example) locks the monitors on two objects (obj1 and obj2)
then calls wait on obj1, then lock on obj1 is only released. Lock on obj2 stays. **?
class A { } // this class is used to create obj1 and obj2
class Test {
static A obj1 = new A();
static A obj2 = new A();
public static void main(String[] args) {
Thread t1 = Thread.currentThread(); // reference to current thread i.e. main
Thread t2 = new B(t1,obj1,obj2); // this thread (t2) is created to check the locks on obj1 and obj2 while t1 is waiting
t2.start(); 
synchronized(obj1) {  // gets lock on obj1
System.out.println("t1 thread holds lock on obj1 : "+t1.holdsLock(obj1)); // true
System.out.println("t1 thread holds lock on obj2 : " +t1.holdsLock(obj2)); // false
synchronized(obj2) {
System.out.println("t1 thread holds lock on obj1 : "+t1.holdsLock(obj1)); // true
System.out.println("t1 thread holds lock on obj2 : " +t1.holdsLock(obj2)); // true
try { obj2.wait(); } catch (InterruptedException ie) { }
}
}
}
}
class B extends Thread {
A obj1; A obj2; Thread t1; Thread t2;
B(Thread t1, A obj1, A obj2) {
this.t1 = t1; this.obj1 = obj1 ; this.obj2 = obj2; this.t2=Thread.currentThread();
}
public void run() {
try { sleep(5000); } catch(InterruptedException ie) { }
synchronized(obj2) {
System.out.println("t2 thread holds lock on obj1 : "+t2.holdsLock(obj1)); // false
System.out.println("t2 thread holds lock on obj2 : "+t2.holdsLock(obj2)); // true
synchronized(obj1) {   // this thread will block here for lock obj1... for ever....
System.out.println("t2 thread holds lock on obj1 : "+t2.holdsLock(obj1)); 
System.out.println("t2 thread holds lock on obj2 : "+t2.holdsLock(obj2)); 
}
}
}
} 
 

[Barkat Mardhani]

posted by Deep:

b) When a thread is in wait() state and is interrupted and it throws InterruptedException, does it relases the lock? My answer is No.

When a thread is in wait state, it has alread given up lock on the object
on which wait() was called.

[Barkat Mardhani]

posted by Deep:

c) When a thread throws an uncaught exception inside a synchronized method of a class, and so before the controls leave the current method and the exception is propagated to the next level, does the thread releases all the locks ? My answer is Yes.

I would say Yes too. Here is an example:

/** This example demonstrates that if a thread (t1 in this example) locks the monitors on two objects (obj1 and obj2)
then throws uncaught exception, then locks on obj1 & obj2  released released. **/
class A { } // this class is used to create obj1 and obj2
class Test {
static A obj1 = new A();
static A obj2 = new A();
public static void main(String[] args) throws Exception{
Thread t1 = Thread.currentThread(); // reference to current thread i.e. main
Thread t2 = new B(t1,obj1,obj2); // this thread (t2) is created to check the locks on obj1 and obj2 while t1 is waiting
t2.start(); 
synchronized(obj1) {  // gets lock on obj1
System.out.println("t1 thread holds lock on obj1 : "+t1.holdsLock(obj1)); // true
System.out.println("t1 thread holds lock on obj2 : " +t1.holdsLock(obj2)); // false
synchronized(obj2) {
System.out.println("t1 thread holds lock on obj1 : "+t1.holdsLock(obj1)); // true
System.out.println("t1 thread holds lock on obj2 : " +t1.holdsLock(obj2)); // true
throw new Exception();
}
}
}
}
class B extends Thread {
A obj1; A obj2; Thread t1; Thread t2;
B(Thread t1, A obj1, A obj2) {
this.t1 = t1; this.obj1 = obj1 ; this.obj2 = obj2; this.t2=Thread.currentThread();
}
public void run() {
try { sleep(5000); } catch(InterruptedException ie) { }
synchronized(obj2) {
System.out.println("t2 thread holds lock on obj1 : "+t2.holdsLock(obj1)); // false
System.out.println("t2 thread holds lock on obj2 : "+t2.holdsLock(obj2)); // true
synchronized(obj1) {   
System.out.println("t2 thread holds lock on obj1 : "+t2.holdsLock(obj1)); // true 
System.out.println("t2 thread holds lock on obj2 : "+t2.holdsLock(obj2)); // true
}
}
}
}