Why does Thread.sleep know which thread.....

[Carol Murphy]

Can someone have a look at this sample code from Osborne's Java 2: The Complete Reference, and anser my question at the end of this post? The
code is question is marked in red.
I need an explanation on the following code snippet from Osborne's
Java 2: The Complete Reference. This code appears as an example of
ThreadGroup. I think the simplest thing to do is post all of the
code, then ask my question. Here it is:

class NewThread extends Thread
{
boolean suspendFlag ;

NewThread ( String threadname, ThreadGroup tgobj )
{
super ( tgobj, threadname ) ;
System.out.println( "New thread: " + this ) ;
suspendFlag = false ;
start () ;
}
public void run()
{
try
{
for( int i = 5 ; i > 0 ; i-- )
{
System.out.println(getName() + ": " + i ) ;
Thread.sleep( 1000 ) ;
synchronized( this )
{
while( suspendFlag )
{
wait() ;
}
}
}
catch ( Exception e )
{
System.out.println( "Exception in "+ getName() ) ;
}
System.out.println( getName() + " exiting." ) ;
}

void mysuspend()
{
suspendFlag = true ;
}
synchronized void myresume()
{
suspendFlag = false ;
notify() ;
}
}
class ThreadGroupDemo
{
public static void main( String args[] )
{
ThreadGroup groupA = new ThreadGroup( "Group A" ) ;
ThreadGroup groupB = new ThreadGroup( "Group B" ) ;
NewThread ob1 = new NewThread( "One", groupA ) ;
NewThread ob2 = new NewThread( "Two", groupA ) ;
NewThread ob3 = new NewThread( "Three", groupB ) ;
NewThread ob4 = new NewThread( "Four", groupB ) ;
System.out.println( "\nHere is output from list():" ) ;
groupA.list() ;
groupB.list() ;
System.out.println() ;
System.out.println( "Suspending Group A" ) ;
Thread tga[] = new Thread[ groupA.activeCount() ] ;
groupA.enumerate( tga ) ;
for( int i = 0 ; i < tga.length ; i++ )
{
( (NewThread)tga[ i ] ).mysuspend() ;
}
<font color=red>
try
{
Thread.sleep( 4000 ) ;
}
catch ( InterruptedException e )
{
System.out.println( "Main thread interrupted." ) ;
}
</font>
System.out.println( "Resuming Group A" ) ;
for( int i = 0 ; i < tga.length ; i++ )
{
( (NewThread)tga[ i ] ).myresume() ;
}

try
{
System.out.println( "Waiting for threads to finish." ) ;
ob1.join() ;
ob2.join() ;
ob3.join() ;
ob4.join() ;
}
catch ( Exception e )
{
System.out.println( "Exception in Main thread" ) ;
}
System.out.println( "Main thread exiting." ) ;
}
}
How does Thread.sleep( 4000 ) know to suspend only the threads in Group A? This bit of code runs after Group A has been suspended by a call to wait() from run(), but I don't see how Thread.sleep( 4000 ) knows which threads to act on. Why doesn't it suspend all threads?

[Aditya Jha]

Hi,
Both methods

Thread.sleep(long milliseconds)

and

Thread.sleep(long milliseconds, int nanoseconds)

are specifically kept

static

because they act on the current executing thread. They have nothing to do with any thread-group or number of live threads at any moment of time. These methods simply act upon the thread which calls them.
Hope this solves your query. Just for reference, please note that the thread on which these methods are invoked does not loose ownership on any monitor held at that moment. Hence, be careful while using this method.
Have fun with threads
- Aditya