Re: multithreading

Michael <>
24 Apr 2007 06:04:24 -0700
On Apr 23, 8:27 pm, Bill <> wrote:

On Apr 23, 11:51 am, "Z." <> wrote:

harsha wrote:

i did't understand the purpose of "JOIN()" method.please help me.
now, i have been learning java.i follow the book called "THE

When thread A join()s thread B, thread A will wait for thread B to
terminate before resuming execution.

Add the Java API URL to your bookmarks:

A thread that has called Thread.join() is waiting for a specified thread
to terminate.

Could you give a simple example why you would want to do this?


For a slightly more concrete example. Imagine that you want to do 5
things in parallel (say download 5 files off the internet), but you
want to tell the user when all 5 things are done. A trivial example
would be a web browser.. You want to stop spinning the wheel when
your'e done with all your tasks.. So you fire up 5 threads, then wait
for them to exit.. The Thread.join() waits until the 'run()' method
exits. So
File[] files = ...
Thread[] threads = new Thread[5];
for (int i = 0; i < 5; i++) { threads[i] = new Thread(new
MyFileReader(files[i])).start(); }
for (int i = 0; i < 5; i++) { threads[i].join(); }

The other reason to join threads is during shut-down, to make sure
that the thread is fully finished before taking down it's resources.
Imagine that you have a thread reading a TCP/IP socket. If the socket
is managed by a differnet thread, how do you coordinate the closing of
the worker threads before disconnecting the socket? Sure you can call
a custom myThreadWorker.shutdown() method, but how can that method
know when the thread is fully finished? Normally it would be
something like:

 Object shutdownLock = new Object();
 boolean running = true;
 boolean injob = false;

void shutdown() {
  synchronized (shutdownLock) {
  running = false;
  if (!injob) { this.interrupt(); }

void run() {
    ... initialize
    while (running) {
   try {
   s = socket.accept();
    synchronized(shutdownLock) { if (!running) { return; } else
{ injob = true; } }
    .... Do something useful
    synchronized (shutdownLock) { injob = false; }
    } catch (InterruptedException e) { if (!running)
{"Interrupted and exiting"); } else { LOG.warn("Interrupted
but not exiting!"); } }

There are other, possibly more elegant ways of handling this

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