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Talking to Threads (and how they answer...)

Talking to Threads (and how they answer...). Corralling Threads. So you can create and synchronize threads... How do you control the threads you have? Look at java.lang.Thread API: Three sets of “control” methods: stop() , suspend() , resume() , destroy()

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Talking to Threads (and how they answer...)

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  1. Talking to Threads(and how they answer...)

  2. Corralling Threads • So you can create and synchronize threads... • How do you control the threads you have? • Look at java.lang.Thread API: • Three sets of “control” methods: • stop(), suspend(), resume(), destroy() • All deprecated or unimplemented. Bleh. • setDaemon(), setPriority() • Changes how scheduler prioritizes threads • yield(), sleep(), interrupt() • These are key control methods

  3. yield() • Tells scheduler “I don’t need to run right now; let someone else go” • Use when waiting for data from another thread, when finished with a big computation and waiting to start next, etc. • Often, purpose better served with wait()

  4. sleep() • Pauses thread for some time period • sleep(n) == pause for at least n milliseconds • Scheduler sets thread aside until time elapses, then puts back in schedule queue • Thread may wake up some random time after specified time • Good for delaying a thread for a while, doing something (roughly) periodically, etc.

  5. interrupt() • Call on a thread to wake up thread, break out of a wait() state, etc. • Generates an InterruptedException on the thread when it is next scheduled (not instantly) • Caveat: only if thread is in a sleep() or wait() call

  6. Control w/ sleep() and interrupt() Thread t=new Thread(new Runnable() { private void run() { boolean done=false; while (!done) { try { sleep(1000); } catch (InterruptedException e) { done=true; } } } } // somewhere else... t.interrupt();

  7. wait()ing for Godot... • sleep(n) is good if you know how long you want to delay • What if a thread wants to wait indefinately, or for some other thread to do something? • Check out Object API: • Object o.wait() -- wait until some other object tells thread to wake up • o.notify() -- wake up one (random) thread that is waiting on o • o.notifyAll() -- wake up every thread that is waiting on o.

  8. Yet more behind-the-scenes Java • Every Object in Java has a (single) “wait list”. • When a thread adds itself to an Object’s wait list, the thread suspends • Some other thread of execution can then call that object’s notify() method • One thread wakes up and can now take action • Does not generate an exception; does not change the status of a sleep()ing Thread • Must be synchronized on target object to wait() or notify() • Prevents collisions while messing w/ wait list

  9. Use of wait lists DataBucket b; Thread t=new Thread(new Runnable(b) { public Runnable(DataBucket buck) { _bucket=b; } public void run() { while (!bored) { while (_bucket.isEmpty()) { synchronized(_bucket) { _bucket.wait(); } } _bucket.getData(); } }}); // elsewhere... b.addData(data); synchronized(b) { b.notify(); }

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