Threads IOutlineHomework #2MidquarterHomework #2Example from HorstmanProblemEvent handlingWhat is a Thread?Thread methodsMulti-threaded executionThread types (origin)Thread types (status)Threads in the JVMWhat we needBounceThreadNoteNoteSummary so farThread LifecycleWays to be non-runnableWays to be runnable but not runningThread prioritiesBounceExpressNoteYieldSynchronizationHorstmann ExampleTransfer methodActual methodConflictTo avoid interruptionSynchronized versionNoteProducer / ConsumerBad solutionsBetter solutionCodeWhat about the producer?CodeScenarioDeadlockThreads IIS 3135.13.2003Outline Homework #2  Grades ThreadsHomework #2 Ave. 10.5Midquarter 2 homeworks 2 quizzes 40% of total pointsHomework #2 SolutionExample from HorstmanProblem Interface freezes during execution Single event handling thread Action event on load menu takes too longEvent handlingaddButton(buttonPanel, "Start",new ActionListener(){ public void actionPerformed(ActionEvent evt){ addBall(); }});... other code ...public void addBall(){ try{ Ball b = new Ball(canvas);canvas.add(b);for (int i = 1; i <= 1000; i++){b.move();Thread.sleep(5);}}catch (InterruptedException exception){ }}What is a Thread? Not a process Threads in JVM share the same memory / objects terminate when application terminates A thread is an object whether or not it is runningThread methodsThread myThread = new Thread ();String name = myThread.getName();myThread.start(); // <-- the most important oneThread.sleep (500); // <-- static method that makes the current thread cease executing for a period of timeMulti-threaded executionThread types (origin) System threads perform system tasks User threads run your programThread types (status) Ordinary threads most user threads Daemon threads most system threads Termination rule when the last non-daemon thread exitsThreads in the JVMWhat we need New Thread to bounce the ball Event handler creates thread starts it running returns right away then other stuff can happenBounceThreadpublic void addBall(){Ball b = new Ball(canvas);canvas.add(b);ballThread = new BallThread(b); // create the threadballThread.start(); // start the thread}... more code ...class BallThread extends Thread{ public BallThread(Ball aBall) { b = aBall; }public void run() // does the actual animation{ try{ for (int i = 1; i <= 1000; i++){ b.move();sleep(5);}}catch (InterruptedException exception){ }}... more code ...Note Subclass of Thread As many threads as we want more BounceThread objects Thread stops when run method exits Call to ballThread.start() not ballThread.run ()Note Call to Thread.sleep(5) this was in initial version Call to canvas.repaint () was canvas.paint(canvas.getGraphics())Summary so far Threads are independent paths of execution share processor time in the Java VM share data already exist in the Java VMThread LifecycleWays to be non-runnable Sleeping time expires Waiting for I/O I/O arrives Blocked In wait stateWays to be runnable but not running Another thread is runningThread priorities Scheduling threads with highest priority run unless they’re unable to run then lower threads run Not ideal! starvation is possible proportional prioritization is betterBounceExpressaddButton(buttonPanel, "Start",new ActionListener(){ public void actionPerformed(ActionEvent evt){addBall(Thread.NORM_PRIORITY, Color.black);}});addButton(buttonPanel, "Express",new ActionListener(){ public void actionPerformed(ActionEvent evt){addBall(Thread.NORM_PRIORITY + 2, Color.red);}});public void addBall(int priority, Color color){ Ball b = new Ball(canvas, color);canvas.add(b);BallThread thread = new BallThread(b);thread.setPriority(priority);thread.start();}Note Why does this happen? black thread only executes when no red thread is availableYield Causes thread to give up the processor Scheduling performed same thread may still be highest priority if so, starts running again Allows threads with same priority to shareSynchronization Switching between threads can happen at any time Inconsistent state if an operation is half-complete (like an incomplete DB transaction)Horstmann Example Couldn’t get it to work as written Machine too fast?Transfer methodpublic void transfer(int from, int to, int amount)throws InterruptedException{ accounts[from] -= amount;accounts[to] += amount;ntransacts++;if (ntransacts % NTEST == 0) test();}Actual methodpublic void transfer(int from, int to, int amount)throws InterruptedException{ int fromAmount = accounts[from];int newAmount = fromAmount – amount;accounts[from] = newAmount;int toAmount = accounts[to];newAmount = toAmount + amount;accounts[to] = newAmount;int newTransacts = ntransacts + 1;ntransacts = newTransacts;if (ntransacts % NTEST == 0) test();}Conflict Thread 1 accounts[from] = 5000 amount = 500 fromAmount = 4500 Thread 2 account[from] = 5000 amount = 200 fromAmount = 4800To avoid interruption An object is “locked” inside a synchronized method or a synchronized block Other threads  calling synchronized methods on the same object will block become non-runnable become runnable when the locking thread leaves the synchronized methodSynchronized versionpublic synchronized void transfer(int from, int to, int amount)throws InterruptedException{ accounts[from] -= amount;accounts[to] += amount;ntransacts++;if (ntransacts % NTEST == 0) test();}Note The synchronized “lock” only affects synchronized methods account.getName() might not be synchronized other threads would not have to wait when calling this methodProducer / Consumer Thread A computes values A calls value.put() Thread B does something with them B calls value.get() What happens: A computes a new value before B has grabbed the current one? B is ready for a new value before A has produced it?Bad solutions Busy waiting B loops until A is ready yuck! Zzzzz B sleeps If A is ready, grabs data otherwise sleep againBetter solution Want B not to run at all until A is ready Solution B calls A.getValue() if no new value is ready, wait() When A has a value A calls notify() or notifyAll() if more than one thread might be waitingCodepublic