Threading Eriq Muhammad Adams J

* Thread ? * When to use threads * Creating Threads * Synchronization * Avoiding Deadlock * Demo * Reference

* A thread is a thread of execution in a program. The Java Virtual Machine allows an application to have multiple threads of execution running concurrently.

* If you have any time consuming tasks or procedures. * For example, use threads in these situations:  When loading lots of files from the local file system  When doing any network communication, such as sending high scores to a server  When doing any massive calculations, such as terrain generation

* Running threads : MyThread myThread = new MyThread(); myThread.start(); Thread myRunnable = new Thread(runnable); myRunnable.start(); * You’re able to create threads by extending Thread class or implement Runnable interface or using anonymous inner class. * Using anonymous inner class : new Thread(){ public void run(){ // do something here … } }.start();

* Implement Runnable interface : public class MyThread implements Runnable{ Public void run(){ // do something here … } * Extending Thread class : public class MyThread extends Thread { Public void run(){ // do something here … }

* Use join() method, If you want your current thread to wait until a thread is finished : myThread.join(); * Use sleep() method, If you want to make your thread sleep : Thread.sleep(100) // sleep thread in 100 ms

* Synchronization problem occurred when you’ve got multiple threads trying to access same objects or variables. Samples: public class Maze { private int playerX; private int playerY; public boolean isAtExit() { return (playerX == 0 && playerY == 0); } public void setPosition(int x, int y) { playerX = x; playerY = y; }

* Problem : Most of the time, this code works fine. But keep in mind that threads can be pre-empted at any time. Imagine this scenario, in which the player moves from (1,0) to (0,1): 1. Starting off, the object's variables are playerX = 1 and playerY = Thread A calls setPosition(0,1). 3. The line playerX = x; is executed. Now playerX = Thread A is pre-empted by Thread B. 5. Thread B calls isAtExit(). 6. Currently, playerX = 0 and playerY = 0, so isAtExit() returns true! In this scenario, the player is reported as solving the maze when it's not the case. To fix this, you need to make sure the setPosition() and isAtExit() methods can't execute at the same time.

* Solution : public class Maze { private int playerX; private int playerY; public synchronized boolean isAtExit() { return (playerX == 0 && playerY == 0); } public synchronized void setPosition(int x, int y) { playerX = x; playerY = y; } Or you can do like this : public void setPosition(int x, int y) { synchronized(this) { playerX = x; playerY = y; }

* Object synchronization : Object myLock = new Object(); synchronized (myLock) {... } * Local synchronization : public void myMethod() { synchronized(this) { // code that needs to be synchronized } // code that is already thread-safe }

* Using wait() and notify() : Case : Thread A waiting a message from Thread B // Thread A public void waitForMessage() { while (hasMessage == false) { Thread.sleep(100); } // Thread B public void setMessage(String message) { hasMessage = true; } It’s work but it’s chuncky

Solution : // Thread A public synchronized void waitForMessage() { try { wait(); } catch (InterruptedException ex) { } } // Thread B public synchronized void setMessage(String message){ notify(); }

* The wait() or wait(timeInMillis) method is used in synchronized blocks of code. When the wait() method executes, the lock is released and the thread waits to be notified. * The notify() method is also used in a synchronized block of code. The notify() method notifies one thread waiting on the same lock. If several threads are waiting on the lock, one of them is notified randomly. * notifyAll(), notifies all threads waiting on the lock, instead of just one. * don't synchronize a method that uses only local variables

* Deadlock is the result of two threads that stall because they are waiting on each other to do something. * Consider this example: 1. Thread A acquires lock Thread B acquires lock Thread B waits for lock 1 to be released. 4. Thread A waits for lock 2 to be released. * Please read article at :

* Demo available in ThreadingDemo.zipThreadingDemo.zip

* David Brackeen, Developing Games in Java, New Riders Publishing, 2003