OPERATING SYSTEMS Frans Sanen

 Recap of threads in Java  Learn to think about synchronization problems in Java  Solve synchronization problems in Java 2

 One of the key success factors of Java  Thread = flow of control  Threads can share state  If a program involves multiple threads, synchronization becomes an issue!  Coordination is needed when several threads access and manipulate shared state 3

class Animation implements Runnable { Thread myThread; Animation (String name) { myThread = new Thread( this ); myThread.start(); } public void run() { while ( true ) { // Draw Frames... repaint(); }

class Animation extends Thread { public void run() { while (true ) { // Draw Frames... repaint(); }

 Wait and notify monitor as the underlying mechanism  Thread can suspend itself by calling wait()  Waiting thread will be suspended if another thread executes notify()  Java also provides notifyAll() method  Methods wait(), notify() and notifyAll() only can be invoked from within monitor regions

 Monitor guarantees mutual exclusion when an object is executing in a monitor region  Synchronized method public void synchronized doSth() {...}  Synchronized code block synchronized (this) {...}

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 Producer produces items and puts them in a shared buffer  Consumer consumes items and gets them out a shared buffer  Producer can’t produce items if the buffer is full.  Consumer can’t consume items if the buffer is empty. 10

 Readers try to read (access) shared data  Writers try to write (manipulate) shared data  Only one writer can write shared data at a given moment in time (“no two writers”).  It never may be the case that both a reader and a writer are working with shared data simultaneously (“no reader and writer at the same time”). 11

 Good luck!

 If two or more threads modify a shared object, declare the methods / code blocks that carry out the modification as synchronized.  If a thread must wait for the state of a shared object to change, it should wait inside the object, not outside, by entering the synchronized method / code block and calling wait().  Whenever a method / code block changes the state of a shared object, it should call notify() to give waiting threads a chance to see if circumstances have changed.  Keep the synchronized granularity as low as possible to maximize parallelism.