Active Objects Threads Active vs Passive Objects 4

Active Objects (Threads)

Active vs. Passive Objects 4 An active object is an object that interacts with its environment, while passive objects only do something when acted upon. 4 A program which contains more than one active object is called a parallel program. 4 A real-time program interacts within its environment under certain time constraints.

Threads 4 Threads are Java’s implementation of active objects. 4 When several activities (processes, or threads) are executing simultaneously, what is actually happening is that the system is executing small portions of each program, one at a time, and switching processes rapidly. 4 So far, all of the programs we have written have had only one thread

Thread Creation 4 In order to create and activate a new thread in a program use: – Thread a 1 = new Thread(); a 1. start(); 4 Thread is defined in java. lang 4 start() Causes this thread to begin execution; the Java Virtual Machine calls the run method of this thread. – The result is that two threads are running concurrently: the current thread (which returns from the call to the start method) and the other thread (which executes its run method). 4 The method run in the Thread class does not do anything, so we must create our own subclasses of Thread & write run

Thread Creation Continued 4 Because all of our active objects must be subclasses of Thread, we cannot make them subclasses of any other class, so we use: – Thread a 2 = new Thread(obj); 4 This connects the object obj and Thread a 2 4 The run method in obj will execute, not a 2’s run 4 obj must implement the interface Runnable, which contains the method run. This means that every class that implements Runnable must have a method run.

Objects with Threads 4 So now, we can define objects that contain an instance variable of class Thread. These classes are said to have their own Thread. – Public Thread activity = new Thread (this); 4 The this connects the Thread to the current object, which must implement the Runnable interface.

Interruption of Threads 4 Because most Threads are designed to execute until stopped, you may call the method interrupt to request that the Thread stop itself. 4 The interruption of a Thread is an exceptional event, so you must put code that might be interrupted in a try statement.

Program Termination 4 A program terminates when all of its Threads have terminated. – If you do not spawn new Threads, then the program ends when main ends, assuming no abnormal program termination elsewhere. – If you spawn new Threads, make sure that you interrupt them before you end main or they will continue to execute.

Join 4 The join method waits until a given Thread is finished before moving on. 4 It can generate an exception, so it must be in at try statement. 4 You may add time to the call to pause the current Thread for a set time only.

Priority 4 In a given Java program, you may assign priority to the Threads that are created. 4 Use get. Priority and set. Priority – valid priority values: Thread. MIN_PRIORITY, Thread. MAX_PRIORITY, Thread. NORM_PRIORITY ( the default).