Java Threading

Before going to discuss about threading in android , you need to learn how threading works in java. This is very important concept in all application development.

All programmers are familiar with writing sequential programs. You've probably written a program that displays "Hello World!", or sorts a list of names, or computes a list of prime numbers. These are sequential programs: each has a beginning, an execution sequence, and an end. At any given time during the runtime of the program there is a single point of execution.

A thread is similar to the sequential programs described above: a single thread also has a beginning, an end, a sequence, and at any given time during the runtime of the thread there is a single point of execution. However, a thread itself is not a program. It cannot run on its own, but runs within a program.

 A thread is a single sequential flow of control within a program.

Life Cycle of a Thread:

Above mentioned stages are explained here:

·         New: A new thread begins its life cycle in the new state. It remains in this state until the program starts the thread. It is also referred to as a born thread.

·         Runnable: After a newly born thread is started, the thread becomes runnable. A thread in this state is considered to be executing its task.

·         Waiting: Sometimes a thread transitions to the waiting state while the thread waits for another thread to perform a task.A thread transitions back to the runnable state only when another thread signals the waiting thread to continue executing.

·         Timed waiting: A runnable thread can enter the timed waiting state for a specified interval of time. A thread in this state transitions back to the runnable state when that time interval expires or when the event it is waiting for occurs.

·         Terminated: A runnable thread enters the terminated state when it completes its task or otherwise terminates.

Thread Priorities:

Every Java thread has a priority that helps the operating system determine the order in which threads are scheduled.

Java priorities are in the range between MIN_PRIORITY (a constant of 1) and MAX_PRIORITY (a constant of 10). By default, every thread is given priority NORM_PRIORITY (a constant of 5).

Threads with higher priority are more important to a program and should be allocated processor time before lower-priority threads. However, thread priorities cannot guarantee the order in which threads execute and very much platform dependentant.

Creating a Thread:

Java defines two ways in which this can be accomplished:

·         You can implement the Runnable interface.

·         You can extend the Thread class, itself.

Create Thread by Implementing Runnable:

The easiest way to create a thread is to create a class that implements the Runnable interface.

To implement Runnable, a class need only implement a single method called run( ), which is declared like this:

public void run( )

You will define the code that constitutes the new thread inside run() method. It is important to understand that run() can call other methods, use other classes, and declare variables, just like the main thread can.

After you create a class that implements Runnable, you will instantiate an object of type Thread from within that class. Thread defines several constructors. The one that we will use is shown here:

Thread(Runnable threadOb, String threadName);

Here threadOb is an instance of a class that implements the Runnable interface and the name of the new thread is specified by threadName.

After the new thread is created, it will not start running until you call its start( ) method, which is declared within Thread. The start( ) method is shown here:

void start( );

Example:

Here is an example that creates a new thread and starts it running:

// Create a new thread. class NewThread implements Runnable {    Thread t;    NewThread() {       // Create a new, second thread       t = new Thread(this, "Demo Thread");       System.out.println("Child thread: " + t);       t.start(); // Start the thread    }       // This is the entry point for the second thread.    public void run() {       try {          for(int i = 5; i > 0; i--) {             System.out.println("Child Thread: " + i);             // Let the thread sleep for a while.             Thread.sleep(500);          }      } catch (InterruptedException e) {          System.out.println("Child interrupted.");      }      System.out.println("Exiting child thread.");    } }   class ThreadDemo {    public static void main(String args[]) {       new NewThread(); // create a new thread       try {          for(int i = 5; i > 0; i--) {            System.out.println("Main Thread: " + i);            Thread.sleep(1000);          }       } catch (InterruptedException e) {          System.out.println("Main thread interrupted.");       }       System.out.println("Main thread exiting.");    } }

This would produce following result:

Child thread: Thread[Demo Thread,5,main] Main Thread: 5 Child Thread: 5 Child Thread: 4 Main Thread: 4 Child Thread: 3 Child Thread: 2 Main Thread: 3 Child Thread: 1 Exiting child thread. Main Thread: 2 Main Thread: 1 Main thread exiting.

Create Thread by Extending Thread:

The second way to create a thread is to create a new class that extends Thread, and then to create an instance of that class.

The extending class must override the run( ) method, which is the entry point for the new thread. It must also call start( ) to begin execution of the new thread.

Example:

Here is the preceding program rewritten to extend Thread:

// Create a second thread by extending Thread class NewThread extends Thread {    NewThread() {       // Create a new, second thread       super("Demo Thread");       System.out.println("Child thread: " + this);       start(); // Start the thread    }      // This is the entry point for the second thread.    public void run() {       try {          for(int i = 5; i > 0; i--) {             System.out.println("Child Thread: " + i);                                               // Let the thread sleep for a while.             Thread.sleep(500);          }       } catch (InterruptedException e) {          System.out.println("Child interrupted.");       }       System.out.println("Exiting child thread.");    } }   class ExtendThread {    public static void main(String args[]) {       new NewThread(); // create a new thread       try {          for(int i = 5; i > 0; i--) {             System.out.println("Main Thread: " + i);             Thread.sleep(1000);          }       } catch (InterruptedException e) {          System.out.println("Main thread interrupted.");       }       System.out.println("Main thread exiting.");    } }

This would produce following result:

Child thread: Thread[Demo Thread,5,main] Main Thread: 5 Child Thread: 5 Child Thread: 4 Main Thread: 4 Child Thread: 3 Child Thread: 2 Main Thread: 3 Child Thread: 1 Exiting child thread. Main Thread: 2 Main Thread: 1 Main thread exiting.