Interthread Communication

 

Consider the classic queuing problem, where one thread is producing some data and another is consuming it. To make the problem more interesting, suppose that the producer has to wait until the consumer is finished before it generates more data.

In a polling system, the consumer would waste many CPU cycles while it waited for the producer to produce. Once the producer was finished, it would start polling, wasting more CPU cycles waiting for the consumer to finish, and so on. Clearly, this situation is undesirable.

To avoid polling, Java includes an elegant interprocess communication mechanism via the following methods:

wait( ): This method tells the calling thread to give up the monitor and go to sleep until some other thread enters the same monitor and calls notify( ).

notify( ): This method wakes up the first thread that called wait( ) on the same object.

notifyAll( ): This method wakes up all the threads that called wait( ) on the same object. The highest priority thread will run first.

These methods are implemented as final methods in Object, so all classes have them. All three methods can be called only from within a synchronized context.

These methods are declared within Object. Various forms of wait( ) exist that allow you to specify a period of time to wait.

Example:

The following sample program consists of four classes: Q, the queue that you're trying to synchronize; Producer, the threaded object that is producing queue entries; Consumer, the threaded object that is consuming queue entries; and PC, the tiny class that creates the single Q, Producer, and Consumer.

The proper way to write this program in Java is to use wait( ) and notify( ) to signal in both directions, as shown here:

class Q {    int n;    boolean valueSet = false;    synchronized int get() {       if(!valueSet)       try {          wait();       } catch(InterruptedException e) {          System.out.println("InterruptedException caught");       }       System.out.println("Got: " + n);       valueSet = false;       notify();       return n;    }    synchronized void put(int n) {       if(valueSet)       try {          wait();       } catch(InterruptedException e) {          System.out.println("InterruptedException caught");       }       this.n = n;       valueSet = true;       System.out.println("Put: " + n);       notify();    } } class Producer implements Runnable {    Q q;    Producer(Q q) {       this.q = q;       new Thread(this, "Producer").start();    }    public void run() {       int i = 0;       while(true) {          q.put(i++);       }    } } class Consumer implements Runnable {     Q q;     Consumer(Q q) {        this.q = q;        new Thread(this, "Consumer").start();     }     public void run() {        while(true) {        q.get();     }   } } class PCFixed {    public static void main(String args[]) {       Q q = new Q();       new Producer(q);       new Consumer(q);       System.out.println("Press Control-C to stop.");    } }

Inside get( ), wait( ) is called. This causes its execution to suspend until the Producer notifies you that some data is ready.

When this happens, execution inside get( ) resumes. After the data has been obtained, get( ) calls notify( ). This tells Producer that it is okay to put more data in the queue.

Inside put( ), wait( ) suspends execution until the Consumer has removed the item from the queue. When execution resumes, the next item of data is put in the queue, and notify( ) is called. This tells the Consumer that it should now remove it.

Here is some output from this program, which shows the clean synchronous behavior:

Put: 1 Got: 1 Put: 2 Got: 2 Put: 3 Got: 3 Put: 4 Got: 4 Put: 5 Got: 5

Thread Synchronization in Java

Thread Synchronization

 

When two or more threads need access to a shared resource, they need some way to ensure that the resource will be used by only one thread at a time.

The process by which this synchronization is achieved is called thread synchronization.

The synchronized keyword in Java creates a block of code referred to as a critical section. Every Java object with a critical section of code gets a lock associated with the object. To enter a critical section, a thread needs to obtain the corresponding object's lock.

This is the general form of the synchronized statement:

synchronized(object) {    // statements to be synchronized }

Here, object is a reference to the object being synchronized. A synchronized block ensures that a call to a method that is a member of object occurs only after the current thread has successfully entered object's monitor.

Here is an example, using a synchronized block within the run( ) method:

// File Name : Callme.java // This program uses a synchronized block. class Callme {    void call(String msg) {       System.out.print("[" + msg);       try {          Thread.sleep(1000);       } catch (InterruptedException e) {          System.out.println("Interrupted");       }       System.out.println("]");    } } // File Name : Caller.java class Caller implements Runnable {    String msg;    Callme target;    Thread t;    public Caller(Callme targ, String s) {       target = targ;       msg = s;       t = new Thread(this);       t.start();    }    // synchronize calls to call()    public void run() {       synchronized(target) { // synchronized block          target.call(msg);       }    } } // File Name : Synch.java class Synch {    public static void main(String args[]) {       Callme target = new Callme();       Caller ob1 = new Caller(target, "Hello");       Caller ob2 = new Caller(target, "Synchronized");       Caller ob3 = new Caller(target, "World");          // wait for threads to end       try {          ob1.t.join();          ob2.t.join();          ob3.t.join();       } catch(InterruptedException e) {          System.out.println("Interrupted");       }    } }

This would produce following result:

[Hello] [World] [Synchronized]

Thread Class Methods Use Example

Example:

The following ThreadClassDemo program demonstrates some of these methods of the Thread class:

// File Name : DisplayMessage.java // Create a thread to implement Runnable public class DisplayMessage implements Runnable {    private String message;    public DisplayMessage(String message)    {       this.message = message;    }    public void run()    {       while(true)       {          System.out.println(message);       }    } } // File Name : GuessANumber.java // Create a thread to extentd Thread public class GuessANumber extends Thread {    private int number;    public GuessANumber(int number)    {       this.number = number;    }    public void run()    {       int counter = 0;       int guess = 0;       do       {           guess = (int) (Math.random() * 100 + 1);           System.out.println(this.getName()                        + " guesses " + guess);           counter++;       }while(guess != number);       System.out.println("** Correct! " + this.getName()                        + " in " + counter + " guesses.**");    } } // File Name : ThreadClassDemo.java public class ThreadClassDemo {    public static void main(String [] args)    {       Runnable hello = new DisplayMessage("Hello");       Thread thread1 = new Thread(hello);       thread1.setDaemon(true);       thread1.setName("hello");       System.out.println("Starting hello thread...");       thread1.start();             Runnable bye = new DisplayMessage("Goodbye");       Thread thread2 = new Thread(hello);       thread2.setPriority(Thread.MIN_PRIORITY);       thread2.setDaemon(true);       System.out.println("Starting goodbye thread...");       thread2.start();       System.out.println("Starting thread3...");       Thread thread3 = new GuessANumber(27);       thread3.start();       try       {          thread3.join();       }catch(InterruptedException e)       {          System.out.println("Thread interrupted.");       }       System.out.println("Starting thread4...");       Thread thread4 = new GuessANumber(75);                   thread4.start();       System.out.println("main() is ending...");    } }

This would produce following result. You can try this example again and again and you would get different result every time.

Starting hello thread... Starting goodbye thread... Hello Hello Hello Hello Hello Hello Hello Hello Hello Thread-2 guesses 27 Hello ** Correct! Thread-2 in 102 guesses.** Hello Starting thread4... Hello Hello ..........remaining result produced.