Java Thread

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What is Thread?

• Thread is a lightweight process.
• Thread is very similar to program that has a single flow of control.
• Thread has a 3 parts- 1.beginning, 2.body and 3.end.
• Thread executes commands squentially.
• Every program have atleast one thread called as main thread.

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The main Thread

When program begins its execution, one thread is started immediately,which is called as main thread of program.

Main thread is required because

• Other child threads will be spawned fro mthe main thread.
• Always, it should be the last thread to finish execution as it performs various shutdown actions.

Main thread is controlled by thread object.

Controlling main thread

• To control main thread, we have need to get the referrence of it by calling currentThread() method.
• currentThread() is a public static member of Thread.
• Syntax - static Thread currentThread()
• Now the main thread can be controlled like other threads using above referrence.

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How to create a Thread?

In general the thread can be created by instantiating an object of type Thread. java defines two ways in which this can be accomplished.

1. By Implementing Runnable Interface
2. By Extending Thread class

1. Implement runnnable interface

The easier way to create a Thread by implementing a class to runnable interface.

The Runnalbe interface have run () method which is required for starting the execution of Thread.

Steps

1. Declare a class which implements the runnable interface.
2. Implement the public void run() method in the class.
3. Create a thread by defining an object that instantiated from this runnable class as the target of the thread.
4. call the thread's start() method to run the thread.
5. The thread will now continue to execute till the main thread ends or we explicitly stop it.

Example

Program to print first 10 numbers using Runnable interface.

class MyThread implements Runnable
{
    public void run()
    {
	System.out.println("Thread started");
	for(int i=1;i<=10;i++)
	{
	    System.out.println("i : "+i);
	}
	System.out.println("Thread exited");
    }
}

public class Creator 
{
    public static void main(String[] args) 
    {	
	MyThread mthread = new MyThread();
	Thread t = new Thread(mthread);
	t.start();
    }
}

Output

Explaination

• In above example we have created a class MyThread which implements runnable interface.
• In class Creator we have created object of class MyThread as mthread and also created object of class Thread.
• The Thread class contains the method called start() to start the Thread hence we have passed mthread object to Thread class constructor and called it's start method using Thread class object.
• The start method calls the run method of mthread and executes the code in it.
• In above example, we have created only one thread hence cpu will execute it without switching to another thread.

Note : This type of approach is used when we have already extended some class to mythread class. That time we cannot extend more than one class to a single class. Hence java allows us to use interface for creating a thread.

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Creating multiple threads using Runnable interface

Example

class A implements Runnable
{
    public void run()
    {
	System.out.println("Thread A started");
	for(int i=1;i<=5;i++)
	    System.out.println("A : "+i);
	System.out.println("Thread A exited");
    }
}
class B implements Runnable
{
    public void run()
    {
	System.out.println("Thread B started");
	for(int i=1;i<=5;i++)
	    System.out.println("B : "+i);
	System.out.println("Thread B exited");
    }
}
class C implements Runnable
{
    public void run()
    {
	System.out.println("Thread C started");
	for(int i=1;i<=5;i++)
	    System.out.println("C : "+i);
	System.out.println("Thread C exited");
    }
}
public class Test 
{
    public static void main(String[] args) 
    {
       A a1 = new A();
       B b1 = new B();
       C c1 = new C();
       new Thread(a1).start();
       new Thread(b1).start();
       new Thread(c1).start();
    }
}

Output

Explaination

• In above example we have created 3 Threads called as A,B,C by implementing runnable interface
• We have called the start method by creating object of Thread class and started all threads.
• The output of above program is not properly from 1 to 5 because,the CPU switches from A,B,C as soon as CPU is available for execution from other tasks.

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2. Extending Thread class

Second way of creating a thread is to extend the Thread class. This approach used when we want to leave a separate class for creating a thread only.

Steps

1. Declare the class and extend the Thread class to it.
2. Implement the public void run() method in the class.
3. Thread a thread object and call the start() method.

Example

Program to print 1 to 5 Numbers by Extending Thread class

class A extends Thread
{
    public void run()
    {
	System.out.println("Thread started");
	for(int i=1;i<=5;i++)
	    System.out.println("i : "+i);
	System.out.println("Thread exited");
    }
}

public class Test {
	public static void main(String[] args) 
       {
	 A a1 = new A();
	 a1.start();
       }
}

Output

Explaination

• In above example we have created seperate thread class A which extends the Thread class.
• We have created an object of thread A and directly called the method start because by extending Thread class we have directly access to its method without creating an object of Thread class.

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Creating multiple threads By exteding Thread class

Example

class A extends Thread
{
    public void run()
    {
	System.out.println("Thread A started");
	for(int i=1;i<=5;i++)
	    System.out.println("A : "+i);
	System.out.println("Thread A exited");
    }
}
class B extends Thread
{
    public void run()
    {
	System.out.println("Thread B started");
	for(int i=1;i<=5;i++)
	    System.out.println("B : "+i);
	System.out.println("Thread B exited");
    }
}
class C extends Thread
{
    public void run()
    {
	System.out.println("Thread C started");
	for(int i=1;i<=5;i++)
	    System.out.println("C : "+i);
	System.out.println("Thread C exited");
    }
}
public class Test {
   public static void main(String[] args) 
   {
        A a1 = new A();
	B b1 = new B();
	C c1 = new C();
	a1.start();
	b1.start();
	c1.start();
    }
}

Output

Explanation

• In above example we have created 3 Threads called as A,B,C by extending Thread class.
• We have called the start method by creating object of Thread class and started all threads.
• The output of above program is not properly from 1 to 5 because,the CPU switches from A,B,C as soon as CPU is available for execution from other tasks.