Multithreading in Java

What is a Java Thread?

A thread is actually a lightweight process. Unlike many other computer languages, Java provides built-in support for multithreaded programming. A multithreaded program contains two or more parts that can run concurrently. Each part of such a program is called thread and each thread defines a separate path of execution. Thus, multithreading is a specialized form of multitasking.

Next concept in this Java Thread blog is integral to the concept Threads and Multithreading.

The Java Thread Model

The Java run-time system depends on threads for many things. Threads reduce inefficiency by preventing the waste of CPU cycles.

Threads exist in several states. Following are those states:  

• New – When we create an instance of Thread class, a thread is in a new state.
• Runnable – The Java thread is in running state.
• Running – A running thread can be suspended, which temporarily suspends its activity. A suspended thread can then be resumed, allowing it to pick up where it left off.
• Waiting – A java thread can be blocked when waiting for a resource.
• Dead – A thread can be terminated, which halts its execution immediately at any given time. Once a thread is terminated, it cannot be resumed. 

Multithreading in Java : Thread Class and Runnable Interface

Java’s multithreading system is built upon the Thread class, its methods, and its companion interface, Runnable. To create a new thread, your program will either extend Thread or implement the Runnableinterface.

The Thread class defines several methods that help manage threads.The table below displays the same:

Method	Meaning
getName()	Obtain thread’s name
getPriority()	Obtain thread’s priority
isAlive()	Determine if a thread is still running
join()	Wait for a thread to terminate
run()	Entry point for the thread
sleep()	Suspend a thread for a period of time
start()	Start a thread by calling its run method

Why is Main Thread so important?

• Because this thread effects the other ‘child’ threads
• Because it performs various shutdown actions
• It is created automatically when your program is started.

How to Create a Java Thread? 

Java lets you create thread in following two ways:- 

• By implementing the Runnable interface.
• By extending the Thread

Runnable Interface

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

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

1	public void run( )

Inside run( ), we will define the code that constitutes the new thread

Example:

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public class MyClass implements Runnable { public void run(){ System.out.println("MyClass running");    } }

To execute the run() method by a thread, pass an instance of MyClass to a Thread in its constructor(A constructor in Java is a block of code similar to a method that’s called when an instance of an object is created). Here is how that is done:

1 2	Thread t1 = new Thread(new MyClass ()); t1.start();

When the thread is started it will call the run() method of the MyClass instance instead of executing its own run() method. The above example would print out the text “MyClass running“.

Extending Java Thread

The second way to create a thread is to create a new class that extends Thread, then override the run() method and then to create an instance of that class. The run() method is what is executed by the thread after you call start(). Here is an example of creating a Java Thread subclass:

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public class MyClass extends Thread {      public void run(){      System.out.println("MyClass running");    } }

To create and start the above thread you can do like this:

1 2	MyClass t1 = new MyClass (); T1.start();

When the run() method executes it will print out the text “MyClass running“.

So far, we have been using only two threads: the main thread and one child thread. However, our program can affect as many threads as it needs. Let’s see how we can create multiple threads.

Creating Multiple Threads

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class MyThread implements Runnable { String name; Thread t;     MyThread String thread){     name = threadname;     t = new Thread(this, name); System.out.println("New thread: " + t); t.start(); }     public void run() {  try {      for(int i = 5; i > 0; i--) {      System.out.println(name + ": " + i);       Thread.sleep(1000); } }catch (InterruptedException e) {      System.out.println(name + "Interrupted"); }      System.out.println(name + " exiting."); } }   class MultiThread { public static void main(String args[]) {      new MyThread("One");      new MyThread("Two");      new NewThread("Three"); try {      Thread.sleep(10000); } catch (InterruptedException e) {       System.out.println("Main thread Interrupted"); }       System.out.println("Main thread exiting.");       } } OUTPUT: New thread: Thread[One,5,main] New thread: Thread[Two,5,main] New thread: Thread[Three,5,main] One: 5 Two: 5 Three: 5 One: 4 Two: 4 Three: 4 One: 3 Three: 3 Two: 3 One: 2 Three: 2 Two: 2 One: 1 Three: 1 Two: 1 One exiting. Two exiting. Three exiting. Main thread exiting.