Multithreading in Java

Java is a multithreaded programming language. That means that we can create a process that can be executed by multiple threads simultaneously.

What is a thread in Java?

A thread is a lightweight smallest part of a process. It can run concurrently with other threads. It is some kind of a mechanism that ensures the independent execution of a single task within a program.

Threads are sharing the same memory. Since the threads are independent, if an exception occurs in one thread, it doesn’t affect other threads.

The life cycle of a thread

During execution, the thread goes through several states. Thread can only be in one state at a time.

Thread life cycle:

• NEW – A thread that has not yet started.
• RUNNABLE – A thread executes in the Java virtual machine (JVM).
• BLOCKED – A thread is blocked
• WAITING – A thread is waiting for another thread to perform a certain action
• TIMED_WAITING – A thread that is waiting for another thread for up to a specified waiting time
• TERMINATED – A thread that has exited

Advantages of Multithreading in Java

• We can significantly speed up program execution time using multiple threads.
• If we get an exception in one thread, that will not affect the execution of other threads since threads run independently of each other.
• Using threads, we can execute multiple operations at the same time, without blocking the main thread that the user started.

How to create a Thread in Java?

We can create a thread in two ways:

1. Extending Thread class
2. Implementing Runnable interface

Creating a Thread by extending the Thread class

Thread class belongs to java.util package. It extends the Object class and implements the Runnable interface. Some of the methods:

• currentThread() – Returns a reference to the currently executing thread object.
• getId() – Returns the identifier of this thread.
• getName() – Returns this thread’s name.
• getPriority() – Returns this thread’s priority.
• isAlive() – Tests if this thread is alive.
• join() – Waits for this thread to finish.
• run() – If this thread was constructed using a separate Runnable object, then that Runnable object’s run method is called; otherwise, this method does nothing and returns.
• sleep() – suspend a thread for a period of time.
• start() – start a thread by calling its run() method

You can check out the official documentation of the Thread class. 

Examples of creating a Thread in Java

Create a thread by extending the Thread class

public class TestMultithreading extends Thread {

  public void run() {
    System.out.println("The thread is running...");
  }

  public static void main(String args[]) {
    TestMultithreading test = new TestMultithreading();
    test.start();
  }

}

Get the id and name of a thread

public class TestMultithreading extends Thread {

  public void run() {
    System.out.println("ID: " + Thread.currentThread().getId());
    System.out.println("Name: " + Thread.currentThread().getName());

    System.out.println("The thread is running...");
  }

  public static void main(String args[]) {
    TestMultithreading test = new TestMultithreading();
    test.start();
  }

}

Put thread to sleep for 5 seconds

public class TestMultithreading extends Thread {

  public void run() {
    long start = System.currentTimeMillis();

    try {
      Thread.sleep(5000); // milliseconds
    } catch (InterruptedException e) {
      e.printStackTrace();
    }

    System.out.println("The thread is running...");
    System.out.println("Execution time: " + ((System.currentTimeMillis()) - start) + " ms");
  }

  public static void main(String args[]) {
    TestMultithreading test = new TestMultithreading();

    test.start();
  }

}

Creating a Thread by implementing a Runnable interface

We can create a thread by implementing a Runnable interface directly. 

The Runnable interface is a functional interface with a single abstract method run() called when we start a thread using the start() method from the Thread class.

We need to instantiate the Thread class using its constructor, which accepts Runnable objects to create a thread.

Example

public class TestMultithreading implements Runnable {

  public void run() {
    System.out.println("The thread is running...");
  }

  public static void main(String args[]) {
    TestMultithreading runnable = new TestMultithreading();

    Thread threadObject = new Thread(runnable); // providing the object of a class that implements Runnable

    threadObject.start();
  }
}