Threads in Java

Overview

When a program is executing it follows a path, that is Thread. Threads are used to perform critical tasks in the background without interrupting the main thread.

There are two ways to create a Thread.

• By extending the class Thread
• By implementing an interface Runnable

Five states of Thread

1. Born
2. Runnable
3. Running
4. Non Runnable
5. Dead

sleep() vs wait()

• sleep() belongs to Thread class whereas wait() belongs to an object.
• sleep() can be called without a synchronized context but wait() should be called in a synchronized context.
• sleep() does not release lock on an object but wait() releases lock on an object during synchronization.
• sleep() is a static method but wait() is not a static method.
• sleep() is executed on a Thread but wait() method is executed on an object. 

interrupt()

• This method is used to interrupt an executing Thread.
• If a Thread is in sleep() or wait() state then the interrupt() method will get called and the catch() statement gets executed. If a Thread doesn't go in sleep() or wait() state then the interrupt() method won't affect the execution of the Thread.

Example of interrupt()

 public class InterruptThreadMain extends Thread {
    @Override
    public void run() {
        super.run();
        try {
            for (int i = 0; i < 10; i++) {
                System.out.println(currentThread() + " " + i);
                Thread.sleep(3000);
            }
        } catch (Exception e) {
            System.out.println(currentThread() + " throws the exception: " + e.getMessage());
        }
    }

    public static void main(String[] args) {
        InterruptThreadMain interruptThreadMain = new InterruptThreadMain();
        interruptThreadMain.start();
        interruptThreadMain.interrupt();

        Thread anonymousThread = new Thread(() -> {
            try {
                for (int j = 0; j < 10; j++) {
                    System.out.println(currentThread() + " " + j);
                    // Thread.sleep(2000);
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
        });
        anonymousThread.start();
        anonymousThread.interrupt();
    }
 }

 Thread[Thread-0,5,main] 0
 Thread[Thread-1,5,main] 0
 Thread[Thread-1,5,main] 1
 Thread[Thread-1,5,main] 2
 Thread[Thread-1,5,main] 3
 Thread[Thread-1,5,main] 4
 Thread[Thread-1,5,main] 5
 Thread[Thread-1,5,main] 6
 Thread[Thread-1,5,main] 7
 Thread[Thread-1,5,main] 8
 Thread[Thread-1,5,main] 9
 Thread[Thread-0,5,main] throws the exception: sleep interrupted

Do's and Dont's 

1. You cannot start a thread again. If you do you will get a runtime exception IllegalThreadStateException

Reference