01. Threads.md

THREADS

• A Multithreaded program contains two or more parts that can run concurrently. Each part of such program is called thread.
• Multithreading enables you to write efficient programs that make maximum use of processing power available in the system.
• Multithreading introduces an asynchronous behaviour to the programs.
• When needed synchronicity can be enforced using interprocess synchronization called the monitor.
• Monitor can be imagined as a box that can only hold single thread. once the thread enters the monitor, all other threads must wait until that thread exits the monitor.

The Main Thread

• when any java programs starts up, one thread begins running immediately, this is the main thread of the program.
• It is created automatically, when the program starts.
• The main thread is the thread from which other "child" threads will be spawned.
• It is the last thread to finish execution since it performs various shutdown actions.

class Main{
    public static void main(String[] args){
        Thread t = Thread.currentThread(); // getting reference of the current thread

        System.out.println(t);

        t.setName("Main Thread"); // setting the name of the thread
        
        System.out.println(t);

        try {
            for(int n=5;n>0;n--){
                System.out.println(n);
                Thread.sleep(1000);
            }
        }catch (InterruptedException ex){
            System.out.println("Thread interrupted");
        }
    }
}

Creating a Thread

• Implementing Runnable Interface
• Extending Thread class

class CustomThread implements Runnable {
    private final Thread t;

    CustomThread(String name){
        t = new Thread(this,name);
    }

    public void start(){
        this.t.start();
    }

    /**
     * Runs this operation.
     */
    @Override
    public void run() {
        try {
            for(int n=5;n>0;n--){
                System.out.println("thread " + t.getName() +" >> " +  n);
                Thread.sleep(500);
            }
        }catch (InterruptedException ex){
            System.out.println("thread " + t.getName() + " interrupted");
        }
        System.out.println("thread " + t.getName() + " exiting");
    }
}

public class Main {

    public static void main(String[] args) {
        Thread t = Thread.currentThread();

        System.out.println("main thread: " + t);

        new CustomThread("t1").start(); // start the new child thread
        new CustomThread("t2").start(); // start the new child thread

        try {
            for(int n=5;n>0;n--){
                System.out.println("main thread: " + n);
                Thread.sleep(1000);
            }
        }catch (InterruptedException ex){
            System.out.println("main thread interrupted");
        }
        System.out.println("Main thread exiting");
    }
}

class CustomThread extends Thread {

    CustomThread(String name){
        super(name);
    }

    /**
     * Runs this operation.
     */
    @Override
    public void run() {
        try {
            for(int n=5;n>0;n--){
                System.out.println("thread " + this.getName() +" >> " +  n);
                Thread.sleep(500);
            }
        }catch (InterruptedException ex){
            System.out.println("thread " + this.getName() + " interrupted");
        }
        System.out.println("thread " + this.getName() + " exiting");
    }
}

public class Main {

    public static void main(String[] args) {
        Thread t = Thread.currentThread();

        System.out.println("main thread: " + t);

        new CustomThread("t1").start(); // start the new child thread
        new CustomThread("t2").start(); // start the new child thread

        try {
            for(int n=5;n>0;n--){
                System.out.println("main thread: " + n);
                Thread.sleep(1000);
            }
        }catch (InterruptedException ex){
            System.out.println("main thread interrupted");
        }
        System.out.println("Main thread exiting");
    }
}