24. Multithreading in Java

 

Java Thread

In Java, threads can be created by using two mechanisms:

1. Extending the Thread class
2. Implementing the Runnable interface

Extending the Thread Class

• We create a class that extends the java.lang.Thread class.
• The java.lang package is imported automatically in out program.
• This class overrides the run() method available in the Thread class.
• A thread begins its life inside run() method.
• We create an object of our class and call start() method to start the execution of a thread.
• start() invokes the run() method on the Thread object.

class Run1 extends Thread {

    @Override

    public void run() {

        for (int i = 1; i <= 5; i++) {

            System.out.println("I am watching coding video.");

            System.out.println("This is very nice video..");

        }

    }

}

class Run2 extends Thread {

    @Override

    public void run() {

        for (int i = 1; i <= 5; i++) {

            System.out.println("I am listening song...");

            System.out.println("This is my favorite song....");

        }

    }

}

class ThreadClass {

    public static void main(String[] arr) {

        Run1 t1 = new Run1();

        Run2 t2 = new Run2();

        t1.start();

        t2.start();

    }

}

Output :

I am watching coding video.
This is very nice video..
I am watching coding video.
This is very nice video..
I am watching coding video.
This is very nice video..
I am watching coding video.
I am listening song...
This is very nice video..
I am watching coding video.
This is very nice video..
This is my favorite song....
I am listening song...
This is my favorite song....
I am listening song...
This is my favorite song....
I am listening song...
This is my favorite song....
I am listening song...
This is my favorite song....

Implementing the Runnable Interface

• We create a new class which implements java.lang.Runnable interface.
• The java.lang package is imported automatically in out program.
• This class overrides the run() method available in the Runnable interface.
• We instantiate a Thread class object and call start() method on this object.

class Run1 implements Runnable {

    @Override

    public void run() {

        for (int i = 1; i <= 5; i++) {

            System.out.println("I am watching coding video.");

            System.out.println("This is very nice video..");

        }

    }

}

class Run2 implements Runnable {

    @Override

    public void run() {

        for (int i = 1; i <= 5; i++) {

            System.out.println("I am listening song...");

            System.out.println("This is my favorite song....");

        }

    }

}

class RunnableInterface {

    public static void main(String[] arr) {

        Run1 r1 = new Run1();

        Run2 r2 = new Run2();

        Thread t1 = new Thread(r1);

        Thread t2 = new Thread(r2);

        t1.start();

        t2.start();

    }

}

Output :

I am listening song...
This is my favorite song....
I am watching coding video.
I am listening song...
This is my favorite song....
I am listening song...
This is my favorite song....
I am listening song...
This is my favorite song....
This is very nice video..
I am watching coding video.
This is very nice video..
I am listening song...
This is my favorite song....
I am watching coding video.
This is very nice video..
I am watching coding video.
This is very nice video..
I am watching coding video.
This is very nice video..

Note: Thread class implements the Runnable interface and the run() method is declared inside the Runnable interface.
We can achieve basic functionality of a thread by extending Thread class because it provides some inbuilt methods like yield(), interrupt() etc. that are not available in Runnable interface.

Thread Priorities

• Whenever we create a thread in Java, it always has some priority assigned to it.
• Priorities in threads is a concept where each thread is having a priority which is represented by numbers ranging from 1 to 10.
• In most cases, the thread scheduler schedules the threads according to their priority (known as primitive scheduling).
• But it is not guaranteed because it depends on JVM specification that which scheduling it chooses.
• The default priority is set to 5 as expected.
• Minimum priority is set to 1.
• Maximum priority is set to 10.

Some of the methods in Thread class are as:

• currentThread(): The method is used to return a reference to the currently executing thread object.
• setName(): This method is used to change the name of the thread.
• getName(): This method is used to return the name of the thread.
• setPriority(): This method is used to change the name of the thread.
• getPriority(): This method is used to return the priority of given thread.
• getId(): This method returns the identifier of the invoked thread. The thread ID is unique, generated when thread is created and remains unchanged during its lifetime.
• join(): The join() method permits one thread to wait until the other thread to finish its execution.

class A extends Thread {

    public void run() {

        System.out.println("Current Thread name is: " + Thread.currentThread().getName());

        System.out.println("Current Thread priority is: " + Thread.currentThread().getPriority());

    }

}

class ThreadPriority {

    public static void main(String[] arr) {

        A obj1 = new A();

        A obj2 = new A();

        A obj3 = new A();

        A obj4 = new A();

        A obj5 = new A();

        A obj6 = new A();

        obj1.setName("First");

        obj6.setName("Last");

        obj1.setPriority(Thread.MIN_PRIORITY);

        obj6.setPriority(Thread.MAX_PRIORITY);

        obj2.setPriority(2);

        obj3.setPriority(7);

        obj1.start();

        obj2.start();

        obj3.start();

        obj4.start();

        obj5.start();

        obj6.start();

    }

}

Output :

Current Thread name is: Thread-4
Current Thread name is: Thread-3
Current Thread name is: Thread-2
Current Thread name is: First
Current Thread priority is: 7
Current Thread priority is: 5
Current Thread priority is: 5
Current Thread priority is: 10
Current Thread name is: Last
Current Thread priority is: 2
Current Thread name is: Thread-5
Current Thread priority is: 1

Java Thread Constructors

Some of the main Constructors of Thread class are explained as:

• Thread(): Allocates a new Thread object.
• Thread(String name): Allocates a new Thread object and sets the name of the Thread.
• Thread(Runnable target): Allocates a new Thread object.
• Thread(Runnable target, String name): Allocates a new Thread object and sets the name of the Thread.

class Run1 extends Thread {

    public Run1(String name) {

        super(name);

    }

    @Override

    public void run() {

        System.out.println("I am extending Thread class.");

        System.out.println("This is inside Run1 class extending Thread class..");

    }

}

class Run2 implements Runnable {

    @Override

    public void run() {

        System.out.println("I am implementing Runnable interface...");

        System.out.println("This is inside Run2 class implementing Runnable interface....");

    }

}

class ThreadConstructors {

    public static void main(String[] arr) {

        Run1 t1 = new Run1("Deepak");

        t1.start();

        System.out.println("The name is " + t1.getName());

        System.out.println("The id is " + t1.getId());

        Run2 r2 = new Run2();

        Thread t2 = new Thread(r2, "Gagandeep");

        t2.start();

        System.out.println("The name is " + t2.getName());

        System.out.println("The id is " + t2.getId());

    }

}

Output :

I am extending Thread class.
This is inside Run1 class extending Thread class..
The name is Deepak
The id is 14
The name is Gagandeep
The id is 15
I am implementing Runnable interface...
This is inside Run2 class implementing Runnable interface....

Note: The above outputs in this page may vary in different runs depending on the JVM (Java Virtual Machine).

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