Introduction to Threads

Multithreading refers to two or more tasks executing concurrently within a single program. A thread is an independent path of execution within a program. Many threads can run concurrently within a program. Every thread in Java is created and controlled by the java.lang.Thread class. A Java program can have many threads, and these threads can run concurrently, either asynchronously or synchronously.

Multithreading has several advantages over Multiprocessing such as;

Threads are lightweight compared to processes
Threads share the same address space and therefore can share both data and code
Context switching between threads is usually less expensive than between processes
Cost of thread intercommunication is relatively low that that of process intercommunication
Threads allow different tasks to be performed concurrently.

Thread Creation

There are two ways to create thread in java;

    Implement the Runnable interface (java.lang.Runnable)
    By Extending the Thread class (java.lang.Thread)

Implementing the Runnable Interface

The Runnable Interface Signature

public interface Runnable {

void run();

}

One way to create a thread in java is to implement the Runnable Interface and then instantiate an object of the class. We need to override the run() method into our class which is the only method that needs to be implemented. The run() method contains the logic of the thread.

The procedure for creating threads based on the Runnable interface is as follows:

1. A class implements the Runnable interface, providing the run() method that will be executed by the thread. An object of this class is a Runnable object.

2. An object of Thread class is created by passing a Runnable object as argument to the Thread constructor. The Thread object now has a Runnable object that implements the run() method.

3. The start() method is invoked on the Thread object created in the previous step. The start() method returns immediately after a thread has been spawned.

4. The thread ends when the run() method ends, either by normal completion or by throwing an uncaught exception.

Below is a program that illustrates instantiation and running of threads using the runnable interface instead of extending the Thread class. To start the thread you need to invoke the start() method on your object.

class RunnableThread implements Runnable {

    Thread runner;
    public RunnableThread() {
    }
    public RunnableThread(String threadName) {
        runner = new Thread(this, threadName); // (1) Create a new thread.
        System.out.println(runner.getName());
        runner.start(); // (2) Start the thread.
    }
    public void run() {
        //Display info about this particular thread
        System.out.println(Thread.currentThread());
    }
}

public class RunnableExample {

    public static void main(String[] args) {
        Thread thread1 = new Thread(new RunnableThread(), "thread1");
        Thread thread2 = new Thread(new RunnableThread(), "thread2");
        RunnableThread thread3 = new RunnableThread("thread3");
        //Start the threads
        thread1.start();
        thread2.start();
        try {
            //delay for one second
            Thread.currentThread().sleep(1000);
        } catch (InterruptedException e) {
        }
        //Display info about the main thread
        System.out.println(Thread.currentThread());
    }
}

Output

thread3
Thread[thread1,5,main]
Thread[thread2,5,main]
Thread[thread3,5,main]
Thread[main,5,main]private