Java Multithreading - Java Technology | BigXStar

What is Multithreading?

Multithreading allows a program to execute multiple threads simultaneously, improving performance and responsiveness. Each thread is a lightweight, independent path of execution.

Creating Threads

Method 1: Extending Thread Class

java

public class MyThread extends Thread {
    @Override
    public void run() {
        for (int i = 1; i <= 5; i++) {
            System.out.println(Thread.currentThread().getName() + ": " + i);
            try { Thread.sleep(500); } catch (InterruptedException e) { }
        }
    }
}

MyThread t1 = new MyThread();
t1.start();  // Starts a new thread

Method 2: Implementing Runnable (Preferred)

java

public class MyTask implements Runnable {
    @Override
    public void run() {
        for (int i = 1; i <= 5; i++) {
            System.out.println(Thread.currentThread().getName() + ": " + i);
        }
    }
}

Thread t1 = new Thread(new MyTask(), "Worker-1");
Thread t2 = new Thread(new MyTask(), "Worker-2");
t1.start();
t2.start();

Method 3: Lambda Expression

java

Thread t = new Thread(() -> {
    System.out.println("Running in: " + Thread.currentThread().getName());
});
t.start();

Thread Lifecycle

State	Description
New	Thread created but not started
Runnable	Ready to run, waiting for CPU
Running	Currently executing
Blocked/Waiting	Waiting for a resource or signal
Terminated	Execution completed

Synchronization

Prevents race conditions when multiple threads access shared data:

java

public class Counter {
    private int count = 0;

    // Synchronized method — only one thread at a time
    public synchronized void increment() {
        count++;
    }

    public int getCount() {
        return count;
    }
}

Counter counter = new Counter();

Thread t1 = new Thread(() -> {
    for (int i = 0; i < 1000; i++) counter.increment();
});

Thread t2 = new Thread(() -> {
    for (int i = 0; i < 1000; i++) counter.increment();
});

t1.start(); t2.start();
t1.join();  t2.join();

System.out.println(counter.getCount());  // Always 2000

Thread Communication

java

// join() — wait for a thread to complete
t1.start();
t1.join();  // Main thread waits for t1 to finish

// sleep() — pause execution
Thread.sleep(1000);  // Sleep for 1 second

// wait() and notify() — inter-thread communication
synchronized (lock) {
    lock.wait();    // Release lock and wait
    lock.notify();  // Wake up one waiting thread
}

Executor Framework (Modern Approach)

java

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

ExecutorService executor = Executors.newFixedThreadPool(3);

for (int i = 1; i <= 5; i++) {
    final int task = i;
    executor.submit(() -> {
        System.out.println("Task " + task + " by " + Thread.currentThread().getName());
    });
}

executor.shutdown();

Practical Example: Parallel Downloader

java

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class ParallelDownloader {
    static void downloadFile(String filename) {
        System.out.printf("[%s] Downloading %s...%n",
            Thread.currentThread().getName(), filename);
        try { Thread.sleep(1000); } catch (InterruptedException e) { }
        System.out.printf("[%s] Completed %s%n",
            Thread.currentThread().getName(), filename);
    }

    public static void main(String[] args) {
        String[] files = {"report.pdf", "image.png", "data.csv", "video.mp4"};

        ExecutorService pool = Executors.newFixedThreadPool(2);
        for (String file : files) {
            pool.submit(() -> downloadFile(file));
        }
        pool.shutdown();
    }
}

Summary

Multithreading runs multiple threads concurrently for better performance
Create threads by extending Thread, implementing Runnable, or using lambdas
Use synchronized to prevent race conditions on shared data
join() waits for thread completion; sleep() pauses execution
The Executor Framework is the modern, preferred way to manage threads
Use thread pools (ExecutorService) instead of creating threads manually