Concurrency and Multithreading in Java: From A to Z

Introduction to Concurrency and Multithreading in Java

Modern applications increasingly require the simultaneous execution of multiple tasks. Whether it's processing server requests, downloading files, or complex calculations — mechanisms of concurrency and multithreading are essential. In Java, these concepts are built into the language and standard library, making it one of the best tools for creating high-performance applications.

In this article, we will cover everything from the basics to advanced tools: how to create threads, synchronize data access, and use modern utilities from the java.util.concurrent package. The material will be useful for both beginners and developers looking to systematize their knowledge.

What is a Thread and why is it needed?

A thread is a lightweight process that executes within an application. Every Java program has at least one thread (main). Multithreading allows multiple threads to execute simultaneously, which is especially important on multi-core processors.

Main advantages of multithreading:

• Improved performance through parallel task execution.
• Enhanced user interface responsiveness.
• Efficient use of system resources.

1. Creating Threads in Java: Classic Approaches

In Java, there are two basic ways to create a thread: extending the Thread class and implementing the Runnable interface. Let's look at both.

Extending the Thread Class

The simplest way is to create a subclass of Thread and override the run() method.

public class MyThread extends Thread {    @Override    public void run() {        System.out.println("Thread " + getName() + " started.");    }

    public static void main(String[] args) {        MyThread thread = new MyThread();        thread.start(); // Start the thread    }}

Important: call start(), not run(). run() will execute the code in the current thread, while start() creates a new one.

Implementing the Runnable Interface

A more flexible approach is to implement Runnable and pass it to the Thread constructor.

public class Task implements Runnable {    @Override    public void run() {        System.out.println("Task is executing in thread: " + Thread.currentThread().getName());    }

    public static void main(String[] args) {        Task task = new Task();        Thread thread = new Thread(task);        thread.start();    }}

This method is preferable because it does not require inheritance and allows the class to extend other classes.

2. Synchronization and Atomicity: How to Avoid Race Conditions

When multiple threads access shared data simultaneously, a race condition occurs. To avoid this, Java provides synchronization mechanisms.

The synchronized Keyword

Using synchronized, you can lock a method or a block of code for a single thread.

public class Counter {    private int count = 0;

    public synchronized void increment() {        count++;    }

    public int getCount() {        return count;    }}

In this example, only one thread can execute increment() at any given moment. However, for simple operations like incrementing, it's better to use atomic classes.

Atomic Classes (Atomic)

The java.util.concurrent.atomic package contains classes that provide lock-free atomic operations.

import java.util.concurrent.atomic.AtomicInteger;

public class AtomicCounter {    private A