Understanding Multithreading in Java

Introduction

Multithreading is a powerful feature in Java that allows multiple threads to run concurrently within a program. this capability can help improve performance, especially in CPU-bound and I/O-bound operations. In this blog, we’ll explore the core concepts of multithreading, the thread lifecycle, how to create and manage threads, common pitfalls, and best practices for using multithreading effectively.

What is Multithreading?

Multithreading refers to the concurrent execution of two or more threads, which are lightweight sub-processes. A thread is the smallest unit of a process that can be scheduled by the operating system. Unlike processes, threads share the same memory space, making it easier to communicate between them but also introducing complexity in terms of synchronization.

Why Use Multithreading?

The main reasons for using multithreading in Java are:

• Concurrency: To allow multiple tasks to be executed simultaneously.
• Resource Efficiency: Threads share the same heap memory, making them more memory-efficient than processes.
• Responsiveness: In GUI-based applications, multithreading can improve responsiveness by handling long-running tasks in the background.

Java Thread Model

Java provides built-in support for multithreading through the java.lang.Thread class and the java.util.concurrent package. The basic building blocks of the Java thread model are:

1. Thread: Represents a single thread.
2. Runnable: A functional interface representing a task that can be executed by a thread.
3. Executors: Provide thread pool management and other utilities.
4. Locks and Synchronization: Ensure proper coordination between threads to avoid issues like data inconsistency.

Create Threads in Java

There are two primary ways to create a thread in Java:

By Extending the Thread Class

You can create a thread by extending the Thread class and overriding its run() methods.

class MyThread extends Thread {
    @Override
    public void run() {
        System.out.println("Thread is running");
    }
}

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

By Implementing the Runnable interface

The preferred approach is to implement the Runnable interface, which separates task definition from the thread execution.

class MyRunnable implements Runnable {
    @Override
    public void run() {
        System.out.println("Runnable is executing");
    }
}

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

Why Runnable is Preferred: Implementing runnable allows more flexibility, such as reusing the task by multiple threads or running it in an executor service.

Thread Lifecycle in Java

A thread can be in one of the following states during its lifecycle:

1. New: When a thread is created but not yet stated.
2. Runnable: When the thread is ready to run or is running.
3. Blocked: When the thread is waiting for a monitor lock (due to synchronization).
4. Waiting: When the thread is waiting indefinitely for another thread to perform an action.
5. Timed Waiting: When the thread is waiting for a specified amount of time.
6. Terminated: When the thread has finished executing.

Thread Synchronization

When multiple threads share the same resources (e.g./ memory of files), there’s a risk of data inconsistency. Synchronization ensures that only one thread can access a critical section at a time.

Using synchronized Keyword

The synchronized keyword ensures that only one thread can execute a method or block of code at a time.

class Counter {
    private int count = 0;

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

    public int getCount() {
        return count;
    }
}

public class Main {
    public static void main(String[] args) throws InterruptedException {
        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("Final count: " + counter.getCount());
    }
}

Thread Safety and Common Pitfalls

While multithreading can significantly boost performance, it also introduces risks like race conditions, deadlock, and livellock. Here’s a quick overview:

• Race Condition: A race condition occurs when multiple threads try to access and modify shared data simultaneously, leading to unpredictable results.
• Deadlock: Deadlock happens when two or more threads are blocked forever because each thread is waiting for a resource that another thread holds.
• Livelock: In livelock, threads are not blocked, they continuously change states and fail to make progress.

Best Practices for Multithreading in Java

• Use Thread Pools: Instead of creating new thread for each task, use the ExecutorService to manage a pool of threads.

ExecutorService executor = Executors.newFixedThreadPool(5);
executor.execute(new MyRunnable());
executor.shutdown();

• Minimize the Use of synchronized: While synchronization is essential for thread safety, overuse can hurt performace.
• Use-Higher-Level Concurrency Utilities: Java provides higher-level concurrency abstractions in java.util.concurrent such as CountDownLatch, CyclicBarrier, Semaphore, Lock, and ReentrantLock.
• Avoid Share Mutable State: Prefer immutability where possible, inherently avoids synchronization issues.
• Handle InterruptedException appropriately to avoid stuck or unresponsive threads.

Advanced Multithreading with java.util.concurrent

ExecutorService and Thread Pools

The ExecutorService provides a higher-level mechanism to manage threads

ExecutorService executor = Executors.newFixedThreadPool(3);
executor.submit(() -> {
    // Task to run in a separate thread
    System.out.println("Task executed by thread pool");
});
executor.shutdown();

Future and Callable

Future allows you to get the result of a computation done by a thread, while Callable is a task that returns a result.

ExecutorService executor = Executors.newSingleThreadExecutor();
Callable<Integer> task = () -> {
    // Compute result in a separate thread
    return 42;
};
Future<Integer> future = executor.submit(task);
System.out.println("Computed result: " + future.get());  // Blocks until result is available
executor.shutdown();

Fork/Join Framework

For tasks that can be recursively split into smaller tasks, Java provides the ForkJoinPool for parallel processing.

ForkJoinPool forkJoinPool = new ForkJoinPool();
forkJoinPool.submit(() -> {
    // Task that can be broken into subtasks
});
forkJoinPool.shutdown();

Conclusion

Multithreading is a powerful feature in Java, allowing developers to create highly responsive, scalable applications. However, it also comes with complexities, such as thread safety, synchronization, and handling race conditions. Using modern Java concurrency utilities like ExecutorService, Future, and ForkJoinPool can simplify the management of threads, leading to more maintainable and efficient code.