Multithread in Java

What is a Thread?

A thread is the smallest unit of a process that can be scheduled for execution. It is a lightweight process that shares the same memory and resources of its parent process, allowing multiple tasks to be performed simultaneously. Java provides built-in support for multithreaded programming, which allows applications to perform multiple tasks at the same time, improving performance and responsiveness.

Key Concepts

• Process: An independent program in execution, with its own memory space.
• Thread: A subset of a process that shares the process’s memory and resources.
• Multithreading: The ability to execute multiple threads simultaneously.

Why Use Multithreading?

Multithreading enables efficient utilization of the CPU and improves the performance of applications by:

• Running tasks concurrently.
• Reducing idle time by utilizing CPU cycles better.
• Enhancing responsiveness in applications, especially in UI-based applications.

Creating a Thread in Java

Java provides two primary ways to create a thread:

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

1. Extending the Thread Class

To create a thread by extending the Thread class, a class must inherit from Thread and override its run() method.

class MyThread extends Thread {
    public void run() {
        System.out.println("Thread is running...");
    }
    
    public static void main(String[] args) {
        MyThread thread = new MyThread();
        thread.start(); // Start the thread
    }
}

2. Implementing the Runnable Interface

Another way to create a thread is by implementing the Runnable interface, which has a single run() method.

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

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

Note: Implementing Runnable is preferred over extending Thread when a class already extends another class, as Java does not support multiple inheritance.

Thread Life Cycle

A thread in Java goes through the following states:

1. New:
   • When a thread is created, but not yet started.
2. Runnable:
   • When the thread is ready to run, but not yet selected by the scheduler.
3. Blocked:
   • When the thread is waiting to acquire a lock.
4. Waiting:
   • When a thread is waiting indefinitely for another thread to perform a particular action.
5. Timed Waiting:
   • When a thread is waiting for another thread to perform an action within a specific time frame.
6. Terminated:
   • When a thread has completed its task and terminates.

Thread Methods

Java provides several methods for controlling and managing threads:

• start():
  • Starts a thread and calls the run() method.
• run():
  • Contains the code to be executed by the thread.
• sleep(long millis):
  • Causes the current thread to sleep for the specified number of milliseconds.
• join():
  • Waits for a thread to die.
• interrupt():
  • Interrupts a sleeping or waiting thread.
• isAlive():
  • Checks if a thread is alive.

class MyRunnable implements Runnable {
    @Override
    public void run() {
        try {
            Thread.sleep(1000); // Sleep for 1 second
            System.out.println("Thread running after 1-second sleep");
        } catch (InterruptedException e) {
            System.out.println("Thread interrupted");
        }
    }
}

public class Main {
    public static void main(String[] args) throws InterruptedException {
        Thread thread = new Thread(new MyRunnable());
        thread.start();
        thread.join(); // Main thread waits for thread to finish
        System.out.println("Main thread execution complete");
    }
}

Thread Synchronization

In multithreading, if multiple threads access a shared resource simultaneously, it can lead to data inconsistency. Synchronization is the solution for this issue. It allows only one thread at a time to access a shared resource.

Java provides various ways to achieve synchronization:

• Synchronized Methods
• Synchronized Blocks
• Static Synchronization

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) {
        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();
        
        try {
            t1.join();
            t2.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Final count: " + counter.getCount()); // Should print 2000
    }
}

Inter-Thread Communication

Java provides methods like wait(), notify(), and notifyAll() to facilitate communication between threads.

• wait(): Tells the current thread to wait until another thread calls notify().
• notify(): Wakes up a single thread that is waiting on the object's monitor.
• notifyAll(): Wakes up all threads that are waiting on the object's monitor.

class SharedResource {
    private boolean flag = false;

    public synchronized void produce() throws InterruptedException {
        while (flag) {
            wait();
        }
        System.out.println("Produced an item");
        flag = true;
        notify();
    }

    public synchronized void consume() throws InterruptedException {
        while (!flag) {
            wait();
        }
        System.out.println("Consumed an item");
        flag = false;
        notify();
    }
}

public class Main {
    public static void main(String[] args) {
        SharedResource resource = new SharedResource();

        Thread producer = new Thread(() -> {
            try {
                for (int i = 0; i < 5; i++) {
                    resource.produce();
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });

        Thread consumer = new Thread(() -> {
            try {
                for (int i = 0; i < 5; i++) {
                    resource.consume();
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });

        producer.start();
        consumer.start();
    }
}

Summary

Multithreading is a powerful feature in Java that allows concurrent execution of tasks, making programs more efficient and responsive. Java provides a rich set of APIs to create and manage threads, handle synchronization, and facilitate inter-thread communication. Properly understanding and utilizing threads can greatly improve the performance and responsiveness of Java applications.