Multithreading in java with examples

Multithreading is one of the most popular feature of Java programming language as it allows the concurrent execution of two or more parts of a program. Concurrent execution means two or more parts of the program are executing at the same time, this maximizes the CPU utilization and gives you better performance. These parts of the program are called threads.

Threads are independent because they all have separate path of execution that’s the reason if an exception occurs in one thread, it doesn’t affect the execution of other threads. All threads of a process share the common memory. The process of executing multiple threads simultaneously is known as multithreading.

Advantages of Multithreading

• Efficient CPU Utilization: As more than one threads run independently, this allows the CPU to perform multiple tasks simultaneously.
• Improved Performance
• Better Resource Sharing: As discussed earlier, threads share common memory, this reduces overhead compared to processes.

Creating Threads in Java

There are two primary ways to create threads:

1. By Extending the Thread Class

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

public class Main {
    public static void main(String[] args) {
        MyThread t1 = new MyThread();
        t1.start(); // Starts the thread and executes the `run` method
    }
}

2. By Implementing the Runnable Interface

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

public class Main {
    public static void main(String[] args) {
        MyRunnable runnable = new MyRunnable();
        Thread t1 = new Thread(runnable);
        t1.start(); // Starts the thread and executes the `run` method
    }
}

Thread Methods

• start(): Starts the thread, calling its run() method.
• run(): Defines the code executed by the thread.
• sleep(milliseconds): Pauses the thread for a specified duration.
• join(): Waits for a thread to finish before continuing.
• isAlive(): Checks if the thread is still running.
• getName() and setName(String name): To retrieve or set a thread’s name.

Thread Synchronization

• Multithreading introduces asynchronous behaviour to the programs. If a thread is writing some data another thread may be reading the same data at that time. This may bring inconsistency.
• When two or more threads need access to a shared resource there should be some way that the resource will be used only by one resource at a time. The process to achieve this is called synchronization.
• To implement the synchronous behavior java has synchronous method. Once a thread is inside a synchronized method, no other thread can call any other synchronized method on the same object. All the other threads then wait until the first thread come out of the synchronized block.

When multiple threads access shared resources, synchronization ensures data consistency:

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("Count: " + counter.getCount());
    }
}

Thread Lifecycle

A thread in Java goes through the following states:

1. New: When a thread object is created. A thread that has not yet started is in this state.
2. Runnable: After calling start(), the thread is ready to run.
3. Running: The thread is executing its run() method. A thread executing in the Java virtual machine is in this state.
4. Blocked/Waiting: The thread is waiting for a resource or signal. A thread that is waiting indefinitely for another thread to perform a particular action is in this state.
5. Timed Waiting: A thread that is waiting for another thread to perform an action for up to a specified waiting time is in this state.
6. Terminated: The thread finishes execution.

Read more about thread states here: Life cycle of threads

Example: Multithreading in Action

class Task1 extends Thread {
    public void run() {
        for (int i = 0; i < 5; i++) {
            System.out.println("Task 1 - Count: " + i);
        }
    }
}

class Task2 extends Thread {
    public void run() {
        for (int i = 0; i < 5; i++) {
            System.out.println("Task 2 - Count: " + i);
        }
    }
}

public class Main {
    public static void main(String[] args) {
        Task1 t1 = new Task1();
        Task2 t2 = new Task2();

        t1.start();
        t2.start();
    }
}

In this example, Task1 and Task2 run concurrently, and their outputs will interleave based on thread scheduling.

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Key Points

• Multithreading is not deterministic; thread execution order may vary.
• Proper synchronization is crucial for handling shared resources to avoid race conditions.
• Java provides the Executor framework for managing thread pools and improving scalability.

If you’d like, I can provide additional details on advanced multithreading concepts like thread pools, Callable, Future, or synchronized blocks.

Multitasking vs Multithreading vs Multiprocessing vs parallel processing

If you are new to java you may get confused among these terms as they are used quite frequently when we discuss multithreading. Let’s talk about them in brief.

Multitasking: Ability to execute more than one task at the same time is known as multitasking.

Multithreading: We already discussed about it. It is a process of executing multiple threads simultaneously. Multithreading is also known as Thread-based Multitasking.

Multiprocessing: It is same as multitasking, however in multiprocessing more than one CPUs are involved. On the other hand one CPU is involved in multitasking.

Parallel Processing: It refers to the utilization of multiple CPUs in a single computer system.

Thread priorities

• Thread priorities are the integers which decide how one thread should be treated with respect to the others.
• Thread priority decides when to switch from one running thread to another, process is called context switching
• A thread can voluntarily release control and the highest priority thread that is ready to run is given the CPU.
• A thread can be preempted by a higher priority thread no matter what the lower priority thread is doing. Whenever a higher priority thread wants to run it does.
• To set the priority of the thread setPriority() method is used which is a method of the class Thread Class.
• In place of defining the priority in integers, we can use MIN_PRIORITY, NORM_PRIORITY or MAX_PRIORITY.

Methods: isAlive() and join()

• In all the practical situations main thread should finish last else other threads which have spawned from the main thread will also finish.
• To know whether the thread has finished we can call isAlive() on the thread which returns true if the thread is not finished.
• Another way to achieve this by using join() method, this method when called from the parent thread makes parent thread wait till child thread terminates.
• These methods are defined in the Thread class.
• We have used isAlive() method in the above examples too.

Inter-thread Communication

We have few methods through which java threads can communicate with each other. These methods are wait(), notify(), notifyAll(). All these methods can only be called from within a synchronized method.
1) To understand synchronization java has a concept of monitor. Monitor can be thought of as a box which can hold only one thread. Once a thread enters the monitor all the other threads have to wait until that thread exits the monitor.
2) wait()  tells the calling thread to give up the monitor and go to sleep until some other thread enters the same monitor and calls notify().
3) notify() wakes up the first thread that called wait() on the same object.
notifyAll() wakes up all the threads that called wait() on the same object. The highest priority thread will run first.