Why should we use run instead of start to launch a thread?

In general, you should not use run() instead of start() to launch a thread in most cases. The start() method is specifically designed to launch a new thread, while run() simply executes the code in the current thread. Here's a detailed explanation of why start() is the preferred method for launching a thread and why using run() can lead to incorrect behavior:

Difference Between run() and start()

• start(): This method creates a new thread and executes the run() method in that new thread. The system manages the creation of the new thread and schedules it for execution.
• run(): This method is just a normal method and does not create a new thread. If you call run() directly, it will execute the code in the current thread rather than in a new, separate thread.

Why Use start() Instead of run()?

Here are the key reasons why you should use start() to launch a thread rather than run():

1. Creating a New Thread

When you use start(), the system creates a new thread, allowing the run() method to execute concurrently with the rest of the program. This is the primary reason for using multithreading in the first place—executing tasks in parallel.

• Using start() Example:

  class MyThread extends Thread {
      public void run() {
          System.out.println("Thread is running.");
      }
  }
  
  public class Main {
      public static void main(String[] args) {
          MyThread t = new MyThread();
          t.start();  // Creates a new thread and runs the code in parallel
      }
  }

  Output:

  Thread is running.
  

  In this example, start() creates a new thread, and the code inside the run() method executes concurrently.

• Using run() Example (Incorrect for Thread Creation):

  class MyThread extends Thread {
      public void run() {
          System.out.println("Thread is running.");
      }
  }
  
  public class Main {
      public static void main(String[] args) {
          MyThread t = new MyThread();
          t.run();  // No new thread is created, and the code runs in the main thread
      }
  }

  Output:

  Thread is running.
  

  In this case, the code inside the run() method runs, but it runs in the main thread, not in a new thread. This defeats the purpose of using threads because no parallelism is achieved.

2. Concurrency and Parallelism

The main advantage of multithreading is to achieve concurrency, where multiple threads run simultaneously and independently of each other. Calling run() directly does not create concurrency because it runs synchronously within the same thread.

• Example: If you need to perform two time-consuming tasks simultaneously (e.g., downloading files and processing data), calling start() allows these tasks to run in parallel, reducing overall execution time.

• Using start() with Multiple Threads:

  class Task extends Thread {
      private String name;
      
      public Task(String name) {
          this.name = name;
      }
  
      public void run() {
          for (int i = 1; i <= 5; i++) {
              System.out.println(name + " is running: " + i);
          }
      }
  }
  
  public class Main {
      public static void main(String[] args) {
          Task t1 = new Task("Task 1");
          Task t2 = new Task("Task 2");
  
          t1.start();  // Runs Task 1 in a new thread
          t2.start();  // Runs Task 2 in a separate thread
      }
  }

  Output (simultaneous execution):

  Task 1 is running: 1
  Task 2 is running: 1
  Task 1 is running: 2
  Task 2 is running: 2
  ...
  

  Here, Task 1 and Task 2 execute concurrently, achieving parallelism.

3. Avoid Blocking the Main Thread

By calling run() directly, you block the main thread because it waits for the run() method to finish executing before proceeding to the next task. This defeats the purpose of multithreading, where the goal is to allow multiple tasks to run simultaneously without one blocking the other.

• Using run() (Blocking Behavior): When you use run(), the main thread will be occupied by the method’s execution and will not be able to execute other code until the run() method completes.

4. Thread Life Cycle Management

Calling start() begins the life cycle of a thread, allowing it to transition through various thread states (e.g., NEW, RUNNABLE, BLOCKED, TERMINATED). This is critical for thread scheduling and management by the operating system.

• start() Method Behavior:

  • When you call start(), the thread transitions from the NEW state to the RUNNABLE state, and the operating system schedules it for execution on a CPU core. The thread’s life cycle is properly managed and handled by the OS.

• run() Method Behavior:

  • When you call run() directly, the thread remains in the NEW state, and the method simply executes like any normal method. There’s no thread life cycle management involved.

5. Achieving True Asynchronous Execution

If you call run() instead of start(), the execution remains synchronous, meaning the tasks are executed one after the other in the same thread. However, calling start() allows asynchronous execution, meaning multiple threads can run independently, without waiting for others to finish.

• Using start() for Asynchronous Execution: Threads will run independently, allowing tasks that don’t depend on each other to complete faster by utilizing system resources efficiently (e.g., CPU cores).

Conclusion

You should use start() instead of run() to launch a thread because start() creates a new thread and runs the run() method in that new thread. This allows for parallel execution and enables the true benefits of multithreading, such as improved performance, concurrency, and better resource utilization. Calling run() directly, on the other hand, runs the method synchronously within the current thread, providing no concurrency or parallelism benefits and leading to inefficient task execution.