Difference between a Thread and an Executor in Java

Even though both Thread and Executor, both are used to execute some code in parallel, there are some key differences between them. The main difference between a Thread and an Executor in Java is that it later provides a thread pool in Java. Along with several concurrency utilities like CountDownLatch, CyclicBarrier, Semaphore, FutureTask, Callable interface, and Conditions, JDK 5 also introduced a built-in thread pool, which provides a set of working threads to run your code in parallel. Since creating, starting, and running a thread is a time-consuming and expensive operation, many Java applications create a spool of thread at start-up and leverage that for executing the task in parallel until Java introduced the built-in thread pool.

This thread-pool is known as the Executor framework which relieved Java application developers from the responsibility of creating and managing threads

The JDK 1.5 Executor framework is a combination of Executor, Executors, and ExecutorService interface to provide a fully functional, feature-rich thread pool in Java.  By the way that was the fundamental difference between the Thread and Executor concept in Java, let's see a couple of more details about Thread and Executor to answer this question better.

Thread vs Executor in Java 

As I said, a Thread is used to run your code in parallel and you can create and start your own thread either by extending java.lang.Thread class or implementing java.lang.Runnable interface. Though both approaches work well in small applications, they have their pros and cons, which you can see here. 

On the other hand, Executor is an interface which also provides parallel execution, but via a thread pool, which is more suitable for large Java application.

1) The first and foremost difference between Thread and Executor is that java.lang.Thread is a class in Java while java.util.concurrent.Executor is an interface.


2) The Executor concept is actually an abstraction over parallel computation. It allows concurrent code to be run in a managed way. On the other hand, Thread is a concrete way to run the code in parallel.

3) The third difference between an Executor and a Thread class is that the former decouples a task (the code which needs to be executed in parallel) from execution, while in the case of a Thread, both task and execution are tightly coupled. 

4) The Executor concept allows your task is to be executed by a worker thread from the thread pool, while Thread itself execute your task.

5) Executor provides a execute() method which accepts a Runnable task, while Thread accepts the Runnable task on its constructor.

6) One more key difference between a Thread and an Executor is that a Thread can only execute one Runnable task but an Executor can execute any number of Runnable tasks.

7) In the case of Thread, the task is executed by the Thread which accepts Runnable instance, but in the case of Execution the command (a Runnable implementation) may be executed in a new thread, a pooled thread, or in the calling thread itself, depending upon the implementation of Executor interface.

8) In the case of a thread, it's the developer's responsibility to create and start the thread, but in the case of Executor, the framework will create and start threads for you. Though you can control the whole process by giving your implementation of the Executor interface. 

Though, with the improvements in ForkJoinPool in Java 7 and 8, you might want to use that instead of Executor.

7) Now, let's see an example of execution of a Runnable task via Executor and via Thread in Java:

import java.util.concurrent.Executor;
import java.util.concurrent.Executors;

public class Main {

  public static void main(String args[]) {

    Runnable task = new Runnable() {
      @Override
      public void run() {
        System.out.println("Task is executed by : "
            + Thread.currentThread().getName());
      }
    };

    Thread t = new Thread(task, "MY_THREAD");
    t.start();

    Executor e = Executors.newSingleThreadExecutor();
    e.execute(task);

  }
}

Output
Task is executed by MY_THREAD
Task is executed by pool-1-thread-1

The difference is quite clear that first is just a thread while later is a pool of threads.

It's worth noting that factory methods of Executors class e.g. newSingleThreadExecutor() return an ExecutorService, which is a sub-interface of Executor and also provides methods to accepts a Callable, terminate, or shut down the thread pool.


That's all about the difference between a Thread and an Executor in Java. You can see that even though both are related to the parallel execution of tasks they are a separate abstraction. A Thread represents something which is responsible for executing your code in parallel, while an Executor is an abstraction for concurrent task execution. 

Most importantly, Executor decouples the task to its execution which means an asynchronous execution is possible, but task and execution are tightly coupled in the case of Thread.

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