What are the advantages of using an ExecutorService?

Question

What is the advantage of using ExecutorService over running threads passing a Runnable into the Thread constructor?

Answer 1

ExecutorService abstracts away many of the complexities associated with the lower-level abstractions like raw Thread. It provides mechanisms for safely starting, closing down, submitting, executing, and blocking on the successful or abrupt termination of tasks (expressed as Runnable or Callable).

From JCiP, Section 6.2, straight from the horse's mouth:

Executor may be a simple interface, but it forms the basis for a flexible and powerful framework for asynchronous task execution that supports a wide variety of task execution policies. It provides a standard means of decoupling task submission from task execution, describing tasks as Runnable. The Executor implementations also provide lifecycle support and hooks for adding statistics gathering, application management, and monitoring. ... Using an Executor is usually the easiest path to implementing a producer-consumer design in your application.

Rather than spending your time implementing (often incorrectly, and with great effort) the underlying infrastructure for parallelism, the j.u.concurrent framework allows you to instead focus on structuring tasks, dependencies, potential parallelism. For a large swath of concurrent applications, it is straightforward to identify and exploit task boundaries and make use of j.u.c, allowing you to focus on the much smaller subset of true concurrency challenges which may require more specialized solutions.

Also, despite the boilerplate look and feel, the Oracle API page summarizing the concurrency utilities includes some really solid arguments for using them, not least:

Developers are likely to already understand the standard library classes, so there is no need to learn the API and behavior of ad-hoc concurrent components. Additionally, concurrent applications are far simpler to debug when they are built on reliable, well-tested components.

This question on SO asks about a good book, to which the immediate answer is JCiP. If you haven't already, get yourself a copy. The comprehensive approach to concurrency presented there goes well beyond this question, and will save you a lot of heartache in the long run.

Answer 2

An advantage I see is in managing/scheduling several threads. With ExecutorService, you don't have to write your own thread manager which can be plagued with bugs. This is especially useful if your program needs to run several threads at once. For example you want to execute two threads at a time, you can easily do it like this:

ExecutorService exec = Executors.newFixedThreadPool(2);

exec.execute(new Runnable() {
  public void run() {
    System.out.println("Hello world");
  }
});

exec.shutdown();

The example may be trivial, but try to think that the "hello world" line consists of a heavy operation and you want that operation to run in several threads at a time in order to improve your program's performance. This is just one example, there are still many cases that you want to schedule or run several threads and use ExecutorService as your thread manager.

For running a single thread, I don't see any clear advantage of using ExecutorService.