The Concurrency API introduces the concept of an ExecutorService as a higher level replacement for working with threads directly. Executors are capable of running asynchronous tasks and typically manage a pool of threads, so we don’t have to create new threads manually. All threads of the internal pool will be reused under the hood for revenant tasks, so we can run as many concurrent tasks as we want throughout the life-cycle of our application with a single executor service.

This is how the first thread-example looks like using executors:

ExecutorService executor = Executors.newSingleThreadExecutor();
executor.submit(() -> {
    String threadName = Thread.currentThread().getName();
    System.out.println(“Hello ” + threadName);
});

// => Hello pool-1-thread-1

The class Executors provides convenient factory methods for creating different kinds of executor services. In this sample we use an executor with a thread pool of size one.

The result looks similar to the above sample but when running the code you’ll notice an important difference: the java process never stops! Executors have to be stopped explicitly – otherwise they keep listening for new tasks.

An ExecutorService provides two methods for that purpose: shutdown() waits for currently running tasks to finish while shutdownNow() interrupts all running tasks and shut the executor down immediately.

This is the preferred way how I typically shutdown executors:

try {
    System.out.println(“attempt to shutdown executor”);
    executor.shutdown();
    executor.awaitTermination(5, TimeUnit.SECONDS);
}
catch (InterruptedException e) {
    System.err.println(“tasks interrupted”);
}
finally {
    if (!executor.isTerminated()) {
        System.err.println(“cancel non-finished tasks”);
    }
    executor.shutdownNow();
    System.out.println(“shutdown finished”);
}