ExecutorService - Java

The java.util.concurrent.ExecutorService interface represents an asynchronous execution mechanism which is capable of executing tasks in the background. An ExecutorService is thus very similar to a thread pool. In fact, the implementation of ExecutorService present in the java.util.concurrentpackage is a thread pool implementation.

ExecutorService Example

Here is a simple Java ExectorService example:

ExecutorService executorService = Executors.newFixedThreadPool(10);

executorService.execute(new Runnable() {
    public void run() {
        System.out.println("Asynchronous task");
    }
});

executorService.shutdown();

First an ExecutorService is created using the newFixedThreadPool() factory method. This creates a thread pool with 10 threads executing tasks.
Second, an anonymous implementation of the Runnable interface is passed to the execute() method. This causes the Runnable to be executed by one of the threads in the ExecutorService.

Task Delegation

Here is a diagram illustrating a thread delegating a task to an ExecutorService for asynchronous execution:

A thread delegating a task to an ExecutorService for asynchronous execution.

Once the thread has delegated the task to the ExecutorService, the thread continues its own execution independent of the execution of that task.

ExecutorService Implementations

Since ExecutorService is an interface, you need to its implementations in order to make any use of it. TheExecutorService has the following implementation in the java.util.concurrent package:

• ThreadPoolExecutor
• ScheduledThreadPoolExecutor

Creating an ExecutorService

How you create an ExecutorService depends on the implementation you use. However, you can use theExecutors factory class to create ExecutorService instances too. Here are a few examples of creating anExecutorService:

ExecutorService executorService1 = Executors.newSingleThreadExecutor();

ExecutorService executorService2 = Executors.newFixedThreadPool(10);

ExecutorService executorService3 = Executors.newScheduledThreadPool(10);

ExecutorService Usage

There are a few different ways to delegate tasks for execution to an ExecutorService:

• execute(Runnable)
• submit(Runnable)
• submit(Callable)
• invokeAny(...)
• invokeAll(...)

I will take a look at each of these methods in the following sections.

execute(Runnable)

The execute(Runnable) method takes a java.lang.Runnable object, and executes it asynchronously. Here is an example of executing a Runnable with an ExecutorService:

ExecutorService executorService = Executors.newSingleThreadExecutor();

executorService.execute(new Runnable() {
    public void run() {
        System.out.println("Asynchronous task");
    }
});

executorService.shutdown();

There is no way of obtaining the result of the executed Runnable, if necessary. You will have to use aCallable for that (explained in the following sections).

submit(Runnable)

The submit(Runnable) method also takes a Runnable implementation, but returns a Future object. ThisFuture object can be used to check if the Runnable as finished executing.
Here is a ExecutorService submit() example:

Future future = executorService.submit(new Runnable() {
    public void run() {
        System.out.println("Asynchronous task");
    }
});

future.get();  //returns null if the task has finished correctly.

submit(Callable)

The submit(Callable) method is similar to the submit(Runnable) method except for the type of parameter it takes. The Callable instance is very similar to a Runnable except that its call() method can return a result. The Runnable.run() method cannot return a result.
The Callable's result can be obtained via the Future object returned by the submit(Callable) method. Here is an ExecutorService Callable example:

Future future = executorService.submit(new Callable(){
    public Object call() throws Exception {
        System.out.println("Asynchronous Callable");
        return "Callable Result";
    }
});

System.out.println("future.get() = " + future.get());

The above code example will output this:

Asynchronous Callable
future.get() = Callable Result

invokeAny()

The invokeAny() method takes a collection of Callable objects, or subinterfaces of Callable. Invoking this method does not return a Future, but returns the result of one of the Callable objects. You have no guarantee about which of the Callable's results you get. Just one of the ones that finish.
If one of the tasks complete (or throws an exception), the rest of the Callable's are cancelled.
Here is a code example:

ExecutorService executorService = Executors.newSingleThreadExecutor();

Set<Callable<String>> callables = new HashSet<Callable<String>>();

callables.add(new Callable<String>() {
    public String call() throws Exception {
        return "Task 1";
    }
});
callables.add(new Callable<String>() {
    public String call() throws Exception {
        return "Task 2";
    }
});
callables.add(new Callable<String>() {
    public String call() throws Exception {
        return "Task 3";
    }
});

String result = executorService.invokeAny(callables);

System.out.println("result = " + result);

executorService.shutdown();

This code example will print out the object returned by one of the Callable's in the given collection. I have tried running it a few times, and the result changes. Sometimes it is "Task 1", sometimes "Task 2" etc.

invokeAll()

The invokeAll() method invokes all of the Callable objects you pass to it in the collection passed as parameter. The invokeAll() returns a list of Future objects via which you can obtain the results of the executions of each Callable.
Keep in mind that a task might finish due to an exception, so it may not have "succeeded". There is no way on aFuture to tell the difference.
Here is a code example:

ExecutorService executorService = Executors.newSingleThreadExecutor();

Set<Callable<String>> callables = new HashSet<Callable<String>>();

callables.add(new Callable<String>() {
    public String call() throws Exception {
        return "Task 1";
    }
});
callables.add(new Callable<String>() {
    public String call() throws Exception {
        return "Task 2";
    }
});
callables.add(new Callable<String>() {
    public String call() throws Exception {
        return "Task 3";
    }
});

List<Future<String>> futures = executorService.invokeAll(callables);

for(Future<String> future : futures){
    System.out.println("future.get = " + future.get());
}

executorService.shutdown();

ExecutorService Shutdown

When you are done using the ExecutorService you should shut it down, so the threads do not keep running.
For instance, if your application is started via a main() method and your main thread exits your application, the application will keep running if you have an active ExexutorService in your application. The active threads inside this ExecutorService prevents the JVM from shutting down.
To terminate the threads inside the ExecutorService you call its shutdown() method. TheExecutorService will not shut down immediately, but it will no longer accept new tasks, and once all threads have finished current tasks, the ExecutorService shuts down. All tasks submitted to the ExecutorServicebefore shutdown() is called, are executed.
If you want to shut down the ExecutorService immediately, you can call the shutdownNow() method. This will attempt to stop all executing tasks right away, and skips all submitted but non-processed tasks. There are no guarantees given about the executing tasks. Perhaps they stop, perhaps the execute until the end. It is a best effort attempt.

Ref: http://tutorials.jenkov.com/java-util-concurrent/executorservice.html