ExecutorService of java.util.concurrent Tutorial

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.

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

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

 

ExecutorService Implementations

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

 

Creating an ExecutorService

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

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 anExecutorService:

  • 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.

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 a Callable for that (explained in the following sections).

 

submit(Runnable)

The submit(Runnable) method also takes a Runnable implementation, but returns a Future object. This Future object can be used to check if theRunnable as finished executing.

Here is a 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. TheRunnable.run() method cannot return a result.

The Callable‘s result can be obtained via the Future object returned by thesubmit(Callable) method. Here is a code 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 Futureobjects 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 a Future 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();

 

Closing an ExecutorService

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 activeExexutorService in your application. The active threads inside thisExecutorService prevents the JVM from shutting down.

To terminate the threads inside the ExecutorService you call its shutdown()method. The ExecutorService will not shut down immediately, but it will no longer accept new tasks, and once all threads have finished current tasks, theExecutorService 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 theshutdownNow() 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.

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

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