When we talk about Concurrency topic in Java, for sure you will come across to this term, Future. In this article, we are going to learn about what is Future, and how can we use it.
java.util.concurrent.Future
Future as the name itself implies, it is a container that store a “future” result which does not exist yet. It is a class that represents a further result of an asynchronous computation. In a long running or computation heavy task, the use of asynchronous processing and Future can be super useful. We can encapsulate the task in Future and run some other process while waiting for the task to be completed.
Implement Future
In this example we are going to stimulate the long running task by creating a task that sleeps for 1 second and return some string.
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public static Future<String> longRunningTask() {
ExecutorService executorService = Executors.newSingleThreadExecutor();
return executorService.submit(() -> {
Thread.sleep(1000);
return "Value from future";
});
}
Consume Future
There is two interesting API in Future class: isDone() and get()
isDone()
Again, the name itself implies, isDone() will tell us if the task has completed. It will return true if completed, otherwise false.
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while(!future.isDone()) {
System.out.println("Waiting future task to be completed");
Thread.sleep(500);
}
get()
This method will return result from the completed task and will block the thread execution if the task is not completed.
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String result = future.get();
System.out.println(result);
Besides, the get() also have an overloaded method that allow us to specify timeout . If the task does not return before the specified timeout, a TimeoutException will be thrown.
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String result = future.get(200, TimeUnit.MILLISECONDS);
System.out.println(result);
Example:
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Exception in thread "main" java.util.concurrent.TimeoutException
at java.util.concurrent.FutureTask.get(FutureTask.java:205)
at com.example.jmeter.test.SimpleFutureExample.main(SimpleFutureExample.java:23)
Cancel Future
There is some case, we started the task and for some reason, we do not need the result anymore. We can simply cancel the task by calling cance(boolean) method. This method does not guarantee the task to be successfully cancelled the task can be already completed / cancelled / cannot be cancelled. The cance(boolean) will only attempt to cancel the task execution, and return true if successfully cancel, otherwise false. The method takes in a boolean argument which serve as a flag to let the program know should the task be interrupted or it is allowed to be completed. After the task is successfully cancelled, the subsequent call from get() will throw a CancellationException. Thus, calling the method isCancelled() can prevent us getting the exception.
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future.cancel(true);
String result = future.get();
System.out.println(result);
Example:
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Exception in thread "main" java.util.concurrent.CancellationException
at java.util.concurrent.FutureTask.report(FutureTask.java:121)
at java.util.concurrent.FutureTask.get(FutureTask.java:192)
at com.example.jmeter.test.MainClass.main(MainClass.java:25)
Conclusion
In this article, we discussed the usage of Future class.
Check out the source code here.