Waiting for background tasks to finish using the CompletableFuture class in Java

Introduction

In this post we saw how to wait for a number background tasks to finish using the CountDownLatch class. The starting point for the discussion was the following situation:

Imagine that you execute a number of long running methods. Also, let’s say that the very last time consuming process depends on the previous processes, let’s call them prerequisites. The dependence is “sequential” meaning that the final stage should only run if the prerequisites have all completed and returned. The first implementation may very well be sequential where the long running methods are called one after the other and each of them blocks the main thread.

However, in case the prerequisites can be executed independently then there’s a much better solution: we can execute them in parallel instead. Independence in this case means that prerequisite A doesn’t need any return value from prerequisite B in which case parallel execution of A and B is not an option.

In this post we’ll look at an alternative solution using the CompletableFuture class. It is way more versatile than CountDownLatch which is really only sort of like a simple lock object. CompletableFuture offers a wide range of possibilities to organise your threads with a fluent API. Here we’ll start off easy with a simple application of this class.

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Sharing primitives across threads in Java using atomic objects

Threading and parallel execution are popular choices when making applications more responsive and resource-efficient. Various tasks are carried out on separate threads where they either produce some result relevant to the main thread or just run in the background “unnoticed”. Often these tasks work autonomously meaning they have their own set of dependencies and variables. That is they do not interfere with a resource that is common to 2 or more threads.

However, that’s not always the case. Imagine that multiple threads are trying to update the same primitive like an integer counter. They perform some action and then update this counter. In this post we’ll see what can go wrong.

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