Getting a result from a parallel task in Java using CompletableFuture

In this post we saw how to start several processes on different threads using the CompletableFuture class. The example concentrated on methods with no return value. We let CompletableFuture finish the tasks in parallel before continuing with another process.

In this post we’ll see a usage of CompletableFuture for functions with a return value. We’ll reuse several elements we saw in the post that concentrated on the Future class.

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Summary of thread-safe collections in .NET

The System.Collections.Concurrent namespace has 4 thread-safe collections that you can use in multi-threaded applications. The starting point is that you have a multi-threaded app where the same collection needs to be accessed by different threads. In that case the well-know collection types, like HashSet, List, Dictionary etc. simply won’t be enough.

If many different threads have access to the same resource then there’s no guarantee on the state of that resource in the moment a thread accesses it in some way: deletion, lookup, insertion or modification. Another thread may have accessed the same resource just milliseconds before that and the other thread will access the resource under the wrong assumptions. You’ll end up with buggy code with unpredictable results and ad-hoc fixes and patches that probably won’t solve the root of the problem.

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

Imagine the situation where 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 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 examine this situation and see how to implement it in Java using the CountDownLatch class.

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Sharing numeric values across threads using Java 8 LongAdder

In this post we saw how to share primitive values across threads using the various atomic objects in the java.util.concurrent.atomic package. The example code demonstrated the AtomicInteger object which is the thread-safe variant of a “normal” integer. Mathematical operations like adding a value to an integer are carried out atomically for that object. This means that the low-level instructions involved in adding two integers are carried out as one unit without the risk of another interfering thread. The same package includes atomic versions of other primitive values such as AtomicBoolean or AtomicLong.

In this post we’ll take a quick look at an addition in Java 8 relevant to sharing integers, longs and doubles.

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Using immutable collections for thread-safe read-only operations in .NET

Sometimes you have a scenario where multiple threads need to read from the same shared collection. We’ve looked at the 4 concurrent, i.e. thread-safe collection types on this blog that are available in the System.Collections.Concurrent namespace. They can be safely used for both concurrent writes and reads.

However, if your threads strictly only need to read from a collection then there’s another option. There are collections in the System.Collections.Immutable namespace that are immutable, i.e. read-only and have been optimisied for concurrent read operations.

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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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