Java Stream API Virtual-Threads-enabled Parallel Collectors - overcoming limitations of standard Parallel Streams

Parallel Collectors is a toolkit that eases parallel collection processing in Java using Stream API without the limitations imposed by standard Parallel Streams.

list.stream()
  .collect(parallel(i -> blockingOp(i), toList()))
    .orTimeout(1000, MILLISECONDS)
    .thenAcceptAsync(System.out::println, executor)
    .thenRun(() -> System.out.println("Finished!"));

They are:

• lightweight, defaulting to Virtual Threads (an alternative to Project Reactor for scenarios where a lighter solution is preferred)
• powerful (the combined power of Stream API and CompletableFutures, allowing for timeout specification, composition with other CompletableFutures, and asynchronous processing)
• configurable (flexibility with customizable Executors and parallelism levels)
• non-blocking (eliminates the need to block the calling thread while awaiting results)
• short-circuiting (if one of the operations raises an exception, remaining tasks will get interrupted)
• non-invasive (they are just custom implementations of Collector interface, no magic inside, zero-dependencies, no Stream API internals hacking)
• versatile (enables easy integration with existing Stream API Collectors)

Maven Dependencies

JDK 21+:

<dependency>
    <groupId>com.pivovarit</groupId>
    <artifactId>parallel-collectors</artifactId>
    <version>3.2.0</version>
</dependency>

JDK 8+:

<dependency>
    <groupId>com.pivovarit</groupId>
    <artifactId>parallel-collectors</artifactId>
    <version>2.6.1</version>
</dependency>

Gradle

JDK 21+:

compile 'com.pivovarit:parallel-collectors:3.2.0'`

JDK 8+:

compile 'com.pivovarit:parallel-collectors:2.6.1'`

Philosophy

Parallel Collectors are intentionally unopinionated, leaving responsibility to users for:

• Proper configuration of provided Executors and their lifecycle management
• Choosing appropriate parallelism levels
• Ensuring the tool is applied in the right context

Review the API documentation before deploying in production.

Basic API

The main entry point is the com.pivovarit.collectors.ParallelCollectors class - which follows the convention established by java.util.stream.Collectors and features static factory methods returning custom java.util.stream.Collector implementations spiced up with parallel processing capabilities.

By design, it's obligatory to supply a custom Executor instance and manage its lifecycle.

All parallel collectors are one-off and must not be reused.

Available Collectors:

• CompletableFuture<Stream<T>> parallel(Function) (uses Virtual Threads)

• CompletableFuture<Collection<T>> parallel(Function, Collector) (uses Virtual Threads)

• CompletableFuture<Stream<T>> parallel(Function, parallelism) (uses Virtual Threads)

• CompletableFuture<Collection<T>> parallel(Function, Collector, parallelism) (uses Virtual Threads)

• CompletableFuture<Stream<T>> parallel(Function, Executor, parallelism)

• CompletableFuture<Collection<T>> parallel(Function, Collector, Executor, parallelism)

• Stream<T> parallelToStream(Function) (uses Virtual Threads)

• Stream<T> parallelToOrderedStream(Function) (uses Virtual Threads)

• Stream<T> parallelToStream(Function, parallelism) (uses Virtual Threads)

• Stream<T> parallelToOrderedStream(Function, parallelism) (uses Virtual Threads)

• Stream<T> parallelToStream(Function, Executor, parallelism)

• Stream<T> parallelToOrderedStream(Function, Executor, parallelism)

Batching Collectors

By default, all ExecutorService threads compete for each task separately - which results in a basic form of work-stealing, which, unfortunately, is not free, but can decrease processing time for subtasks with varying processing times.

However, if the processing time for all subtasks is similar, it might be better to distribute tasks in batches to avoid excessive contention.

Batching alternatives are available under the ParallelCollectors.Batching namespace.

Leveraging CompletableFuture

Parallel Collectors™ expose results wrapped in CompletableFuture instances which provides great flexibility and the possibility of working with them in a non-blocking fashion:

CompletableFuture<List<String>> result = list.stream()
  .collect(parallel(i -> foo(i), toList(), executor));

This makes it possible to conveniently apply callbacks and compose with other CompletableFutures:

list.stream()
  .collect(parallel(i -> foo(i), toSet(), executor))
  .thenAcceptAsync(System.out::println, otherExecutor)
  .thenRun(() -> System.out.println("Finished!"));

Or just join() if you just want to block the calling thread and wait for the result:

List<String> result = list.stream()
  .collect(parallel(i -> foo(i), toList(), executor))
  .join();

What's more, since JDK9, you can even provide your own timeout easily.

Examples

1. Apply i -> foo(i) in parallel on a custom Executor and collect to List

Executor executor = ...

CompletableFuture<List<String>> result = list.stream()
  .collect(parallel(i -> foo(i), toList(), executor));

2. Apply i -> foo(i) in parallel on a custom Executor with max parallelism of 4 and collect to Set

Executor executor = ...

CompletableFuture<Set<String>> result = list.stream()
  .collect(parallel(i -> foo(i), toSet(), executor, 4));

3. Apply i -> foo(i) in parallel on a custom Executor and collect to LinkedList

Executor executor = ...

CompletableFuture<List<String>> result = list.stream()
  .collect(parallel(i -> foo(i), toCollection(LinkedList::new), executor));

4. Apply i -> foo(i) in parallel on a custom Executor and stream results in completion order

Executor executor = ...

list.stream()
  .collect(parallelToStream(i -> foo(i), executor))
  .forEach(i -> ...);

5. Apply i -> foo(i) in parallel on a custom Executor and stream results in the original order

Executor executor = ...

list.stream()
  .collect(parallelToOrderedStream(i -> foo(i), executor))
  .forEach(i -> ...);

Rationale

Stream API is a great tool for collection processing, especially if you need to parallelize the execution of CPU-intensive tasks, for example:

public static void parallelSetAll(int[] array, IntUnaryOperator generator) {
    Objects.requireNonNull(generator);
    IntStream.range(0, array.length).parallel().forEach(i -> { array[i] = generator.applyAsInt(i); });
}

However, Parallel Streams execute tasks on a shared ForkJoinPool instance.

Unfortunately, it's not the best choice for running blocking operations even when using ManagedBlocker - as explained here by Tagir Valeev - this could easily lead to the saturation of the common pool, and to a performance degradation of everything that uses it.

For example:

List<String> result = list.parallelStream()
  .map(i -> foo(i)) // runs implicitly on ForkJoinPool.commonPool()
  .toList();

To avoid such problems, the solution is to isolate blocking tasks and run them on a separate thread pool... but there's a catch.

Sadly, Streams can only run parallel computations on the common ForkJoinPool, which effectively restricts their applicability to CPU-bound jobs.

However, there's a trick that allows running parallel Streams in a custom FJP instance... but it's not considered reliable (and can still induce oversubscription issues while competing with the common pool for resources)

Note, however, that this technique of submitting a task to a fork-join pool to run the parallel stream in that pool is an implementation "trick" and is not guaranteed to work. Indeed, the threads or thread pool that is used for the execution of parallel streams is unspecified. By default, the common fork-join pool is used, but in different environments, different thread pools might end up being used.

Says Stuart Marks on StackOverflow.

Not even mentioning that this approach was seriously flawed before JDK-10 - if a Stream was targeted towards another pool, splitting would still need to adhere to the parallelism of the common pool and not the one of the targeted pool [JDK8190974].

Dependencies

None - the library is implemented using core Java libraries.

Limitations

• Upstream Stream is always evaluated as a whole, even if the following operation is short-circuiting. This means that none of these should be used to work with infinite streams. The design of the Collector API imposes this limitation.

• Never use Parallel Collectors with Executors with RejectedExecutionHandler that discards tasks - this might result in a deadlock.

Good Practices

• Consider providing reasonable timeouts for CompletableFutures in order to not block for unreasonably long in case when something bad happens (how-to)
• Name your thread pools - it makes debugging easier
• Limit the size of a working queue of your thread pool (source)
• Limit the level of parallelism (source)
• A no-longer-used ExecutorService should be shut down to allow reclamation of its resources
• Keep in mind that CompletableFuture#then(Apply|Combine|Consume|Run|Accept) might be executed by the calling thread. If this is not suitable, use CompletableFuture#then(Apply|Combine|Consume|Run|Accept)Async instead, and provide a custom Executor instance.

Words of Caution

While Parallel Collectors and Virtual Threads make parallelization easy, it doesn't always mean it's the best choice. Platform threads are resource-intensive, and parallelism comes with a cost.

Before opting for parallel processing, consider addressing the root cause through alternatives like DB-level JOIN statements, batching, data reorganization, or... simply selecting a more suitable API method.

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See CHANGELOG.MD for a complete version history.