63-parallel-stream

Example 63 — Parallel Stream

Demonstrates ParallelStreamDetector catching a parallelStream() that feeds results into a shared non-thread-safe ArrayList, causing ConcurrentModificationException or silently lost updates.

The Problem

parallelStream() distributes work across multiple threads from the common ForkJoinPool. If those threads all write to the same non-thread-safe collection, the result is undefined behavior:

List<String> results = new ArrayList<>();   // NOT thread-safe

orders.parallelStream()
      .forEach(o -> results.add(process(o))); // concurrent add() → data races

ArrayList.add() is not atomic. Concurrent writes can corrupt the internal array, lose elements, or throw ConcurrentModificationException. Use Collectors.toList() or a ConcurrentLinkedQueue instead.

OrderAggregator.processOrders() has exactly this bug: it accumulates into a shared ArrayList field from a parallelStream().forEach().

How to Reproduce

Remove the @Disabled annotation from test_concurrent_detectsBug in OrderAggregatorTest. The ParallelStreamDetector will report the non-thread-safe side effect inside the parallel stream.

@AsyncTest(threads = 8, invocations = 50, includes = DetectorType.PARALLEL_STREAMS)
void test_concurrent_detectsBug() { ... }

Run with Maven:

mvn test

Or with Gradle:

./gradlew test