idea-plugin-strayCompletableFuture — Stray CompletableFuture Detector

An IntelliJ IDEA plugin that highlights stray (uncollected) CompletableFuture calls in Java: an expression that produces a java.util.concurrent.CompletableFuture whose result is discarded as a statement. Such futures are never awaited, chained, returned, or stored, so exceptions they complete with are silently swallowed and the async work goes unobserved.

CompletableFuture.runAsync(this::work);   // ⚠ flagged — result discarded
service.fetchAsync(id);                   // ⚠ flagged — returns CompletableFuture, discarded
future.thenApply(x -> x + 1);             // ⚠ flagged — new future discarded

CompletableFuture<Void> f = CompletableFuture.runAsync(this::work); // ok — stored
return service.fetchAsync(id);                                      // ok — returned
allFutures.add(service.fetchAsync(id));                             // ok — passed on
service.fetchAsync(id).join();                                      // ok — awaited

How it works

StrayCompletableFutureInspection is a LocalInspectionTool (registered as localInspection in plugin.xml). It visits every PsiExpressionStatement — the PSI node for an expression whose value is thrown away — and reports it when the expression is a method call or constructor whose type is CompletableFuture or a subtype (checked with InheritanceUtil.isInheritor). Any use that collects the future (assignment, return, passing as an argument, method chaining) is a different PSI shape, so it is not flagged.

Suppress a deliberate fire-and-forget call with //noinspection StrayCompletableFuture or via the standard quick-fix menu.

Project layout

build.gradle.kts                                            Gradle build (IntelliJ Platform plugin 1.17.4)
src/main/java/com/example/cfd/StrayCompletableFutureInspection.java
src/main/resources/META-INF/plugin.xml
src/main/resources/inspectionDescriptions/StrayCompletableFuture.html
samples/StrayFutureSamples.java                             Manual test cases

Building

Requirements: JDK 17 (the 2023.2 platform is compiled for Java 17) and network access to download the platform.

This scaffold ships without the gradle/wrapper/gradle-wrapper.jar binary. Create it once with a system Gradle, then build:

gradle wrapper --gradle-version 8.7   # one-time, if you don't already have the wrapper jar
./gradlew buildPlugin                 # produces build/distributions/idea-plugin-strayCompletableFuture-0.1.0.zip

Alternatively, just open the folder in IntelliJ IDEA — it provisions Gradle automatically, so no wrapper jar is needed.

Trying it

./gradlew runIde

launches a sandbox IDE with the plugin installed. Open samples/StrayFutureSamples.java in it and confirm the four flagged cases show warnings and the "ok" cases do not.

Installing into your IDE

Settings → Plugins → ⚙ → Install Plugin from Disk… and pick the built build/distributions/idea-plugin-strayCompletableFuture-0.1.0.zip.