Introduce `EmptyReactivePublisher` bug checker #1560

mohamedsamehsalah · 2025-02-21T13:12:24Z

Suggested commit message

Introduce `EmptyReactivePublisher` bug checker

mohamedsamehsalah · 2025-02-21T13:13:57Z

...one-contrib/src/test/java/tech/picnic/errorprone/bugpatterns/EmptyReactivePublisherTest.java

+            "    Mono.just(16).subscribe(null, t -> {});",
+            "",
+            "    // BUG: Diagnostic contains:",
+            "    Mono.just(17).then().subscribe(i -> {});",


This is the statement that inspired this bug checker; this consumer is never called.

sonarqubecloud · 2025-02-21T13:15:04Z

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
87.5% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

github-actions · 2025-02-21T13:15:45Z

Surviving mutants in this change: 8
Killed mutants in this change: 14

class	surviving	killed
🧟`tech.picnic.errorprone.bugpatterns.EmptyReactivePublisher`	8	14

Mutation testing report by Pitest. Review any surviving mutants by inspecting the line comments under Files changed.

github-actions · 2025-12-03T14:20:16Z

Suggested commit message:

Introduce `EmptyReactivePublisher` bug checker (#1560)

Add `EmptyReactivePublisher` to warn on vacuous Reactor operators
invoked on empty `Mono`/`Flux` instances or empty subscriptions.

github-actions · 2025-12-03T14:29:03Z

...r-prone-contrib/src/main/java/tech/picnic/errorprone/bugpatterns/EmptyReactivePublisher.java

+              String.format(
+                  "Operator `%s` on an empty `Mono`s is a no-op",
+                  ASTHelpers.getSymbol(tree).getSimpleName()))


[Error Prone] _{reported by reviewdog 🐶}

Suggested change

String.format(

"Operator `%s` on an empty `Mono`s is a no-op",

ASTHelpers.getSymbol(tree).getSimpleName()))

"Operator `%s` on an empty `Mono`s is a no-op"

.formatted(ASTHelpers.getSymbol(tree).getSimpleName()))

Will apply.

github-actions · 2025-12-03T14:51:47Z

Surviving mutants in this change: 8
Killed mutants in this change: 18

class	surviving	killed
🧟`tech.picnic.errorprone.bugpatterns.EmptyReactivePublisher`	8	18

Mutation testing report by Pitest. Review any surviving mutants by inspecting the line comments under Files changed.

rickie

Pushing a commit with many small suggestions. PTAL and feel free to tweak 😄.

Very elegant PR. The testing makes refactoring easy 🚀.

rickie · 2025-12-03T14:31:08Z

...r-prone-contrib/src/main/java/tech/picnic/errorprone/bugpatterns/EmptyReactivePublisher.java

+
+/**
+ * A {@link BugChecker} that flags {@link Publisher} operations that are known to be vacuous, given
+ * that they are invoked on a {@link Mono} or {@link Flux} that do not emit next signals.


Suggested change

* that they are invoked on a {@link Mono} or {@link Flux} that do not emit next signals.

* that they are invoked on a {@link Mono} or {@link Flux} that does not emit next signals.

Right?

rickie · 2025-12-03T14:31:27Z

...r-prone-contrib/src/main/java/tech/picnic/errorprone/bugpatterns/EmptyReactivePublisher.java

+public final class EmptyReactivePublisher extends BugChecker
+    implements MethodInvocationTreeMatcher {
+  private static final long serialVersionUID = 1L;
+


Suggested change

rickie · 2025-12-05T08:11:13Z

...r-prone-contrib/src/main/java/tech/picnic/errorprone/bugpatterns/EmptyReactivePublisher.java

+
+  private static final Matcher<ExpressionTree> SUBSCRIBE =
+      instanceMethod()
+          .onDescendantOf(Suppliers.typeFromString("org.reactivestreams.Publisher"))


Suggested change

.onDescendantOf(Suppliers.typeFromString("org.reactivestreams.Publisher"))

.onDescendantOf(type("org.reactivestreams.Publisher"))

Also works :) ?

rickie · 2025-12-05T08:18:29Z

...r-prone-contrib/src/main/java/tech/picnic/errorprone/bugpatterns/EmptyReactivePublisher.java

+      Suppliers.typeFromString("reactor.core.publisher.Flux");
+  private static final Matcher<Tree> CONSUMER =
+      isSubtypeOf(type(Consumer.class.getCanonicalName()));
+  private static final Supplier<Type> VOID = type(Void.class.getCanonicalName());


There is one in Error Prone itself we can use :)

rickie · 2025-12-05T08:18:47Z

...r-prone-contrib/src/main/java/tech/picnic/errorprone/bugpatterns/EmptyReactivePublisher.java

+
+    if (SUBSCRIBE.matches(tree, state)) {
+      // First argument passed to `#subscribe` is always an on next signal `Consumer`.
+      ExpressionTree firstArgument = Iterables.getFirst(tree.getArguments(), null);


perhaps simply .getFirst() on the collection?

rickie · 2025-12-05T09:52:15Z

...r-prone-contrib/src/main/java/tech/picnic/errorprone/bugpatterns/EmptyReactivePublisher.java

+    if (EMPTY_MONO.matches(receiver, state) && VACUOUS_EMPTY_MONO_OPERATORS.matches(tree, state)) {
+      return buildDescription(tree)
+          .setMessage(
+              "Operator `%s` on an empty `Mono`s is a no-op"


Can extract this into a method :)

rickie · 2025-12-05T09:52:25Z

...r-prone-contrib/src/main/java/tech/picnic/errorprone/bugpatterns/EmptyReactivePublisher.java

+          .build();
+    }
+
+    if (EMPTY_MONO.matches(receiver, state) && VACUOUS_EMPTY_MONO_OPERATORS.matches(tree, state)) {


I think we can omit this first part of the if-statement

rickie · 2025-12-05T09:52:42Z

...r-prone-contrib/src/main/java/tech/picnic/errorprone/bugpatterns/EmptyReactivePublisher.java

+      isSubtypeOf(type(Consumer.class.getCanonicalName()));
+  private static final Supplier<Type> VOID = type(Void.class.getCanonicalName());
+
+  private static final Matcher<ExpressionTree> EMPTY_FLUX =


We can probably combine this one and the next

rickie · 2025-12-05T09:53:09Z

...r-prone-contrib/src/main/java/tech/picnic/errorprone/bugpatterns/EmptyReactivePublisher.java

+      Suppliers.typeFromString("reactor.core.publisher.Flux");
+  private static final Matcher<Tree> CONSUMER =
+      isSubtypeOf(type(Consumer.class.getCanonicalName()));
+  private static final Supplier<Type> VOID = type(Void.class.getCanonicalName());


There is one provided by EP :D

rickie · 2025-12-05T09:54:17Z

...one-contrib/src/test/java/tech/picnic/errorprone/bugpatterns/EmptyReactivePublisherTest.java

+            "    Mono.just(6).handle((i, j) -> {});",
+            "    Mono.just(7).map(i -> i);",
+            "",
+            "    // BUG: Diagnostic contains: doOnNext",


Let's expand the message to be more accurate in testing :)

github-actions · 2025-12-05T09:58:25Z

Surviving mutants in this change: 5
Killed mutants in this change: 17

class	surviving	killed
🧟`tech.picnic.errorprone.bugpatterns.EmptyReactivePublisher`	5	17

Mutation testing report by Pitest. Review any surviving mutants by inspecting the line comments under Files changed.

github-actions · 2025-12-05T12:34:07Z

Surviving mutants in this change: 5
Killed mutants in this change: 17

class	surviving	killed
🧟`tech.picnic.errorprone.bugpatterns.EmptyReactivePublisher`	5	17

Mutation testing report by Pitest. Review any surviving mutants by inspecting the line comments under Files changed.

sonarqubecloud · 2025-12-05T14:32:33Z

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
96.1% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

github-actions · 2025-12-05T14:33:16Z

Surviving mutants in this change: 1
Killed mutants in this change: 21

class	surviving	killed
🧟`tech.picnic.errorprone.bugpatterns.EmptyReactivePublisher`	1	21

Mutation testing report by Pitest. Review any surviving mutants by inspecting the line comments under Files changed.

mohamedsamehsalah requested review from Stephan202 and rickie February 21, 2025 13:12

mohamedsamehsalah added the new feature label Feb 21, 2025

mohamedsamehsalah added this to the 0.21.0 milestone Feb 21, 2025

mohamedsamehsalah commented Feb 21, 2025

View reviewed changes

rickie modified the milestones: 0.21.0, 0.22.0 Mar 19, 2025

Stephan202 modified the milestones: 0.22.0, 0.23.0, 0.24.0 Apr 14, 2025

rickie modified the milestones: 0.24.0, 0.25.0 Jul 21, 2025

Stephan202 mentioned this pull request Aug 17, 2025

Recognize and simplify more "empty" expressions #1819

Merged

rickie modified the milestones: 0.25.0, 0.26.0 Sep 22, 2025

Stephan202 modified the milestones: 0.26.0, 0.28.0 Oct 26, 2025

Introduce EmptyReactivePublisher bug checker

a83f846

rickie force-pushed the mohamedsamehsalah/empty-reactive-publisher branch from e8a8e37 to a83f846 Compare December 3, 2025 14:19

rickie temporarily deployed to codex December 3, 2025 14:19 — with GitHub Actions Inactive

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rickie approved these changes Dec 5, 2025

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Revert one change

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Kill mutants

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rickie temporarily deployed to codex December 5, 2025 14:30 — with GitHub Actions Inactive

	* that they are invoked on a {@link Mono} or {@link Flux} that do not emit next signals.
	* that they are invoked on a {@link Mono} or {@link Flux} that does not emit next signals.

	.onDescendantOf(Suppliers.typeFromString("org.reactivestreams.Publisher"))
	.onDescendantOf(type("org.reactivestreams.Publisher"))

Introduce EmptyReactivePublisher bug checker #1560

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Introduce EmptyReactivePublisher bug checker #1560

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Introduce `EmptyReactivePublisher` bug checker #1560

Introduce `EmptyReactivePublisher` bug checker #1560

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