Fix `MigrateCollections*` template errors on complex expression contexts by knutwannheden · Pull Request #1067 · openrewrite/rewrite-migrate-java

knutwannheden · 2026-04-21T08:37:01Z

Motivation

The MigrateCollections{Empty,Singleton,Unmodifiable}* recipes threw
IllegalArgumentException: Expected a template that would generate exactly one statement to replace one statement, but generated {0,2} whenever the Collections.xxx() call sat in an expression context that the block-statement stub generator doesn't handle. Observed in the wild on Spring, Spring Data, and Conductor codebases — for example:

// AbstractSockJsSession.java — switch expression arm
return switch (type) {
    case 0 -> Collections.emptyList();
    ...
};

// StaticListableBeanFactory.java — parenthesized ternary branch
return (beanType != null ? ... : Collections.emptySet());

// SQSObservableQueue.java — method argument
return Observable.from(Collections.emptyList());

// AlpsJsonHttpMessageConverter.java — ternary branch with nested method invocation
return body instanceof RootResourceInformation
        ? Collections.singletonMap("alps", converter.convert((RootResourceInformation) body))
        : body;

The recipes also blew up on Kotlin .kt sources (three MigrateCollectionsSingletonMap failures in Spring Security), because JavaTemplate's stub generator produces a Java-shaped stub that doesn't line up with a K-tree.

Summary

Drop .contextSensitive() from all JavaTemplate.builder(...) calls in the MigrateCollections* family. The templates (List.of(), Map.of(), Set.of(), Set.of(#{any()}), Map.of(#{any()}, #{any()})) reference only static factories and their arguments — no local scope — so the context-sensitive block-statement stub generator was doing nothing useful and just exposing gaps (no handling for J.SwitchExpression, substitution quirks for method-invocation values). With context-free templating, the stub is always class Template {{ Object o = List.of(); }} and parses to exactly one statement regardless of the surrounding syntax.
Add KotlinFileChecker / GroovyFileChecker preconditions to every MigrateCollections* recipe, matching the pattern already used by ReplaceUnusedVariablesWithUnderscore and IfElseIfConstructToSwitch. Idiomatic Kotlin already has emptyList()/mapOf()/setOf() and Groovy has []/[:]; if those languages ever warrant migration, they belong in separate language-specific recipes.

Test plan

New regression tests for switch expressions, ternaries, method arguments, and parenthesized expressions in MigrateCollectionsEmptyListTest, MigrateCollectionsEmptyMapTest, MigrateCollectionsEmptySetTest, and MigrateCollectionsSingletonMapTest.
New Kotlin no-op test in MigrateCollectionsSingletonMapTest to lock in the precondition.
Existing tests for all eight recipes still pass.
Full ./gradlew test passes locally.

The `MigrateCollections{Empty,Singleton,Unmodifiable}*` family applied `JavaTemplate` in `.contextSensitive()` mode. When the `Collections.emptyList()` (etc.) call sat inside a switch expression (`case 0 -> Collections.emptyList()`), a ternary branch, a method argument, or similar non-statement contexts, the block-statement stub generator in rewrite-core either produced 0 statements (for `J.SwitchExpression` parents, which it does not handle) or 2 statements (for some substitutions with method-invocation values), causing `IllegalArgumentException: Expected a template that would generate exactly one statement to replace one statement, but generated {0,2}`. Dropping `.contextSensitive()` makes these simple static-factory templates context-free, so the stub is a trivial `class Template {{ Object o = List.of(); }}` that always parses to one statement regardless of surrounding syntax. Also guard every `MigrateCollections*` recipe with `KotlinFileChecker`/`GroovyFileChecker` preconditions — consistent with `ReplaceUnusedVariablesWithUnderscore` and `IfElseIfConstructToSwitch` — since `JavaTemplate` cannot reliably target non-Java trees, and the idiomatic replacement in Kotlin (`emptyList()`) and Groovy (`[]`) isn't `List.of()` anyway.

moderne-meeseeks Bot added this to OpenRewrite Apr 21, 2026

moderne-meeseeks Bot assigned knutwannheden Apr 21, 2026

github-project-automation Bot moved this to In Progress in OpenRewrite Apr 21, 2026

This was referenced Apr 21, 2026

KotlinPrinter NPE on J.ClassDeclaration with null body openrewrite/rewrite#7442

Closed

BlockStatementTemplateGenerator does not handle J.SwitchExpression parent openrewrite/rewrite#7443

Closed

knutwannheden changed the title ~~Fix MigrateCollections* template errors on complex expression contexts~~ Fix MigrateCollections* template errors on complex expression contexts Apr 21, 2026

knutwannheden merged commit eac4344 into main Apr 21, 2026
1 check passed

github-project-automation Bot moved this from In Progress to Done in OpenRewrite Apr 21, 2026

knutwannheden deleted the fix-migratecollections-template-errors branch April 21, 2026 08:56

This was referenced Apr 21, 2026

Fix NPE in KotlinPrinter when class body is null openrewrite/rewrite#7444

Closed

Handle J.SwitchExpression in BlockStatementTemplateGenerator openrewrite/rewrite#7445

Merged

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Fix `MigrateCollections*` template errors on complex expression contexts#1067

Fix `MigrateCollections*` template errors on complex expression contexts#1067
knutwannheden merged 1 commit intomainfrom
fix-migratecollections-template-errors

knutwannheden commented Apr 21, 2026

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knutwannheden commented Apr 21, 2026

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