ImportLayoutStyle: cache classpath conflict data per JavaSourceSet

[knutwannheden]

Motivation

ImportLayoutStyle.ImportLayoutConflictDetection walks the entire classpath for every file in setJVMClassNames() and mapNamesInPackageToPackages() — a fresh detector is created per file by OrderImports/AddImport, so the cost is O(files × classpath) of getPackageName()/getFullyQualifiedName() string work plus HashMap churn. CPU profiling of a static-analysis run over a large multi-module project showed ImportLayoutStyle at ~15% of the whole run. With a lazily-resolved classpath it is worse, because the full per-file walk forces materializing the entire lazy classpath on every file.

• A fast path along these lines was added in Introduce JavaTypeFactory and extend JavaSourceSet with classpath fast paths #7528 (a JavaSourceSet.ClasspathIndex SPI) and later reverted in Type signature, marker, and SPI changes across rewrite-java and rewrite-maven #7845 as an unused SPI surface. This restores the optimization without any SPI — a concrete, lazily-cached method on JavaSourceSet with a live consumer — so it can't be dropped again as "unused".

What changed

JavaSourceSet gains a lazily-built, per-instance index of classpath types grouped by package, exposed as a single generic accessor (import-conflict policy stays in ImportLayoutStyle):

public List<JavaType.FullyQualified> typesInPackage(String packageName);

It is a Lombok @Getter(lazy = true) field: thread-safe lazy init, and excluded from serialization, equals/hashCode, toString, and the with* constructor threading — so a source set produced by withClasspath (e.g. addTypesForGav/removeTypesForGav) rebuilds the index for its new classpath rather than reusing a stale one.

ImportLayoutConflictDetection resolves only the imported packages through that index when a source set is available, instead of walking the whole classpath:

// new overloads; the existing ones delegate with sourceSet = null (unchanged full-walk behavior)
style.orderImports(imports, classpath, classpathDirty, sourceSet);
style.addImport(imports, toAdd, pkg, classpath, classpathDirty, sourceSet);

OrderImports/AddImport pass the CU's JavaSourceSet. The classpathDirty safe path and the cheap classpath.isEmpty() guard are unchanged, and the cached path is byte-for-byte equivalent to the full walk (mirrors the per-type semantics of the original loop, including the static-member/FQN branch).

Summary

• Add JavaSourceSet.typesInPackage(String) backed by a lazily-built Map<packageName, List<FullyQualified>> (single classpath walk per instance, @Getter(lazy = true)).
• ImportLayoutConflictDetection uses the per-source-set index when present; falls back to the existing full walk when no source set is supplied.
• New orderImports/addImport overloads carrying the JavaSourceSet; old overloads retained and delegate with null.
• OrderImports and AddImport thread the source set through.

Scope note

The cache is keyed to a JavaSourceSet instance, so it benefits every file that shares that instance (the in-memory parse/recipe path). On serialized pipelines where each file is re-hydrated into its own marker instance, the transient index is cold per file (same as the existing transient typeFactory); a follow-up can add an ExecutionContext-id-keyed backstop if profiling shows that path still hot. Memoizing JavaType.FullyQualified.getPackageName() is a complementary, independently valuable follow-up.

Test plan

• JavaSourceSetTest: typesInPackage returns the types in a package; the classpath is walked exactly once across repeated calls (counting collection); the lazy index is excluded from the Jackson payload and the marker round-trips.
• ImportLayoutStyleTest: the new orderImports(..., sourceSet) overload is byte-for-byte equal to the full-walk path, both when a java.lang collision must suppress folding and when folding must proceed.
• Regression green: OrderImportsTest, AddImportTest (incl. doNotFoldPackageWithJavaLangClassNames, now exercising the cached path), full :rewrite-java, DirtyJavaSourceSetsProducerTest, ChangeTypeTest/ChangePackageTest/RemoveImportTest/RemoveUnusedImportsTest.