Support multi-release sources in Eclipse

Prior to this commit, the Java 21 and Java 24 multi-release sources in
spring-core could not be developed or tested within Eclipse, since an
Eclipse project supports only a single Java compliance level.

This commit configures Eclipse projects that use the multiReleaseJar
plugin with the highest multi-release version as their Java baseline
(Java 24 for spring-core) and includes the corresponding multi-release
source folders. Types which are overridden by a multi-release source
folder are excluded from lower source folders, and higher source
folders are placed first so that the debugger's source lookup resolves
overriding types.

A lower baseline can be configured via the "eclipseJavaBaseline"
project property: for example, -PeclipseJavaBaseline=17.

See gh-37397

gradle/ide.gradle

@@ -68,13 +68,109 @@ eclipse.classpath.file.whenMerged {
 	}
 }
 
-// Remove classpath entries for multi-release source sets (such as "src/main/java21")
-// and their Gradle output (such as "build/classes/java/java21"), since an Eclipse
-// project supports only a single Java compliance level. Consequently, features
-// managed via the multiReleaseJar plugin cannot be built or tested within Eclipse.
+// Since an Eclipse project supports only a single Java compliance level, projects
+// that use the multiReleaseJar plugin are configured with a single Java baseline
+// within Eclipse. By default, the highest multi-release version is used (such as
+// Java 24 for spring-core), so that the corresponding sources can be developed
+// and tested within Eclipse. A different baseline can be configured via the
+// "eclipseJavaBaseline" project property: for example, -PeclipseJavaBaseline=17.
+// Multi-release source folders for versions above the baseline are not configured
+// as source folders in Eclipse (the sources themselves remain untouched), and types
+// which are overridden by a multi-release source folder are excluded from lower
+// source folders in Eclipse.
+def javaBaseline = 17
+
+def multiReleaseVersions = {
+	sourceSets.names.findAll { it ==~ /java\d+/ }.collect { (it - 'java') as int }
+}
+
+def eclipseJavaBaseline = {
+	def releaseVersions = multiReleaseVersions()
+	if (releaseVersions.isEmpty()) {
+		return javaBaseline
+	}
+	return (project.hasProperty('eclipseJavaBaseline') ?
+			(project.property('eclipseJavaBaseline') as int) : releaseVersions.max())
+}
+
+// Return the Java version of the supplied source folder: 0 for "src/main/java",
+// 21 for "src/main/java21", or null if it is not a Java source folder.
+def javaVersionOf = { SourceFolder folder ->
+	def matcher = (folder.path =~ /^src\/(main|test)\/java(\d*)$/)
+	if (!matcher.matches()) {
+		return null
+	}
+	return (matcher.group(2) ? (matcher.group(2) as int) : 0)
+}
+
+// Return the paths of all Java source files in the supplied directory, relative to
+// that directory: for example, "org/springframework/core/task/VirtualThreadDelegate.java"
+// for "src/main/java21/org/springframework/core/task/VirtualThreadDelegate.java".
+def relativeJavaFiles = { String dir ->
+	def root = file(dir).toPath()
+	return fileTree(dir).matching { include '**/*.java' }.collect {
+		root.relativize(it.toPath()).toString().replace('\\', '/')
+	}
+}
+
+// Remove existing source folders, since their exclusion patterns depend on the
+// configured baseline and would otherwise result in duplicate entries.
+eclipse.classpath.file.beforeMerged { classpath ->
+	if (!multiReleaseVersions().isEmpty()) {
+		classpath.entries.removeAll { it instanceof SourceFolder }
+	}
+}
+
+eclipse.jdt.file.whenMerged { jdt ->
+	def baseline = JavaVersion.toVersion(eclipseJavaBaseline())
+	jdt.sourceCompatibility = baseline
+	jdt.targetCompatibility = baseline
+}
+
 eclipse.classpath.file.whenMerged { classpath ->
-	classpath.entries.removeAll { it.path =~ /src\/(main|test)\/java\d+/ }
+	def baseline = eclipseJavaBaseline()
+
+	// Remove Gradle output for multi-release source sets, such as
+	// "build/classes/java/java21".
 	classpath.entries.removeAll { it.path =~ /build\/classes\/[^\/]+\/java\d+(Test)?$/ }
+
+	// Remove classpath entries for multi-release source folders above the baseline.
+	classpath.entries.removeAll {
+		it instanceof SourceFolder && (javaVersionOf(it) ?: 0) > baseline
+	}
+
+	// Mark multi-release test source folders (such as "src/test/java21") as test
+	// sources, since they share the output folder with "src/test/java".
+	classpath.entries.findAll { it instanceof SourceFolder && it.path ==~ /src\/test\/java\d+/ }.each {
+		it.entryAttributes['test'] = 'true'
+	}
+
+	// Exclude types from lower source folders which are overridden in higher ones.
+	['main', 'test'].each { scope ->
+		def sourceFolders = classpath.entries.findAll {
+			it instanceof SourceFolder && it.path.startsWith("src/${scope}/") &&
+					javaVersionOf(it) != null
+		}
+		sourceFolders.each { folder ->
+			def overriddenTypes = sourceFolders
+					.findAll { javaVersionOf(it) > javaVersionOf(folder) }
+					.collectMany { relativeJavaFiles(it.path) }
+					.findAll { file("${folder.path}/${it}").exists() }
+			if (!overriddenTypes.isEmpty()) {
+				folder.excludes = (folder.excludes + overriddenTypes).unique()
+			}
+		}
+
+		// Place higher source folders before lower ones, so that the debugger's source
+		// lookup finds overriding types before excluded ones.
+		def multiReleaseFolders = sourceFolders.findAll { javaVersionOf(it) > 0 }
+				.sort { -javaVersionOf(it) }
+		def baseFolder = sourceFolders.find { javaVersionOf(it) == 0 }
+		if (baseFolder != null && !multiReleaseFolders.isEmpty()) {
+			classpath.entries.removeAll(multiReleaseFolders)
+			classpath.entries.addAll(classpath.entries.indexOf(baseFolder), multiReleaseFolders)
+		}
+	}
 }
 
 // Remove classpath entries for non-existent libraries added by the multiReleaseJar

import-into-eclipse.md

@@ -53,11 +53,15 @@ within your locally cloned `spring-framework` working directory._
      compile `spring-core` and generate JAXB types for `spring-oxm`.
 1. `spring-aspects` does not compile due to references to aspect types unknown to Eclipse.
    - If you installed _AJDT_ into Eclipse it should work.
-1. The Java 21 and Java 24 multi-release sources in `spring-core` (`src/main/java21`,
-   `src/main/java24`, and `src/test/java21`) are not configured as source folders in
-   Eclipse, since an Eclipse project supports only a single Java compliance level.
-   - Use `./gradlew :spring-core:java21Test` or `./gradlew :spring-core:java24Test` to
-     build and test those sources from the command line.
+1. Since an Eclipse project supports only a single Java compliance level, `spring-core`
+   is configured with a Java 24 baseline in Eclipse, which includes the Java 21 and
+   Java 24 multi-release sources (`src/main/java21`, `src/main/java24`, and
+   `src/test/java21`). Types overridden by those sources are excluded from
+   `src/main/java`. Eclipse does not report usage of Java features newer than the
+   project-wide Java 17 baseline; however, such usage will be caught by the Gradle build.
+   - To work on the Java 17 or Java 21 variants instead, run
+     `./gradlew eclipse -PeclipseJavaBaseline=17` (or `21`) and refresh the projects in
+     Eclipse (or close and reopen them if the compiler settings are not updated).
 1. While JUnit tests pass from the command line with Gradle, some may fail when run from
    the IDE.
    - Resolving this is a work in progress.