Introduce Gradle Toolchain support in build

Prior to this commit, the Spring Framework build would rely on
setting a custom Java HOME for building all sources and tests
with that JDK.

This approach is not flexible enough, since we would be testing
the source compatibility against a recent JDK, but not a common
case experienced by the community: compiling and running
application code with a recent JDK and the official, JDK8-based
Framework artifacts.
This method is also limiting our choice of JDKs to the ones
currently supported by Gradle itself.

This commit introduces the support of Gradle JVM Toolchains in
the Spring Framework build.

We can now select a specific JDK for compiling the main
SourceSets (Java, Groovy and Kotlin) and another one for
compiling and running the test SourceSets:

`./gradlew check -PmainToolChain=8 -PtestToolchain=15`

Gradle will automatically find the JDKs present on the host or
download one automcatically. You can find out about the ones
installed on your host using:

`./gradlew -q javaToolchains`

Finally, this commit also refactors the CI infrastructure to:

* only have a single CI image (with all the supported JDKs)
* use this new feature to compile with JDK8 but test it
against JDK11 and JDK15.

Closes gh-25787

build.gradle

@@ -310,14 +310,13 @@ configure([rootProject] + javaProjects) { project ->
 	apply plugin: "java-test-fixtures"
 	apply plugin: "checkstyle"
 	apply plugin: 'org.springframework.build.compile'
-	apply from: "${rootDir}/gradle/custom-java-home.gradle"
+	apply from: "${rootDir}/gradle/toolchains.gradle"
 	apply from: "${rootDir}/gradle/ide.gradle"
 
 	pluginManager.withPlugin("kotlin") {
 		apply plugin: "org.jetbrains.dokka"
 		compileKotlin {
 			kotlinOptions {
-				jvmTarget = "1.8"
 				languageVersion = "1.3"
 				apiVersion = "1.3"
 				freeCompilerArgs = ["-Xjsr305=strict"]
@@ -326,7 +325,6 @@ configure([rootProject] + javaProjects) { project ->
 		}
 		compileTestKotlin {
 			kotlinOptions {
-				jvmTarget = "1.8"
 				freeCompilerArgs = ["-Xjsr305=strict"]
 			}
 		}

buildSrc/README.md

@@ -8,24 +8,18 @@ They are declared in the `build.gradle` file in this folder.
 ### Compiler conventions
 
 The `org.springframework.build.compile` plugin applies the Java compiler conventions to the build.
-By default, the build compiles sources with Java `1.8` source and target compatibility.
-You can test a different source compatibility version on the CLI with a project property like:
-
-```
-./gradlew test -PjavaSourceVersion=11
-```
 
 ## Build Plugins
 
-## Optional dependencies
+### Optional dependencies
 
 The `org.springframework.build.optional-dependencies` plugin creates a new `optional`
 Gradle configuration - it adds the dependencies to the project's compile and runtime classpath
 but doesn't affect the classpath of dependent projects.
 This plugin does not provide a `provided` configuration, as the native `compileOnly` and `testCompileOnly`
 configurations are preferred.
 
-## API Diff
+### API Diff
 
 This plugin uses the [Gradle JApiCmp](https://github.com/melix/japicmp-gradle-plugin) plugin
 to generate API Diff reports for each Spring Framework module. This plugin is applied once on the root

buildSrc/src/main/java/org/springframework/build/compile/CompilerConventionsPlugin.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -20,31 +20,21 @@
 import java.util.Arrays;
 import java.util.List;
 
-import org.gradle.api.JavaVersion;
 import org.gradle.api.Plugin;
 import org.gradle.api.Project;
+import org.gradle.api.plugins.JavaLibraryPlugin;
 import org.gradle.api.plugins.JavaPlugin;
 import org.gradle.api.plugins.JavaPluginConvention;
 import org.gradle.api.tasks.compile.JavaCompile;
 
 /**
  * {@link Plugin} that applies conventions for compiling Java sources in Spring Framework.
- * <p>One can override the default Java source compatibility version
- * with a dedicated property on the CLI: {@code "./gradlew test -PjavaSourceVersion=11"}.
  *
  * @author Brian Clozel
  * @author Sam Brannen
  */
 public class CompilerConventionsPlugin implements Plugin<Project> {
 
-	/**
-	 * The project property that can be used to switch the Java source
-	 * compatibility version for building source and test classes.
-	 */
-	public static final String JAVA_SOURCE_VERSION_PROPERTY = "javaSourceVersion";
-
-	public static final JavaVersion DEFAULT_COMPILER_VERSION = JavaVersion.VERSION_1_8;
-
 	private static final List<String> COMPILER_ARGS;
 
 	private static final List<String> TEST_COMPILER_ARGS;
@@ -69,7 +59,7 @@ public class CompilerConventionsPlugin implements Plugin<Project> {
 
 	@Override
 	public void apply(Project project) {
-		project.getPlugins().withType(JavaPlugin.class, javaPlugin -> applyJavaCompileConventions(project));
+		project.getPlugins().withType(JavaLibraryPlugin.class, javaPlugin -> applyJavaCompileConventions(project));
 	}
 
 	/**
@@ -79,15 +69,6 @@ public void apply(Project project) {
 	 */
 	private void applyJavaCompileConventions(Project project) {
 		JavaPluginConvention java = project.getConvention().getPlugin(JavaPluginConvention.class);
-		if (project.hasProperty(JAVA_SOURCE_VERSION_PROPERTY)) {
-			JavaVersion javaSourceVersion = JavaVersion.toVersion(project.property(JAVA_SOURCE_VERSION_PROPERTY));
-			java.setSourceCompatibility(javaSourceVersion);
-		}
-		else {
-			java.setSourceCompatibility(DEFAULT_COMPILER_VERSION);
-		}
-		java.setTargetCompatibility(DEFAULT_COMPILER_VERSION);
-
 		project.getTasks().withType(JavaCompile.class)
 				.matching(compileTask -> compileTask.getName().equals(JavaPlugin.COMPILE_JAVA_TASK_NAME))
 				.forEach(compileTask -> {

ci/images/ci-image/Dockerfile

@@ -4,5 +4,8 @@ ADD setup.sh /setup.sh
 ADD get-jdk-url.sh /get-jdk-url.sh
 RUN ./setup.sh java8
 
-ENV JAVA_HOME /opt/openjdk
+ENV JAVA_HOME /opt/openjdk/java8
+ENV JDK11 /opt/openjdk/java11
+ENV JDK15 /opt/openjdk/java15
+
 ENV PATH $JAVA_HOME/bin:$PATH

ci/images/setup.sh

@@ -19,13 +19,20 @@ curl --output /opt/concourse-release-scripts.jar https://repo.spring.io/release/
 ###########################################################
 # JAVA
 ###########################################################
-JDK_URL=$( ./get-jdk-url.sh $1 )
 
 mkdir -p /opt/openjdk
-cd /opt/openjdk
-curl -L ${JDK_URL} | tar zx --strip-components=1
-test -f /opt/openjdk/bin/java
-test -f /opt/openjdk/bin/javac
+pushd /opt/openjdk > /dev/null
+for jdk in java8 java11 java15
+do
+  JDK_URL=$( /get-jdk-url.sh $jdk )
+  mkdir $jdk
+  pushd $jdk > /dev/null
+  curl -L ${JDK_URL} | tar zx --strip-components=1
+  test -f bin/java
+  test -f bin/javac
+  popd > /dev/null
+done
+popd
 
 ###########################################################
 # GRADLE ENTERPRISE

ci/pipeline.yml

@@ -85,18 +85,6 @@ resources:
   source:
     <<: *docker-resource-source
     repository: ((docker-hub-organization))/spring-framework-ci
-- name: ci-image-jdk11
-  type: docker-image
-  icon: docker
-  source:
-    <<: *docker-resource-source
-    repository: ((docker-hub-organization))/spring-framework-ci-jdk11
-- name: ci-image-jdk15
-  type: docker-image
-  icon: docker
-  source:
-    <<: *docker-resource-source
-    repository: ((docker-hub-organization))/spring-framework-ci-jdk15
 - name: artifactory-repo
   type: artifactory-resource
   icon: package-variant
@@ -161,14 +149,6 @@ jobs:
       params:
         build: ci-images-git-repo/ci/images
         dockerfile: ci-images-git-repo/ci/images/ci-image/Dockerfile
-    - put: ci-image-jdk11
-      params:
-        build: ci-images-git-repo/ci/images
-        dockerfile: ci-images-git-repo/ci/images/ci-image-jdk11/Dockerfile
-    - put: ci-image-jdk15
-      params:
-        build: ci-images-git-repo/ci/images
-        dockerfile: ci-images-git-repo/ci/images/ci-image-jdk15/Dockerfile
 - name: build
   serial: true
   public: true
@@ -227,16 +207,19 @@ jobs:
   serial: true
   public: true
   plan:
-  - get: ci-image-jdk11
+  - get: ci-image
   - get: git-repo
   - get: every-morning
     trigger: true
   - put: repo-status-jdk11-build
     params: { state: "pending", commit: "git-repo" }
   - do:
     - task: check-project
-      image: ci-image-jdk11
+      image: ci-image
       file: git-repo/ci/tasks/check-project.yml
+      params:
+        MAIN_TOOLCHAIN: 8
+        TEST_TOOLCHAIN: 11
       <<: *build-project-task-params
     on_failure:
       do:
@@ -251,16 +234,19 @@ jobs:
   serial: true
   public: true
   plan:
-    - get: ci-image-jdk15
+    - get: ci-image
     - get: git-repo
     - get: every-morning
       trigger: true
     - put: repo-status-jdk15-build
       params: { state: "pending", commit: "git-repo" }
     - do:
       - task: check-project
-        image: ci-image-jdk15
+        image: ci-image
         file: git-repo/ci/tasks/check-project.yml
+        params:
+          MAIN_TOOLCHAIN: 8
+          TEST_TOOLCHAIN: 15
         <<: *build-project-task-params
       on_failure:
         do:

ci/scripts/check-project.sh

@@ -4,5 +4,6 @@ set -e
 source $(dirname $0)/common.sh
 
 pushd git-repo > /dev/null
-./gradlew -Dorg.gradle.internal.launcher.welcomeMessageEnabled=false --no-daemon --max-workers=4 check
+./gradlew -Dorg.gradle.internal.launcher.welcomeMessageEnabled=false -Dorg.gradle.java.installations.fromEnv=JDK11,JDK15 \
+  -PmainToolchain=$MAIN_TOOLCHAIN -PtestToolchain=$TEST_TOOLCHAIN --no-daemon --max-workers=4 check
 popd > /dev/null

ci/tasks/check-project.yml

@@ -10,6 +10,8 @@ caches:
 params:
   BRANCH:
   CI: true
+  MAIN_TOOLCHAIN:
+  TEST_TOOLCHAIN:
   GRADLE_ENTERPRISE_ACCESS_KEY:
   GRADLE_ENTERPRISE_CACHE_USERNAME:
   GRADLE_ENTERPRISE_CACHE_PASSWORD:

gradle/spring-module.gradle

@@ -1,3 +1,4 @@
+apply plugin: 'java-library'
 apply plugin: 'org.springframework.build.compile'
 apply plugin: 'org.springframework.build.optional-dependencies'
 // Uncomment the following for Shadow support in the jmhJar block.