Use JApiCmp Gradle plugin for API diffs

This commit removes JDiff from the Spring Framework build and instead,
adds a Gradle plugin that configure JApiCmp tasks on the framework
modules.

Fixes gh-22942
See gh-23282

buildSrc/src/main/java/org/springframework/build/api/ApiDiffPlugin.java

@@ -0,0 +1,115 @@
+/*
+ * Copyright 2002-2019 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.build.api;
+
+import java.io.File;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.Collections;
+import java.util.List;
+
+import me.champeau.gradle.japicmp.JapicmpPlugin;
+import me.champeau.gradle.japicmp.JapicmpTask;
+import org.gradle.api.Plugin;
+import org.gradle.api.Project;
+import org.gradle.api.artifacts.Configuration;
+import org.gradle.api.artifacts.Dependency;
+import org.gradle.api.plugins.JavaBasePlugin;
+import org.gradle.api.plugins.JavaPlugin;
+import org.gradle.api.tasks.TaskProvider;
+import org.gradle.jvm.tasks.Jar;
+
+/**
+ * {@link Plugin} that applies the {@code "japicmp-gradle-plugin"}
+ * and create tasks for all subprojects, diffing the public API one by one
+ * and creating the reports in {@code "build/reports/api-diff/$OLDVERSION_to_$NEWVERSION/"}.
+ * <p>{@code "./gradlew apiDiff -PbaselineVersion=5.1.0.RELEASE"} will output the
+ * reports for the API diff between the baseline version and the current one for all modules.
+ * You can limit the report to a single module with
+ * {@code "./gradlew :spring-core:apiDiff -PbaselineVersion=5.1.0.RELEASE"}.
+ *
+ * @author Brian Clozel
+ */
+public class ApiDiffPlugin implements Plugin<Project> {
+
+	public static final String TASK_NAME = "apiDiff";
+
+	private static final String BASELINE_VERSION_PROPERTY = "baselineVersion";
+
+	private static final List<String> PACKAGE_INCLUDES = Collections.singletonList("org.springframework.*");
+
+	@Override
+	public void apply(Project project) {
+		if (project.hasProperty(BASELINE_VERSION_PROPERTY) && project.equals(project.getRootProject())) {
+			project.getPluginManager().apply(JapicmpPlugin.class);
+			project.getPlugins().withType(JapicmpPlugin.class,
+					plugin -> applyApiDiffConventions(project));
+		}
+	}
+
+	private void applyApiDiffConventions(Project project) {
+		String baselineVersion = project.property(BASELINE_VERSION_PROPERTY).toString();
+		project.subprojects(subProject -> createApiDiffTask(baselineVersion, subProject));
+	}
+
+	private void createApiDiffTask(String baselineVersion, Project project) {
+		if (isProjectEligible(project)) {
+			JapicmpTask apiDiff = project.getTasks().create(TASK_NAME, JapicmpTask.class);
+			apiDiff.setDescription("Generates an API diff report with japicmp");
+			apiDiff.setGroup(JavaBasePlugin.DOCUMENTATION_GROUP);
+
+			apiDiff.setOldClasspath(project.files(createBaselineConfiguration(baselineVersion, project)));
+			TaskProvider<Jar> jar = project.getTasks().withType(Jar.class).named("jar");
+			apiDiff.setNewArchives(project.getLayout().files(jar.get().getArchiveFile().get().getAsFile()));
+			apiDiff.setNewClasspath(getRuntimeClassPath(project));
+			apiDiff.setPackageIncludes(PACKAGE_INCLUDES);
+			apiDiff.setOnlyModified(true);
+			apiDiff.setIgnoreMissingClasses(true);
+			// Ignore Kotlin metadata annotations since they contain
+			// illegal HTML characters and fail the report generation
+			apiDiff.setAnnotationExcludes(Collections.singletonList("@kotlin.Metadata"));
+
+			apiDiff.setHtmlOutputFile(getOutputFile(baselineVersion, project));
+
+			apiDiff.dependsOn(project.getTasks().getByName("jar"));
+		}
+	}
+
+	private boolean isProjectEligible(Project project) {
+		return project.getPlugins().hasPlugin(JavaPlugin.class)
+				&& !project.getName().equals("spring-core-coroutines")
+				&& !project.getName().equals("spring-framework-bom");
+	}
+
+	private Configuration createBaselineConfiguration(String baselineVersion, Project project) {
+		String baseline = String.join(":",
+				project.getGroup().toString(), project.getName(), baselineVersion);
+		Dependency baselineDependency = project.getDependencies().create(baseline + "@jar");
+		return project.getRootProject().getConfigurations().detachedConfiguration(baselineDependency);
+	}
+
+	private Configuration getRuntimeClassPath(Project project) {
+		return project.getConfigurations().getByName(JavaPlugin.RUNTIME_CLASSPATH_CONFIGURATION_NAME);
+	}
+
+	private File getOutputFile(String baseLineVersion, Project project) {
+		Path outDir = Paths.get(project.getRootProject().getBuildDir().getAbsolutePath(),
+				"reports", "api-diff",
+				baseLineVersion + "_to_" + project.getRootProject().getVersion());
+		return project.file(outDir.resolve(project.getName() + ".html").toString());
+	}
+
+}