Configure RuntimeHintsAgent in test suite

This commit configures the `RuntimeHintsAgent` in the Spring Framework
test suite.
Instead of applying the agent to the entire test suite, and possibly
interfering with other tests, this commit adds a new custom Gradle
plugin that does the following:

* create a new test task named `"runtimeHintsTest"`
* run this task with the runtime hints java agent
* only execute tests tagged with `"RuntimeHintsTests"`

See gh-27981

buildSrc/src/main/java/org/springframework/build/hint/RuntimeHintsAgentExtension.java

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2002-2022 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.hint;
+
+import java.util.Collections;
+
+import org.gradle.api.model.ObjectFactory;
+import org.gradle.api.provider.SetProperty;
+
+/**
+ * Entry point to the DSL extension for the {@link RuntimeHintsAgentPlugin} Gradle plugin.
+ * @author Brian Clozel
+ */
+public class RuntimeHintsAgentExtension {
+
+	private final SetProperty<String> includedPackages;
+
+	private final SetProperty<String> excludedPackages;
+
+	public RuntimeHintsAgentExtension(ObjectFactory objectFactory) {
+		this.includedPackages = objectFactory.setProperty(String.class).convention(Collections.singleton("org.springframework"));
+		this.excludedPackages = objectFactory.setProperty(String.class).convention(Collections.emptySet());
+	}
+
+	public SetProperty<String> getIncludedPackages() {
+		return this.includedPackages;
+	}
+
+	public SetProperty<String> getExcludedPackages() {
+		return this.excludedPackages;
+	}
+
+	String asJavaAgentArgument() {
+		StringBuilder builder = new StringBuilder();
+		this.includedPackages.get().forEach(packageName -> builder.append('+').append(packageName).append(','));
+		this.excludedPackages.get().forEach(packageName -> builder.append('-').append(packageName).append(','));
+		return builder.toString();
+	}
+}

buildSrc/src/main/java/org/springframework/build/hint/RuntimeHintsAgentPlugin.java

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2002-2022 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.hint;
+
+import org.gradle.api.Plugin;
+import org.gradle.api.Project;
+import org.gradle.api.plugins.JavaPlugin;
+import org.gradle.api.tasks.bundling.Jar;
+import org.gradle.api.tasks.testing.Test;
+
+/**
+ * {@link Plugin} that configures the {@code RuntimeHints} Java agent to test tasks.
+ *
+ * @author Brian Clozel
+ */
+public class RuntimeHintsAgentPlugin implements Plugin<Project> {
+
+	public static final String RUNTIMEHINTS_TEST_TASK = "runtimeHintsTest";
+	private static final String EXTENSION_NAME = "runtimeHintsAgent";
+
+
+	@Override
+	public void apply(Project project) {
+
+		project.getPlugins().withType(JavaPlugin.class, javaPlugin -> {
+			RuntimeHintsAgentExtension agentExtension = project.getExtensions().create(EXTENSION_NAME,
+					RuntimeHintsAgentExtension.class, project.getObjects());
+			Test agentTest = project.getTasks().create(RUNTIMEHINTS_TEST_TASK, Test.class, test -> {
+				test.useJUnitPlatform(options -> {
+					options.includeTags("RuntimeHintsTests");
+				});
+				test.include("**/*Tests.class", "**/*Test.class");
+				test.systemProperty("java.awt.headless", "true");
+			});
+			project.afterEvaluate(p -> {
+				Jar jar = project.getRootProject().project("spring-core-test").getTasks().withType(Jar.class).named("jar").get();
+				agentTest.jvmArgs("-javaagent:" + jar.getArchiveFile().get().getAsFile() + "=" + agentExtension.asJavaAgentArgument());
+			});
+			project.getTasks().getByName("check", task -> task.dependsOn(agentTest));
+		});
+	}
+}