Add build DSL extension to enable Java preview features

This commit adds a DSL Gradle extension for optionally enabling Java
preview features in a specific project module. The "--enable-preview"
JVM flag will be configured automatically for compile and test tasks
where this is applied:

```
springFramework {
	enableJavaPreviewFeatures = true
}
```

See gh-33616

buildSrc/README.md

@@ -11,6 +11,16 @@ The `org.springframework.build.conventions` plugin applies all conventions to th
 * Configuring the Kotlin compiler, see `KotlinConventions`
 * Configuring testing in the build with `TestConventions` 
 
+This plugin also provides a DSL extension to optionally enable Java preview features for
+compiling and testing sources in a module. This can be applied with the following in a
+module build file:
+
+```groovy
+springFramework {
+	enableJavaPreviewFeatures = true
+}
+```
+
 
 ## Build Plugins
 

buildSrc/src/main/java/org/springframework/build/ConventionsPlugin.java

@@ -36,6 +36,7 @@ public class ConventionsPlugin implements Plugin<Project> {
 
 	@Override
 	public void apply(Project project) {
+		project.getExtensions().create("springFramework", SpringFrameworkExtension.class);
 		new CheckstyleConventions().apply(project);
 		new JavaConventions().apply(project);
 		new KotlinConventions().apply(project);

buildSrc/src/main/java/org/springframework/build/JavaConventions.java

@@ -75,19 +75,25 @@ public class JavaConventions {
 			toolchain.getVendor().set(JvmVendorSpec.BELLSOFT);
 			toolchain.getLanguageVersion().set(JavaLanguageVersion.of(17));
 		});
-		project.getTasks().withType(JavaCompile.class)
-				.matching(compileTask -> compileTask.getName().equals(JavaPlugin.COMPILE_JAVA_TASK_NAME))
-				.forEach(compileTask -> {
-					compileTask.getOptions().setCompilerArgs(COMPILER_ARGS);
-					compileTask.getOptions().setEncoding("UTF-8");
-				});
-		project.getTasks().withType(JavaCompile.class)
-				.matching(compileTask -> compileTask.getName().equals(JavaPlugin.COMPILE_TEST_JAVA_TASK_NAME)
-						|| compileTask.getName().equals("compileTestFixturesJava"))
-				.forEach(compileTask -> {
-					compileTask.getOptions().setCompilerArgs(TEST_COMPILER_ARGS);
-					compileTask.getOptions().setEncoding("UTF-8");
-				});
+		SpringFrameworkExtension frameworkExtension = project.getExtensions().getByType(SpringFrameworkExtension.class);
+		project.afterEvaluate(p -> {
+			p.getTasks().withType(JavaCompile.class)
+					.matching(compileTask -> compileTask.getName().startsWith(JavaPlugin.COMPILE_JAVA_TASK_NAME))
+					.forEach(compileTask -> {
+						compileTask.getOptions().setCompilerArgs(COMPILER_ARGS);
+						compileTask.getOptions().getCompilerArgumentProviders().add(frameworkExtension.asArgumentProvider());
+						compileTask.getOptions().setEncoding("UTF-8");
+					});
+			p.getTasks().withType(JavaCompile.class)
+					.matching(compileTask -> compileTask.getName().startsWith(JavaPlugin.COMPILE_TEST_JAVA_TASK_NAME)
+							|| compileTask.getName().equals("compileTestFixturesJava"))
+					.forEach(compileTask -> {
+						compileTask.getOptions().setCompilerArgs(TEST_COMPILER_ARGS);
+						compileTask.getOptions().getCompilerArgumentProviders().add(frameworkExtension.asArgumentProvider());
+						compileTask.getOptions().setEncoding("UTF-8");
+					});
+
+		});
 	}
 
 }

buildSrc/src/main/java/org/springframework/build/SpringFrameworkExtension.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2002-2024 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;
+
+import java.util.Collections;
+import java.util.List;
+
+import org.gradle.api.Project;
+import org.gradle.api.provider.Property;
+import org.gradle.process.CommandLineArgumentProvider;
+
+public class SpringFrameworkExtension {
+
+	private final Property<Boolean> enableJavaPreviewFeatures;
+
+	public SpringFrameworkExtension(Project project) {
+		this.enableJavaPreviewFeatures = project.getObjects().property(Boolean.class);
+	}
+
+	public Property<Boolean> getEnableJavaPreviewFeatures() {
+		return this.enableJavaPreviewFeatures;
+	}
+
+	public CommandLineArgumentProvider asArgumentProvider() {
+		return () -> {
+			if (getEnableJavaPreviewFeatures().getOrElse(false)) {
+				return List.of("--enable-preview");
+			}
+			return Collections.emptyList();
+		};
+	}
+}

buildSrc/src/main/java/org/springframework/build/TestConventions.java

@@ -67,6 +67,8 @@ class TestConventions {
 				"--add-opens=java.base/java.util=ALL-UNNAMED",
 				"-Xshare:off"
 		);
+		test.getJvmArgumentProviders().add(project.getExtensions()
+				.getByType(SpringFrameworkExtension.class).asArgumentProvider());
 	}
 
 	private void configureTestRetryPlugin(Project project, Test test) {