Introduce CLI for triggering AOT test context processing

This commit introduces ProcessTestsAheadOfTimeCommand, a command-line
application (CLI) that scans the provided classpath roots for Spring
integration test classes and then generates AOT artifacts for those
test classes in the provided output directories.

This CLI is only intended to be used by build tools.

Closes gh-28825

spring-test/src/main/java/org/springframework/test/context/aot/ProcessTestsAheadOfTimeCommand.java

@@ -0,0 +1,91 @@
+/*
+ * 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.test.context.aot;
+
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.Set;
+import java.util.concurrent.Callable;
+import java.util.stream.Stream;
+
+import picocli.CommandLine;
+import picocli.CommandLine.Command;
+import picocli.CommandLine.Option;
+import picocli.CommandLine.Parameters;
+
+import org.springframework.aot.generate.FileSystemGeneratedFiles;
+import org.springframework.aot.generate.GeneratedFiles;
+import org.springframework.aot.nativex.FileNativeConfigurationWriter;
+import org.springframework.aot.nativex.NativeConfigurationWriter;
+
+/**
+ * Command-line application that scans the provided classpath roots for Spring
+ * integration test classes and then generates AOT artifacts for those test
+ * classes in the provided output directories.
+ *
+ * @author Sam Brannen
+ * @since 6.0
+ * @see TestClassScanner
+ * @see TestContextAotGenerator
+ * @see FileNativeConfigurationWriter
+ */
+@Command(mixinStandardHelpOptions = true, description = "Process test classes ahead of time")
+public class ProcessTestsAheadOfTimeCommand implements Callable<Integer> {
+
+	@Parameters(index = "0", arity = "1..*", description = "Classpath roots for compiled test classes.")
+	private Path[] testClasspathRoots;
+
+	@Option(names = {"--packages"}, required = false, description = "Test packages to scan. This is optional any only intended for testing purposes.")
+	private String[] packagesToScan = new String[0];
+
+	@Option(names = {"--sources-out"}, required = true, description = "Output path for the generated sources.")
+	private Path sourcesOutputPath;
+
+	@Option(names = {"--resources-out"}, required = true, description = "Output path for the generated resources.")
+	private Path resourcesOutputPath;
+
+
+	@Override
+	public Integer call() throws Exception {
+		TestClassScanner testClassScanner = new TestClassScanner(Set.of(this.testClasspathRoots));
+		Stream<Class<?>> testClasses = testClassScanner.scan(this.packagesToScan);
+
+		// TODO Determine if we need to support CLASS output path.
+		Path tempDir = Files.createTempDirectory("classes");
+		GeneratedFiles generatedFiles = new FileSystemGeneratedFiles(kind -> switch(kind) {
+				case SOURCE -> this.sourcesOutputPath;
+				case RESOURCE -> this.resourcesOutputPath;
+				case CLASS -> tempDir;
+		});
+		TestContextAotGenerator generator = new TestContextAotGenerator(generatedFiles);
+		generator.processAheadOfTime(testClasses);
+
+		NativeConfigurationWriter writer = new FileNativeConfigurationWriter(this.resourcesOutputPath);
+		writer.write(generator.getRuntimeHints());
+
+		return 0;
+	}
+
+	static int execute(String[] args) throws Exception {
+		return new CommandLine(new ProcessTestsAheadOfTimeCommand()).execute(args);
+	}
+
+	public static void main(String[] args) throws Exception {
+		System.exit(execute(args));
+	}
+
+}