Deleting existing output before running AOT processing

Closes gh-30981

Author: wilkinsona

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/main/java/org/springframework/boot/maven/AotGenerateMojo.java

@@ -45,7 +45,6 @@
 import org.apache.maven.plugins.annotations.ResolutionScope;
 
 import org.springframework.boot.loader.tools.RunProcess;
-import org.springframework.util.FileSystemUtils;
 
 /**
  * Invoke the AOT engine on the application.
@@ -83,7 +82,6 @@ public class AotGenerateMojo extends AbstractRunMojo {
 	protected void run(File workingDirectory, String startClassName, Map<String, String> environmentVariables)
 			throws MojoExecutionException, MojoFailureException {
 		try {
-			deletePreviousAotAssets();
 			generateAotAssets(workingDirectory, startClassName, environmentVariables);
 			compileSourceFiles(getClassPathUrls());
 			copyNativeConfiguration(this.generatedResources.toPath());
@@ -93,12 +91,6 @@ protected void run(File workingDirectory, String startClassName, Map<String, Str
 		}
 	}
 
-	private void deletePreviousAotAssets() {
-		FileSystemUtils.deleteRecursively(this.generatedSources);
-		FileSystemUtils.deleteRecursively(this.generatedResources);
-		FileSystemUtils.deleteRecursively(this.generatedClasses);
-	}
-
 	private void generateAotAssets(File workingDirectory, String startClassName,
 			Map<String, String> environmentVariables) throws MojoExecutionException {
 		List<String> args = new ArrayList<>();

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/AotProcessor.java

@@ -38,6 +38,7 @@
 import org.springframework.context.support.GenericApplicationContext;
 import org.springframework.javapoet.ClassName;
 import org.springframework.util.Assert;
+import org.springframework.util.FileSystemUtils;
 
 /**
  * Entry point for AOT processing of a {@link SpringApplication}.
@@ -95,6 +96,7 @@ public AotProcessor(Class<?> application, String[] applicationArgs, Path sourceO
 	 * Trigger the processing of the application managed by this instance.
 	 */
 	public void process() {
+		deleteExistingOutput();
 		AotProcessingHook hook = new AotProcessingHook();
 		SpringApplicationHooks.withHook(hook, this::callApplicationMainMethod);
 		GenericApplicationContext applicationContext = hook.getApplicationContext();
@@ -103,6 +105,21 @@ public void process() {
 		performAotProcessing(applicationContext);
 	}
 
+	private void deleteExistingOutput() {
+		deleteExistingOutput(this.sourceOutput, this.resourceOutput, this.classOutput);
+	}
+
+	private void deleteExistingOutput(Path... paths) {
+		for (Path path : paths) {
+			try {
+				FileSystemUtils.deleteRecursively(path);
+			}
+			catch (IOException ex) {
+				throw new RuntimeException("Failed to delete existing output in '" + path + "'");
+			}
+		}
+	}
+
 	private void callApplicationMainMethod() {
 		try {
 			this.application.getMethod("main", String[].class).invoke(null, new Object[] { this.applicationArgs });

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/AotProcessorTests.java

@@ -16,6 +16,8 @@
 
 package org.springframework.boot;
 
+import java.io.IOException;
+import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.function.Consumer;
 
@@ -76,6 +78,29 @@ void invokeMainWithMissingArguments() {
 				.withMessageContaining("Usage:");
 	}
 
+	@Test
+	void processingDeletesExistingOutput(@TempDir Path directory) throws IOException {
+		Path sourceOutput = directory.resolve("source");
+		Path resourceOutput = directory.resolve("resource");
+		Path classOutput = directory.resolve("class");
+		Path existingSourceOutput = createExisting(sourceOutput);
+		Path existingResourceOutput = createExisting(resourceOutput);
+		Path existingClassOutput = createExisting(classOutput);
+		AotProcessor processor = new AotProcessor(SampleApplication.class, new String[0], sourceOutput, resourceOutput,
+				classOutput, "com.example", "example");
+		processor.process();
+		assertThat(existingSourceOutput).doesNotExist();
+		assertThat(existingResourceOutput).doesNotExist();
+		assertThat(existingClassOutput).doesNotExist();
+	}
+
+	private Path createExisting(Path directory) throws IOException {
+		Path existing = directory.resolve("existing");
+		Files.createDirectories(directory);
+		Files.createFile(existing);
+		return existing;
+	}
+
 	private Consumer<Path> hasGeneratedAssetsForSampleApplication() {
 		return (directory) -> {
 			assertThat(directory.resolve(