Expose a system property when AOT processing is running

This commit exposes a "spring.aot.processing" system property when the
AOT engine is running. This can be used by code that need to react
differently when the application is being refreshed for AOT processing.

Closes gh-29340

Author: snicoll

Diff for: spring-context/src/main/java/org/springframework/context/aot/AbstractAotProcessor.java

@@ -30,20 +30,28 @@
 /**
  * Abstract base class for filesystem-based ahead-of-time (AOT) processing.
  *
- * <p>Concrete implementations are typically used to kick off optimization of an
- * application or test suite in a build tool.
+ * <p>Concrete implementations should override {@link #doProcess()} that kicks
+ * off the optimization of the target, usually an application.
  *
  * @author Stephane Nicoll
  * @author Andy Wilkinson
  * @author Phillip Webb
  * @author Sam Brannen
  * @since 6.0
+ * @param <T> the type of the processing result
  * @see FileSystemGeneratedFiles
  * @see FileNativeConfigurationWriter
  * @see org.springframework.context.aot.ContextAotProcessor
  * @see org.springframework.test.context.aot.TestAotProcessor
  */
-public abstract class AbstractAotProcessor {
+public abstract class AbstractAotProcessor<T> {
+
+	/**
+	 * The name of a system property that is made available when the processor
+	 * runs.
+	 * @see #doProcess()
+	 */
+	private static final String AOT_PROCESSING = "spring.aot.processing";
 
 	private final Settings settings;
 
@@ -64,6 +72,22 @@ protected Settings getSettings() {
 		return this.settings;
 	}
 
+	/**
+	 * Run AOT processing.
+	 * @return the result of the processing.
+	 */
+	public final T process() {
+		try {
+			System.setProperty(AOT_PROCESSING, "true");
+			return doProcess();
+		}
+		finally {
+			System.clearProperty(AOT_PROCESSING);
+		}
+	}
+
+	protected abstract T doProcess();
+
 	/**
 	 * Delete the source, resource, and class output directories.
 	 */

Diff for: spring-context/src/main/java/org/springframework/context/aot/ContextAotProcessor.java

@@ -46,7 +46,7 @@
  * @since 6.0
  * @see org.springframework.test.context.aot.TestAotProcessor
  */
-public abstract class ContextAotProcessor extends AbstractAotProcessor {
+public abstract class ContextAotProcessor extends AbstractAotProcessor<ClassName> {
 
 	private final Class<?> applicationClass;
 
@@ -64,7 +64,7 @@ protected ContextAotProcessor(Class<?> applicationClass, Settings settings) {
 
 
 	/**
-	 * Get the the application entry point (typically a class with a {@code main()} method).
+	 * Get the application entry point (typically a class with a {@code main()} method).
 	 */
 	protected Class<?> getApplicationClass() {
 		return this.applicationClass;
@@ -77,7 +77,8 @@ protected Class<?> getApplicationClass() {
 	 * @return the {@code ClassName} of the {@code ApplicationContextInitializer}
 	 * entry point
 	 */
-	public ClassName process() {
+	@Override
+	protected ClassName doProcess() {
 		deleteExistingOutput();
 		GenericApplicationContext applicationContext = prepareApplicationContext(getApplicationClass());
 		return performAotProcessing(applicationContext);

Diff for: spring-context/src/test/java/org/springframework/context/aot/AotProcessorTests.java

@@ -27,13 +27,26 @@
 import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 
 /**
- * Tests for {@link AbstractAotProcessor}, settings, and builder.
+ * Tests for {@link AbstractAotProcessor}.
  *
  * @author Sam Brannen
+ * @author Stephane Nicoll
  * @since 6.0
  */
 class AotProcessorTests {
 
+	@Test
+	void springAotProcessingIsAvailableInDoProcess(@TempDir Path tempDir) {
+		Settings settings = createTestSettings(tempDir);
+		assertThat(new AbstractAotProcessor<String>(settings) {
+			@Override
+			protected String doProcess() {
+				assertThat(System.getProperty("spring.aot.processing")).isEqualTo("true");
+				return "Hello";
+			}
+		}.process()).isEqualTo("Hello");
+	}
+
 	@Test
 	void builderRejectsMissingSourceOutput() {
 		assertThatIllegalArgumentException()
@@ -123,4 +136,14 @@ void builderAcceptsRequiredSettings(@TempDir Path tempDir) {
 		assertThat(settings.getArtifactId()).isEqualTo("my-artifact");
 	}
 
+	private static Settings createTestSettings(Path tempDir) {
+		return Settings.builder()
+				.sourceOutput(tempDir)
+				.resourceOutput(tempDir)
+				.classOutput(tempDir)
+				.groupId("my-group")
+				.artifactId("my-artifact")
+				.build();
+	}
+
 }

Diff for: spring-test/src/main/java/org/springframework/test/context/aot/TestAotProcessor.java

@@ -36,7 +36,7 @@
  * @see TestContextAotGenerator
  * @see org.springframework.context.aot.ContextAotProcessor
  */
-public abstract class TestAotProcessor extends AbstractAotProcessor {
+public abstract class TestAotProcessor extends AbstractAotProcessor<Void> {
 
 	private final Set<Path> classpathRoots;
 
@@ -66,9 +66,11 @@ protected Set<Path> getClasspathRoots() {
 	 * {@linkplain #deleteExistingOutput() clearing output directories} first and
 	 * then {@linkplain #performAotProcessing() performing AOT processing}.
 	 */
-	public void process() {
+	@Override
+	protected Void doProcess() {
 		deleteExistingOutput();
 		performAotProcessing();
+		return null;
 	}
 
 	/**