Add AotDetector to reliably opt-in for optimizations

This commit adds a central utility to figure out if the application
must run with Ahead-Of-Time optimizations. This is mandatory for running
in a native image but can be selected on the JVM using the
"spring.aot.enabled" property.

This commit also introduces a utility that can be used to initialize a
context with generated artifacts. This represents the runtime
counterpart of ApplicationContextAotGenerator.

Closes gh-28474

spring-context/src/main/java/org/springframework/context/aot/ApplicationContextAotInitializer.java

@@ -0,0 +1,77 @@
+/*
+ * 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.context.aot;
+
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
+
+import org.springframework.beans.BeanUtils;
+import org.springframework.context.ApplicationContextInitializer;
+import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.lang.Nullable;
+import org.springframework.util.ClassUtils;
+
+/**
+ * Initializes a {@link ConfigurableApplicationContext} using AOT optimizations.
+ *
+ * @author Stephane Nicoll
+ * @since 6.0
+ */
+public class ApplicationContextAotInitializer {
+
+	private static final Log logger = LogFactory.getLog(ApplicationContextAotInitializer.class);
+
+	/**
+	 * Initialize the specified application context using the specified
+	 * {@link ApplicationContextInitializer} class names. Each class name is
+	 * expected to have a default constructor.
+	 * @param context the context to initialize
+	 * @param initializerClassNames the application context initializer class names
+	 */
+	public void initialize(ConfigurableApplicationContext context, String... initializerClassNames) {
+		if (logger.isDebugEnabled()) {
+			logger.debug("Initializing ApplicationContext with AOT");
+		}
+		for (String initializerClassName : initializerClassNames) {
+			if (logger.isTraceEnabled()) {
+				logger.trace("Applying " + initializerClassName);
+			}
+			loadInitializer(initializerClassName, context.getClassLoader()).initialize(context);
+		}
+	}
+
+	@SuppressWarnings("unchecked")
+	private ApplicationContextInitializer<ConfigurableApplicationContext> loadInitializer(
+			String className, @Nullable ClassLoader classLoader) {
+		Object initializer = instantiate(className, classLoader);
+		if (!(initializer instanceof ApplicationContextInitializer)) {
+			throw new IllegalArgumentException("Not an ApplicationContextInitializer: " + className);
+		}
+		return (ApplicationContextInitializer<ConfigurableApplicationContext>) initializer;
+	}
+
+	private static Object instantiate(String className, @Nullable ClassLoader classLoader) {
+		try {
+			Class<?> type = ClassUtils.forName(className, classLoader);
+			return BeanUtils.instantiateClass(type);
+		}
+		catch (Exception ex) {
+			throw new IllegalArgumentException("Failed to instantiate ApplicationContextInitializer: " + className, ex);
+		}
+	}
+
+}

spring-context/src/test/java/org/springframework/context/aot/ApplicationContextAotInitializerTests.java

@@ -0,0 +1,101 @@
+/*
+ * 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.context.aot;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.beans.factory.support.RootBeanDefinition;
+import org.springframework.context.ApplicationContextInitializer;
+import org.springframework.context.support.GenericApplicationContext;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
+
+/**
+ * Tests for {@link ApplicationContextAotInitializer}.
+ *
+ * @author Stephane Nicoll
+ */
+class ApplicationContextAotInitializerTests {
+
+	private final ApplicationContextAotInitializer initializer = new ApplicationContextAotInitializer();
+
+	@Test
+	void initializeInvokeApplicationContextInitializer() {
+		GenericApplicationContext context = new GenericApplicationContext();
+		initializer.initialize(context, TestApplicationContextInitializer.class.getName());
+		assertThat(context.getBeanDefinitionNames()).containsExactly("test");
+	}
+
+	@Test
+	void initializeInvokeApplicationContextInitializersInOrder() {
+		GenericApplicationContext context = new GenericApplicationContext();
+		initializer.initialize(context, AnotherApplicationContextInitializer.class.getName(),
+				TestApplicationContextInitializer.class.getName());
+		assertThat(context.getBeanDefinitionNames()).containsExactly("another", "test");
+	}
+
+	@Test
+	void initializeFailWithNonApplicationContextInitializer() {
+		GenericApplicationContext context = new GenericApplicationContext();
+		assertThatIllegalArgumentException()
+				.isThrownBy(() -> initializer.initialize(context, "java.lang.String"))
+				.withMessageContaining("Not an ApplicationContextInitializer: ")
+				.withMessageContaining("java.lang.String");
+	}
+
+	@Test
+	void initializeFailWithApplicationContextInitializerAndNonDefaultConstructor() {
+		GenericApplicationContext context = new GenericApplicationContext();
+		assertThatIllegalArgumentException()
+				.isThrownBy(() -> initializer.initialize(context,
+						ConfigurableApplicationContextInitializer.class.getName()))
+				.withMessageContaining("Failed to instantiate ApplicationContextInitializer: ")
+				.withMessageContaining(ConfigurableApplicationContextInitializer.class.getName());
+	}
+
+
+	static class TestApplicationContextInitializer implements ApplicationContextInitializer<GenericApplicationContext> {
+
+		@Override
+		public void initialize(GenericApplicationContext applicationContext) {
+			applicationContext.registerBeanDefinition("test", new RootBeanDefinition());
+		}
+
+	}
+
+	static class AnotherApplicationContextInitializer implements ApplicationContextInitializer<GenericApplicationContext> {
+
+		@Override
+		public void initialize(GenericApplicationContext applicationContext) {
+			applicationContext.registerBeanDefinition("another", new RootBeanDefinition());
+		}
+
+	}
+
+	static class ConfigurableApplicationContextInitializer implements ApplicationContextInitializer<GenericApplicationContext> {
+
+		public ConfigurableApplicationContextInitializer(ClassLoader classLoader) {
+		}
+
+		@Override
+		public void initialize(GenericApplicationContext applicationContext) {
+
+		}
+	}
+
+}

spring-core/src/main/java/org/springframework/aot/AotDetector.java

@@ -0,0 +1,49 @@
+/*
+ * 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.aot;
+
+import org.springframework.core.NativeDetector;
+import org.springframework.core.SpringProperties;
+
+/**
+ * Determine if AOT-processed optimizations must be used rather than the
+ * regular runtime. Strictly for internal use within the framework.
+ *
+ * @author Stephane Nicoll
+ * @since 6.0
+ */
+public abstract class AotDetector {
+
+	/**
+	 * System property that indicates the application should run with AOT
+	 * generated artifacts. If such optimizations are not available, it is
+	 * recommended to throw an exception rather than falling back to the
+	 * regular runtime behavior.
+	 */
+	public static final String AOT_ENABLED = "spring.aot.enabled";
+
+	/**
+	 * Return whether AOT optimizations must be considered at runtime. This
+	 * is mandatory in a native image but can be triggered on the JVM using
+	 * the {@value AOT_ENABLED} spring property.
+	 * @return whether AOT optimizations must be considered
+	 */
+	public static boolean useGeneratedArtifacts() {
+		return (NativeDetector.inNativeImage() || SpringProperties.getFlag(AOT_ENABLED));
+	}
+
+}

spring-core/src/main/resources/META-INF/native-image/org.springframework/spring-core/native-image.properties

@@ -1,2 +1,3 @@
-Args = --initialize-at-build-time=org.springframework.core.NativeDetector \
+Args = --initialize-at-build-time=org.springframework.aot.AotDetector \
+--initialize-at-build-time=org.springframework.core.NativeDetector \
 --initialize-at-build-time=org.springframework.util.unit.DataSize