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/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) {
+
+		}
+	}
+
+}