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);
+		}
+	}
+
+}