Replace AotApplicationContextInitializer with interface

Replace the `ApplicationContextAotInitializer` class with an
`AotApplicationContextInitializer` interface so that its use can be
detected using a simple `instanceof` check. The existing functionality
has been moved to a factory method on the interface allowing:

	`new ApplicationContextAotInitializer()
		.initialize(context, names);`

To now be written as:

	`AotApplicationContextInitializer.forInitializerClasses(names)
		.initialize(context);`

See gh-29157

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

@@ -0,0 +1,119 @@
+/*
+ * 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 java.util.List;
+
+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 AotApplicationContextInitializer}.
+ *
+ * @author Stephane Nicoll
+ */
+class AotApplicationContextInitializerTests {
+
+	@Test
+	void initializeInvokesApplicationContextInitializer() {
+		GenericApplicationContext context = new GenericApplicationContext();
+		AotApplicationContextInitializer.forInitializerClasses(
+				TestApplicationContextInitializer.class.getName())
+				.initialize(context);
+		assertThat(context.getBeanDefinitionNames()).containsExactly("test");
+	}
+
+	@Test
+	void initializeInvokesApplicationContextInitializersInOrder() {
+		GenericApplicationContext context = new GenericApplicationContext();
+		AotApplicationContextInitializer.forInitializerClasses(
+				AnotherApplicationContextInitializer.class.getName(),
+				TestApplicationContextInitializer.class.getName())
+				.initialize(context);
+		assertThat(context.getBeanDefinitionNames()).containsExactly("another", "test");
+	}
+
+	@Test
+	void initializeWhenClassIsNotApplicationContextInitializerThrowsException() {
+		GenericApplicationContext context = new GenericApplicationContext();
+		assertThatIllegalArgumentException()
+				.isThrownBy(() -> AotApplicationContextInitializer.forInitializerClasses("java.lang.String")
+						.initialize(context))
+				.withMessageContaining("not assignable")
+				.withMessageContaining("ApplicationContextInitializer")
+				.withMessageContaining("java.lang.String");
+	}
+
+	@Test
+	void initializeWhenInitializerHasNoDefaultConstructorThrowsException() {
+		GenericApplicationContext context = new GenericApplicationContext();
+		assertThatIllegalArgumentException()
+				.isThrownBy(() -> AotApplicationContextInitializer.forInitializerClasses(
+						ConfigurableApplicationContextInitializer.class.getName()).initialize(context))
+				.withMessageContaining("Failed to instantiate ApplicationContextInitializer: ")
+				.withMessageContaining(ConfigurableApplicationContextInitializer.class.getName());
+	}
+
+	@Test
+	void getAotInitializersReturnsOnlyAotInitializers() {
+		ApplicationContextInitializer<GenericApplicationContext> l1 = context -> { };
+		ApplicationContextInitializer<GenericApplicationContext> l2 = context -> { };
+		AotApplicationContextInitializer<GenericApplicationContext> a1 = context -> { };
+		AotApplicationContextInitializer<GenericApplicationContext> a2 = l2::initialize;
+		List<ApplicationContextInitializer<GenericApplicationContext>> initializers = List.of(l1, l2, a1, a2);
+		List<AotApplicationContextInitializer<GenericApplicationContext>> aotInitializers = AotApplicationContextInitializer
+				.getAotInitializers(initializers);
+		assertThat(aotInitializers).containsExactly(a1, a2);
+	}
+
+
+	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-context/src/test/java/org/springframework/context/aot/ApplicationContextAotInitializerTests.java

@@ -30,6 +30,7 @@ import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException
  *
  * @author Stephane Nicoll
  */
+@Deprecated
 class ApplicationContextAotInitializerTests {
 
 	private final ApplicationContextAotInitializer initializer = new ApplicationContextAotInitializer();
@@ -54,7 +55,8 @@ class ApplicationContextAotInitializerTests {
 		GenericApplicationContext context = new GenericApplicationContext();
 		assertThatIllegalArgumentException()
 				.isThrownBy(() -> initializer.initialize(context, "java.lang.String"))
-				.withMessageContaining("Not an ApplicationContextInitializer: ")
+				.withMessageContaining("not assignable")
+				.withMessageContaining("ApplicationContextInitializer")
 				.withMessageContaining("java.lang.String");
 	}