Add support for functional registration of application listener

This commit adds a functional style registration of an application
listener for a particular event. Rather than introducing another method
at the ConfigurableApplicationContext interface level, this commit
provides a factory method in GenericApplicationListener.

Closes gh-21411

Author: snicoll

spring-context/src/main/java/org/springframework/context/event/GenericApplicationListener.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -16,6 +16,8 @@
 
 package org.springframework.context.event;
 
+import java.util.function.Consumer;
+
 import org.springframework.context.ApplicationEvent;
 import org.springframework.context.ApplicationListener;
 import org.springframework.core.ResolvableType;
@@ -53,4 +55,16 @@ default boolean supportsEventType(Class<? extends ApplicationEvent> eventType) {
 	 */
 	boolean supportsEventType(ResolvableType eventType);
 
+	/**
+	 * Create a new {@code ApplicationListener} for the given event type.
+	 * @param eventType the event to listen to
+	 * @param consumer the consumer to invoke when a matching event is fired
+	 * @param <E> the specific {@code ApplicationEvent} subclass to listen to
+	 * @return a corresponding {@code ApplicationListener} instance
+	 * @since 6.1.3
+	 */
+	static <E extends ApplicationEvent> GenericApplicationListener forEventType(Class<E> eventType, Consumer<E> consumer) {
+		return new GenericApplicationListenerDelegate<>(eventType, consumer);
+	}
+
 }

spring-context/src/main/java/org/springframework/context/event/GenericApplicationListenerDelegate.java

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2002-2024 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.event;
+
+import java.util.function.Consumer;
+
+import org.springframework.context.ApplicationEvent;
+import org.springframework.core.ResolvableType;
+
+/**
+ * A {@link GenericApplicationListener} implementation that supports a single
+ * event type.
+ *
+ * @author Stephane Nicoll
+ * @since 6.1.3
+ * @param <E> the specific {@code ApplicationEvent} subclass to listen to
+ */
+class GenericApplicationListenerDelegate<E extends ApplicationEvent> implements GenericApplicationListener {
+
+	private final Class<E> supportedEventType;
+
+	private final Consumer<E> consumer;
+
+	GenericApplicationListenerDelegate(Class<E> supportedEventType, Consumer<E> consumer) {
+		this.supportedEventType = supportedEventType;
+		this.consumer = consumer;
+	}
+
+	@Override
+	public void onApplicationEvent(ApplicationEvent event) {
+		this.consumer.accept(this.supportedEventType.cast(event));
+	}
+
+	@Override
+	public boolean supportsEventType(ResolvableType eventType) {
+		return this.supportedEventType.isAssignableFrom(eventType.toClass());
+	}
+
+}

spring-context/src/test/java/org/springframework/context/event/ApplicationContextEventTests.java

@@ -21,9 +21,11 @@
 import java.util.List;
 import java.util.Set;
 import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.function.Consumer;
 
 import org.aopalliance.intercept.MethodInvocation;
 import org.junit.jupiter.api.Test;
+import org.mockito.ArgumentCaptor;
 
 import org.springframework.aop.framework.ProxyFactory;
 import org.springframework.beans.BeansException;
@@ -65,6 +67,7 @@
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.verifyNoMoreInteractions;
 import static org.springframework.context.support.AbstractApplicationContext.APPLICATION_EVENT_MULTICASTER_BEAN_NAME;
 
 /**
@@ -534,6 +537,20 @@ public void lambdaAsListenerWithJava9StyleClassCastMessage() {
 		context.close();
 	}
 
+	@Test
+	@SuppressWarnings("unchecked")
+	void addListenerWithConsumer() {
+		Consumer<ContextRefreshedEvent> consumer = mock(Consumer.class);
+		GenericApplicationContext context = new GenericApplicationContext();
+		context.addApplicationListener(GenericApplicationListener.forEventType(
+				ContextRefreshedEvent.class, consumer));
+		context.refresh();
+		ArgumentCaptor<ContextRefreshedEvent> captor = ArgumentCaptor.forClass(ContextRefreshedEvent.class);
+		verify(consumer).accept(captor.capture());
+		assertThat(captor.getValue().getApplicationContext()).isSameAs(context);
+		verifyNoMoreInteractions(consumer);
+	}
+
 	@Test
 	public void beanPostProcessorPublishesEvents() {
 		GenericApplicationContext context = new GenericApplicationContext();

spring-context/src/test/java/org/springframework/context/event/GenericApplicationListenerTests.java

@@ -0,0 +1,73 @@
+/*
+ * Copyright 2002-2024 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.event;
+
+import java.util.function.Consumer;
+
+import org.junit.jupiter.api.Test;
+import org.mockito.InOrder;
+
+import org.springframework.core.ResolvableType;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.inOrder;
+import static org.mockito.Mockito.mock;
+
+/**
+ * Tests for {@link GenericApplicationListener}.
+ *
+ * @author Stephane Nicoll
+ */
+class GenericApplicationListenerTests extends AbstractApplicationEventListenerTests {
+
+	@Test
+	void forEventTypeWithStrictTypeMatching() {
+		GenericApplicationListener listener = GenericApplicationListener
+				.forEventType(StringEvent.class, event -> {});
+		assertThat(listener.supportsEventType(ResolvableType.forClass(StringEvent.class))).isTrue();
+	}
+
+	@Test
+	void forEventTypeWithSubClass() {
+		GenericApplicationListener listener = GenericApplicationListener
+				.forEventType(GenericTestEvent.class, event -> {});
+		assertThat(listener.supportsEventType(ResolvableType.forClass(StringEvent.class))).isTrue();
+	}
+
+	@Test
+	void forEventTypeWithSuperClass() {
+		GenericApplicationListener listener = GenericApplicationListener
+				.forEventType(StringEvent.class, event -> {});
+		assertThat(listener.supportsEventType(ResolvableType.forClass(GenericTestEvent.class))).isFalse();
+	}
+
+	@Test
+	@SuppressWarnings("unchecked")
+	void forEventTypeInvokesConsumer() {
+		Consumer<StringEvent> consumer = mock(Consumer.class);
+		GenericApplicationListener listener = GenericApplicationListener
+				.forEventType(StringEvent.class, consumer);
+		StringEvent event = new StringEvent(this, "one");
+		StringEvent event2 = new StringEvent(this, "two");
+		listener.onApplicationEvent(event);
+		listener.onApplicationEvent(event2);
+		InOrder ordered = inOrder(consumer);
+		ordered.verify(consumer).accept(event);
+		ordered.verify(consumer).accept(event2);
+	}
+
+}