Introduce TransactionalApplicationListener interface (with callback support)

Includes forPayload methods and common adapter classes for programmatic usage.
Aligns default order values for event handling delegates to LOWEST_PRECEDENCE.

Closes gh-24163

spring-context/src/main/java/org/springframework/context/ApplicationListener.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -17,6 +17,7 @@
 package org.springframework.context;
 
 import java.util.EventListener;
+import java.util.function.Consumer;
 
 /**
  * Interface to be implemented by application event listeners.
@@ -45,4 +46,17 @@ public interface ApplicationListener<E extends ApplicationEvent> extends EventLi
 	 */
 	void onApplicationEvent(E event);
 
+
+	/**
+	 * Create a new {@code ApplicationListener} for the given payload consumer.
+	 * @param consumer the event payload consumer
+	 * @param <T> the type of the event payload
+	 * @return a corresponding {@code ApplicationListener} instance
+	 * @since 5.3
+	 * @see PayloadApplicationEvent
+	 */
+	static <T> ApplicationListener<PayloadApplicationEvent<T>> forPayload(Consumer<T> consumer) {
+		return event -> consumer.accept(event.getPayload());
+	}
+
 }

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

@@ -37,6 +37,7 @@ import org.springframework.context.ApplicationEvent;
 import org.springframework.context.PayloadApplicationEvent;
 import org.springframework.context.expression.AnnotatedElementKey;
 import org.springframework.core.BridgeMethodResolver;
+import org.springframework.core.Ordered;
 import org.springframework.core.ReactiveAdapter;
 import org.springframework.core.ReactiveAdapterRegistry;
 import org.springframework.core.ResolvableType;
@@ -95,6 +96,12 @@ public class ApplicationListenerMethodAdapter implements GenericApplicationListe
 	private EventExpressionEvaluator evaluator;
 
 
+	/**
+	 * Construct a new ApplicationListenerMethodAdapter.
+	 * @param beanName the name of the bean to invoke the listener method on
+	 * @param targetClass the target class that the method is declared on
+	 * @param method the listener method to invoke
+	 */
 	public ApplicationListenerMethodAdapter(String beanName, Class<?> targetClass, Method method) {
 		this.beanName = beanName;
 		this.method = BridgeMethodResolver.findBridgedMethod(method);
@@ -135,7 +142,7 @@ public class ApplicationListenerMethodAdapter implements GenericApplicationListe
 
 	private static int resolveOrder(Method method) {
 		Order ann = AnnotatedElementUtils.findMergedAnnotation(method, Order.class);
-		return (ann != null ? ann.value() : 0);
+		return (ann != null ? ann.value() : Ordered.LOWEST_PRECEDENCE);
 	}
 
 
@@ -332,6 +339,14 @@ public class ApplicationListenerMethodAdapter implements GenericApplicationListe
 		return this.applicationContext.getBean(this.beanName);
 	}
 
+	/**
+	 * Return the target listener method.
+	 * @since 5.3
+	 */
+	protected Method getTargetMethod() {
+		return this.targetMethod;
+	}
+
 	/**
 	 * Return the condition to use.
 	 * <p>Matches the {@code condition} attribute of the {@link EventListener}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -29,6 +29,7 @@ import org.springframework.core.Ordered;
  *
  * @author Stephane Nicoll
  * @since 4.2
+ * @see ApplicationListenerMethodAdapter
  */
 public class DefaultEventListenerFactory implements EventListenerFactory, Ordered {