GH-1982: Boot 2.6 Compatibility (Retryable Topics)

Resolves #1982

Spring Boot 2.6 disables circular bean references by default.

When using `RetryTopicConfigurationBuilder.dltHandlerMethod` to specify the location of
the bean handler, a circular reference is caused if the method is in the same bean as
the listener (because the `EndpointHandlerMethod` attempts to resolve it while the listener
bean is being post processed).

Deprecate `RetryTopicConfigurationBuilder.dltHandlerMethod(Class<?> clazz, String methodName)`.
As an aside, it is antithetical in Spring to expect a single bean with a type.
Replace with `dltHandlerMethod(String beanName, String methodName)`.
This allows deferring the bean lookup if a `BeanCurrentlyInCreationException` is thrown.
When that happens, return the "endpoint" as the "bean" and a dummy `Method` to satisfy
nullable requirements.
When the registry detects that the "bean" is an "endpoint", resolve the bean and method
at that time and re-populate the `MethodKafkaListenerEndpoint`.

Move `EndpointHandlerMethod` to `support` to avoid a package tangle.

Add a test to simulate Boot 2.6 default behavior.

* Remove deprecated method from docs.

Author: garyrussell

spring-kafka-docs/src/main/asciidoc/retrytopic.adoc

@@ -500,8 +500,7 @@ public void processMessage(MyPojo message) {
 ----
 ====
 
-The DLT handler method can also be provided through the RetryTopicConfigurationBuilder.dltHandlerMethod(Class, String) method, passing as arguments the class and method name that should process the DLT's messages.
-If a bean instance of the provided class is found in the application context that bean is used for Dlt processing, otherwise an instance is created with full dependency injection support.
+The DLT handler method can also be provided through the RetryTopicConfigurationBuilder.dltHandlerMethod(String, String) method, passing as arguments the bean name and method name that should process the DLT's messages.
 
 ====
 [source, java]
@@ -510,7 +509,7 @@ If a bean instance of the provided class is found in the application context tha
 public RetryTopicConfiguration myRetryTopic(KafkaTemplate<Integer, MyPojo> template) {
     return RetryTopicConfigurationBuilder
             .newInstance()
-            .dltProcessor(MyCustomDltProcessor.class, "processDltMessage")
+            .dltProcessor("myCustomDltProcessor", "processDltMessage")
             .create(template);
 }
 

spring-kafka/src/main/java/org/springframework/kafka/annotation/RetryableTopicAnnotationProcessor.java

@@ -32,12 +32,12 @@
 import org.springframework.core.annotation.AnnotationUtils;
 import org.springframework.expression.spel.support.StandardEvaluationContext;
 import org.springframework.kafka.core.KafkaOperations;
-import org.springframework.kafka.retrytopic.EndpointHandlerMethod;
 import org.springframework.kafka.retrytopic.RetryTopicConfiguration;
 import org.springframework.kafka.retrytopic.RetryTopicConfigurationBuilder;
 import org.springframework.kafka.retrytopic.RetryTopicConfigurer;
 import org.springframework.kafka.retrytopic.RetryTopicConstants;
 import org.springframework.kafka.retrytopic.RetryTopicInternalBeanNames;
+import org.springframework.kafka.support.EndpointHandlerMethod;
 import org.springframework.retry.annotation.Backoff;
 import org.springframework.retry.backoff.ExponentialBackOffPolicy;
 import org.springframework.retry.backoff.ExponentialRandomBackOffPolicy;

spring-kafka/src/main/java/org/springframework/kafka/config/KafkaListenerEndpointRegistry.java

@@ -42,6 +42,7 @@
 import org.springframework.kafka.listener.ContainerGroup;
 import org.springframework.kafka.listener.ListenerContainerRegistry;
 import org.springframework.kafka.listener.MessageListenerContainer;
+import org.springframework.kafka.support.EndpointHandlerMethod;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
@@ -214,6 +215,16 @@ public void registerListenerContainer(KafkaListenerEndpoint endpoint, KafkaListe
 	protected MessageListenerContainer createListenerContainer(KafkaListenerEndpoint endpoint,
 			KafkaListenerContainerFactory<?> factory) {
 
+		if (endpoint instanceof MethodKafkaListenerEndpoint) {
+			MethodKafkaListenerEndpoint<?, ?> mkle = (MethodKafkaListenerEndpoint<?, ?>) endpoint;
+			Object bean = mkle.getBean();
+			if (bean instanceof EndpointHandlerMethod) {
+				EndpointHandlerMethod ehm = (EndpointHandlerMethod) bean;
+				ehm = new EndpointHandlerMethod(ehm.resolveBean(this.applicationContext), ehm.getMethodName());
+				mkle.setBean(ehm.resolveBean(this.applicationContext));
+				mkle.setMethod(ehm.getMethod());
+			}
+		}
 		MessageListenerContainer listenerContainer = factory.createListenerContainer(endpoint);
 
 		if (listenerContainer instanceof InitializingBean) {

spring-kafka/src/main/java/org/springframework/kafka/retrytopic/EndpointCustomizerFactory.java

@@ -23,6 +23,7 @@
 
 import org.springframework.beans.factory.BeanFactory;
 import org.springframework.kafka.config.MethodKafkaListenerEndpoint;
+import org.springframework.kafka.support.EndpointHandlerMethod;
 import org.springframework.kafka.support.TopicPartitionOffset;
 
 /**

spring-kafka/src/main/java/org/springframework/kafka/retrytopic/EndpointHandlerMethod.java

@@ -19,6 +19,7 @@
 import java.util.List;
 
 import org.springframework.kafka.support.AllowDenyCollectionManager;
+import org.springframework.kafka.support.EndpointHandlerMethod;
 
 /**
  * Contains the provided configuration for the retryable topics.

spring-kafka/src/main/java/org/springframework/kafka/retrytopic/RetryTopicConfiguration.java

@@ -24,6 +24,7 @@
 import org.springframework.kafka.config.ConcurrentKafkaListenerContainerFactory;
 import org.springframework.kafka.core.KafkaOperations;
 import org.springframework.kafka.support.AllowDenyCollectionManager;
+import org.springframework.kafka.support.EndpointHandlerMethod;
 import org.springframework.lang.Nullable;
 import org.springframework.retry.backoff.BackOffPolicy;
 import org.springframework.retry.backoff.ExponentialBackOffPolicy;
@@ -80,11 +81,31 @@ public class RetryTopicConfigurationBuilder {
 	private Boolean autoStartDltHandler;
 
 	/* ---------------- DLT Behavior -------------- */
+	/**
+	 * Configure a DLT handler method.
+	 * @param clazz the class containing the method.
+	 * @param methodName the method name.
+	 * @return the builder.
+	 * @deprecated in favor of {@link #dltHandlerMethod(String, String)}.
+	 */
+	@Deprecated
 	public RetryTopicConfigurationBuilder dltHandlerMethod(Class<?> clazz, String methodName) {
 		this.dltHandlerMethod = RetryTopicConfigurer.createHandlerMethodWith(clazz, methodName);
 		return this;
 	}
 
+	/**
+	 * Configure a DLT handler method.
+	 * @param beanName the bean name.
+	 * @param methodName the method name.
+	 * @return the builder.
+	 * @since 2.8
+	 */
+	public RetryTopicConfigurationBuilder dltHandlerMethod(String beanName, String methodName) {
+		this.dltHandlerMethod = RetryTopicConfigurer.createHandlerMethodWith(beanName, methodName);
+		return this;
+	}
+
 	public RetryTopicConfigurationBuilder dltHandlerMethod(
 			EndpointHandlerMethod endpointHandlerMethod) {
 

spring-kafka/src/main/java/org/springframework/kafka/retrytopic/RetryTopicConfigurationBuilder.java

@@ -34,6 +34,7 @@
 import org.springframework.kafka.config.MethodKafkaListenerEndpoint;
 import org.springframework.kafka.config.MultiMethodKafkaListenerEndpoint;
 import org.springframework.kafka.listener.ListenerUtils;
+import org.springframework.kafka.support.EndpointHandlerMethod;
 import org.springframework.lang.Nullable;
 
 
@@ -387,8 +388,8 @@ private void throwIfMultiMethodEndpoint(MethodKafkaListenerEndpoint<?, ?> mainEn
 		}
 	}
 
-	public static EndpointHandlerMethod createHandlerMethodWith(Class<?> beanClass, String methodName) {
-		return new EndpointHandlerMethod(beanClass, methodName);
+	public static EndpointHandlerMethod createHandlerMethodWith(Object beanOrClass, String methodName) {
+		return new EndpointHandlerMethod(beanOrClass, methodName);
 	}
 
 	public static EndpointHandlerMethod createHandlerMethodWith(Object bean, Method method) {

spring-kafka/src/main/java/org/springframework/kafka/retrytopic/RetryTopicConfigurer.java

@@ -0,0 +1,134 @@
+/*
+ * Copyright 2021 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.kafka.support;
+
+import java.lang.reflect.Method;
+import java.util.Arrays;
+
+import org.springframework.beans.factory.BeanCurrentlyInCreationException;
+import org.springframework.beans.factory.BeanFactory;
+import org.springframework.beans.factory.NoSuchBeanDefinitionException;
+import org.springframework.beans.factory.support.BeanDefinitionRegistry;
+import org.springframework.beans.factory.support.RootBeanDefinition;
+import org.springframework.util.Assert;
+import org.springframework.util.ReflectionUtils;
+
+/**
+ * Handler method for retrying endpoints.
+ *
+ * @author Tomaz Fernandes
+ * @author Gary Russell
+ * @since 2.7
+ *
+ */
+public class EndpointHandlerMethod {
+
+	private final Object beanOrClass;
+
+	private final String methodName;
+
+	private Object bean;
+
+	private Method method;
+
+	public EndpointHandlerMethod(Object beanOrClass, String methodName) {
+		Assert.notNull(beanOrClass, () -> "No destination bean or class provided!");
+		Assert.notNull(methodName, () -> "No method name for destination bean class provided!");
+		this.beanOrClass = beanOrClass;
+		this.methodName = methodName;
+	}
+
+	public EndpointHandlerMethod(Object bean, Method method) {
+		Assert.notNull(bean, () -> "No bean for destination provided!");
+		Assert.notNull(method, () -> "No method for destination bean class provided!");
+		this.method = method;
+		this.bean = bean;
+		this.beanOrClass = bean.getClass();
+		this.methodName = method.getName();
+	}
+
+	/**
+	 * Return the method.
+	 * @return the method.
+	 */
+	public Method getMethod() {
+		if (this.beanOrClass instanceof Class) {
+			return forClass((Class<?>) this.beanOrClass);
+		}
+		Assert.state(this.bean != null, "Bean must be resolved before accessing its method");
+		if (this.bean instanceof EndpointHandlerMethod) {
+			try {
+				return Object.class.getMethod("toString");
+			}
+			catch (NoSuchMethodException | SecurityException e) {
+			}
+		}
+		return forClass(this.bean.getClass());
+	}
+
+	/**
+	 * Return the method name.
+	 * @return the name.
+	 * @since 2.8
+	 */
+	public String getMethodName() {
+		Assert.state(this.methodName != null, "Unexpected call to getMethodName()");
+		return this.methodName;
+	}
+
+	public Object resolveBean(BeanFactory beanFactory) {
+		if (this.bean instanceof EndpointHandlerMethod) {
+			return ((EndpointHandlerMethod) this.bean).beanOrClass;
+		}
+		if (this.bean == null) {
+			try {
+				if (this.beanOrClass instanceof Class) {
+					Class<?> clazz = (Class<?>) this.beanOrClass;
+					try {
+						this.bean = beanFactory.getBean(clazz);
+					}
+					catch (NoSuchBeanDefinitionException e) {
+						String beanName = clazz.getSimpleName() + "-handlerMethod";
+						((BeanDefinitionRegistry) beanFactory).registerBeanDefinition(beanName,
+								new RootBeanDefinition(clazz));
+						this.bean = beanFactory.getBean(beanName);
+					}
+				}
+				else {
+					String beanName = (String) this.beanOrClass;
+					this.bean = beanFactory.getBean(beanName);
+				}
+			}
+			catch (BeanCurrentlyInCreationException ex) {
+				this.bean = this;
+			}
+		}
+		return this.bean;
+	}
+
+	private Method forClass(Class<?> clazz) {
+		if (this.method == null) {
+			this.method = Arrays.stream(ReflectionUtils.getDeclaredMethods(clazz))
+					.filter(mthd -> mthd.getName().equals(this.methodName))
+					.findFirst()
+					.orElseThrow(() -> new IllegalArgumentException(
+							String.format("No method %s in class %s", this.methodName, clazz)));
+		}
+		return this.method;
+	}
+
+}