Add support for CacheRequestHandlerAdvice

* Fix `AbstractMessageProcessingTransformer` to react for the
`AbstractIntegrationMessageBuilder` invocation result and don't wrap it
into the `Message`
* Demonstrate functionality in the `CacheRequestHandlerAdviceTests`

* Polishing and Docs

* Fix JavaDocs warnings

Doc polishing.

spring-integration-core/src/test/java/org/springframework/integration/handler/advice/CacheRequestHandlerAdviceTests.java

@@ -0,0 +1,146 @@
+/*
+ * Copyright 2019 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.integration.handler.advice;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.atomic.AtomicInteger;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.cache.Cache;
+import org.springframework.cache.CacheManager;
+import org.springframework.cache.concurrent.ConcurrentMapCacheManager;
+import org.springframework.cache.interceptor.CacheEvictOperation;
+import org.springframework.cache.interceptor.CachePutOperation;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.integration.annotation.ServiceActivator;
+import org.springframework.integration.annotation.Transformer;
+import org.springframework.integration.config.EnableIntegration;
+import org.springframework.integration.support.MessageBuilder;
+import org.springframework.messaging.Message;
+import org.springframework.messaging.MessageChannel;
+import org.springframework.messaging.support.GenericMessage;
+import org.springframework.test.annotation.DirtiesContext;
+import org.springframework.test.context.junit.jupiter.SpringJUnitConfig;
+
+/**
+ * @author Artem Bilan
+ *
+ * @since 5.2
+ */
+@SpringJUnitConfig
+@DirtiesContext
+public class CacheRequestHandlerAdviceTests {
+
+	private static final String TEST_CACHE = "testCache";
+
+	private static final String TEST_PUT_CACHE = "testPutCache";
+
+	@Autowired
+	private AtomicInteger cachedMethodCounter;
+
+	@Autowired
+	private MessageChannel transformerChannel;
+
+	@Autowired
+	private MessageChannel serviceChannel;
+
+	@Autowired
+	private CacheManager cacheManager;
+
+	@Test
+	@SuppressWarnings("unchecked")
+	void testCacheRequestHandlerAdvice() {
+		GenericMessage<String> testMessage1 = new GenericMessage<>("foo");
+		this.transformerChannel.send(testMessage1);
+		GenericMessage<String> testMessage2 = new GenericMessage<>("foo");
+		this.transformerChannel.send(testMessage2);
+		this.transformerChannel.send(new GenericMessage<>("foo"));
+
+		assertThat(this.cachedMethodCounter.get()).isEqualTo(1);
+		Cache testCache = cacheManager.getCache(TEST_CACHE);
+		assertThat(testCache).isNotNull();
+
+		ConcurrentMap<Object, Object> nativeCache = (ConcurrentMap<Object, Object>) testCache.getNativeCache();
+		assertThat(nativeCache).hasSize(1);
+		assertThat(nativeCache.values()).element(0).isSameAs(testMessage1);
+
+		this.serviceChannel.send(testMessage1);
+		this.serviceChannel.send(testMessage2);
+
+		assertThat(nativeCache).hasSize(0);
+
+		testCache = cacheManager.getCache(TEST_PUT_CACHE);
+		assertThat(testCache).isNotNull();
+
+		nativeCache = (ConcurrentMap<Object, Object>) testCache.getNativeCache();
+		assertThat(nativeCache).hasSize(1);
+		assertThat(nativeCache.values()).element(0).isSameAs(testMessage2);
+	}
+
+	@Configuration
+	@EnableIntegration
+	public static class Config {
+
+		@Bean
+		public CacheManager cacheManager() {
+			return new ConcurrentMapCacheManager();
+		}
+
+		@Bean
+		public CacheRequestHandlerAdvice cacheAdvice() {
+			CacheRequestHandlerAdvice cacheRequestHandlerAdvice = new CacheRequestHandlerAdvice(TEST_CACHE);
+			cacheRequestHandlerAdvice.setKeyExpressionString("payload");
+			return cacheRequestHandlerAdvice;
+		}
+
+		@Bean
+		public AtomicInteger cachedMethodCounter() {
+			return new AtomicInteger();
+		}
+
+		@Transformer(inputChannel = "transformerChannel", outputChannel = "nullChannel", adviceChain = "cacheAdvice")
+		public Object transform(Message<?> message) {
+			cachedMethodCounter().getAndIncrement();
+			return MessageBuilder.fromMessage(message);
+		}
+
+		@Bean
+		public CacheRequestHandlerAdvice cachePutAndEvictAdvice() {
+			CacheRequestHandlerAdvice cacheRequestHandlerAdvice = new CacheRequestHandlerAdvice();
+			cacheRequestHandlerAdvice.setKeyExpressionString("payload");
+			CachePutOperation.Builder cachePutBuilder = new CachePutOperation.Builder();
+			cachePutBuilder.setCacheName(TEST_PUT_CACHE);
+			CacheEvictOperation.Builder cacheEvictBuilder = new CacheEvictOperation.Builder();
+			cacheEvictBuilder.setCacheName(TEST_CACHE);
+			cacheRequestHandlerAdvice.setCacheOperations(cachePutBuilder.build(), cacheEvictBuilder.build());
+			return cacheRequestHandlerAdvice;
+		}
+
+		@ServiceActivator(inputChannel = "serviceChannel", outputChannel = "nullChannel",
+				adviceChain = "cachePutAndEvictAdvice")
+		public Message<?> service(Message<?> message) {
+			return message;
+		}
+
+	}
+
+}