Leniently support treating input Strings as JSON

Fixes #871

* Only support conversion based on `text/plain` header if target type is String;

spring-cloud-stream-integration-tests/src/test/java/org/springframework/cloud/stream/config/TextPlainConversionTest.java

@@ -43,7 +43,8 @@ import static org.assertj.core.api.Assertions.assertThat;
  * @since 1.2
  */
 @RunWith(SpringJUnit4ClassRunner.class)
-@SpringBootTest(classes = TextPlainConversionTest.FooProcessor.class)
+@SpringBootTest(classes = TextPlainConversionTest.FooProcessor.class,
+		webEnvironment = SpringBootTest.WebEnvironment.NONE)
 public class TextPlainConversionTest {
 
 	@Autowired

spring-cloud-stream-integration-tests/src/test/java/org/springframework/cloud/stream/config/TextPlainToJsonConversionTest.java

@@ -0,0 +1,114 @@
+/*
+ * Copyright 2017 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
+ *
+ *      http://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.cloud.stream.config;
+
+import java.util.concurrent.TimeUnit;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.cloud.stream.annotation.EnableBinding;
+import org.springframework.cloud.stream.annotation.StreamListener;
+import org.springframework.cloud.stream.binder.BinderFactory;
+import org.springframework.cloud.stream.messaging.Processor;
+import org.springframework.cloud.stream.test.binder.TestSupportBinder;
+import org.springframework.integration.support.MessageBuilder;
+import org.springframework.messaging.Message;
+import org.springframework.messaging.MessageChannel;
+import org.springframework.messaging.MessageHeaders;
+import org.springframework.messaging.handler.annotation.SendTo;
+import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author Marius Bogoevici
+ * @since 1.2
+ */
+@RunWith(SpringJUnit4ClassRunner.class)
+@SpringBootTest(classes = TextPlainToJsonConversionTest.FooProcessor.class, webEnvironment = SpringBootTest.WebEnvironment.NONE)
+public class TextPlainToJsonConversionTest {
+
+	@Autowired
+	private Processor testProcessor;
+
+	@Autowired
+	private BinderFactory binderFactory;
+
+	@Test
+	public void testNoContentTypeToJsonConversionOnInput() throws Exception {
+		testProcessor.input().send(MessageBuilder.withPayload("{\"name\":\"Bar\"}").build());
+		@SuppressWarnings("unchecked")
+		Message<?> received = ((TestSupportBinder) binderFactory.getBinder(null, MessageChannel.class))
+				.messageCollector().forChannel(testProcessor.output()).poll(1, TimeUnit.SECONDS);
+		assertThat(received).isNotNull();
+		assertThat(((Foo) received.getPayload()).getName()).isEqualTo("transformed-Bar");
+	}
+
+	@Test
+	public void testTextPlainToJsonConversionOnInput() throws Exception {
+		testProcessor.input().send(MessageBuilder.withPayload("{\"name\":\"Bar\"}")
+				.setHeader(MessageHeaders.CONTENT_TYPE, "text/plain").build());
+		@SuppressWarnings("unchecked")
+		Message<?> received = ((TestSupportBinder) binderFactory.getBinder(null, MessageChannel.class))
+				.messageCollector().forChannel(testProcessor.output()).poll(1, TimeUnit.SECONDS);
+		assertThat(received).isNotNull();
+		assertThat(((Foo) received.getPayload()).getName()).isEqualTo("transformed-Bar");
+	}
+
+	@EnableBinding(Processor.class)
+	@EnableAutoConfiguration
+	public static class FooProcessor {
+
+		@StreamListener("input")
+		@SendTo("output")
+		public Foo consume(Foo foo) {
+			return new Foo("transformed-" + foo.getName());
+		}
+
+	}
+
+	public static class Foo {
+
+		private String name;
+
+		public Foo() {
+		}
+
+		public Foo(String name) {
+			this.name = name;
+		}
+
+		public String getName() {
+			return name;
+		}
+
+		public void setName(String name) {
+			this.name = name;
+		}
+
+		@Override
+		public String toString() {
+			return "Foo{name='" + name + "'}";
+		}
+
+	}
+
+}

spring-cloud-stream/src/main/java/org/springframework/cloud/stream/converter/ObjectStringMessageConverter.java

@@ -44,6 +44,12 @@ public class ObjectStringMessageConverter extends AbstractMessageConverter {
 		return true;
 	}
 
+	@Override
+	protected boolean canConvertFrom(Message<?> message, Class<?> targetClass) {
+		// only supports the conversion to String
+		return supportsMimeType(message.getHeaders()) && String.class == targetClass;
+	}
+
 	protected Object convertFromInternal(Message<?> message, Class<?> targetClass, Object conversionHint) {
 		if (message.getPayload() != null) {
 			if (message.getPayload() instanceof byte[]) {