To better understand the mechanics and the necessity behind content-type negotiation, we take a look at a very simple use case by using the following message handler as an example:

@StreamListener(Processor.INPUT)
@SendTo(Processor.OUTPUT)
public String handle(Person person) {..}

	Note
	For simplicity, we assume that this is the only handler in the application (we assume there is no internal pipeline).

The handler shown in the preceding example expects a Person object as an argument and produces a String type as an output. In order for the framework to succeed in passing the incoming Message as an argument to this handler, it has to somehow transform the payload of the Message type from the wire format to a Person type. In other words, the framework must locate and apply the appropriate MessageConverter. To accomplish that, the framework needs some instructions from the user. One of these instructions is already provided by the signature of the handler method itself (Person type). Consequently, in theory, that should be (and, in some cases, is) enough. However, for the majority of use cases, in order to select the appropriate MessageConverter, the framework needs an additional piece of information. That missing piece is contentType.

Spring Cloud Stream provides three mechanisms to define contentType (in order of precedence):

As mentioned earlier, the preceding list also demonstrates the order of precedence in case of a tie. For example, a header-provided content type takes precedence over any other content type. The same applies for a content type set on a per-binding basis, which essentially lets you override the default content type. However, it also provides a sensible default (which was determined from community feedback).

Another reason for making application/json the default stems from the interoperability requirements driven by distributed microservices architectures, where producer and consumer not only run in different JVMs but can also run on different non-JVM platforms.

When the non-void handler method returns, if the the return value is already a Message, that Message becomes the payload. However, when the return value is not a Message, the new Message is constructed with the return value as the payload while inheriting headers from the input Message minus the headers defined or filtered by SpringIntegrationProperties.messageHandlerNotPropagatedHeaders. By default, there is only one header set there: contentType. This means that the new Message does not have contentType header set, thus ensuring that the contentType can evolve. You can always opt out of returning a Message from the handler method where you can inject any header you wish.

If there is an internal pipeline, the Message is sent to the next handler by going through the same process of conversion. However, if there is no internal pipeline or you have reached the end of it, the Message is sent back to the output destination.

9.1.1 Content Type versus Argument Type

As mentioned earlier, for the framework to select the appropriate MessageConverter, it requires argument type and, optionally, content type information. The logic for selecting the appropriate MessageConverter resides with the argument resolvers (HandlerMethodArgumentResolvers), which trigger right before the invocation of the user-defined handler method (which is when the actual argument type is known to the framework). If the argument type does not match the type of the current payload, the framework delegates to the stack of the pre-configured MessageConverters to see if any one of them can convert the payload. As you can see, the Object fromMessage(Message<?> message, Class<?> targetClass); operation of the MessageConverter takes targetClass as one of its arguments. The framework also ensures that the provided Message always contains a contentType header. When no contentType header was already present, it injects either the per-binding contentType header or the default contentType header. The combination of contentType argument type is the mechanism by which framework determines if message can be converted to a target type. If no appropriate MessageConverter is found, an exception is thrown, which you can handle by adding a custom MessageConverter (see “Section 9.3, “User-defined Message Converters””).

But what if the payload type matches the target type declared by the handler method? In this case, there is nothing to convert, and the payload is passed unmodified. While this sounds pretty straightforward and logical, keep in mind handler methods that take a Message<?> or Object as an argument. By declaring the target type to be Object (which is an instanceof everything in Java), you essentially forfeit the conversion process.

	Note
	Do not expect Message to be converted into some other type based only on the contentType. Remember that the contentType is complementary to the target type. If you wish, you can provide a hint, which MessageConverter may or may not take into consideration.

Object fromMessage(Message<?> message, Class<?> targetClass);

Message<?> toMessage(Object payload, @Nullable MessageHeaders headers);

Object fromMessage(Message<?> message, Class<?> targetClass);

Message<byte[]> toMessage(Object payload, @Nullable MessageHeaders headers);

As mentioned earlier, the framework already provides a stack of MessageConverters to handle most common use cases. The following list describes the provided MessageConverters, in order of precedence (the first MessageConverter that works is used):

When no appropriate converter is found, the framework throws an exception. When that happens, you should check your code and configuration and ensure you did not miss anything (that is, ensure that you provided a contentType by using a binding or a header). However, most likely, you found some uncommon case (such as a custom contentType perhaps) and the current stack of provided MessageConverters does not know how to convert. If that is the case, you can add custom MessageConverter. See Section 9.3, “User-defined Message Converters”.

Spring Cloud Stream exposes a mechanism to define and register additional MessageConverters. To use it, implement org.springframework.messaging.converter.MessageConverter, configure it as a @Bean, and annotate it with @StreamMessageConverter. It is then apended to the existing stack of `MessageConverter`s.

	Note
	It is important to understand that custom MessageConverter implementations are added to the head of the existing stack. Consequently, custom MessageConverter implementations take precedence over the existing ones, which lets you override as well as add to the existing converters.

The following example shows how to create a message converter bean to support a new content type called application/bar:

@EnableBinding(Sink.class)
@SpringBootApplication
public static class SinkApplication {

    ...

    @Bean
    @StreamMessageConverter
    public MessageConverter customMessageConverter() {
        return new MyCustomMessageConverter();
    }
}

public class MyCustomMessageConverter extends AbstractMessageConverter {

    public MyCustomMessageConverter() {
        super(new MimeType("application", "bar"));
    }

    @Override
    protected boolean supports(Class<?> clazz) {
        return (Bar.class.equals(clazz));
    }

    @Override
    protected Object convertFromInternal(Message<?> message, Class<?> targetClass, Object conversionHint) {
        Object payload = message.getPayload();
        return (payload instanceof Bar ? payload : new Bar((byte[]) payload));
    }
}

Spring Cloud Stream also provides support for Avro-based converters and schema evolution. See “Chapter 10, Schema Evolution Support” for details.