Message Transformers play a very important role in enabling the loose-coupling of Message Producers and Message Consumers. Rather than requiring every Message-producing component to know what type is expected by the next consumer, Transformers can be added between those components. Generic transformers, such as one that converts a String to an XML Document, are also highly reusable.

For some systems, it may be best to provide a Canonical Data Model, but Spring Integration's general philosophy is not to require any particular format. Rather, for maximum flexibility, Spring Integration aims to provide the simplest possible model for extension. As with the other endpoint types, the use of declarative configuration in XML and/or Annotations enables simple POJOs to be adapted for the role of Message Transformers. These configuration options will be described below.

The <transformer> element is used to create a Message-transforming endpoint. In addition to "input-channel" and "output-channel" attributes, it requires a "ref". The "ref" may either point to an Object that contains the @Transformer annotation on a single method (see below) or it may be combined with an explicit method name value provided via the "method" attribute.

<transformer id="testTransformer" ref="testTransformerBean" input-channel="inChannel"
             method="transform" output-channel="outChannel"/>
<beans:bean id="testTransformerBean" class="org.foo.TestTransformer" />

Using a "ref" attribute is generally recommended if the custom transformer handler implementation can be reused in other <transformer> definitions. However if the custom transformer handler implementation should be scoped to a single definition of the <transformer>, you can define an inner bean definition:

<transformer id="testTransformer" input-channel="inChannel" method="transform"
                output-channel="outChannel">
  <beans:bean class="org.foo.TestTransformer"/>
</transformer>

The method that is used for transformation may expect either the Message type or the payload type of inbound Messages. It may also accept Message header values either individually or as a full map by using the @Header and @Headers parameter annotations respectively. The return value of the method can be any type. If the return value is itself a Message, that will be passed along to the transformer's output channel. If the return type is a Map, and the original Message payload was not a Map, the entries in that Map will be added to the Message headers of the original Message (the keys must be Strings). If the return value is null, then no reply Message will be sent (effectively the same behavior as a Message Filter returning false). Otherwise, the return value will be sent as the payload of an outbound reply Message.

There are a also a few Transformer implementations available out of the box. Because, it is fairly common to use the toString() representation of an Object, Spring Integration provides an ObjectToStringTransformer whose output is a Message with a String payload. That String is the result of invoking the toString operation on the inbound Message's payload.

 <object-to-string-transformer input-channel="in" output-channel="out"/>

A potential example for this would be sending some arbitrary object to the 'outbound-channel-adapter' in the file namespace. Whereas that Channel Adapter only supports String, byte-array, or java.io.File payloads by default, adding this transformer immediately before the adapter will handle the necessary conversion. Of course, that works fine as long as the result of the toString() call is what you want to be written to the File. Otherwise, you can just provide a custom POJO-based Transformer via the generic 'transformer' element shown previously.

If you need to serialize an Object to a byte array or deserialize a byte array back into an Object, Spring Integration provides symmetrical serialization transformers.

 <payload-serializing-transformer input-channel="objectsIn" output-channel="bytesOut"/>

 <payload-deserializing-transformer input-channel="bytesIn" output-channel="objectsOut"/>

If you only need to add headers to a Message, and they are not dynamically determined by Message content, then referencing a custom implementation may be overkill. For that reason, Spring Integration provides the 'header-enricher' element.

 <header-enricher input-channel="in" output-channel="out">
     <header name="foo" value="123"/>
     <header name="bar" ref="someBean"/>
 </header-enricher>

As added convenience, Spring Integration also provides Object-to-Map and Map-to-Object transformers which utilize Spring Expression Language (SpEL) to serialize and de-serialize the object graphs. Object hierarchy is introspected to the most primitive types (e.g., String, int etc.). The path to this type is described via SpEL, which becomes the keykey in the transformed Map with primitive type being the value.

For example:

public class Parent{
    private Child child;
    private String name; 
    // setters and getters are omitted
}

public class Child{
   private String name; 
   private List<String> nickNames;
   // setters and getters are omitted
}

... will be transformed to a Map which looks like this: {person.name=George, person.child.name=Jenna, person.child.nickNames[0]=Bimbo . . . etc}

SpEL-based Map allows you to describe the object structure without sharing the actual types allowing you to restore/rebuild the object graph into a differently typed Object graph as long as you maintain the structure.

For example: The above structure could be easily restored back to the following Object graph via Map-to-Object transformer:

public class Father{
    private Kid child;
    private String name; 
    // setters and getters are omitted
}

public class Kid{
   private String name; 
   private List<String> nickNames;
   // setters and getters are omitted
}

To configure these transformers, Spring Integration provides namespace support Object-to-Map:

<object-to-map-transformer input-channel="directInput" output-channel="output"/>

Map-to-Object

<int:map-to-object-transformer input-channel="input" 
                         output-channel="output" 
                          type="org.foo.Person"/>

or

<int:map-to-object-transformer input-channel="inputA" 
                                output-channel="outputA" 
                                ref="person"/>
<bean id="person" class="org.foo.Person" scope="prototype"/>

The @Transformer annotation can also be added to methods that expect either the Message type or the message payload type. The return value will be handled in the exact same way as described above in the section describing the <transformer> element.

@Transformer
Order generateOrder(String productId) {
    return new Order(productId);
}

Transformer methods may also accept the @Header and @Headers annotations that is documented in Section B.5, “Annotation Support”

@Transformer
Order generateOrder(String productId, @Header("customerName") String customer) {
    return new Order(productId, customer);
}