6. Message Transformation

6.1 Transformer

6.1.1 Introduction

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.

For the same reason of maximizing flexibility, Spring does not require XML-based Message payloads. Nevertheless, the framework does provide some convenient Transformers for dealing with XML-based payloads if that is indeed the right choice for your application. For more information on those transformers, see Chapter 24, XML Support - Dealing with XML Payloads.

6.1.2 Configuring Transformer

Configuring Transformer with XML

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"
  <beans:bean class="org.foo.TestTransformer"/>


Using both the "ref" attribute and an inner handler definition in the same <transformer> configuration is not allowed, as it creates an ambiguous condition and will result in an Exception being thrown.

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.

Transformers and Spring Expression Language (SpEL)

Just like Routers, Aggregators and other components, as of Spring Integration 2.0 Transformers can also benefit from SpEL support (http://static.springsource.org/spring/docs/3.0.x/spring-framework-reference/html/expressions.html) whenever transformation logic is relatively simple.

<int:transformer input-channel="inChannel" 
	expression="payload.toUpperCase() + '- [' + T(java.lang.System).currentTimeMillis() + ']'"/>

In the above configuration we are achieving a simple transformation of the payload with a simple SpEL expression and without writing a custom transformer. Our payload (assuming String) will be upper-cased and concatenated with the current timestamp with some simple formatting.

Common Transformers

There are 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.

When debugging, this transformer is not typically necessary since the 'logging-channel-adapter' is capable of logging the Message payload. Refer to the section called “Wire Tap” for more detail.

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. These will use standard Java serialization by default, but you can provide an implementation of Spring 3.0's Serializer or Deserializer strategies via the 'serializer' and 'deserializer' attributes, respectively.

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

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

Object-to-Map Transformer

Spring Integration also provides Object-to-Map and Map-to-Object transformers which utilize the Spring Expression Language (SpEL) to serialize and de-serialize the object graphs. The object hierarchy is introspected to the most primitive types (String, int, etc.). The path to this type is described via SpEL, which becomes the key in the transformed Map. The primitive type becomes 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}

The 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 the 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"/>


<int:map-to-object-transformer input-channel="input" 


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

NOTE: 'ref' and 'type' attributes are mutually exclusive. You can only use one. Also, if using the 'ref' attribute, you must point to a 'prototype' scoped bean, otherwise a BeanCreationException will be thrown. 

Configuring a Transformer with Annotations

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.

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”

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

6.1.3 Header Filter

Some times your transformation use case might be as simple as removing a few headers. For such a use case, Spring Integration provides a Header Filter which allows you to specify certain header names that should be removed from the output Message (e.g. for security reasons or a value that was only needed temporarily). Basically the Header Filter is the opposite of the Header Enricher. The latter is discussed in Section 6.2.2, “Header Enricher”
<int:header-filter input-channel="inputChannel" 
		output-channel="outputChannel" header-names="lastName, state"/>
As you can see, configuration of a Header Filter is quite simple. It is a typical endpoint with input/output channels and a header-names attribute. That attribute accepts the names of the header(s) (delimited by commas if there are multiple) that need to be removed. So, in the above example the headers named 'lastName' and 'state' will not be present on the outbound Message.

6.2 Content Enricher

6.2.1 Introduction

At times you may have a requirement to enhance a request with more information than was provided by the target system. The Content Enricher pattern describes various scenarios as well as the component (Enricher), which allows you to address such requirements.

6.2.2 Header Enricher

If you only need to add headers to a Message, and they are not dynamically determined by the Message content, then referencing a custom implementation of a Transformer may be overkill. For that reason, Spring Integration provides support for the Header Enricher pattern. It is exposed via the <header-enricher> element.

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

The Header Enricher also provides helpful sub-elements to set well-known header names.

<int:header-enricher input-channel="in" output-channel="out">
		<int:error-channel ref="applicationErrorChannel"/>
		<int:reply-channel ref="quoteReplyChannel"/>
		<int:correlation-id value="123"/>
		<int:priority value="HIGHEST"/>
		<int:header name="bar" ref="someBean"/>

In the above configuration you can clearly see that for well-known headers such as errorChannel, correlationId, priority, replyChanneletc., instead of using generic <header> sub-elements where you would have to provide both header 'name' and 'value', you can use convenient sub-elements to set those values directly.

POJO Support

Often a header value cannot be defined statically and has to be determined dynamically based on some content in the Message. That is why Header Enricher allows you to also specify a bean 'ref' and 'method' that will calculate the header value. Let's look at the following configuration:

<int:header-enricher input-channel="in" output-channel="out">
    <int:header name="foo" method="computeValue" ref="myBean"/>

<bean id="myBean" class="foo.bar.MyBean"/>

public class MyBean {
    public String computeValue(String payload){
        return payload.toUpperCase() + "_US";


SpEL Support

In Spring Integration 2.0 we have introduced the convenience of the Spring Expression Language (SpEL) to help configure many different components. The Header Enricher is one of them. Looking again at the POJO example above, you can see that the computation logic to determine the header value is actually pretty simple. A natural question would be: "is there a simpler way to accomplish this?". That is where SpEL shows its true power.

<int:header-enricher input-channel="in" output-channel="out">
	<int:header name="foo" expression="payload.toUpperCase() + '_US'"/>

As you can see, by using SpEL for such simple cases, we no longer have to provide a separate class and configure it in the application context. All we need is the expression attribute configured with a valid SpEL expression. The 'payload' and 'headers' variables are bound to the SpEL Evaluation Context, giving you full access to the incoming Message.

Adapter specific Header Enrichers

As you go through the manual, you will see that as an added convenience, Spring Integration also provides adapter specific Header Enrichers (e.g., MAIL, XMPP, etc.)

6.3 Claim Check

6.3.1 Introduction

In the earlier sections we've covered several Content Enricher type components that help you deal with situations where a message is missing a piece of data. We also discussed Content Filtering which lets you remove data items from a message. However there are times when we want to hide data temporarily. For example, in a distributed system we may receive a Message with a very large payload. Some intermittent message processing steps may not need access to this payload and some may only need to access certain headers, so carrying the large Message payload through each processing step may cause performance degradation, may produce a security risk, and may make debugging more difficult.

The Claim Check pattern describes a mechanism that allows you to store data in a well known place while only maintaining a pointer (Claim Check) to where that data is located. You can pass that pointer around as a payload of a new Message thereby allowing any component within the message flow to get the actual data as soon as it needs it. This approach is very similar to the Certified Mail process where you'll get a Claim Check in your mailbox and would have to go to the Post Office to claim your actual package. Of course it's also the same idea as baggage-claim on a flight or in a hotel.

Spring Integration provides two types of Claim Check transformers: Incoming Claim Check Transformer and Outgoing Claim Check Transformer. Convenient namespace-based mechanisms are available to configure them.

Incoming Claim Check Transformer

An Incoming Claim Check Transformer will transform an incoming Message by storing it in the Message Store identified by its message-store attribute.

<int:claim-check-in id="checkin"

In the above configuration the Message that is received on the input-channel will be persisted to the Message Store identified with the message-store attribute and indexed with generated ID. That ID is the Claim Check for that Message. The Claim Check will also become the payload of the new (transformed) Message that will be sent to the output-channel.

Now, lets assume that at some point you do need access to the actual Message. You can of course access the Message Store manually and get the contents of the Message, or you can use the same approach as before except now you will be transforming the Claim Check to the actual Message by using an Outgoing Claim Check Transformer.

Outgoing Claim Check Transformer

An Outgoing Claim Check Transformer allows you to transform a Message with a Claim Check payload into a Message with the original content as its payload.

<claim-check-out id="checkout"

In the above configuration, the Message that is received on the input-channel should have a Claim Check as its payload and the Outgoing Claim Check Transformer will transform it into a Message with the original payload by simply querying the Message store for a Message identified by the provided Claim Check. It then sends the newly checked-out Message to the output-channel.

Although we rarely care about the details of the claim checks as long as they work, it is still worth knowing that the current implementation of the actual Claim Check (the pointer) in Spring Integration is a UUID to ensure uniqueness.

A word on Message Store

org.springframework.integration.store.MessageStore is a strategy interface for storing and retrieving messages. Spring Integration provides two convenient implementations of it. SimpleMessageStore: an in-memory, Map-based implementation (the default, good for testing) and JdbcMessageStore: an implementation that uses a relational database via JDBC.