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.

<int: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:

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

	Note
	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.

As of Spring Integration 2.0, a Message Transformer's transformation method can no longer return null. Returning null will result in an exception since a Message Transformer should always be expected to transform each source Message into a valid target Message. In other words, a Message Transformer should not be used as a Message Filter since there is a dedicated <filter> option for that. However, if you do need this type of behavior (where a component might return NULL and that should not be considered an error), a service-activator could be used. Its requires-reply value is FALSE by default, but that can be set to TRUE in order to have Exceptions thrown for NULL return values as with the transformer.

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"
	output-channel="outChannel"
	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.

<int: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.

	Tip
	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.

Note

The object-to-string-transformer is very simple; it invokes toString() on the inbound payload. There are two exceptions to this (since 3.0): if the payload is a char[], it invokes new String(payload); if the payload is a byte[], it invokes new String(payload, charset), where charset is "UTF-8" by default. The charset can be modified by supplying the charset attribute on the transformer. For more sophistication (such as selection of the charset dynamically, at runtime), you can use a SpEL expression-based transformer instead; for example: <int:transformer input-channel="in" output-channel="out" expression="new java.lang.String(payload, headers['myCharset']" />

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.

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

<int: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
}

If you need to create a "structured" map, you can provide the 'flatten' attribute. The default value for this attribute is 'true' meaning the default behavior; if you provide a 'false' value, then the structure will be a map of maps.

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: {name=George, child={name=Jenna, nickNames=[Bimbo, ...]}}

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

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

or

<int:object-to-map-transformer input-channel="directInput" output-channel="output" flatten="false"/>

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"/>

	Note
	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.

JSON Transformers

Object to JSON and JSON to Object transformers are provided.

<int:object-to-json-transformer input-channel="objectMapperInput"/>

<int:json-to-object-transformer input-channel="objectMapperInput"
    type="foo.MyDomainObject"/>

These use a vanilla JsonObjectMapper by default based on implementation from classpath. You can provide your own custom JsonObjectMapper implementation with appropriate options or based on required library (e.g. GSON).

<int:json-to-object-transformer input-channel="objectMapperInput"
    type="foo.MyDomainObject" object-mapper="customObjectMapper"/>

Note

Beginning with version 3.0, the object-mapper attribute references an instance of a new strategy interface JsonObjectMapper. This abstraction allows multiple implementations of json mappers to be used. Implementations that wrap Boon and Jackson 2 are provided, with the version being detected on the classpath. These classes are BoonJsonObjectMapper and Jackson2JsonObjectMapper. Note, BoonJsonObjectMapper is provided since version 4.1.

Important

If there are requirements to use both Jackson libraries and/or Boon in the same application, keep in mind that before version 3.0, the JSON transformers used only Jackson 1.x. From 4.1 on, the framework will select Jackson 2 by default ahead of the Boon implementation if both are on the classpath. Jackson 1.x is no longer supported by the framework internally but, of course, you can still use it within your code. To avoid unexpected issues with JSON mapping features, when using annotations, there may be a need to apply annotations from both Jacksons and/or Boon on domain classes: @org.codehaus.jackson.annotate.JsonIgnoreProperties(ignoreUnknown=true) @com.fasterxml.jackson.annotation.JsonIgnoreProperties(ignoreUnknown=true) @org.boon.json.annotations.JsonIgnoreProperties("foo") public class Foo { @org.codehaus.jackson.annotate.JsonProperty("fooBar") @com.fasterxml.jackson.annotation.JsonProperty("fooBar") @org.boon.json.annotations.JsonProperty("fooBar") public Object bar; }

You may wish to consider using a FactoryBean or simple factory method to create the JsonObjectMapper with the required characteristics.

public class ObjectMapperFactory {

    public static Jackson2JsonObjectMapper getMapper() {
        ObjectMapper mapper = new ObjectMapper();
        mapper.configure(JsonParser.Feature.ALLOW_COMMENTS, true);
        return new Jackson2JsonObjectMapper(mapper);
    }
}

<bean id="customObjectMapper" class="foo.ObjectMapperFactory"
            factory-method="getMapper"/>

Important

Beginning with version 2.2, the object-to-json-transformer sets the content-type header to application/json, by default, if the input message does not already have that header present. It you wish to set the content type header to some other value, or explicitly overwrite any existing header with some value (including application/json), use the content-type attribute. If you wish to suppress the setting of the header, set the content-type attribute to an empty string (""). This will result in a message with no content-type header, unless such a header was present on the input message.

Beginning with version 3.0, the ObjectToJsonTransformer adds headers, reflecting the source type, to the message. Similarly, the JsonToObjectTransformer can use those type headers when converting the JSON to an object. These headers are mapped in the AMQP adapters so that they are entirely compatible with the Spring-AMQP JsonMessageConverter.

This enables the following flows to work without any special configuration...

...->amqp-outbound-adapter---->

---->amqp-inbound-adapter->json-to-object-transformer->...

Where the outbound adapter is configured with a JsonMessageConverter and the inbound adapter uses the default SimpleMessageConverter.

...->object-to-json-transformer->amqp-outbound-adapter---->

---->amqp-inbound-adapter->...

Where the outbound adapter is configured with a SimpleMessageConverter and the inbound adapter uses the default JsonMessageConverter.

...->object-to-json-transformer->amqp-outbound-adapter---->

---->amqp-inbound-adapter->json-to-object-transformer->

Where both adapters are configured with a SimpleMessageConverter.

	Note
	When using the headers to determine the type, you should not provide a class attribute, because it takes precedence over the headers.

In addition to JSON Transformers, Spring Integration provides a built-in #jsonPath SpEL function for use in expressions. For more information see Appendix A, Spring Expression Language (SpEL).

#xpath SpEL Function

Since version 3.0, Spring Integration also provides a built-in #xpath SpEL function for use in expressions. For more information see Section 33.9, “#xpath SpEL Function”.

Beginning with version 4.0, the ObjectToJsonTransformer supports the resultType property, to specify the node JSON representation. The result node tree representation depends on the implementation of the provided JsonObjectMapper. By default, the ObjectToJsonTransformer uses a Jackson2JsonObjectMapper and delegates the conversion of the object to the node tree to the ObjectMapper#valueToTree method. The node JSON representation provides efficiency for using the JsonPropertyAccessor, when the downstream message flow uses SpEL expressions with access to the properties of the JSON data. See Section A.4, “PropertyAccessors”. When using Boon, the NODE representation is a Map<String, Object>

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 F.5, “Annotation Support”

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

Also see Section 7.7.5, “Advising Endpoints Using Annotations”.

Starting with Spring Integration 3.0, a new sub-element <int:header-channels-to-string/> is available; it has no attributes. This converts existing replyChannel and errorChannel headers (when they are a MessageChannel) to a String and stores the channel(s) in a registry for later resolution when it is time to send a reply, or handle an error. This is useful for cases where the headers might be lost; for example when serializing a message into a message store or when transporting the message over JMS. If the header does not already exist, or it is not a MessageChannel, no changes are made.

Use of this functionality requires the presence of a HeaderChannelRegistry bean. By default, the framework creates a DefaultHeaderChannelRegistry with the default expiry (60 seconds). Channels are removed from the registry after this time. To change this, simply define a bean with id integrationHeaderChannelRegistry and configure the required default delay using a constructor argument (milliseconds).

Since version 4.1, you can set a property removeOnGet to true on the <bean/> definition, and the mapping entry will be removed immediately on first use. This might be useful in a high-volume environment and when the channel is only used once, rather than waiting for the reaper to remove it.

The HeaderChannelRegistry has a size() method to determine the current size of the registry. The runReaper() method cancels the current scheduled task and runs the reaper immediately; the task is then scheduled to run again based on the current delay. These methods can be invoked directly by getting a reference to the registry, or you can send a message with, for example, the following content to a control bus:

"@integrationHeaderChannelRegistry.runReaper()"

This sub-element is a convenience only, and is the equivalent of specifying:

<int:reply-channel
	expression="@integrationHeaderChannelRegistry.channelToChannelName(headers.replyChannel)"
	overwrite="true" />
<int:error-channel
	expression="@integrationHeaderChannelRegistry.channelToChannelName(headers.errorChannel)"
	overwrite="true" />

Starting with version 4.1, you can now override the registry's configured reaper delay, so the the channel mapping is retained for at least the specified time, regardless of the reaper delay:

<int:header-enricher input-channel="inputTtl" output-channel="next">
	<int:header-channels-to-string time-to-live-expression="120000" />
</int:header-enricher>

<int:header-enricher input-channel="inputCustomTtl" output-channel="next">
	<int:header-channels-to-string
		time-to-live-expression="headers['channelTTL'] ?: 120000" />
</int:header-enricher>

In the first case, the time to live for every header channel mapping will be 2 minutes; in the second case, the time to live is specified in the message header and uses an elvis operator to use 2 minutes if there is no header.

Below, please find an overview of all available configuration options that are available for the payload enricher:

<int:enricher request-channel=""                          
              auto-startup="true"                         
              id=""                                       
              order=""                                    
              output-channel=""                           
              request-payload-expression=""               
              reply-channel=""                            
              error-channel=""                            
              send-timeout=""                             
              should-clone-payload="false">               
    <int:poller></int:poller>                             
    <int:property name="" expression="" null-result-expression="'Could not determine the name'"/>  
    <int:property name="" value="23" type="java.lang.Integer" null-result-expression="'0'"/>
    <int:header name="" expression="" null-result-expression=""/>  
    <int:header name="" value="" overwrite="" type="" null-result-expression=""/>
</int:enricher>

Below, please find several examples of using a Payload Enricher in various situations.

In the following example, a User object is passed as the payload of the Message. The User has several properties but only the username is set initially. The Enricher's request-channel attribute below is configured to pass the User on to the findUserServiceChannel.

Through the implicitly set reply-channel a User object is returned and using the property sub-element, properties from the reply are extracted and used to enrich the original payload.

<int:enricher id="findUserEnricher"
              input-channel="findUserEnricherChannel"
              request-channel="findUserServiceChannel">
    <int:property name="email"    expression="payload.email"/>
    <int:property name="password" expression="payload.password"/>
</int:enricher>

	Note
	The code samples shown here, are part of the Spring Integration Samples project. Please feel free to check it out at: https://github.com/SpringSource/spring-integration-samples

How do I pass only a subset of data to the request channel?

Using a request-payload-expression attribute a single property of the payload can be passed on to the request channel instead of the full message. In the example below on the username property is passed on to the request channel. Keep in mind, that alwhough only the username is passed on, the resulting message send to the request channel will contain the full set of MessageHeaders.

<int:enricher id="findUserByUsernameEnricher"
              input-channel="findUserByUsernameEnricherChannel"
              request-channel="findUserByUsernameServiceChannel"
              request-payload-expression="payload.username">
    <int:property name="email"    expression="payload.email"/>
    <int:property name="password" expression="payload.password"/>
</int:enricher>

How can I enrich payloads that consist of Collection data?

In the following example, instead of a User object, a Map is passed in. The Map contains the username under the map key username. Only the username is passed on to the request channel. The reply contains a full User object, which is ultimately added to the Map under the user key.

<int:enricher id="findUserWithMapEnricher"
              input-channel="findUserWithMapEnricherChannel"
              request-channel="findUserByUsernameServiceChannel"
              request-payload-expression="payload.username">
    <int:property name="user"    expression="payload"/>
</int:enricher>

How can I enrich payloads with static information without using a request channel?

Here is an example that does not use a request channel at all, but solely enriches the message's payload with static values. But please be aware that the word 'static' is used loosly here. You can still use SpEL expressions for setting those values.

<int:enricher id="userEnricher"
              input-channel="input">
    <int:property name="user.updateDate" expression="new java.util.Date()"/>
    <int:property name="user.firstName" value="foo"/>
    <int:property name="user.lastName"  value="bar"/>
    <int:property name="user.age"       value="42"/>
</int:enricher>