38. XML Support - Dealing with XML Payloads

Spring Integration’s XML support extends the core of Spring Integration with the following components:

You need to include this dependency into your project:

Maven. 

<dependency>
    <groupId>org.springframework.integration</groupId>
    <artifactId>spring-integration-xml</artifactId>
    <version>5.1.3.BUILD-SNAPSHOT</version>
</dependency>

Gradle. 

compile "org.springframework.integration:spring-integration-xml:5.1.3.BUILD-SNAPSHOT"

These components make working with XML messages in Spring Integration simpler. The messaging components work with XML that is represented in a range of formats, including instances of java.lang.String, org.w3c.dom.Document, and javax.xml.transform.Source. However, where a DOM representation is required (for example, in order to evaluate an XPath expression), the String payload is converted into the required type and then converted back to String. Components that require an instance of DocumentBuilder create a namespace-aware instance if you do not provide one. When you require greater control over document creation, you can provide an appropriately configured instance of DocumentBuilder.

38.1 Namespace Support

All components within the Spring Integration XML module provide namespace support. In order to enable namespace support, you need to import the schema for the Spring Integration XML Module. The following example shows a typical setup:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:int="http://www.springframework.org/schema/integration"
  xmlns:int-xml="http://www.springframework.org/schema/integration/xml"
  xsi:schemaLocation="http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans.xsd
    http://www.springframework.org/schema/integration
    http://www.springframework.org/schema/integration/spring-integration.xsd
    http://www.springframework.org/schema/integration/xml
    http://www.springframework.org/schema/integration/xml/spring-integration-xml.xsd">
</beans>

38.1.1 XPath Expressions

Many of the components within the Spring Integration XML module work with XPath Expressions. Each of those components either references an XPath Expression that has been defined as a top-level element or uses a nested <xpath-expression/> element.

All forms of XPath expressions result in the creation of an XPathExpression that uses the Spring org.springframework.xml.xpath.XPathExpressionFactory. When XPath expressions are created, the best XPath implementation that is available on the classpath is used (either JAXP 1.3+ or Jaxen, with JAXP being preferred).

[Note]Note

Internally, Spring Integration uses the XPath functionality provided by the Spring Web Services project (http://www.spring.io/spring-ws). Specifically, we use the Spring Web Services XML module (spring-xml-x.x.x.jar). For a deeper understanding, see the respective documentation at http://docs.spring.io/spring-ws/docs/current/reference/html/common.html#xpath.

Here is an overview of all available configuration parameters of the xpath-expression element: The following listing shows the available attributes for the xpath-expression element:

<int-xml:xpath-expression expression="" 1
          id=""                         2
          namespace-map=""              3
          ns-prefix=""                  4
          ns-uri="">                    5
    <map></map>                         6
</int-xml:xpath-expression>

1

Defines an XPath expression. Required.

2

The identifier of the underlying bean definition. It is an instance of org.springframework.xml.xpath.XPathExpression. Optional.

3

Reference to a map that contains namespaces. The key of the map defines the namespace prefix, and the value of the map sets the namespace URI. It is not valid to specify both this attribute and the map element or the ns-prefix and ns-uri attributes. Optional.

4

Lets you set the namespace prefix directly as an attribute on the XPath expression element. If you set ns-prefix, you must also set the ns-uri attribute. Optional.

5

Lets you directly set the namespace URI as an attribute on the XPath expression element. If you set ns-uri, you must also set the ns-prefix attribute. Optional.

6

Defines a map that contains namespaces. Only one map child element is allowed. The key of the map defines the namespace prefix, and the value of the map sets the namespace URI. It is not valid to specify both this element and the map attribute or set the ns-prefix and ns-uri attributes. Optional.

Providing Namespaces (Optional) to XPath Expressions

For the XPath Expression Element, you can provide namespace information as configuration parameters. You can define namespaces by using one of the following choices:

  • Reference a map by using the namespace-map attribute
  • Provide a map of namespaces by using the map sub-element
  • Specify the ns-prefix and ns-uri attributes

All three options are mutually exclusive. Only one option can be set.

The following example shows several different ways to use XPath expressions, including the options for setting the XML namespaces mentioned earlier:

<int-xml:xpath-filter id="filterReferencingXPathExpression"
                      xpath-expression-ref="refToXpathExpression"/>

<int-xml:xpath-expression id="refToXpathExpression" expression="/name"/>

<int-xml:xpath-filter id="filterWithoutNamespace">
    <int-xml:xpath-expression expression="/name"/>
</int-xml:xpath-filter>

<int-xml:xpath-filter id="filterWithOneNamespace">
    <int-xml:xpath-expression expression="/ns1:name"
                              ns-prefix="ns1" ns-uri="www.example.org"/>
</int-xml:xpath-filter>

<int-xml:xpath-filter id="filterWithTwoNamespaces">
    <int-xml:xpath-expression expression="/ns1:name/ns2:type">
        <map>
            <entry key="ns1" value="www.example.org/one"/>
            <entry key="ns2" value="www.example.org/two"/>
        </map>
    </int-xml:xpath-expression>
</int-xml:xpath-filter>

<int-xml:xpath-filter id="filterWithNamespaceMapReference">
    <int-xml:xpath-expression expression="/ns1:name/ns2:type"
                              namespace-map="defaultNamespaces"/>
</int-xml:xpath-filter>

<util:map id="defaultNamespaces">
    <util:entry key="ns1" value="www.example.org/one"/>
    <util:entry key="ns2" value="www.example.org/two"/>
</util:map>

Using XPath Expressions with Default Namespaces

When working with default namespaces, you may run into situations that behave differently than you might expect. Assume we have the following XML document (which represents an order of two books):

<?xml version="1.0" encoding="UTF-8"?>
<order>
    <orderItem>
        <isbn>0321200683</isbn>
        <quantity>2</quantity>
    </orderItem>
    <orderItem>
        <isbn>1590596439</isbn>
        <quantity>1</quantity>
    </orderItem>
</order>

This document does not declare a namespace. Therefore, applying the following XPath Expression works as expected:

<int-xml:xpath-expression expression="/order/orderItem" />

You might expect that the same expression also works for the following XML file:

<?xml version="1.0" encoding="UTF-8"?>
<order xmlns="http://www.example.org/orders">
	<orderItem>
		<isbn>0321200683</isbn>
		<quantity>2</quantity>
	</orderItem>
	<orderItem>
		<isbn>1590596439</isbn>
		<quantity>1</quantity>
	</orderItem>
</order>

The preceding example looks exactly the same as the previous example but declares a default namespace.

However, the previous XPath expression (/order/orderItem) fails in this case.

In order to solve this issue, you must provide a namespace prefix and a namespace URI either by setting the ns-prefix and ns-uri attributes or by setting the namespace-map attribute. The namespace URI must match the namespace declared in your XML document. In the preceding example, that is http://www.example.org/orders.

You can, however, arbitrarily choose the namespace prefix. In fact, providing an empty string actually works. (However, null is not allowed.) In the case of a namespace prefix consisting of an empty string, your Xpath expression must use a colon (":") to indicate the default namespace. If you leave off the colon, the XPath expression does not match. The following XPath Expression matches against the XML document in the preceding example:

<int-xml:xpath-expression expression="/:order/:orderItem"
    ns-prefix="" ns-uri="http://www.example.org/prodcuts"/>

You can also provide any other arbitrarily chosen namespace prefix. The following XPath expression (which use the myorder namespace prefix) also matches:

<int-xml:xpath-expression expression="/myorder:order/myorder:orderItem"
    ns-prefix="myorder" ns-uri="http://www.example.org/prodcuts"/>

The namespace URI is the really important piece of information, not the prefix. The Jaxen FAQ summarizes the point very well:

In XPath 1.0, all unprefixed names are unqualified. There is no requirement that the prefixes used in the XPath expression are the same as the prefixes used in the document being queried. Only the namespace URIs need to match, not the prefixes.

38.2 Transforming XML Payloads

This section covers how to transform XML payloads

38.2.1 Configuring Transformers as Beans

This section will explain the workings of the following transformers and how to configure them as beans:

All of the XML transformers extend either AbstractTransformer or AbstractPayloadTransformer and therefore implement Transformer. When configuring XML transformers as beans in Spring Integration, you would normally configure the Transformer in conjunction with a MessageTransformingHandler. This lets the transformer be used as an endpoint. Finally, we discuss the namespace support , which allows for configuring the transformers as elements in XML.

UnmarshallingTransformer

An UnmarshallingTransformer lets an XML Source be unmarshalled by using implementations of the Spring OXM Unmarshaller. Spring’s Object/XML Mapping support provides several implementations that support marshalling and unmarshalling by using JAXB, Castor, JiBX, and others. The unmarshaller requires an instance of Source. If the message payload is not an instance of Source, conversion is still attempted. Currently, String, File, byte[] and org.w3c.dom.Document payloads are supported. To create a custom conversion to a Source, you can inject an implementation of a SourceFactory.

[Note]Note

If you do not explicitly set a SourceFactory, the property on the UnmarshallingTransformer is, by default, set to a DomSourceFactory.

Starting with version 5.0, the UnmarshallingTransformer also supports an org.springframework.ws.mime.MimeMessage as the incoming payload. This can be useful when we receive a raw WebServiceMessage with MTOM attachments over SOAP . See Section 37.6, “MTOM Support” for more information.

The following example shows how to define an unmarshalling transformer:

<bean id="unmarshallingTransformer" class="o.s.i.xml.transformer.UnmarshallingTransformer">
    <constructor-arg>
        <bean class="org.springframework.oxm.jaxb.Jaxb2Marshaller">
            <property name="contextPath" value="org.example" />
        </bean>
    </constructor-arg>
</bean>

Using MarshallingTransformer

The MarshallingTransformer lets an object graph be converted into XML by using a Spring OXM Marshaller. By default, the MarshallingTransformer returns a DomResult. However, you can control the type of result by configuring an alternative ResultFactory, such as StringResultFactory. In many cases, it is more convenient to transform the payload into an alternative XML format. To do so, configure a ResultTransformer. Spring integration provides two implementations, one that converts to String and another that converts to Document. The following example configures a marshalling transformer that transforms to a document:

<bean id="marshallingTransformer" class="o.s.i.xml.transformer.MarshallingTransformer">
    <constructor-arg>
        <bean class="org.springframework.oxm.jaxb.Jaxb2Marshaller">
            <property name="contextPath" value="org.example"/>
        </bean>
    </constructor-arg>
    <constructor-arg>
        <bean class="o.s.i.xml.transformer.ResultToDocumentTransformer"/>
    </constructor-arg>
</bean>

By default, the MarshallingTransformer passes the payload object to the Marshaller. However, if its boolean extractPayload property is set to false, the entire Message instance is passed to the Marshaller instead. That may be useful for certain custom implementations of the Marshaller interface, but, typically, the payload is the appropriate source object for marshalling when you delegate to any of the various Marshaller implementations.

XsltPayloadTransformer

The XsltPayloadTransformer transforms XML payloads by using Extensible Stylesheet Language Transformations (XSLT). The transformer’s constructor requires an instance of either Resource or Templates to be passed in. Passing in a Templates instance allows for greater configuration of the TransformerFactory used to create the template instance.

As with the UnmarshallingTransformer, the XsltPayloadTransformer does the actual XSLT transformation against instances of Source. Therefore, if the message payload is not an instance of Source, conversion is still attempted. String and Document payloads are supported directly.

To create a custom conversion to a Source, you can inject an implementation of a SourceFactory.

[Note]Note

If a SourceFactory is not set explicitly, the property on the XsltPayloadTransformer is, by default, set to a DomSourceFactory.

By default, the XsltPayloadTransformer creates a message with a Result payload, similar to the XmlPayloadMarshallingTransformer. You can customize this by providing a ResultFactory or a ResultTransformer.

The following example configures a bean that works as an XSLT payload transformer:

<bean id="xsltPayloadTransformer" class="o.s.i.xml.transformer.XsltPayloadTransformer">
  <constructor-arg value="classpath:org/example/xsl/transform.xsl"/>
  <constructor-arg>
    <bean class="o.s.i.xml.transformer.ResultToDocumentTransformer"/>
  </constructor-arg>
</bean>

Starting with Spring Integration 3.0, you can specify the transformer factory class name by using a constructor argument. You can do so by using the transformer-factory-class attribute when you use the namespace.

Using ResultTransformer Implementations

Both the MarshallingTransformer and the XsltPayloadTransformer let you specify a ResultTransformer. Thus, if the marshalling or XSLT transformation returns a Result, you have the option to also use a ResultTransformer to transform the Result into another format. Spring Integration provides two concrete ResultTransformer implementations:

By default, the MarshallingTransformer always returns a Result. By specifying a ResultTransformer, you can customize the type of payload returned.

The behavior is slightly more complex for the XsltPayloadTransformer. By default, if the input payload is an instance of String or Document the resultTransformer property is ignored.

However, if the input payload is a Source or any other type, the resultTransformer property is applied. Additionally, you can set the alwaysUseResultFactory property to true, which also causes the specified resultTransformer to be used.

For more information and examples, see Section 38.2.3, “Namespace Configuration and Result Transformers”.

38.2.2 Namespace Support for XML Transformers

Namespace support for all XML transformers is provided in the Spring Integration XML namespace, a template for which was shown earlier. The namespace support for transformers creates an instance of either EventDrivenConsumer or PollingConsumer, according to the type of the provided input channel. The namespace support is designed to reduce the amount of XML configuration by allowing the creation of an endpoint and transformer that use one element.

Using an UnmarshallingTransformer

The namespace support for the UnmarshallingTransformer is shown below. Since the namespace create an endpoint instance rather than a transformer, you can nest a poller within the element to control the polling of the input channel. The following example shows how to do so:

<int-xml:unmarshalling-transformer id="defaultUnmarshaller"
    input-channel="input" output-channel="output"
    unmarshaller="unmarshaller"/>

<int-xml:unmarshalling-transformer id="unmarshallerWithPoller"
    input-channel="input" output-channel="output"
    unmarshaller="unmarshaller">
    <int:poller fixed-rate="2000"/>
<int-xml:unmarshalling-transformer/>

Using a MarshallingTransformer

The namespace support for the marshalling transformer requires an input-channel, an output-channel, and a reference to a marshaller. You can use the optional result-type attribute to control the type of result created. Valid values are StringResult or DomResult (the default). The following example configures a marshalling transformer:

<int-xml:marshalling-transformer
     input-channel="marshallingTransformerStringResultFactory"
     output-channel="output"
     marshaller="marshaller"
     result-type="StringResult" />

<int-xml:marshalling-transformer
    input-channel="marshallingTransformerWithResultTransformer"
    output-channel="output"
    marshaller="marshaller"
    result-transformer="resultTransformer" />

<bean id="resultTransformer" class="o.s.i.xml.transformer.ResultToStringTransformer"/>

Where the provided result types do not suffice, you can provide a reference to a custom implementation of ResultFactory as an alternative to setting the result-type attribute by using the result-factory attribute. The result-type and result-factory attributes are mutually exclusive.

[Note]Note

Internally, the StringResult and DomResult result types are represented by the ResultFactory implementations: StringResultFactory and DomResultFactory respectively.

Using an XsltPayloadTransformer

Namespace support for the XsltPayloadTransformer lets you either pass in a Resource (in order to create the Templates instance) or pass in a pre-created Templates instance as a reference. As with the marshalling transformer, you can control the type of the result output by specifying either the result-factory or the result-type attribute. When you need to convert result before sending, you can use a result-transformer attribute to reference an implementation of ResultTransformer.

[Important]Important

If you specify the result-factory or the result-type attribute, the alwaysUseResultFactory property on the underlying XsltPayloadTransformer is set to true by the XsltPayloadTransformerParser.

The following example configures two XSLT transformers:

<int-xml:xslt-transformer id="xsltTransformerWithResource"
    input-channel="withResourceIn" output-channel="output"
    xsl-resource="org/springframework/integration/xml/config/test.xsl"/>

<int-xml:xslt-transformer id="xsltTransformerWithTemplatesAndResultTransformer"
    input-channel="withTemplatesAndResultTransformerIn" output-channel="output"
    xsl-templates="templates"
    result-transformer="resultTransformer"/>

You may need to have access to Message data, such as the Message headers, in order to assist with transformation. For example, you may need to get access to certain Message headers and pass them on as parameters to a transformer (for example, transformer.setParameter(..)). Spring Integration provides two convenient ways to accomplish this, as the following example shows:

<int-xml:xslt-transformer id="paramHeadersCombo"
    input-channel="paramHeadersComboChannel" output-channel="output"
    xsl-resource="classpath:transformer.xslt"
    xslt-param-headers="testP*, *foo, bar, baz">

    <int-xml:xslt-param name="helloParameter" value="hello"/>
    <int-xml:xslt-param name="firstName" expression="headers.fname"/>
</int-xml:xslt-transformer>

If message header names match one-to-one to parameter names, you can use the xslt-param-headers attribute. In it, you can use wildcards for simple pattern matching. It supports the following simple pattern styles: xxx*, *xxx, *xxx*, and xxx*yyy.

You can also configure individual XSLT parameters by using the <xslt-param/> element. On that element, you can set the expression attribute or the value attribute. The expression attribute should be any valid SpEL expression with the Message being the root object of the expression evaluation context. The value attribute (as with any value in Spring beans) lets you specify simple scalar values. You can also use property placeholders (such as ${some.value}). So, with the expression and value attributes, you can map XSLT parameters to any accessible part of the Message as well as any literal value.

Starting with Spring Integration 3.0, you can now specify the transformer factory class name by setting the transformer-factory-class attribute.

38.2.3 Namespace Configuration and Result Transformers

We cover using result transformers in the section called “Using ResultTransformer Implementations”. The examples in this section use XML namespace configuration to illustrates several special use cases. First, we define the ResultTransformer, as the following example shows:

<beans:bean id="resultToDoc" class="o.s.i.xml.transformer.ResultToDocumentTransformer"/>

This ResultTransformer accepts either a StringResult or a DOMResult as input and converts the input into a Document.

Now we can declare the transformer, as follows:

<int-xml:xslt-transformer input-channel="in" output-channel="fahrenheitChannel"
    xsl-resource="classpath:noop.xslt" result-transformer="resultToDoc"/>

If the incoming message’s payload is of type Source, then, as a first step, the Result is determined by using the ResultFactory. As we did not specify a ResultFactory, the default DomResultFactory is used, meaning that the transformation yields a DomResult.

However, as we specified a ResultTransformer, it is used and the resulting Message payload is of type Document.

[Important]Important

The specified ResultTransformer is ignored with String or Document payloads. If the incoming message’s payload is of type String, the payload after the XSLT transformation is a String. Similarly, if the incoming message’s payload is of type Document, the payload after the XSLT transformation is a`Document`.

If the message payload is not a Source, a String, or a Document, as a fallback option, we try to create a`Source` by using the default SourceFactory. As we did not specify a SourceFactory explicitly by using the source-factory attribute, the default DomSourceFactory is used. If successful, the XSLT transformation is executed as if the payload was of type Source, as described in the previous paragraphs.

[Note]Note

The DomSourceFactory supports the creation of a DOMSource from a Document, a File, or a String payload.

The next transformer declaration adds a result-type attribute that uses StringResult as its value. The result-type is internally represented by the StringResultFactory. Thus, you could have also added a reference to a StringResultFactory, by using the result-factory attribute, which would have been the same. The following example shows that transformer declaration:

<int-xml:xslt-transformer input-channel="in" output-channel="fahrenheitChannel"
		xsl-resource="classpath:noop.xslt" result-transformer="resultToDoc"
		result-type="StringResult"/>

Because we use a ResultFactory, the alwaysUseResultFactory property of the XsltPayloadTransformer class is implicitly set to true. Consequently, the referenced ResultToDocumentTransformer is used.

Therefore, if you transform a payload of type String, the resulting payload is of type Document.

XsltPayloadTransformer and <xsl:output method="text"/>

<xsl:output method="text"/> tells the XSLT template to produce only text content from the input source. In this particular case, we have no reason to use a DomResult. Therefore, the XsltPayloadTransformer defaults to StringResult if the output property called method of the underlying javax.xml.transform.Transformer returns text. This coercion is performed independently from the inbound payload type. This behavior is available only you set the if the result-type attribute or the result-factory attribute for the <int-xml:xslt-transformer> component.

38.3 Transforming XML Messages with XPath

When it comes to message transformation, XPath is a great way to transform messages that have XML payloads. You can do so by defining XPath transformers with the <xpath-transformer/> element.

38.3.1 Simple XPath Transformation

Consider following transformer configuration:

<int-xml:xpath-transformer input-channel="inputChannel" output-channel="outputChannel"
      xpath-expression="/person/@name" />

Also consider the following Message:

Message<?> message =
  MessageBuilder.withPayload("<person name='John Doe' age='42' married='true'/>").build();

After sending this message to the inputChannel, the XPath transformer configured earlier transforms this XML Message to a simple Message with a payload of John Doe, all based on the simple XPath Expression specified in the xpath-expression attribute.

XPath also lets you perform simple conversion of an extracted element to a desired type. Valid return types are defined in javax.xml.xpath.XPathConstants and follow the conversion rules specified by the javax.xml.xpath.XPath interface.

The following constants are defined by the XPathConstants class: BOOLEAN, DOM_OBJECT_MODEL, NODE, NODESET, NUMBER, and STRING.

You can configure the desired type by using the evaluation-type attribute of the <xpath-transformer/> element, as the following example shows (twice):

<int-xml:xpath-transformer input-channel="numberInput" xpath-expression="/person/@age"
                           evaluation-type="NUMBER_RESULT" output-channel="output"/>

<int-xml:xpath-transformer input-channel="booleanInput"
                           xpath-expression="/person/@married = 'true'"
                           evaluation-type="BOOLEAN_RESULT" output-channel="output"/>

38.3.2 Node Mappers

If you need to provide custom mapping for the node extracted by the XPath expression, you can provide a reference to the implementation of the org.springframework.xml.xpath.NodeMapper (an interface used by XPathOperations implementations for mapping Node objects on a per-node basis). To provide a reference to a NodeMapper, you can use the node-mapper attribute, as the following example shows:

<int-xml:xpath-transformer input-channel="nodeMapperInput" xpath-expression="/person/@age"
                           node-mapper="testNodeMapper" output-channel="output"/>

The following example shows a NodeMapper implementation that works with the preceding example:

class TestNodeMapper implements NodeMapper {
  public Object mapNode(Node node, int nodeNum) throws DOMException {
    return node.getTextContent() + "-mapped";
  }
}

38.3.3 XML Payload Converter

You can also use an implementation of the org.springframework.integration.xml.XmlPayloadConverter to provide more granular transformation. The following example shows how to define one:

<int-xml:xpath-transformer input-channel="customConverterInput"
                           output-channel="output" xpath-expression="/test/@type"
                           converter="testXmlPayloadConverter" />

The following example shows an XmlPayloadConverter implementation that works with the preceding example:

class TestXmlPayloadConverter implements XmlPayloadConverter {
  public Source convertToSource(Object object) {
    throw new UnsupportedOperationException();
  }
  //
  public Node convertToNode(Object object) {
    try {
      return DocumentBuilderFactory.newInstance().newDocumentBuilder().parse(
          new InputSource(new StringReader("<test type='custom'/>")));
    }
    catch (Exception e) {
      throw new IllegalStateException(e);
    }
  }
  //
  public Document convertToDocument(Object object) {
    throw new UnsupportedOperationException();
  }
}

If you do not provide this reference, the DefaultXmlPayloadConverter is used. It should suffice in most cases, because it can convert from Node, Document, Source, File, String, InputStream, and byte[] payloads. If you need to extend beyond the capabilities of that default implementation, an upstream Transformer is probably a better option than providing a reference to a custom implementation of this strategy here.

38.4 Splitting XML Messages

XPathMessageSplitter supports messages with either String or Document payloads. The splitter uses the provided XPath expression to split the payload into a number of nodes. By default, this results in each Node instance becoming the payload of a new message. When each message should be a Document, you can set the createDocuments flag. Where a String payload is passed in, the payload is converted and then split before being converted back to a number of String messages. The XPath splitter implements MessageHandler and should therefore be configured in conjunction with an appropriate endpoint (see the namespace support example after the following example for a simpler configuration alternative). The following example configures a bean that uses an XPathMessageSplitter:

<bean id="splittingEndpoint"
      class="org.springframework.integration.endpoint.EventDrivenConsumer">
    <constructor-arg ref="orderChannel" />
    <constructor-arg>
        <bean class="org.springframework.integration.xml.splitter.XPathMessageSplitter">
            <constructor-arg value="/order/items" />
            <property name="documentBuilder" ref="customisedDocumentBuilder" />
            <property name="outputChannel" ref="orderItemsChannel" />
        </bean>
    </constructor-arg>
</bean>

XPath splitter namespace support lets you create a message endpoint with an input channel and output channel, as the following example shows:

<!-- Split the order into items and create a new message for each item node -->
<int-xml:xpath-splitter id="orderItemSplitter"
                       input-channel="orderChannel"
                       output-channel="orderItemsChannel">
    <int-xml:xpath-expression expression="/order/items"/>
</int-xml:xpath-splitter>

<!-- Split the order into items, create a new document for each item-->
<int-xml:xpath-splitter id="orderItemDocumentSplitter"
                       input-channel="orderChannel"
                       output-channel="orderItemsChannel"
                       create-documents="true">
    <int-xml:xpath-expression expression="/order/items"/>
    <int:poller fixed-rate="2000"/>
</int-xml:xpath-splitter>

Starting with version 4.2, the XPathMessageSplitter exposes the outputProperties (such as OutputKeys.OMIT_XML_DECLARATION) property for an javax.xml.transform.Transformer instance when a request payload is not of type org.w3c.dom.Node. The following example defines a property and uses it with the output-properties property:

<util:properties id="outputProperties">
	<beans:prop key="#{T (javax.xml.transform.OutputKeys).OMIT_XML_DECLARATION}">yes</beans:prop>
</util:properties>

<xpath-splitter input-channel="input"
             output-properties="outputProperties">
    <xpath-expression expression="/orders/order"/>
</xpath-splitter>

Starting with version 4.2, the XPathMessageSplitter exposes an iterator option as a boolean flag (defaults to true). This allows the "streaming" of split nodes in the downstream flow. With the iterator mode set to true, each node is transformed while iterating. When false, all entries are first transformed, before the split nodes start being sent to the output channel. (You can think of the difference as "transform, send, transform, send" versus "transform, transform, send, send".) See Section 8.3, “Splitter” for more information.

38.5 Routing XML Messages with XPath

Similar to SpEL-based routers, Spring Integration provides support for routing messages based on XPath expressions, which lets you create a message endpoint with an input channel but no output channel. Instead, one or more output channels are determined dynamically. The following example shows how to create such a router:

<int-xml:xpath-router id="orderTypeRouter" input-channel="orderChannel">
    <int-xml:xpath-expression expression="/order/type"/>
</int-xml:xpath-router>
[Note]Note

For an overview of attributes that are common among Routers, see Section 8.1.2, “Common Router Parameters”.

Internally, XPath expressions are evaluated as type NODESET and converted to a List<String> that represents channel names. Typically, such a list contains a single channel name. However, based on the results of an XPath Expression, the XPath router can also take on the characteristics of a recipient list router if the XPath expression returns more than one value. In that case, the List<String> contains more than one channel name. Consequently, messages are sent to all the channels in the list.

Thus, assuming that the XML file passed to the following router configuration contains many responder sub-elements that represent channel names, the message is sent to all of those channels:

<!-- route the order to all responders-->
<int-xml:xpath-router id="responderRouter" input-channel="orderChannel">
    <int-xml:xpath-expression expression="/request/responders"/>
</int-xml:xpath-router>

If the returned values do not represent the channel names directly, you can specify additional mapping parameters to map those returned values to actual channel names. For example if the /request/responders expression results in two values (responderA and responderB), but you do not want to couple the responder names to channel names, you can provide additional mapping configuration, such as the following:

<!-- route the order to all responders-->
<int-xml:xpath-router id="responderRouter" input-channel="orderChannel">
    <int-xml:xpath-expression expression="/request/responders"/>
    <int-xml:mapping value="responderA" channel="channelA"/>
    <int-xml:mapping value="responderB" channel="channelB"/>
</int-xml:xpath-router>

As already mentioned, the default evaluation type for XPath expressions is NODESET, which is converted to a List<String> of channel names, which handles single channel scenarios as well as multiple channel scenarios.

Nonetheless, certain XPath expressions may evaluate as type String from the very beginning. Consider, for example, the following XPath Expression:

name(./node())

This expression returns the name of the root node. If the default evaluation type NODESET is being used, it results in an exception.

For these scenarios, you can use the evaluate-as-string attribute, which lets you manage the evaluation type. It is FALSE by default. However, if you set it to TRUE, the String evaluation type is used.

[Note]Note

XPath 1.0 specifies 4 data types:

  • Node-sets
  • Strings
  • Number
  • Boolean

When the XPath Router evaluates expressions by using the optional evaluate-as-string attribute, the return value is determined by the string() function, as defined in the XPath specification. This means that, if the expression selects multiple nodes, it return the string value of the first node.

For further information, see:

For example, if we want to route based on the name of the root node, we can use the following configuration:

<int-xml:xpath-router id="xpathRouterAsString"
        input-channel="xpathStringChannel"
        evaluate-as-string="true">
    <int-xml:xpath-expression expression="name(./node())"/>
</int-xml:xpath-router>

38.5.1 XML Payload Converter

For XPath Routers, you can also specify the Converter to use when converting payloads prior to XPath evaluation. As such, the XPath Router supports custom implementations of the XmlPayloadConverter strategy, and when configuring an xpath-router element in XML, a reference to such an implementation may be provided via the converter attribute.

If this reference is not explicitly provided, the DefaultXmlPayloadConverter is used. It should be sufficient in most cases, since it can convert from Node, Document, Source, File, and String typed payloads. If you need to extend beyond the capabilities of that default implementation, then an upstream Transformer is generally a better option in most cases, rather than providing a reference to a custom implementation of this strategy here.

38.6 XPath Header Enricher

The XPath header enricher defines a header enricher message transformer that evaluates an XPath expression against the message payload and inserts the result of the evaluation into a message header.

The following listing shows all the available configuration parameters:

<int-xml:xpath-header-enricher default-overwrite="true"    1
                               id=""                       2
                               input-channel=""            3
                               output-channel=""           4
                               should-skip-nulls="true">   5
    <int:poller></int:poller>                              6
    <int-xml:header name=""                                7
                    evaluation-type="STRING_RESULT"        8
                    header-type="int"                      9
                    overwrite="true"                       10
                    xpath-expression=""                    11
                    xpath-expression-ref=""/>              12
</int-xml:xpath-header-enricher>

1

Specifies the default boolean value for whether to overwrite existing header values. This takes effect only for child elements that do not provide their own overwrite attribute. If you do not set the default- overwrite attribute, the specified header values do not overwrite any existing ones with the same header names. Optional.

2

ID for the underlying bean definition. Optional.

3

The receiving message channel of this endpoint. Optional.

4

Channel to which enriched messages are sent. Optional.

5

Specifies whether null values, such as might be returned from an expression evaluation, should be skipped. The default value is true. If a null value should trigger removal of the corresponding header, set this to false. Optional.

6

A poller to use with the header enricher. Optional.

7

The name of the header to be enriched. Mandatory.

8

The result type expected from the XPath evaluation. If you did not set a header-type attribute, this is the type of the header value. The following values are allowed: BOOLEAN_RESULT, STRING_RESULT, NUMBER_RESULT, NODE_RESULT, and NODE_LIST_RESULT. If not set, it defaults internally to XPathEvaluationType.STRING_RESULT. Optional.

9

The fully qualified class name for the header value type. The result of the XPath evaluation is converted to this type by ConversionService. This allows, for example, a NUMBER_RESULT (a double) to be converted to an Integer. The type can be declared as a primitive (such as int), but the result is always the equivalent wrapper class (such as Integer). The same integration ConversionService discussed in Section 10.1.6, “Payload Type Conversion” is used for the conversion, so conversion to custom types is supported by adding a custom converter to the service. Optional.

10

Boolean value to indicate whether this header value should overwrite an existing header value for the same name if already present on the input Message.

11

The XPath expression as a String. You must set either this attribute or xpath-expression-ref, but not both.

12

The XPath expression reference. You must set either this attribute or xpath-expression, but not both.

38.7 Using the XPath Filter

This component defines an XPath-based message filter. Internally, this components uses a MessageFilter that wraps an instance of AbstractXPathMessageSelector.

[Note]Note

See Section 8.2, “Filter” for further details.

to use the XPath filter you must, at a minimum, provide an XPath expression either by declaring the xpath-expression element or by referencing an XPath Expression in the xpath-expression-ref attribute.

If the provided XPath expression evaluates to a boolean value, no further configuration parameters are necessary. However, if the XPath expression evaluates to a String, you should set the match-value attribute, against which the evaluation result is matched.

match-type has three options:

  • exact: Correspond to equals on java.lang.String. The underlying implementation uses a StringValueTestXPathMessageSelector
  • case-insensitive: Correspond to equals-ignore-case on java.lang.String. The underlying implementation uses a StringValueTestXPathMessageSelector
  • regex: Matches operations one java.lang.String. The underlying implementation uses a RegexTestXPathMessageSelector

When providing a match-type value of regex, the value provided with the match-value attribute must be a valid regular expression.

The following example shows all the available attributes for the xpath-filter element:

<int-xml:xpath-filter discard-channel=""                      1
                      id=""                                   2
                      input-channel=""                        3
                      match-type="exact"                      4
                      match-value=""                          5
                      output-channel=""                       6
                      throw-exception-on-rejection="false"    7
                      xpath-expression-ref="">                8
    <int-xml:xpath-expression ... />                          9
    <int:poller ... />                                        10
</int-xml:xpath-filter>

1

Message channel where you want rejected messages to be sent. Optional.

2

ID for the underlying bean definition. Optional.

3

The receiving message channel of this endpoint. Optional.

4

Type of match to apply between the XPath evaluation result and the match-value. The default is exact. Optional.

5

String value to be matched against the XPath evaluation result. If you do not set this attribute, the XPath evaluation must produce a boolean result. Optional.

6

The channel to which messages that matched the filter criteria are dispatched. Optional.

7

By default, this property is set to false and rejected messages (messages that did not match the filter criteria) are silently dropped. However, if set to true, message rejection results in an error condition and an exception being propagated upstream to the caller. Optional.

8

Reference to an XPath expression instance to evaluate.

9

This child element sets the XPath expression to be evaluated. If you do not include this element, you must set the xpath-expression-ref attribute. Also, you can include only one xpath-expression element.

10

A poller to use with the XPath filter. Optional.

38.8 #xpath SpEL Function

Spring Integration, since version 3.0, provides the built-in #xpath SpEL function, which invokes the XPathUtils.evaluate(...) static method. This method delegates to an org.springframework.xml.xpath.XPathExpression. The following listing shows some usage examples:

<transformer expression="#xpath(payload, '/name')"/>

<filter expression="#xpath(payload, headers.xpath, 'boolean')"/>

<splitter expression="#xpath(payload, '//book', 'document_list')"/>

<router expression="#xpath(payload, '/person/@age', 'number')">
    <mapping channel="output1" value="16"/>
    <mapping channel="output2" value="45"/>
</router>

The #xpath() also supports a third optional parameter for converting the result of the XPath evaluation. It can be one of the String constants (string, boolean, number, node, node_list and document_list) or an org.springframework.xml.xpath.NodeMapper instance. By default, the #xpath SpEL function returns a String representation of the XPath evaluation.

[Note]Note

To enable the #xpath SpEL function, you can add the spring-integration-xml.jar to the classpath. You need no declare any components from the Spring Integration XML Namespace.

For more information, see "`Appendix A, Spring Expression Language (SpEL).

38.9 XML Validating Filter

The XML Validating Filter lets you validate incoming messages against provided schema instances. The following schema types are supported:

Messages that fail validation can either be silently dropped or be forwarded to a definable discard-channel. Furthermore, you can configure this filter to throw an Exception in case validation fails.

The following listing shows all the available configuration parameters:

<int-xml:validating-filter discard-channel=""                    1
                           id=""                                 2
                           input-channel=""                      3
                           output-channel=""                     4
                           schema-location=""                    5
                           schema-type="xml-schema"              6
                           throw-exception-on-rejection="false"  7
                           xml-converter=""                      8
                           xml-validator="">                     9
    <int:poller .../>                                            10
</int-xml:validating-filter>

1

Message channel where you want rejected messages to be sent. Optional.

2

ID for the underlying bean definition. Optional.

3

The receiving message channel of this endpoint. Optional.

4

Message channel where you want accepted messages to be sent. Optional.

5

Sets the location of the schema to validate the message’s payload against. Internally uses the org.springframework.core.io.Resource interface. You can set this attribute or the xml-validator attribute but not both. Optional.

6

Sets the schema type. Can be either xml-schema or relax-ng. Optional. If not set, it defaults to xml-schema, which internally translates to org.springframework.xml.validation.XmlValidatorFactory#SCHEMA_W3C_XML.

7

If true, a MessageRejectedException is thrown if validation fails for the provided Message’s payload. Defaults to false if not set. Optional.

8

Reference to a custom org.springframework.integration.xml.XmlPayloadConverter strategy. Optional.

9

Reference to a custom sorg.springframework.xml.validation.XmlValidator strategy. You can set this attribute or the schema-location attribute but not both. Optional.

10

A poller to use with the XPath filter. Optional.