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Chapter 168. XSLT

XSLT

The xslt: component allows you to process a message using an XSLT template. This can be ideal when using Templating to generate respopnses for requests.

URI format

xslt:templateName[?options]

Where templateName is the classpath-local URI of the template to invoke; or the complete URL of the remote template. Refer to the Spring Documentation for more detail of the URI syntax

You can append query options to the URI in the following format, ?option=value&option=value&...

Here are some example URIs

URI	Description
`xslt:com/acme/mytransform.xsl`	Refers to the file, `com/acme/mytransform.xsl`, on the classpath.
`xslt:file:///foo/bar.xsl`	Refers to the file, `/foo/bar.xsl`.
`xslt:http://acme.com/cheese/foo.xsl`	Refers to the remote HTTP resource.

From Camel 2.9 onwards the XSLT component is provided directly in the camel-core.

Options

Name	Default Value	Description
`converter`	`null`	Option to override default XmlConverter. Will lookup for the converter in the Registry. The provided converted must be of type org.apache.camel.converter.jaxp.XmlConverter.
`transformerFactory`	`null`	Option to override default TransformerFactory. Will lookup for the transformerFactory in the Registry. The provided transformer factory must be of type `javax.xml.transform.TransformerFactory`.
`transformerFactoryClass`	`null`	Option to override default TransformerFactory. Will create a `TransformerFactoryClass` instance and set it to the converter.
`uriResolver`	`null`	Camel 2.3: Allows you to use a custom `javax.xml.transformation.URIResolver`. Camel will by default use its own implementation `org.apache.camel.builder.xml.XsltUriResolver` which is capable of loading from classpath.
`resultHandlerFactory`	`null`	Camel 2.3: Allows you to use a custom `org.apache.camel.builder.xml.ResultHandlerFactory` which is capable of using custom `org.apache.camel.builder.xml.ResultHandler` types.
`failOnNullBody`	`true`	Camel 2.3: Whether or not to throw an exception if the input body is null.
`deleteOutputFile`	`false`	Camel 2.6: If you have `output=file` then this option dictates whether or not the output file should be deleted when the Exchange is done processing. For example suppose the output file is a temporary file, then it can be a good idea to delete it after use.
`output`	`string`	Camel 2.3: Option to specify which output type to use. Possible values are: `string, bytes, DOM, file`. The first three options are all in memory based, where as `file` is streamed directly to a `java.io.File`. For `file` you must specify the filename in the IN header with the key `Exchange.XSLT_FILE_NAME` which is also `CamelXsltFileName`. Also any paths leading to the filename must be created beforehand, otherwise an exception is thrown at runtime.
`contentCache`	`true`	Camel 2.6: Cache for the resource content (the stylesheet file) when it is loaded. If set to `false` Camel will reloader the stylesheet file on each message processing. This is good for development.
`allowStAX`	`true`	Camel 2.8.3/2.9: Whether to allow using StAX as the `javax.xml.transform.Source`.
`transformerCacheSize`	`0`	Camel 2.9.3/2.10.1: The number of `javax.xml.transform.Transformer` object that are cached for reuse to avoid calls to `Template.newTransformer()`.
`saxon`	`false`	Camel 2.11: Whether to use Saxon as the `transformerFactoryClass`. If enabled then the class `net.sf.saxon.TransformerFactoryImpl`. You would need to add Saxon to the classpath.
`errorListener`		Camel 2.14: Allows to configure to use a custom `javax.xml.transform.ErrorListener`. Beware when doing this then the default error listener which captures any errors or fatal errors and store information on the Exchange as properties is not in use. So only use this option for special use-cases.

Using XSLT endpoints

For example you could use something like

from("activemq:My.Queue").
  to("xslt:com/acme/mytransform.xsl");

To use an XSLT template to forumulate a response for a message for InOut message exchanges (where there is a JMSReplyTo header).

If you want to use InOnly and consume the message and send it to another destination you could use the following route:

from("activemq:My.Queue").
  to("xslt:com/acme/mytransform.xsl").
  to("activemq:Another.Queue");

Getting Parameters into the XSLT to work with

By default, all headers are added as parameters which are available in the XSLT. To do this you will need to declare the parameter so it is then useable.

<setHeader headerName="myParam"><constant>42</constant></setHeader>
<to uri="xslt:MyTransform.xsl"/>

And the XSLT just needs to declare it at the top level for it to be available:

<xsl: ...... >

   <xsl:param name="myParam"/>
  
    <xsl:template ...>

Spring XML versions

To use the above examples in Spring XML you would use something like

  <camelContext xmlns="http://activemq.apache.org/camel/schema/spring">
    <route>
      <from uri="activemq:My.Queue"/>
      <to uri="xslt:org/apache/camel/spring/processor/example.xsl"/>
      <to uri="activemq:Another.Queue"/>
    </route>
  </camelContext>

There is a test case along with its Spring XML if you want a concrete example.

Using xsl:include

Camel provides its own implementation of URIResolver which allows Camel to load included files from the classpath and more intelligent than before.

For example this include:

<xsl:include href="staff_template.xsl"/>

Will now be located relative from the starting endpoint, which for example could be:

.to("xslt:org/apache/camel/component/xslt/staff_include_relative.xsl")

Which means Camel will locate the file in the classpath as org/apache/camel/component/xslt/staff_template.xsl. This allows you to use xsl include and have xsl files located in the same folder such as we do in the example org/apache/camel/component/xslt.

You can use the following two prefixes classpath: or file: to instruct Camel to look either in classpath or file system. If you omit the prefix then Camel uses the prefix from the endpoint configuration. If that neither has one, then classpath is assumed.

You can also refer back in the paths such as

    <xsl:include href="../staff_other_template.xsl"/>

Which then will resolve the xsl file under org/apache/camel/component.

Using xsl:include and default prefix

When using xsl:include such as:

<xsl:include href="staff_template.xsl"/>

Then in Camel 2.10.3 and older, then Camel will use "classpath:" as the default prefix, and load the resource from the classpath. This works for most cases, but if you configure the starting resource to load from file,

.to("xslt:file:etc/xslt/staff_include_relative.xsl")

.. then you would have to prefix all your includes with "file:" as well.

<xsl:include href="file:staff_template.xsl"/>

From Camel 2.10.4 onwards we have made this easier as Camel will use the prefix from the endpoint configuration as the default prefix. So from Camel 2.10.4 onwards you can do:

<xsl:include href="staff_template.xsl"/>

Which will load the staff_template.xsl resource from the file system, as the endpoint was configured with "file:" as prefix. You can still though explicit configure a prefix, and then mix and match. And have both file and classpath loading. But that would be unusual, as most people either use file or classpath based resources.

Dynamic stylesheets

To provide a dynamic stylesheet at runtime you can define a dynamic URI. For example, you can do this using the Recipient List Enterprise Integration Pattern (EIP), which is invoked using the .recipientList command in the Java DSL.

Accessing warnings, errors and fatalErrors from XSLT ErrorListener

Available as of Camel 2.14

From Camel 2.14 onwards, any warning/error or fatalError is stored on the current Exchange as a property with the keys Exchange.XSLT_ERROR, Exchange.XSLT_FATAL_ERROR, or Exchange.XSLT_WARNING which allows end users to get hold of any errors happening during transformation.

For example in the stylesheet below, we want to terminate if a staff has an empty dob field. And to include a custom error message using xsl:message.

  <xsl:template match="/">
    <html>
      <body>
        <xsl:for-each select="staff/programmer">
          <p>Name: <xsl:value-of select="name"/><br />
            <xsl:if test="dob=''">
              <xsl:message terminate="yes">Error: DOB is an empty string!</xsl:message>
            </xsl:if>
          </p>
        </xsl:for-each>
      </body>
    </html>
  </xsl:template>

This information is not available on the Exchange stored as an Exception that contains the message in the getMessage() method on the exception. The exception is stored on the Exchange as a warning with the key Exchange.XSLT_WARNING.