Apache Camel: XQuery

XQuery

Camel supports XQuery to allow an Expression or Predicate to be used in the DSL or Xml Configuration. For example you could use XQuery to create an Predicate in a Message Filter or as an Expression for a Recipient List.

To enable XQuery support you need the camel-saxon library to be on your classpath.

From 1.3 of Camel onwards you can use XQuery expressions directly using smart completion in your IDE as follows

from("queue:foo").filter().
  xquery("//foo")).
  to("queue:bar")

In earlier versions of Camel you had to use theXQueryBuilder methods then you can use the xquery() method inside your rules.

import static org.apache.camel.builder.saxon.XQueryBuilder.*;
...
from("queue:foo").
  filter(xquery("//foo")).
  to("queue:bar")

You can also use functions inside your query, in which case you need an explicit type conversion (or you will get a org.w3c.dom.DOMException: HIERARCHY_REQUEST_ERR) by passing the Class as a second argument to the xquery() method.

from("direct:start").
  recipientList().xquery("concat('mock:foo.', /person/@city)", String.class);

Using XML configuration

If you prefer to configure your routes in your Spring XML file then you can use XPath expressions as follows

<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:foo="http://example.com/person"
       xsi:schemaLocation="
       http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
       http://activemq.apache.org/camel/schema/spring http://activemq.apache.org/camel/schema/spring/camel-spring.xsd">

  <camelContext id="camel" xmlns="http://activemq.apache.org/camel/schema/spring">
    <route>
      <from uri="activemq:MyQueue"/>
      <filter>
        <xquery>/foo:person[@name='James']</xquery>
        <to uri="mqseries:SomeOtherQueue"/>
      </filter>
    </route>
  </camelContext>
</beans>

Notice how we can reuse the namespace prefixes, foo in this case, in the XPath expression for easier namespace based XQuery expressions!

When you use functions in your XQuery expression you need an explicit type conversion which is done in the xml configuration via the @type attribute:

<xquery type="java.lang.String">concat('mock:foo.', /person/@city)</xquery>

Using XQuery as an endpoint

Sometimes an XQuery expression can be quite large; it can essentally be used for Templating. So you may want to use an XQuery Endpoint so you can route using XQuery templates.

The following example shows how to take a message of an ActiveMQ queue (MyQueue) and transform it using XQuery and send it to MQSeries.

<camelContext id="camel" xmlns="http://activemq.apache.org/camel/schema/spring">
    <route>
      <from uri="activemq:MyQueue"/>
      <to uri="xquery:com/acme/someTransform.xquery"/>
      <to uri="mqseries:SomeOtherQueue"/>
    </route>
  </camelContext>

Examples

Here is a simple example using an XQuery expression as a predicate in a Message Filter

from("direct:start").filter().xquery("/person[@name='James']").to("mock:result");

This example uses XQuery with namespaces as a predicate in a Message Filter

Namespaces ns = new Namespaces("c", "http://acme.com/cheese");

from("direct:start").
        filter().xquery("/c:person[@name='James']", ns).
        to("mock:result");

Learning XQuery

XQuery is a very powerful language for querying, searching, sorting and returning XML. For help learning XQuery try these tutorials

Mike Kay's XQuery Primer
the W3Schools XQuery Tutorial

You might also find the XQuery function reference useful

XQuery

Using XML configuration

Using XQuery as an endpoint

Examples

Learning XQuery

Overview

Documentation

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