XPath

XPath

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

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

from("queue:foo").
  choice().xpath("//foo")).to("queue:bar").
  otherwise().to("queue:others");

Namespaces

In 1.3 onwards you can easily use namespaces with XPath expressions using the Namespaces helper class.

Variables

Variables in XPath is defined in different namespaces. The default namespace is http://camel.apache.org/schema/spring.

Namespace URI	Local part	Type	Description
http://camel.apache.org/xml/in/	in	Message	the exchange.in message
http://camel.apache.org/xml/out/	out	Message	the exchange.out message
http://camel.apache.org/xml/functions/	functions	Object	Camel 2.5: Additional functions
http://camel.apache.org/xml/variables/environment-variables	env	Object	OS environment variables
http://camel.apache.org/xml/variables/system-properties	system	Object	Java System properties
http://camel.apache.org/xml/variables/exchange-property		Object	the exchange property

Camel will resolve variables according to either:

• namespace given
• no namespace given

Namespace given

If the namespace is given then Camel is instructed exactly what to return. However when resolving either in or out Camel will try to resolve a header with the given local part first, and return it. If the local part has the value body then the body is returned instead.

No namespace given

If there is no namespace given then Camel resolves only based on the local part. Camel will try to resolve a variable in the following steps:

• from variables that has been set using the variable(name, value) fluent builder
• from message.in.header if there is a header with the given key
• from exchange.properties if there is a property with the given key

Functions

Camel adds the following XPath functions that can be used to access the exchange:

Here's an example showing some of these functions in use.

And the new functions introduced in Camel 2.5:

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"
       xsi:schemaLocation="
       http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
       http://camel.apache.org/schema/spring http://camel.apache.org/schema/spring/camel-spring.xsd">

  <camelContext id="camel" xmlns="http://activemq.apache.org/camel/schema/spring" xmlns:foo="http://example.com/person">
    <route>
      <from uri="activemq:MyQueue"/>
      <filter>
        <xpath>/foo:person[@name='James']</xpath>
        <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 XPath expressions!

See also this discussion on the mailinglist about using your own namespaces with xpath

Setting result type

The XPath expression will return a result type using native XML objects such as org.w3c.dom.NodeList. But many times you want a result type to be a String. To do this you have to instruct the XPath which result type to use.

In Java DSL:

xpath("/foo:person/@id", String.class)

In Spring DSL you use the resultType attribute to provide a fully qualified classname:

<xpath resultType="java.lang.String">/foo:person/@id</xpath>

In @XPath:
Available as of Camel 2.1

@XPath(value = "concat('foo-',//order/name/)", resultType = String.class) String name)

Where we use the xpath function concat to prefix the order name with foo-. In this case we have to specify that we want a String as result type so the concat function works.

Examples

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

If you have a standard set of namespaces you wish to work with and wish to share them across many different XPath expressions you can use the NamespaceBuilder as shown in this example

In this sample we have a choice construct. The first choice evaulates if the message has a header key type that has the value Camel.
The 2nd choice evaluates if the message body has a name tag <name> which values is Kong.
If neither is true the message is routed in the otherwise block:

And the spring XML equivalent of the route:

XPath injection

You can use Bean Integration to invoke a method on a bean and use various languages such as XPath to extract a value from the message and bind it to a method parameter.

The default XPath annotation has SOAP and XML namespaces available. If you want to use your own namespace URIs in an XPath expression you can use your own copy of the XPath annotation to create whatever namespace prefixes you want to use.

i.e. cut and paste upper code to your own project in a different package and/or annotation name then add whatever namespace prefix/uris you want in scope when you use your annotation on a method parameter. Then when you use your annotation on a method parameter all the namespaces you want will be available for use in your XPath expression.

NOTE this feature is supported from Camel 1.6.1.

For example

public class Foo {
	
    @MessageDriven(uri = "activemq:my.queue")
    public void doSomething(@XPath("/ns1:foo/ns2:bar/text()") String correlationID, @Body String body) {
		// process the inbound message here
    }
}

Using XPathBuilder without an Exchange

Available as of Camel 2.3

You can now use the org.apache.camel.builder.XPathBuilder without the need for an Exchange. This comes handy if you want to use it as a helper to do custom xpath evaluations.

It requires that you pass in a CamelContext since a lot of the moving parts inside the XPathBuilder requires access to the Camel Type Converter and hence why CamelContext is needed.

For example you can do something like this:

boolean matches = XPathBuilder.xpath("/foo/bar/@xyz").matches(context, "<foo><bar xyz='cheese'/></foo>"));

This will match the given predicate.

You can also evaluate for example as shown in the following three examples:

    String name = XPathBuilder.xpath("foo/bar").evaluate(context, "<foo><bar>cheese</bar></foo>", String.class);
    Integer number = XPathBuilder.xpath("foo/bar").evaluate(context, "<foo><bar>123</bar></foo>", Integer.class);
    Boolean bool = XPathBuilder.xpath("foo/bar").evaluate(context, "<foo><bar>true</bar></foo>", Boolean.class);

Evaluating with a String result is a common requirement and thus you can do it a bit simpler:

    String name = XPathBuilder.xpath("foo/bar").evaluate(context, "<foo><bar>cheese</bar></foo>");

Using Saxon with XPathBuilder

Available as of Camel 2.3

You need to add camel-saxon as dependency to your project.

Its now easier to use Saxon with the XPathBuilder which can be done in several ways as shown below.
Where as the latter ones are the easiest ones.

Using a factory

Using ObjectModel

The easy one

Setting a custom XPathFactory using System Property

Available as of Camel 2.3

Camel now supports reading the JVM system property javax.xml.xpath.XPathFactory that can be used to set a custom XPathFactory to use.

This unit test shows how this can be done to use Saxon instead:

Camel will log at INFO level if it uses a non default XPathFactory such as:

XPathBuilder  INFO  Using system property javax.xml.xpath.XPathFactory:http://saxon.sf.net/jaxp/xpath/om with value:
                    net.sf.saxon.xpath.XPathFactoryImpl when creating XPathFactory

Dependencies

The XPath language is part of camel-core.