Apache Camel: 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.

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

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

Variables

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

Function	Argument	Type	Description
in:body	none	Object	Will return the in message body.
in:header	the header name	Object	Will return the in message header.
out:body	none	Object	Will return the out message body.
out:header	the header name	Object	Will return the out message header.

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

from("direct:start").choice()
  .when().xpath("in:header('foo') = 'bar'").to("mock:x")
  .when().xpath("in:body() = '<two/>'").to("mock:y")
  .otherwise().to("mock:z");

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>
        <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!

Examples

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

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

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

// lets define the namespaces we'll need in our filters
Namespaces ns = new Namespaces("c", "http://acme.com/cheese")
        .add("xsd", "http://www.w3.org/2001/XMLSchema");

// now lets create an xpath based Message Filter
from("direct:start").
        filter(ns.xpath("/c:person[@name='James']")).
        to("mock:result");

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:

from("direct:in").choice()
    // using $headerName is special notation in Camel to get the header key
    .when().xpath("$type = 'Camel'")
        .to("mock:camel")
    // here we test for the body name tag
    .when().xpath("//name = 'Kong'")
        .to("mock:donkey")
    .otherwise()
        .to("mock:other")
    .end();

And the spring XML equivalent of the route:

<camelContext id="camel" xmlns="http://activemq.apache.org/camel/schema/spring">
    <route>
        <from uri="direct:in"/>
        <choice>
            <when>
                <xpath>$type = 'Camel'</xpath>
                <to uri="mock:camel"/>
            </when>
            <when>
                <xpath>//name = 'Kong'</xpath>
                <to uri="mock:donkey"/>
            </when>
            <otherwise>
                <to uri="mock:other"/>
            </otherwise>
        </choice>
    </route>
</camelContext>

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 the XPath annotation 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.

For example

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

Dependencies

The XPath language is part of camel-core.

XPath

Namespaces

Variables

Namespace given

No namespace given

Functions

Using XML configuration

Examples

XPath injection

Dependencies

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