Chapter 80. XPath

The XPath language supports 10 options, which are listed below.

You can easily use namespaces with XPath expressions using the Namespaces helper class.

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

Camel will resolve variables according to either:

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.

<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.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>

If the message body is stream based, which means the input it receives is submitted to Camel as a stream. That means you will only be able to read the content of the stream once. So often when you use XPath as Message Filter or Content Based Router then you need to access the data multiple times, and you should use Stream Caching or convert the message body to a String prior which is safe to be re-read multiple times.

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

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

The XPath expression will return a result type using native XML objects such as org.w3c.dom.NodeList. However, 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 XML DSL you use the resultType attribute to provide the fully qualified classname.

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

Using @XPath annotation:

@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.

Some users may have XML stored in a header. To apply an XPath to a header’s value you can do this by defining the 'headerName' attribute.

<xpath headerName="invoiceDetails">/invoice/@orderType = 'premium'</xpath>

And in Java DSL you specify the headerName as the 2nd parameter as shown:

xpath("/invoice/@orderType = 'premium'", "invoiceDetails")

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");

And in XML

<route>
  <from uri="direct:start"/>
  <filter>
    <xpath>/person[@name='James']</xpath>
    <to uri="mock:result"/>
  </filter>
</route>

If you have a standard set of namespaces you wish to work with and wish to share them across many XPath expressions you can use the org.apache.camel.support.builder.Namespaces when using Java DSL as shown:

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

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

Notice how the namespaces are provided to xpath with the ns variable that are passed in as the 2nd parameter.

Each namespace is a key=value pair, where the prefix is the key. In the XPath expression then the namespace is used by its prefix, eg:

/c:person[@name='James']

The namespace builder supports adding multiple namespaces as shown:

Namespaces ns = new Namespaces("c", "http://acme.com/cheese")
                     .add("w", "http://acme.com/wine")
                     .add("b", "http://acme.com/beer");

When using namespaces in XML DSL then its different, as you setup the namespaces in the XML root tag (or one of the camelContext, routes, route tags).

In the XML example below we use Spring XML where the namespace is declared in the root tag beans, in the line with xmlns:foo="http://example.com/person":

<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.xsd
       http://camel.apache.org/schema/spring http://camel.apache.org/schema/spring/camel-spring.xsd
    ">

  <camelContext xmlns="http://camel.apache.org/schema/spring">
    <route>
      <from uri="direct:start"/>
      <filter>
        <xpath logNamespaces="true">/foo:person[@name='James']</xpath>
        <to uri="mock:result"/>
      </filter>
    </route>
  </camelContext>

</beans>

This namespace uses foo as prefix, so the <xpath> expression uses /foo: to use this namespace.

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.

public class Foo {

    @Consume(uri = "activemq:my.queue")
    public void doSomething(@XPath("/person/@name") String name, String xml) {
        // process the inbound message here
    }
}

You can now use the org.apache.camel.language.xpath.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 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 make this simpler:

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

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

It’s now easier to use Saxon with the XPathBuilder which can be done in several ways as shown below

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

-Djavax.xml.xpath.XPathFactory=org.apache.xpath.jaxp.XPathFactoryImpl

<xpath factoryRef="saxonFactory" resultType="java.lang.String">current-dateTime()</xpath>

<bean id="saxonFactory" class="net.sf.saxon.xpath.XPathFactoryImpl"/>

<xpath objectModel="http://saxon.sf.net/jaxp/xpath/om" resultType="java.lang.String">current-dateTime()</xpath>

<xpath saxon="true" resultType="java.lang.String">current-dateTime()</xpath>

Many XPath-related issues that users frequently face are linked to the usage of namespaces. You may have some misalignment between the namespaces present in your message, and those that your XPath expression is aware of or referencing. XPath predicates or expressions that are unable to locate the XML elements and attributes due to namespaces issues may simply look like they are not working, when in reality all there is to it is a lack of namespace definition.

Namespaces in XML are completely necessary, and while we would love to simplify their usage by implementing some magic or voodoo to wire namespaces automatically, truth is that any action down this path would disagree with the standards and would greatly hinder interoperability.

Therefore, the utmost we can do is assist you in debugging such issues by adding two new features to the XPath Expression Language and are thus accessible from both predicates and expressions.

[me: {prefix -> namespace}, {prefix -> namespace}], [parent: [me: {prefix -> namespace}, {prefix -> namespace}], [parent: [me: {prefix -> namespace}]]]

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

<xpath logNamespaces="true" resultType="String">/foo:person/@id</xpath>

2012-01-16 13:23:45,878 [stSaxonWithFlag] INFO  XPathBuilder  - Namespaces discovered in message:
{xmlns:a=[http://apache.org/camel], DEFAULT=[http://apache.org/default],
xmlns:b=[http://apache.org/camelA, http://apache.org/camelB]}

You can externalize the script and have Camel load it from a resource such as "classpath:", "file:", or "http:". This is done using the following syntax: "resource:scheme:location", eg to refer to a file on the classpath you can do:

.setHeader("myHeader").xpath("resource:classpath:myxpath.txt", String.class)

To use XPath in your camel routes you need to add the dependency on camel-xpath which implements the XPath language.

If you use maven you could just add the following to your pom.xml, substituting the version number for the latest & greatest release (see the download page for the latest versions).

<dependency>
  <groupId>org.apache.camel</groupId>
  <artifactId>camel-xpath</artifactId>
  <version>{CamelSBProjectVersion}</version>
</dependency>

When using xpath with Spring Boot make sure to use the following Maven dependency to have support for auto configuration:

<dependency>
  <groupId>org.apache.camel.springboot</groupId>
  <artifactId>camel-xpath-starter</artifactId>
</dependency>

The component supports 9 options, which are listed below.