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.
If the message body is stream based, which means the input is received by Camel as a stream, then you will only be able to read the content of the stream once. Oftentimes when using XPath as Message Filter or Content Based Router the data will be accessed multiple times. Therefore use Stream caching or convert the message body to a String
beforehand. This makes it safe to be re-read multiple times.
Namespaces
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/ |
|
| The |
http://camel.apache.org/xml/out/ |
|
| The |
http://camel.apache.org/xml/function/ |
|
| Camel 2.5: Additional functions. |
http://camel.apache.org/xml/variables/environment-variables |
| Object | OS environment variables. |
http://camel.apache.org/xml/variables/system-properties |
| Object | Java System properties. |
http://camel.apache.org/xml/variables/exchange-property |
|
| 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 thevariable(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 |
---|---|---|---|
| none |
| Will return the |
| the header name | Object | Will return the |
| none |
| Will return the |
| the header name |
| Will return the |
| key for property |
| Camel 2.5: To lookup a property using the Properties component (property placeholders). |
| simple expression | Object | Camel 2.5: To evaluate a Simple expression. |
function:properties
and function:simple
is not supported when the return type is a NodeSet
, such as when using with a Splitter EIP.Here's an example showing some of these functions in use.
Using XML Configuration
If you prefer to configure your routes in your Spring XML file then you can use XPath expressions as follows
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 the 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:
In Spring DSL you use the resultType
attribute to provide a fully qualified classname:
In @XPath
:
Available as of Camel 2.1
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.
Using XPath on Headers
Available as of Camel 2.11
Some users may have XML stored in a header. To apply an XPath statement to a header's value you can do this by defining the headerName
attribute.
In XML DSL:headerName
as the 2nd parameter as shown:
Examples
Here is a simple example using an XPath expression as a predicate in a Message FilterNamespaceBuilder
as shown in this examplechoice
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:
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.
Example:
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:
This will match the given predicate.
You can also evaluate for example as shown in the following three examples:
Evaluating with a String result is a common requirement and thus you can do it a bit simpler:
Using Saxon with XPathBuilder
Available as of Camel 2.3
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. Where as the latter ones are the easiest ones.
Using a factory
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:INFO
level if it uses a non default XPathFactory
such as:
To use Apache Xerces you can configure the system property:
Enabling Saxon from Spring DSL
Available as of Camel 2.10
Similarly to Java DSL, to enable Saxon from Spring DSL you have three options:
Specifying the factory
Specifying the object model
Shortcut
Namespace Auditing to Aid Debugging
Available as of Camel 2.10
A large number of 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.
Logging the Namespace Context of Your XPath Expression/Predicate
Every time a new XPath expression is created in the internal pool, Camel will log the namespace context of the expression under the org.apache.camel.builder.xml.XPathBuilder
logger. Since Camel represents Namespace Contexts in a hierarchical fashion (parent-child relationships), the entire tree is output in a recursive manner with the following format:
Any of these options can be used to activate this logging:
- Enable
TRACE
logging on theorg.apache.camel.builder.xml.XPathBuilder
logger, or some parent logger such asorg.apache.camel
or the root logger. - Enable the
logNamespaces
option as indicated in Auditing Namespaces, in which case the logging will occur on theINFO
level.
Auditing namespaces
Camel is able to discover and dump all namespaces present on every incoming message before evaluating an XPath expression, providing all the richness of information you need to help you analyse and pinpoint possible namespace issues. To achieve this, it in turn internally uses another specially tailored XPath expression to extract all namespace mappings that appear in the message, displaying the prefix and the full namespace URI(s) for each individual mapping.
Some points to take into account:
- The implicit XML namespace (xmlns:xml="http://www.w3.org/XML/1998/namespace") is suppressed from the output because it adds no value.
- Default namespaces are listed under the
DEFAULT
keyword in the output. - Keep in mind that namespaces can be remapped under different scopes. Think of a top-level 'a' prefix which in inner elements can be assigned a different namespace, or the default namespace changing in inner scopes. For each discovered prefix, all associated URIs are listed.
You can enable this option in Java DSL and Spring DSL.
Java DSL:
Spring DSL:
The result of the auditing will be appear at the INFO
level under the org.apache.camel.builder.xml.XPathBuilder
logger and will look like the following:
Loading Script from External Resource
Available as of Camel 2.11
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
, e.g., to refer to a file on the classpath you can do:
Dependencies
The XPath language is part of camel-core.