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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.
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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.
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Namespaces
You can easily use namespaces with XPath expressions using the Namespaces helper class.
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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 The |
http://camel.apache.org/xml/out/![]() |
|
| the 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 The exchange property. |
Camel will resolve variables according to either:
- namespace given
- no namespace given
Namespace
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Given
If the namespace is given then Camel is instructed exactly what to return. However when resolving either in or out 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 value body
then the body is returned instead.
No
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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:
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- From
variables
that has been set using thevariable(name, value)
fluent builder.
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- From
message.in.header
if there is a header with the given key.
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- 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 in the |
| the header name | Object | Will return the in the |
| none |
| Will return the out the |
| the header name |
| Will return the out 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. |
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Here's an example showing some of these functions in use.
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And the new functions introduced in Camel 2.5:
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Using XML
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Configuration
If you prefer to configure your routes in your Spring XML file then you can use XPath expressions as follows
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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 xpathXPath.
Setting
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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:
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In Spring DSL you use the the resultType
attribute to provide a fully qualified classname:
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In @XPath
:
Available as of Camel 2.1
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Where we use the xpath function XPath function concat
to prefix the order name with foo-
. In this case we have to specify that we want a a String
as result type so the the concat
function works.
Using XPath on Headers
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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 'the headerName
' attribute.
In XML DSL:
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headerName
as the 2nd parameter as shown:
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Examples
Here is a simple example using an XPath expression as a predicate in a Message Filter
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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 the NamespaceBuilder
as shown in this example
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In this sample we have a a choice
construct. The first choice evaulates if the message has a header key key type
that has the value Camel
.
The 2nd 2nd choice
evaluates if the message body has a name tag tag <name>
which values is Kong
.
If neither is true the message is routed in the otherwise block:
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And the spring XML equivalent of the route:
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XPath
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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.
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to use.
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ie.eg., cut and paste upper code to your own project in a different package and/or annotation name then add whatever namespace prefix/uris 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
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Example:
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Using XPathBuilder
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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 XPath evaluations. It requires that you pass in a CamelContext since a lot of the moving parts inside the the XPathBuilder
requires access to the Camel Type Converter and hence why CamelContext is needed.
For example you can do something like this:
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This will match the given predicate.
You can also evaluate for example as shown in the following three examples:
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Evaluating with a String result is a common requirement and thus you can do it a bit simpler:
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Using Saxon with XPathBuilder
Available as of Camel 2.3
You need to add camel-saxon
as dependency to your project. Its It's now easier to use Saxon with the the XPathBuilder
which can be done in several ways as shown below.
Where as the latter ones are the easiest ones.
Using a factory
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Using ObjectModel
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Using the object model
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The easy one
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Setting a
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Custom XPathFactory
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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 custom XPathFactory
to use.
This unit test shows how this can be done to use Saxon instead:
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Camel will log at INFO
level if it uses a non default default XPathFactory
such as:
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To use Apache Xerces you can configure the system property
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:
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Enabling Saxon from Spring DSL
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Similarly to Java DSL, to enable Saxon from Spring DSL you have three options:
Specifying the factory
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Specifying the object model
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Shortcut
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Namespace
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Auditing to
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Aid Debugging
Available as of Camel 2.10
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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 accesible accessible from both predicates and expressions.
Logging the Namespace Context of
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Your XPath
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Expression/
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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:
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Any of these options can be used to activate this logging:
- Enable 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 the theINFO
level
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- .
AuditingNamespaces
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.
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- 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 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:
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Spring DSL:
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The result of the auditing will be appear at the the INFO
level under the org.apache.camel.builder.xml.XPathBuilder
logger and will look like the following:
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Loading
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Script from
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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., eg to refer to a file on the classpath you can do:
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Dependencies
The XPath language is part of camel-core.