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29.8. Using Variables and Functions

Evaluating variables in a route
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When evaluating XPath expressions inside a route, you can use XPath variables to access the contents of the current exchange, as well as O/S environment variables and Java system properties. The syntax to access a variable value is $VarName or $Prefix:VarName, if the variable is accessed through an XML namespace.
For example, you can access the In message's body as $in:body and the In message's header value as $in:HeaderName. O/S environment variables can be accessed as $env:EnvVar and Java system properties can be accessed as $system:SysVar.
In the following example, the first route extracts the value of the /person/city element and inserts it into the city header. The second route filters exchanges using the XPath expression, $in:city = 'London', where the $in:city variable is replaced by the value of the city header. 
from("file:src/data?noop=true")
    .setHeader("city").xpath("/person/city/text()")
    .to("direct:tie");
        
from("direct:tie")
    .filter().xpath("$in:city = 'London'").to("file:target/messages/uk");
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Evaluating functions in a route
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In addition to the standard XPath functions, the XPath language defines additional functions. These additional functions (which are listed in Table 29.4, “XPath Custom Functions”) can be used to access the underlying exchange, to evaluate a simple expression or to look up a property in the Apache Camel property placeholder component.
For example, the following example uses the in:header() function and the in:body() function to access a head and the body from the underlying exchange: 
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");
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Notice the similarity between theses functions and the corresponding in:HeaderName or in:body variables. The functions have a slightly different syntax however: in:header('HeaderName') instead of in:HeaderName; and in:body() instead of in:body.

Evaluating variables in XPathBuilder
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You can also use variables in expressions that are evaluated using the XPathBuilder class. In this case, you cannot use variables such as $in:body or $in:HeaderName, because there is no exchange object to evaluate against. But you can use variables that are defined inline using the variable(Name, Value) fluent builder method.
For example, the following XPathBuilder construction evaluates the $test variable, which is defined to have the value, London: 
String var = XPathBuilder.xpath("$test")
               .variable("test", "London")
               .evaluate(getContext(), "<name>foo</name>");
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Note that variables defined in this way are automatically entered into the global namespace (for example, the variable, $test, uses no prefix).
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