Constant

Since Camel 1.5

The Constant Expression Language is really just a way to use a constant value or object.

This is a fixed constant value (or object) that is only set once during starting up the route, do not use this if you want dynamic values during routing.

Constant Options

The Constant language supports 2 options, which are listed below.

Name Default Java Type Description

resultType (common)

String

Sets the class of the result type (type from output).

trim (advanced)

true

Boolean

Whether to trim the source code to remove leading and trailing whitespaces and line breaks. For example when using DSLs where the source will span across multiple lines and there may be additional line breaks at both the beginning and end.

Example

The setHeader EIP can utilize a constant expression like:

<route>
  <from uri="seda:a"/>
  <setHeader name="theHeader">
    <constant>the value</constant>
  </setHeader>
  <to uri="mock:b"/>
</route>

in this case, the message coming from the seda:a endpoint will have the header with key theHeader set its value as the value (string type).

And the same example using Java DSL:

from("seda:a")
  .setHeader("theHeader", constant("the value"))
  .to("mock:b");

Specifying type of value

The option resultType can be used to specify the type of the value, when the value is given as a String value, which happens when using XML or YAML DSL:

For example to set a header with int type you can do:

<route>
  <from uri="seda:a"/>
  <setHeader name="zipCode">
    <constant resultType="int">90210</constant>
  </setHeader>
  <to uri="mock:b"/>
</route>

Loading constant from external resource

You can externalize the constant 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").constant("resource:classpath:constant.txt")

Dependencies

The Constant language is part of camel-core.

Back to top