Camel Components

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 the following options which are listed below.

Name Default Java Type Description

resultType (common)

String

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.

resolveResource (advanced)

false

Boolean

Whether a result of the expression that is a String starting with resource: is loaded as a resource and its content becomes the result, e.g. a script that returns resource:file:order.json or resource:classpath:templates/order.json (a name without a scheme is a classpath resource). Off by default; the resource: prefix on the expression text itself is always resolved. Applies to the expression used as a value, not as a predicate.

Example

The setHeader EIP can utilize a constant expression like:

  • Java

  • XML

  • YAML

from("seda:a")
  .setHeader("theHeader", constant("the value"))
  .to("mock:b");
<route>
  <from uri="seda:a"/>
  <setHeader name="theHeader">
    <constant>the value</constant>
  </setHeader>
  <to uri="mock:b"/>
</route>
- route:
    from:
      uri: seda:a
      steps:
        - setHeader:
            name: theHeader
            expression:
              constant:
                expression: the value
        - to:
            uri: mock:b

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).

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:

  • XML

  • YAML

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

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.