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Chapter 83. MVEL Component

MVEL Component
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Available as of Camel 2.12
The mvel: component allows you to process a message using an MVEL template. This can be ideal when using Templating to generate responses for requests.
Maven users will need to add the following dependency to their pom.xml for this component: 
<dependency>
    <groupId>org.apache.camel</groupId>
    <artifactId>camel-mvel</artifactId>
    <version>x.x.x</version>
    <!-- use the same version as your Camel core version -->
</dependency>
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URI format
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mvel:templateName[?options]
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Where templateName is the classpath-local URI of the template to invoke; or the complete URL of the remote template (eg: file://folder/myfile.mvel).
You can append query options to the URI in the following format, ?option=value&option=value&...

Options
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Expand
Option Default Description
contentCache true Cache for the resource content when it is loaded. The cached resource content can be cleared via JMX using the endpoint's clearContentCache operation.
encoding null Character encoding of the resource content.

Message Headers
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The mvel component sets a couple headers on the message.
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Header Description
CamelMvelResourceUri The templateName as a String object.

MVEL Context
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Camel will provide exchange information in the MVEL context (just a Map). The Exchange is transfered as:
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key value
exchange The Exchange itself.
exchange.properties The Exchange properties.
headers The headers of the In message.
camelContext The Camel Context intance.
request The In message.
in The In message.
body The In message body.
out The Out message (only for InOut message exchange pattern).
response The Out message (only for InOut message exchange pattern).

Hot reloading
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The mvel template resource is, by default, hot reloadable for both file and classpath resources (expanded jar). If you set contentCache=true, Camel will only load the resource once, and thus hot reloading is not possible. This scenario can be used in production, when the resource never changes.

Dynamic templates
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Camel provides two headers by which you can define a different resource location for a template or the template content itself. If any of these headers is set then Camel uses this over the endpoint configured resource. This allows you to provide a dynamic template at runtime.
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Header Type Description
CamelMvelResourceUri String A URI for the template resource to use instead of the endpoint configured.
CamelMvelTemplate String The template to use instead of the endpoint configured.

Samples
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For example you could use something like 
from("activemq:My.Queue").
  to("mvel:com/acme/MyResponse.mvel");
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To use a MVEL template to formulate a response to a message for InOut message exchanges (where there is a JMSReplyTo header).
To specify what template the component should use dynamically via a header, so for example: 
from("direct:in").
  setHeader("CamelMvelResourceUri").constant("path/to/my/template.mvel").
  to("mvel:dummy");
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To specify a template directly as a header the component should use dynamically via a header, so for example: 
from("direct:in").
  setHeader("CamelMvelTemplate").constant("@{\"The result is \" + request.body * 3}\" }").
  to("velocity:dummy");
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