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h2. Velocity

The *velocity:* component allows you to process a message using an [Apache Velocity|http://velocity.apache.org/] 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:
{code:xml}
<dependency>
    <groupId>org.apache.camel</groupId>
    <artifactId>camel-velocity</artifactId>
    <version>x.x.x</version>
    <!-- use the same version as your Camel core version -->
</dependency>
{code}

h3. URI format

{code}
velocity:templateName[?options]
{code}

Where *templateName* is the classpath-local URI of the template to invoke; or the complete URL of the remote template (eg: file://folder/myfile.vm).

You can append query options to the URI in the following format, {{?option=value&option=value&...}}

h3. Options
{div:class=confluenceTableSmall}
|| Option || Default || Description ||
| {{loaderCache}} | {{true}} | Velocity based file loader cache. |
| {{contentCache}} |  | New option in Camel 1.4: Cache for the resource content when it is loaded. By default, it's {{false}} in Camel 1.x. By default, it's {{true}} in Camel 2.x.  \\ Note : as of Camel 2.9 cached resource content can be cleared via JMX using the endpoint's {{clearContentCache}} operation. |
| {{encoding}} | {{null}} | New option in Camel 1.6: Character encoding of the resource content. |
| {{propertiesFile}} | {{null}} | New option in Camel 2.1: The URI of the properties file which is used for VelocityEngine initialization. | 
{div}

h3. Message Headers

The velocity component sets a couple headers on the message (you can't set these yourself and from Camel 2.1 velocity component will not set these headers which will cause some side effect on the dynamic template support):
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|| Header || Description ||
| {{org.apache.camel.velocity.resource}} | Camel 1.x: The resource as an {{org.springframework.core.io.Resource}} object. |
| {{org.apache.camel.velocity.resourceUri}} | Camel 1.x: The *templateName* as a {{String}} object. |
| {{CamelVelocityResource}} | Camel 2.0: The resource as an {{org.springframework.core.io.Resource}} object. |
| {{CamelVelocityResourceUri}} | Camel 2.0: The *templateName* as a {{String}} object. |
{div}

In Camel 1.4 headers set during the Velocity evaluation are returned to the message and added as headers. Then its kinda possible to return values from Velocity to the Message.

For example, to set the header value of {{fruit}} in the Velocity template {{.tm}}:
{code}
$in.setHeader('fruit', 'Apple')
{code}

The {{fruit}} header is now accessible from the {{message.out.headers}}.

h3. Velocity Context
Camel will provide exchange information in the Velocity context (just a {{Map}}). The {{Exchange}} is transfered as:
{div:class=confluenceTableSmall}
|| 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). |
{div}

h3. Hot reloading
The Velocity 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.

h3. Dynamic templates
*Available as of Camel 2.1*
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.
{div:class=confluenceTableSmall}
|| Header || Type || Description ||
| CamelVelocityResourceUri | String | *Camel 2.1:* A URI for the template resource to use instead of the endpoint configured. |
| CamelVelocityTemplate | String | *Camel 2.1:* The template to use instead of the endpoint configured. |
{div}

h3. Samples

For example you could use something like

{code}
from("activemq:My.Queue").
  to("velocity:com/acme/MyResponse.vm");
{code}

To use a Velocity template to formulate a response to a message for InOut message exchanges (where there is a {{JMSReplyTo}} header).

If you want to use InOnly and consume the message and send it to another destination, you could use the following route:

{code}
from("activemq:My.Queue").
  to("velocity:com/acme/MyResponse.vm").
  to("activemq:Another.Queue");
{code}

And to use the content cache, e.g. for use in production, where the {{.vm}} template never changes:

{code}
from("activemq:My.Queue").
  to("velocity:com/acme/MyResponse.vm?contentCache=true").
  to("activemq:Another.Queue");
{code}

And a file based resource:
{code}
from("activemq:My.Queue").
  to("velocity:file://myfolder/MyResponse.vm?contentCache=true").
  to("activemq:Another.Queue");
{code}

In *Camel 2.1* it's possible to specify what template the component should use dynamically via a header, so for example:

{code}
from("direct:in").
  setHeader("CamelVelocityResourceUri").constant("path/to/my/template.vm").
  to("velocity:dummy");
{code}

In *Camel 2.1* it's possible to specify a template directly as a header the component should use dynamically via a header, so for example:

{code}
from("direct:in").
  setHeader("CamelVelocityTemplate").constant("Hi this is a velocity template that can do templating ${body}").
  to("velocity:dummy");
{code}

h3. The Email Sample
In this sample we want to use Velocity templating for an order confirmation email. The email template is laid out in Velocity as:
{code}
Dear ${headers.lastName}, ${headers.firstName}

Thanks for the order of ${headers.item}.

Regards Camel Riders Bookstore
${body}
{code}

And the java code:

{snippet:id=e1|lang=java|url=camel/trunk/components/camel-velocity/src/test/java/org/apache/camel/component/velocity/VelocityLetterTest.java}

{include:Endpoint See Also}