Velocity

Since Camel 1.2

The velocity: component allows you to process a message using an Apache Velocity 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-velocity</artifactId>
    <version>x.x.x</version>
    <!-- use the same version as your Camel core version -->
</dependency>

URI format

velocity:templateName[?options]

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&…​

Options

The Velocity component supports 2 options, which are listed below.

Name Description Default Type

velocityEngine (advanced)

To use the VelocityEngine otherwise a new engine is created

VelocityEngine

resolveProperty Placeholders (advanced)

Whether the component should resolve property placeholders on itself when starting. Only properties which are of String type can use property placeholders.

true

boolean

The Velocity endpoint is configured using URI syntax:

velocity:resourceUri

with the following path and query parameters:

Path Parameters (1 parameters):

Name Description Default Type

resourceUri

Required Path to the resource. You can prefix with: classpath, file, http, ref, or bean. classpath, file and http loads the resource using these protocols (classpath is default). ref will lookup the resource in the registry. bean will call a method on a bean to be used as the resource. For bean you can specify the method name after dot, eg bean:myBean.myMethod.

String

Query Parameters (5 parameters):

Name Description Default Type

contentCache (producer)

Sets whether to use resource content cache or not

false

boolean

encoding (producer)

Character encoding of the resource content.

String

loaderCache (producer)

Enables / disables the velocity resource loader cache which is enabled by default

true

boolean

propertiesFile (producer)

The URI of the properties file which is used for VelocityEngine initialization.

String

synchronous (advanced)

Sets whether synchronous processing should be strictly used, or Camel is allowed to use asynchronous processing (if supported).

false

boolean

Spring Boot Auto-Configuration

When using Spring Boot make sure to use the following Maven dependency to have support for auto configuration:

<dependency>
  <groupId>org.apache.camel</groupId>
  <artifactId>camel-velocity-starter</artifactId>
  <version>x.x.x</version>
  <!-- use the same version as your Camel core version -->
</dependency>

The component supports 3 options, which are listed below.

Name Description Default Type

camel.component.velocity.enabled

Enable velocity component

true

Boolean

camel.component.velocity.resolve-property-placeholders

Whether the component should resolve property placeholders on itself when starting. Only properties which are of String type can use property placeholders.

true

Boolean

camel.component.velocity.velocity-engine

To use the VelocityEngine otherwise a new engine is created. The option is a org.apache.velocity.app.VelocityEngine type.

String

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):

Header Description

CamelVelocityResourceUri

The templateName as a String object.

CamelVelocitySupplementalContext

Camel 2.16: To add additional information to the used VelocityContext. The value of this header should be a Map with key/values that will added (override any existing key with the same name).

This can be used to pre setup some common key/values you want to reuse in your velocity endpoints.

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:

$in.setHeader("fruit", "Apple")

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

Velocity Context

Camel will provide exchange information in the Velocity context (just a Map). The Exchange is transfered as:

key value

exchange

The Exchange itself.

exchange.properties

The Exchange properties.

headers

The headers of the In message.

camelContext

The Camel Context instance.

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).

Since Camel-2.14, you can setup a custom Velocity Context yourself by setting the message header *CamelVelocityContext *just like this

   VelocityContext velocityContext = new VelocityContext(variableMap);
   exchange.getIn().setHeader("CamelVelocityContext", velocityContext);

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.

Dynamic templates

Since 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.
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.

Samples

For example you could use something like

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

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:

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

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

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

And a file based resource:

from("activemq:My.Queue").
  to("velocity:file://myfolder/MyResponse.vm?contentCache=true").
  to("activemq:Another.Queue");

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

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

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:

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

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:

Dear ${headers.lastName}, ${headers.firstName}

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

Regards Camel Riders Bookstore
${body}

And the java code:

Back to top