# Velocity

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.

## 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 (for example, `file://folder/myfile.vm`).

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

## Options

Option | Default | Description
--- | --- | ---
`loaderCache` | `true` | Velocity based file loader cache.
`contentCache` |  | New option in Apache Camel 1.4: Cache for the resource content when it is loaded. By default, it's `false` in Apache Camel 1.x. By default, it's `true` in Apache Camel 2.x.
`encoding` | `null` | New option in Apache 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.

## Message Headers

The velocity component sets some headers on the message (you cannot set these yourself):

Header | Description
--- | ---
`org.apache.camel.velocity.resource` | Apache Camel 1.x: The resource as an `org.springframework.core.io.Resource` object.
`org.apache.camel.velocity.resourceUri` | Apache Camel 1.x: The templateName as a `String` object.
`CamelVelocityResource` | Apache Camel 2.0: The resource as an `org.springframework.core.io.Resource` object.
`CamelVelocityResourceUri` | Apache Camel 2.0: The templateName as a `String` object.

In Apache Camel 1.4 headers set during the Velocity evaluation are returned to the message and added as headers. This makes it 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

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

key | value
--- | ---
`exchange` | The `Exchange` itself.
`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

The Velocity template resource is, by default, hot reloadable for both file and classpath resources (expanded jar). If you set `contentCache=true`, Apache 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

Available as of Apache 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 | Apache Camel 2.1: A URI for the template resource to use instead of the endpoint configured.
CamelVelocityTemplate | String | Apache 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 Apache 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 Apache 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:

    private Exchange createLetter() {
        Exchange exchange = context.getEndpoint("direct:a").createExchange();
        Message msg = exchange.getIn();
        msg.setHeader("firstName", "Claus");
        msg.setHeader("lastName", "Ibsen");
        msg.setHeader("item", "Camel in Action");
        msg.setBody("PS: Next beer is on me, James");
        return exchange;
    }

    @Test
    public void testVelocityLetter() throws Exception {
        MockEndpoint mock = getMockEndpoint("mock:result");
        mock.expectedMessageCount(1);
        mock.expectedBodiesReceived("Dear Ibsen, Claus\n\nThanks for the order of Camel in Action.\n\nRegards Camel Riders Bookstore\nPS: Next beer is on me, James");

        template.send("direct:a", createLetter());

        mock.assertIsSatisfied();
    }

    protected RouteBuilder createRouteBuilder() throws Exception {
        return new RouteBuilder() {
            public void configure() throws Exception {
                from("direct:a").to("velocity:org/apache/camel/component/velocity/letter.vm").to("mock:result");
            }
        };
    }