Velocity component

The velocity component renders an Apache Velocity (VTL) template against the message and sets the rendered text as the new body.

Scheme: velocity: · Crate: cameleer-core · Cargo feature: templates · Consumer: no · Producer: yes · Apache Camel: Velocity

When to use it

• Render emails, notifications and reports from a message with loops and conditions.
• Build a few-shot prompt for a model: loop over example pairs and append the new input.
• Reuse Velocity templates from an existing Java Camel application.

URI format

velocity:templateResource[?options]

templateResource is a file path relative to the working directory, file:path, classpath:path (read as a relative path) or ref:beanName (a String in the registry).

Example

A few-shot classification prompt: the examples come from the request, the count from $foreach.count, and the instruction changes with a header.

#set($n = $body.examples.size())
Classify the sentiment. $n examples follow.
#foreach($e in $body.examples)
$foreach.count. "$e.text" => $e.label
#end
Text: "$body.input"
#if($headers.strict == "true")
Answer with exactly one word.
#end

- route:
    id: velocity-few-shot
    from:
      uri: direct:velocity-few-shot
      steps:
        - unmarshal:
            json: {}
        - setHeader:
            name: strict
            constant: "true"
        - to: velocity:templates/few-shot.vm

{"examples":[{"text":"Great support, thanks!","label":"positive"},{"text":"Still broken after a week.","label":"negative"}],"input":"The update fixed it."}

Classify the sentiment. 2 examples follow.
1. "Great support, thanks!" => positive
2. "Still broken after a week." => negative
Text: "The update fixed it."
Answer with exactly one word.

Supplemental context

The CamelVelocitySupplementalContext header (a map) adds entries to the data model; the CamelVelocityContext header replaces it entirely.

Hello $name from $team, quiet missing value: [$!missing]

- route:
    id: velocity-supplemental
    from:
      uri: direct:velocity-supplemental
      steps:
        - setHeader:
            name: CamelVelocitySupplementalContext
            simple: "${map('name','Ada','team','platform')}"
        - to: velocity:templates/hello.vm

ignored

Hello Ada from platform, quiet missing value: []

Rust DSL

b.from("direct:velocity-rust")
    .set_header("CamelVelocitySupplementalContext", simple("${map('name','Ada','team','ops')}"))
    .to("velocity:templates/hello.vm");

Options

The data model: $body, $headers, $variables, $exchange ($exchange.properties, $exchange.exchangeId, $exchange.getIn().getHeader("x")), $request / $in, and $camelContext with allowContextMapAll=true.

Supported VTL

References $x, ${x}, quiet $!x, properties, method calls ($s.toUpperCase(), $headers.get("x")), index access $list[0] and $map["k"]; #set, #if/#elseif/#else, #foreach with $foreach.count/index/hasNext/first/last, #break, #stop, #macro and calls, #include, #parse, #evaluate, #define, comments ## and #* *#, unparsed blocks #[[ ]]#, escapes, arithmetic, comparisons and logical operators. Conditions follow Velocity 2: null, false, empty strings, empty collections and zero are false.

Known difference: a #end line directly followed by another directive line (such as #if(...)) can leave an empty line in the output where Velocity 2 removes both lines. Put a text line between the two directives, as the example does.

Message headers

Run it with Docker

docker run --rm -v "$PWD:/routes:ro" pmsudhi/cameleer run /routes/few-shot.camel.yaml

Common questions

How do I print nothing for a missing value?

Use a quiet reference: $!name. A plain $name that is null renders as the literal text $name, as in Velocity.

How do I get the loop index?

$foreach.count starts at 1, $foreach.index at 0; $foreach.hasNext helps with separators.

Can I pass extra values without changing the body?

Yes: set CamelVelocitySupplementalContext to a map (for example with Simple's ${map(...)}), and its keys become top-level template variables.

Related

• Mustache, FreeMarker, StringTemplate, MVEL.
• Simple language for the map(...) function.
• Apache Camel Velocity component

Found a problem?

If something on this page doesn't match what Cameleer does, or you hit a bug while building with it, please help fix it, whether you are a person or a coding agent: open an issue that describes what you saw, then fork the repository, fix it, add or update a test, run cargo test --workspace, and open a pull request. CONTRIBUTING.md explains the ground rules.