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Apache Velocity
Apache Velocity logo.svg
Developer(s)Apache Software Foundation
Stable release
2.3 / February 27, 2021; 15 months ago (2021-02-27)[1]
RepositoryVelocity Repository
Written inJava
Operating systemCross-platform
TypeTemplate engine
LicenseApache License 2.0
Websitevelocity.apache.org

Apache Velocity first released in April 2001, is a Java-based template engine that provides a template language to reference objects defined in Java code. It aims to ensure clean separation between the presentation tier and business tiers in a Web application (the model–view–controller design pattern).

Velocity is an open source software project hosted by the Apache Software Foundation. It is released under the Apache License. Jon Scott Stevens derived the name from the AltiVec Velocity Engine in the PowerPC G4 chip.

Uses

Some common types of applications that use Velocity are:

Code examples

The following template:

## Velocity Hello World
<html>
    <body>
        #set( $foo = "Velocity" )
        ## followed by
        Hello $foo World!
    </body>
</html>

processed by Velocity produces the following HTML:

<html>
    <body>
        Hello Velocity World!
    </body>
</html>

Another example using conditions:

#if( $foo < 10 )
    **Go North**
#elseif( $foo == 10 )
    **Go East**
#elseif( $bar == 6 )
    **Go South**
#else
    **Go West**
#end

AWS API Gateway body mapping template, for DynamoDB HTTP:

{
    "TableName": "happy-marketer",
    "KeyConditionExpression": "pk = :pk AND begins_with(sk, :sk)",
    #if ($input.params('projectStatus') == 'archived')
    "FilterExpression": "projectStatus = :Archived",
    #else
    "FilterExpression": "projectStatus = :InProgress OR projectStatus = :Completed",
    #end
    "ExpressionAttributeValues": {
        ":pk": {
            "S": "project"
        },
        ":sk": {
            "S": "$context.authorizer.claims.email"
        },
        #if ($input.params('projectStatus') == 'archived')
        ":Archived": {
            "S": "Archived"
        }
        #else
        ":InProgress": {
            "S": "In Progress"
        },
        ":Completed": {
            "S": "Completed"
        }
        #end
    }
}

Another example with optional array; without condition will fail because of invalid resulting JSON:

{
    "email": "$context.authorizer.claims.email"
    #if ($input.params('countries') != "")
    , "countries": $input.params('countries')
    #end
}

The syntax and overall concept of the Apache Velocity templates is similar to the syntax of the older WebMacro template engine, which is now also an open source project.[citation needed]

See also

References

  1. ^ "Apache Velocity - Project News". velocity.apache.org. Apache Software Foundation. Retrieved 2021-04-02.
  2. ^ "PoweredByVelocity". Velocity Wiki. Wiki.apache.org. 2009-12-30. Archived from the original on 23 March 2010. Retrieved 2010-03-29.

Bibliography