Grails is a project from SpringSource and VMware led by Graeme Rocher. Grails, is defined in its website as:
an advanced and innovative open source web application platform that delivers new levels of developer productivity by applying principles like Convention over Configuration. Grails helps development teams embrace agile methodologies, deliver quality applications in reduced amounts of time, and focus on what really matters: creating high quality, easy to use applications that delight users. Grails naturally complements Java application development since it is built on Spring and based on Groovy, the leading dynamic language for the Java platform.
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