Play is a Scala (with Java support as well) based web framework that eschews traditional Java web-development practices and embraces a more modern approach, analogous to the Rails or Django.
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Pro Asynchronous Core
It's like Java, but more Haskell-y.
Pro Interoperable with Java
Pro Good Websocket Support
Pro Play is an extensive ecosystem
Play uses Akka for concurrency, Scala for a templating engine, Netty as a client-server framework and SBT (Simple Build Tool) for building. And they all come out of the box.
Play also comes with the option to scaffold your applications. Play is an all-embracing ecosystem designed to increase developer productivity and shorten development times.
Pro Good documentation and a great community
Play has quite a large community which provides numerous tutorials and videos related to developing with Play.
The Play official documentation covers many things, such as the framework itself but also specific stuff such as Akka, SBT and Netty.
There are also many big companies that base their main sites around Play, one of them is LinkedIn which provides third-party documentation on a regular basis.
Pro Simple for beginners
Play is very simple to get started. The documentation is very helpful for beginners and advanced users alike and the official website has a great "Getting Started" tutorial to begin developing with Play.
Pro Readable code
Play framework's convention over configuration methodology makes most Play projects have a very similar structure. This means that the code written for the framework is very readable. This enables a developer to switch between applications without having to relearn the ecosystem for every project. The built-in templating system also helps with code and makes it possible to have a very low count of lines of code.
Pro Can use Java, one of the most widely known languages
Java is one of the most widely known languages, so people coming from that background can jump right in and not have to learn a new language syntax.
Con Not as many resources to learn
Other languages and frameworks have countless tutorials, books, moocs, etc. Java and Play does not have nearly as much.
Con Backward incompatibility
The jump from Play 1 to Play 2.x caused a lot of confusion. While it is important to have some kind of evolution, sometimes it causes backward incompatibility which can create some problems. It makes tutorials or modules made for the old version obsolete. This can make it hard for beginners to find useful resources. The template engine which used Groovy now uses Scala.