Content Web application is out box feature available in SDL Tridion, which allows developer to build application using MVC framework using JAVA, Same is also possible using DD4T Java.
As far as I know DD4T is not supported by SDL Tridion.
What are the advantages of using DD4T instead of CWA? Is it to save license cost?
Good question. I have not used CWA in a long time, but I will do my best to explain.
CWA is a product/module/architecture which allows you to store all of your content in the Broker Database. You then have a CWA enabled web application which uses OOTB JAR files to retrieve the content (i.e. Pages and their component presentations as well as binary files like images).
CWA still requires you to write templates on the Content Management side in order to render the component presentations (and optionally the page views)
DD4T on the other hand, is (as you state) an open source project which is not supported by SDL (but I believe they do use it themselves). DD4T partly grew out of the lack of a .NET based CWA offering. One thing it offers that CWA does not, is a standard set of templates, which means that developers need little or no knowledge of the Tridion Content Manager to start developing.
Personally I have not used the Java version of DD4T, but the .NET version is great. One big plus of it being open source, is that there are a lot of people sharing their ideas on it. You might notice, at this point in time there are only 4 cwa questions on here, and over 160 dd4t ones.
At the end of the day, you need to build your own web application which will never be supported by SDL, so the only thing you get from using CWA is a few extra JAR files, whereas DD4T uses only supported methods of retrieving data from the Content Broker anyway (so really is no riskier, but you benefit from not needing to right as much code yourself).
Chris hits a few of the main differences between the CWA and DD4T project. My view is a bit simpler, so I'll share it too.
In a Tridion implementation your content lives (at least) three places:
Tridion systems architecture +----------+ +----------+ +-----------+ | Content | Publish | Content | Deliver | Visitor's | | Manager | ----------> | Delivery | ----------> | Browser | | Database | | Broker | | | +----------+ +----------+ +-----------+
We all know that the Tridion Content Manager stores content in an XML format. And we also know that the vast majority of the content that reaches the visitor's browser is HTML. The difference between CWA and DD4T is where the transformation from XML into HTML happens.
Typical CWA applications render their XML content into HTML snippets during publishing. where Components are rendered into HTML snippets by templates. These HTML snippets (Dynamic Component Presentations) are stored in the broker database. During delivery the correct HTML snippets are loaded from the broker and sent to the visitor.
CWA +----------+ +----------+ +-----------+ | Content | Publish | Content | Deliver | Visitor's | | Manager | ----------> | Delivery | ----------> | Browser | | Database | XML -> HTML | Broker | | | +----------+ +----------+ +-----------+
Typical DD4T application render the XML content into HTML when the visitor hits the page. So during publishing they keep the Components in XML format and they store those XML snippets in the broker database (as Dynamic Component Presentation). During delivery the connect XML components are loaded and are rendered into HTML for the visitor.
DD4T +----------+ +----------+ +-----------+ | Content | Publish | Content | Deliver | Visitor's | | Manager | ----------> | Delivery | ----------> | Browser | | Database | | Broker | XML -> HTML | | +----------+ +----------+ +-----------+
