Remove Servlet API dependency (#73)

[chkal]

This pull request addresses #73, which was discussed quite some time ago. JAX-RS does NOT depend on the Servlet API and can therefore be used in other environments. As MVC is built on top of JAX-RS, it makes no sense to introduce a dependency on the Servlet API.

The only class which currently depends on the Servlet API is ViewEngineContext, which exposes the HttpServletRequest and the HttpServletResponse. Beside the fact that this is the only reason for us to depend on Servlet, exposing these Servlet specific classes is bad for other reasons:

• It allows to write the result of processing the view directly to HttpServletResponse.getOutputStream() which basically completely bypasses JAX-RS.
• It allows to write to HttpServletResponse.getWriter() which may use a completely different encoding from the one negotiated via @Produces.

This pull request contains the following changes:

• The Servlet API is removed from our pom.xml.
• Removed a reference to the Servlet API for the MvcContext javadocs.
• Remove ViewEngineContext.getRequest() and ViewEngineContext.getResponse().
• Added ViewEngineContext.getOutputStream() which will be the primary way to obtain the stream to write the view to.
• Added ViewEngineContext.getResponseHeaders() which exposes a mutable map which can be used to modify the HTTP response headers. I don't really the mutable map approach, but that's the way JAX-RS exposes such headers for MessageBodyWriter, so we should be consistent with JAX-RS here.
• Added ViewEngineContext.getMediaType() to provide the ViewEngine with all required information about the negotiated media type and character encoding.
• I understand that there may be reasons for ViewEngine implementation to need the HttpServletRequest or HttpServletResponse. For these cases I added ViewEngineContext.getRequest(Class<T>) and ViewEngineContext.getResponse(Class<T>). These methods don't have a hard dependency on the Servlet API but allow to obtain the request via getRequest(HttpServletRequest.class). It is a bit similar to JSF's ExternalContext.getRequest() but more type safe like EntityManager.unwrap(Class). The nice thing about these methods is that they also support other request/response objects in non-servlet environments.

I already modified Ozark to support this new API on a private branch. It worked great! And I was even able to fix some really serve encoding issues in some of the 3rd party view engines.

This is a very huge change. But it is worth the effort. So any feedback is welcome.

[ivargrimstad]

I like it!

[Daniel-Dos]

very good.

[lefloh]

Great work, thanks a lot!

[Daniel-Dos]

It was excellent.