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Writing a Jetty Handler

 The org.mortbay.jetty.Handler interface provides Jetty's core of content generation or manipulation. Classes that implement this interface are used to coordinate requests, filter requests and generate content.

The Handler API

The core API of the Handler interface is

The target

The target of a handler is an identifier for the resource that should handle the passed request. This is normally the URI that is parsed from a HTTP Request. However, in two key circumstances the target may differ from the URI of the passed request:

  1. If the request has been dispatched to a named resource, such as a named servlet, then the target is the name of that resource.
  2. If the request is being made by a call to RequestDispatcher.include(...)
    then the target is the URI of the included resource and will be different to the URI of the actual request.

The Request and Response

The request and response objects used in the signature of the handle method are HttpServletRequest and HttpServletResponse. These are the standard APIs and are moderately restricted in what can be done to the request and response. More often than not, access to the jetty implementations of these classes is required: Request and Response. However, as the request and response may be wrapped by handlers, filters and servlets, it is not possible to pass the implementation directly. The following mantra will retrieve the core implementation objects from under any wrappers:

Note that if the handler passes the request on to another handler, it should use the
request/response objects passed in and not the base objects. This is to preserve any wrapping done by up stream handlers.

The dispatch

The dispatch argument indicates the state of the handling of the call and may be:

  • REQUEST == 1 - An original request received from a connector.
  • FORWARD == 2 - A request being forwarded by a RequestDispatcher
  • INCLUDE == 4 - A request being included by a RequestDispatcher
  • ERROR == 8 - A request being forwarded to a error handler by the container.

These mostly have significance for servlet and related handlers. For example, the security handler only applies authentication and authorization to REQUEST dispatches.

h2 Handling Requests

A Handler may handle a request by:

Generating a Response

Modifying the Request and/or Response

Passing the Request and Response to another handler

A combination of the above.

Other stuff

 Handlers are usually arranged in a list and a request presented to each handler in turn until (or at most) one indicates that the request has been handled. In this situation, this will allow handlers to: 

  • Ignore requests that are not applicable
  • Handle requests by populating the response and/or generating content
  • Modify the request but allow it to pass onto the next handler(s).  Headers and attributes may be modified or an InputStream filter added
  • Modify the response but allow the request to pass onto the next handler(s).  Headers may be modified or OutputStream filters added

These are the handlers that implement the org.mortbay.jetty.Handler interface package:

  • Abstract Handler - Abstract handler.
  • Context Handler - This handler wraps a call to handle by setting the context and servlet path, plus setting the context classloader.
  • Error Handler - Handler for error pages.  It is registered at the org.mortbay.jetty.handler.ErrorHandler context attributed and called by the HttpResponse.sendError method to write an error page.
  • Handler Collection - Handler collection.
  • Default Handler - Handler for resources that were not found.  Implements OPTIONS and TRACE methods for the server.
  • Request Log Handler - This handler can be used to wrap an individual context for context logging.
  • Handler wrapper - Handler collection.

There are some examples of implementing custom handlers in the Embedded Examples.

See also Newbie Guide to Jetty.

TBD.

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