表示层模式:Intercepting Filter—拦截过滤器模式

Context
        The presentation-tier request handling mechanism receives many different types of
requests, which require varied types of processing. Some requests are simply forwarded to
the appropriate handler component, while other requests must be modified, audited, or
uncompressed before being further processed.
Problem
        Preprocessing and post-processing of a client Web request and response are required.
When a request enters a Web application, it often must pass several entrance tests
prior to the main processing stage. For example,
Has the client been authenticated?
Does the client have a valid session?
Is the client's IP address from a trusted network?
Does the request path violate any constraints?
What encoding does the client use to send the data?
Do we support the browser type of the client?
Some of these checks are tests, resulting in a yes or no answer that determines
whether processing will continue. Other checks manipulate the incoming data stream into a
form suitable for processing.
        The classic solution consists of a series of conditional checks, with any failed check
aborting the request. Nested if/else statements are a standard strategy, but this solution leads
to code fragility and a copy-and-paste style of programming, because the flow of the
filtering and the action of the filters is compiled into the application.
The key to solving this problem in a flexible and unobtrusive manner is to have a
simple mechanism for adding and removing processing components, in which each
component completes a specific filtering action.
Forces
        Common processing, such as checking the data-encoding scheme or logging
information about each request, completes per request.
Centralization of common logic is desired.
Services should be easy to add or remove unobtrusively without affecting existing
components, so that they can be used in a variety of combinations, such as
Logging and authentication
Debugging and transformation of output for a specific client
Uncompressing and converting encoding scheme of input
Solution
        Create pluggable filters to process common services in a standard manner without
requiring changes to core request processing code. The filters intercept incoming requests
and outgoing responses, allowing preprocessing and post-processing. We are able to add
and remove these filters unobtrusively, without requiring changes to our existing code.
We are able, in effect, to decorate our main processing with a variety of common
services, such as security, logging, debugging, and so forth. These filters are components
that are independent of the main application code, and they may be added or removed
declaratively. For example, a deployment configuration file may be modified to set up a
chain of filters. The same configuration file might include a mapping of specific URLs to
this filter chain. When a client requests a resource that matches this configured URL
mapping, the filters in the chain are each processed in order before the requested target
resource is invoked.