Filter learning

A Filter is an object that can perform filtering tasks on resource requests (resources: servlet or static content) or resource responses, or both.

The filter object performs the filtering task in the doFilter method. Each Filter has access to the FilterConfig object, from which it can get its initialization parameters, such as a reference to the ServletContext, which can be used to load the resources needed for the filtering task.

Some filter examples, such as:

1) Authentication Filters 
2) Logging and Auditing Filters 
3) Image conversion Filters 
4) Data compression Filters 
5) Encryption Filters 
6) Tokenizing Filters 
7) Filters that trigger resource access events 
8) XSL/T filters 
9) Mime-type chain Filter 

default void	destroy() Called by the web container to indicate to a filter that it is being taken out of service.
void	doFilter(ServletRequest request, ServletResponse response, FilterChain chain) The  doFilter  method of the Filter is called by the container each time a request/response pair is passed through the chain due to a client request for a resource at the end of the chain.
default void	init(FilterConfig filterConfig) Called by the web container to indicate to a filter that it is being placed into service.

void doFilter(ServletRequest request,
              ServletResponse response,
              FilterChain chain)
       throws java.io.IOException,
              ServletException

The  doFilter  method of the Filter is called by the container each time a request/response pair is passed through the chain due to a client request for a resource at the end of the chain. The FilterChain passed in to this method allows the Filter to pass on the request and response to the next entity in the chain.

A typical implementation of this method would follow the following pattern:- 
1. Examine the request
2. Optionally wrap the request object with a custom implementation to filter content or headers for input filtering 
3. Optionally wrap the response object with a custom implementation to filter content or headers for output filtering 
4. a) Either invoke the next entity in the chain using the FilterChain object (chain.doFilter()), 
4. b) or not pass on the request/response pair to the next entity in the filter chain to block the request processing
5. Directly set headers on the response after invocation of the next entity in the filter chain.

Parameters:

request  - The request to process

response  - The response associated with the request

chain  - Provides access to the next filter in the chain for this filter to pass the request and response to for further processing