public interface Filter
A filter is an object that performs filtering tasks on either the request to a resource (a servlet or static content), or on the response from a resource, or both.
Filters perform filtering in the doFilter
method. Every Filter has access to a FilterConfig object from
which it can obtain its initialization parameters, and a reference to the ServletContext which it can use, for
example, to load resources needed for filtering tasks.
Filters are configured in the deployment descriptor of a web application.
Examples that have been identified for this design are:
Modifier and Type | Method and Description |
---|---|
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.
|
default void init(FilterConfig filterConfig) throws ServletException
Called by the web container to indicate to a filter that it is being placed into service.
The servlet container calls the init method exactly once after instantiating the filter. The init method must complete successfully before the filter is asked to do any filtering work.
The web container cannot place the filter into service if the init method either
filterConfig
- a FilterConfig
object containing the filter's configuration and initialization
parametersServletException
- if an exception has occurred that interferes with the filter's normal operationvoid doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException
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:
chain.doFilter()
),
request
- the ServletRequest
object contains the client's requestresponse
- the ServletResponse
object contains the filter's responsechain
- the FilterChain
for invoking the next filter or the resourceIOException
- if an I/O related error has occurred during the processingServletException
- if an exception occurs that interferes with the filter's normal operationUnavailableException
default void destroy()
Called by the web container to indicate to a filter that it is being taken out of service.
This method is only called once all threads within the filter's doFilter method have exited or after a timeout period has passed. After the web container calls this method, it will not call the doFilter method again on this instance of the filter.
This method gives the filter an opportunity to clean up any resources that are being held (for example, memory, file handles, threads) and make sure that any persistent state is synchronized with the filter's current state in memory.
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