Filter

The funciton of filter: filter the resouce and it's one of the Serlvet technology. When the user access to the resource, the filter will make some process for the request and response.

How to use Filter?

　　1. Create a class implements the Filter interface

　　2. Modify the web.xml file's configuration and configure the filter's mapping:

　　web.xml:

    <filter>
        <filter-name>FilterDemo1</filter-name>
        <filter-class>com.pp.filter.FilterDemo1</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>FilterDemo1</filter-name>
        <!-- Filter all the resource in the application -->
        <url-pattern>/*</url-pattern>
    </filter-mapping>

　　3. Call the chain.doFilter(request, response) method to execute the resourece

The life circle of Filter:

　The Filter instance is created by server and the method in Filter also called by server.

　The Filter will be complete the instantiation and call the init() method when the application loaded. Every request sent by user, the doFilter() method will be called. And the server will call the destroy() method when the application was removed or the server stopped.

public class FilterDemo1 implements Filter {

    public FilterDemo1() {
        System.out.println("FilterDemo1's constructor");
    }

    public void destroy() {
        System.out.println("destroy the FilterDemo1");
    }

    // this method will be called when access resource
    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException {
        System.out.println("filter the FilterDemo1");
        // access to other resource
        chain.doFilter(request, response);
    }

    public void init(FilterConfig config) throws ServletException {
        System.out.println("initialize the FilterDemo1");
    }
}

　Feature: the instance variables in Filter will lead to thread security problmes.

The filter order: 

The multi-filter execution order: the filter execution is executed by the filter-mapping configuring order in web.xml

Servlet & Filter:

　　1. The Filter has the same function of Servlet, in addition: the Filter has the chain.doFilter() function.

　　2. In MVC, the Servlet acts as the Controller, and Filter also coule works as Controller.

　　3. Struts1: Servlet serves as Controller　　Struts2: Filter serves as Controller