过滤器Filter

public class Servlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        resp.getWriter().print("你好,世界");
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

    <servlet>
        <servlet-name>servlet</servlet-name>
        <servlet-class>per.lc.Servlet.Servlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>servlet</servlet-name>
        <url-pattern>/hi</url-pattern>
    </servlet-mapping>

public class CharacterEncodingFilter implements Filter {
    private String characterEncoding="utf-8";
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("拦截开始");
        if(filterConfig.getInitParameter("Encoding")!=null){
            characterEncoding=filterConfig.getInitParameter("Encoding");
        }
    }
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletRequest request=(HttpServletRequest)servletRequest;       //servletRequest是HttpServletRequest的超类，这里主要是要处理http协议，就要强转为HttpServletReqest类
        HttpServletResponse response=(HttpServletResponse) servletResponse;
        request.setCharacterEncoding(characterEncoding);
        response.setContentType("text/html;charset="+characterEncoding);
        filterChain.doFilter(request,response);
    }
    public void destroy() {
        System.out.println("拦截结束");
        characterEncoding=null;
    }

//ServletReqest 、HttpServletRequest只是接口，并不去实现接口的方法，方法是由第三方厂商实现的，比如Tomcat，在Catalina.jar 包由对应的方法的实现。

　　　<filter>
        <filter-name>characterEncoding</filter-name>
        <filter-class>per.lc.FilterDemo.CharacterEncodingFilter</filter-class>
        <init-param>
            <param-name>Encoding</param-name>
            <param-value>utf-8</param-value>
        </init-param>
    </filter>
    <filter-mapping>
        <filter-name>characterEncoding</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
将这段添加到web.xml

<url-pattern>/*</url-pattern>
The /* on a servlet overrides all other servlets, including all servlets provided by the servletcontainer such as the default servlet and the JSP servlet. Whatever request you fire, it will end up in that servlet. 
This is thus a bad URL pattern for servlets. Usually, you‘d like to use /* on a Filter only. It is able to let the request continue to any of the servlets listening on a more specific URL pattern by calling FilterChain#doFilter().

<url-pattern>/</url-pattern>
The / doesn‘t override any other servlet. It only replaces the servletcontainer‘s builtin default servlet for all requests which doesn‘t match any other registered servlet. 
This is normally only invoked on static resources (CSS/JS/image/etc) and directory listings. The servletcontainer‘s builtin default servlet is also capable of dealing with HTTP cache requests, media (audio/video) streaming and file download resumes. 
Usually, you don‘t want to override the default servlet as you would otherwise have to take care of all its tasks, which is not exactly trivial (JSF utility library OmniFaces has an open source example). This is thus also a bad URL pattern for servlets. 
As to why JSP pages doesn‘t hit this servlet, it‘s because the servletcontainer‘s builtin JSP servlet will be invoked, which is already by default mapped on the more specific URL pattern *.jsp