过滤器Filter

Filter就像一个一个哨卡，用户的请求需要经过Filter,
并且可以有多个过滤器

利用过滤器我们可以实现例如自动登录、解决中文乱码问题等。

1.编写Filter：实现Filter接口，重写方法

public class MyFilter implements Filter {

    public void destroy() {
        System.out.println("过滤器销毁");
    }

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        HttpServletRequest hrequest = (HttpServletRequest)request;
        HttpServletResponse hresponse = (HttpServletResponse)response;
        hrequest.setCharacterEncoding("utf-8");
        hresponse.setContentType("text/html;charset=utf-8");
        System.out.println("过滤器执行");
        //放行
        chain.doFilter(request, response);
        //filter执行按照配置顺序来进行
    }

    public void init(FilterConfig fConfig) throws ServletException {
        // TODO Auto-generated method stub
    }

}

这里要注意chain.doFilter，执行后即意味着当前过滤器放行，执行下一个过滤器

2.XML配置：

  <filter>
    <display-name>MyFilter</display-name>
    <filter-name>MyFilter</filter-name>
    <filter-class>web.filter.MyFilter</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>MyFilter</filter-name>
    <url-pattern>/*</url-pattern>
  </filter-mapping>

配置方式和Servlet类似，不同的是过滤器的执行顺序是按照配置文件的先后执行，即在配置文件上方的先被执行。

3.应用

解决中文乱码问题

/**
 * 装饰者：
 * 1.增强类与被增强类实现统一接口
 * 2.增强类中传入被增强类
 * 3.重写需要增强的方法
 * */
class EnhancedRequest extends HttpServletRequestWrapper{
    private HttpServletRequest request;
    public EnhancedRequest(HttpServletRequest request) {
        super(request);
        this.request = request;
    }
    //getParameter重写
    @Override
    public String getParameter(String name) {
        String parameter = request.getParameter(name);
        try {
            parameter=new String(parameter.getBytes("iso8859-1"),"utf-8");//转码
        } catch (UnsupportedEncodingException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return parameter;
    }
    
}
public class CNFilter implements Filter {

    public void destroy() {
        // TODO Auto-generated method stub
    }

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        HttpServletRequest req = (HttpServletRequest) request;
        HttpServletResponse rep = (HttpServletResponse) response;
        //增强类对象
        EnhancedRequest enhancedRequest = new EnhancedRequest(req);
        rep.setContentType("text/html;charset=utf-8");//设置响应编码
        chain.doFilter(enhancedRequest, rep);
    }

    public void init(FilterConfig fConfig) throws ServletException {
        // TODO Auto-generated method stub
    }

}