javaweb基本組件之filter的使用

1、filter在web容器中的作用主要有以下這些：
①對用戶拋過來的請求進行字符編碼上的轉換，及爲對用戶的響應對象填充公用的頭信息；
②驗證用戶的身份是否已經是登錄狀態和訪問權限的控制；
③其它一些想在用戶訪問之前做的操作

2、使用實例，爲請求內容指定編碼
①編寫自己的filter類EncodingFilter

package com.demo.filter;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpServletRequest;

public class EncodingFilter implements Filter{

     protected FilterConfig filterConfig = null;   
     protected String encoding = "";   

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

    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException {
        // TODO Auto-generated method stub
         ((HttpServletRequest) request).setCharacterEncoding(encoding);
         ((HttpServletResponse) response).setHeader("Cache-Control","no-cache");
         ((HttpServletResponse) response).setHeader("Pragma","no-cache");
         ((HttpServletResponse) response).setDateHeader ("Expires", -1);
         ((HttpServletResponse) response).setCharacterEncoding(encoding);
         chain.doFilter(request, response);
    }

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        // TODO Auto-generated method stub
        this.filterConfig = filterConfig;   
        this.encoding = filterConfig.getInitParameter("encoding");   
    }

}

EncodingFilter類主要實現了Filter接口，實現了destroy, doFilter, init等filter接口中的方法，init方法在web容器加載filter時執行一次，destory在web容器關閉時被調用釋放佔用的資源，對於每個請求，都會調用一次doFilter過濾方法

②裝配到容器當中去

<filter>
    <filter-name>encoding</filter-name>
    <filter-class>com.demo.filter.EncodingFilter</filter-class>
    <init-param>
        <param-name>encoding</param-name>
        <param-value>utf-8</param-value>
    </init-param>
  </filter>
  <filter-mapping>
    <filter-name>encoding</filter-name>
    <url-pattern>*</url-pattern>
  </filter-mapping>

這樣在每次請求過來的時候就可以將請求的內容轉換爲utf-8的編碼了，並且設置響應中內容爲不緩存