原理，过滤所有请求中含有非法的字符，例如：, & <  select delete 等关键字，黑客可以利用这些字符进行注入攻击，原理是后台实现使用拼接字符串，案例：

某个网站的登入验证的SQL查询代码为

      strSQL = "SELECT * FROM users WHERE (name = '" + userName + "') and (pw = '"+ passWord +"');"

恶意填入

      userName = "' OR '1'='1";与passWord = "' OR '1'='1";时，将导致原本的SQL字符串被填为
       

      strSQL = "SELECT * FROM users WHERE (name = '' OR '1'='1') and (pw = '' OR '1'='1');"

也就是实际上运行的SQL命令会变成下面这样的

        strSQL = "SELECT * FROM users;"

因此达到无帐号密码，亦可登入网站。所以SQL注入攻击被俗称为黑客的填空游戏。

实现三个步骤：

1，编写filter

2，配置xml

3，配置error.jsp

filter代码；

package cn.kepu.filter; 
 
import java.io.IOException; 
import java.util.ArrayList; 
import java.util.Arrays; 
import java.util.List; 
import java.util.Map; 
import java.util.Set; 
 
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.HttpServletRequest; 
import javax.servlet.http.HttpServletResponse; 
/**
 * 防止sql注入，自定义filter www.2cto.com
 * cn.kepu.filter.SqlInjectFilter.java
 * @author ffr
 * created at 2012-7-12
 */ 
public class SqlInjectFilter implements Filter { 
     
    private static List<String> invalidsql = new ArrayList<String>(); 
    private static String error = "/error.jsp"; 
    private static boolean debug = false; 
     
    public void destroy() { 
         
    } 
    public void doFilter(ServletRequest req, ServletResponse res, 
            FilterChain fc) throws IOException, ServletException { 
        if(debug){ 
            System.out.println("prevent sql inject filter works"); 
        } 
        HttpServletRequest request = (HttpServletRequest)req; 
        HttpServletResponse response = (HttpServletResponse)res; 
        Map<String, String> params = request.getParameterMap(); 
        Set<String> keys = params.keySet(); 
        for(String key : keys){ 
            String value = request.getParameter(key); 
            if(debug){ 
                System.out.println("process params <key, value>: <"+key+", "+value+">"); 
            } 
            for(String word : invalidsql){ 
                if(word.equalsIgnoreCase(value) || value.contains(word)){ 
                    if(value.contains("<")){ 
                        value = value.replace("<", "<"); 
                    } 
                    if(value.contains(">")){ 
                        value = value.replace(">", ">"); 
                    } 
                    request.getSession().setAttribute("sqlInjectError", "the request parameter \""+value+"\" contains keyword: \""+word+"\""); 
                    response.sendRedirect(request.getContextPath()+error); 
                    return; 
                } 
            } 
        } 
        fc.doFilter(req, res); 
    } 
    public void init(FilterConfig conf) throws ServletException { 
        String sql = conf.getInitParameter("invalidsql"); 
        String errorpage = conf.getInitParameter("error"); 
        String de = conf.getInitParameter("debug"); 
        if(errorpage != null){ 
            error = errorpage; 
        } 
        if(sql != null){ 
            invalidsql = Arrays.asList(sql.split(" ")); 
        } 
        if(de != null && Boolean.parseBoolean(de)){ 
            debug = true; 
            System.out.println("PreventSQLInject Filter staring..."); 
            System.out.println("print filter details"); 
            System.out.println("invalid words as fllows (split with blank):"); 
            for(String s : invalidsql){ 
                System.out.print(s+" "); 
            } 
            System.out.println(); 
            System.out.println("error page as fllows"); 
            System.out.println(error); 
            System.out.println(); 
        } 
    } 
} 
2.web.xml中添加如下配置：

[html]
<filter> 
    <filter-name>PreventSqlInject</filter-name> 
    <filter-class>cn.kepu.filter.SqlInjectFilter</filter-class> 
    <!-- filter word, split with blank --> 
    <init-param> 
        <param-name>invalidsql</param-name> 
        <param-value>select insert delete from update create destory drop alter and or like exec count chr mid master truncate char declare ; - ' % < ></param-value> 
    </init-param> 
    <!-- error page --> 
    <init-param> 
        <param-name>error</param-name> 
        <param-value>/error.jsp</param-value> 
    </init-param> 
    <!-- debug -->     
    <init-param> 
        <param-name>debug</param-name> 
        <param-value>true</param-value> 
    </init-param> 
  </filter> 
  <filter-mapping> 
    <filter-name>PreventSqlInject</filter-name> 
    <url-pattern>/*</url-pattern> 
  </filter-mapping> 

3，在根目录下添加error.jsp
[plain]
<%@ page language="java" import="java.util.*" pageEncoding="utf-8"%> 
<% 
String path = request.getContextPath(); 
%> 
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 
<html> 
  <head> 
    <title>防sql注入系统</title> 
  </head> 
   
  <body> 
    这个是防sql注入系统，自动过滤您的请求，请更换请求字符串。 
    <%=session.getAttribute("sqlInjectError")%> 
    <p><a href="<%=path%>">点此返回</a></p> 
  </body> 
</html> 
作者：fufengrui