11、过滤器Filter
了解Servlet过滤器的概念
了解Servlet过滤器的工作过程
掌握创建Servlet过滤器的方法
掌握发布Servlet过滤器的方法
1、Servlet过滤器的概念
Servlet过滤器是在Java Servlet规范2.3中定义的，它能够对Servlet容器的请求和响应对象进行检查和修改。
Servlet过滤器本身并不生成请求和响应对象，他只提供过滤作用。
Servlet过滤器能够在Servlet被调用之前检查Request对象，修改Request Header和Request内容；
在Servlet被调用之后检查Response对象，修改responseHeader和Response内容。Servlet过滤器负责过滤的Web组件可以是Servlet、JSP或HTML文件
2、Servlet过滤器的过滤过程
3、Filter接口
所有的Servlet过滤器类都必须实现javax.servlet.Filter接口。这个接口含有3个过滤器类必须实现的方法：
init（）、doFilter（）、destory（）
每一个过滤器都要在web容器部署描述符中，就是web.xml中进行部署
- init（FilterConfig）：这是Servlet过滤器的初始化方法，Servlet容器创建Servlet过滤器实例后将调用这个方法。在这个方法中可以读取web.xml文件中Servlet过滤器的初始化参数
- doFilter（ServletRequest，ServletResponse，FilterChain）：这个方法完成实际的过滤操作。当客户请求访问与过滤器关联的URL时，Servlet容器将先调用过滤器的doFilter方法。FilterChain参数用于访问后续过滤器
- destroy（）：Servlet容器在销毁过滤器实例前调用该方法，在这个方法中可以释放Servlet过滤器占用的资源。
过滤器的链式请求处理过程FilterChain示意图：
过滤器如果出现错误，整个服务就将失败
4、一个具体的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.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
public class LoginFilter implements Filter
{
public void destroy()
{
}
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException
{
HttpServletRequest r = (HttpServletRequest)request;
HttpSession session = r.getSession();
String requestURI = r.getRequestURI();
if(requestURI.endsWith("login.jsp") || requestURI.endsWith("MyLoginServlet"))
{
chain.doFilter(request, response);
return;
}
if(null == session.getAttribute("user"))
{
((HttpServletResponse)response).sendRedirect("filter/login.jsp");
return;
}
else
{
chain.doFilter(request, response);
}
}
public void init(FilterConfig filterConfig) throws ServletException
{
System.out.println("filter init invoke!");
}
}
部署描述符中的配置：
<filter> <filter-name>LoginFilter</filter-name> <filter-class>com.cdtax.filter.LoginFilter</filter-class> </filter> <filter-mapping> <filter-name>LoginFilter</filter-name> <url-pattern>/*</url-pattern> </filter-mapping>
一个使用filter过滤器的实例：一个输入页面comment.jsp，有username和comment两个输入域，提交后由一个servlet处理转向一个显示页面，一个过滤器对输入的内容进行过滤，username中含有123则替换为abc，comment中有wokao替换为****
comment.jsp
page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath%>">
<title>My JSP 'comment.jsp' starting page</title>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
<!--
<link rel="stylesheet" type="text/css" href="styles.css">
-->
</head>
<body>
<form action="CommentServlet" method="post">
username:<input type="text" name="username"/><br/>
commnet:<textarea name="comment" rows="7" cols="7"></textarea><br/>
<input type="submit" value="submit"/>
</form>
</body>
</html>
CommentServlet：
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class CommentServlet extends HttpServlet
{
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
req.getRequestDispatcher("commentResult.jsp").forward(req, resp);
}
}
显示页面：commentResult.jsp
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath%>">
<title>My JSP 'commentResult.jsp' starting page</title>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
<!--
<link rel="stylesheet" type="text/css" href="styles.css">
-->
</head>
<body>
username:<%= request.getAttribute("username") %><br/>
comment:<%= request.getAttribute("comment") %>
</body>
</html>
过滤器：CommentFilter
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.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class CommentFilter implements Filter
{
public void destroy()
{
}
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException
{
HttpServletRequest req = (HttpServletRequest)request;
HttpServletResponse resp = (HttpServletResponse)response;
String username = req.getParameter("username");
String comment = req.getParameter("comment");
username = username.replace("123", "abc");
comment = comment.replace("wokao", "****");
req.setAttribute("username", username);
req.setAttribute("comment", comment);
chain.doFilter(request, response);
}
public void init(FilterConfig filterConfig) throws ServletException
{
}
}
web.xml中的部署
<filter> <filter-name>CommentFilter</filter-name> <filter-class>com.cdtax.filter.CommentFilter</filter-class> </filter> <filter-mapping> <filter-name>CommentFilter</filter-name> <url-pattern>/CommentServlet</url-pattern> </filter-mapping>
5、串联Servlet过滤器的工作流程
客户并不知道过滤器的存在，过滤器完全是服务器端的行为
过滤器链的实例
info.jsp
<body>
<form action="InfoServlet" method="post">
username:<input type="text" name="username"/><br/>
<input type="submit" value="submit"/>"
</form>
</body>
InfoServlet:
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class InfoServlet extends HttpServlet
{
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
System.out.println(req.getParameter("username"));
req.getRequestDispatcher("infoResult.jsp").forward(req, resp);
}
}
infoResult.jsp：
<body>
<%= request.getParameter("username") %>
</body>
过滤器MyFilter1和MyFilter2
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;
public class MyFilter1 implements Filter
{
public void destroy()
{
}
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException
{
System.out.println("Filter1 before chain.doFilter");
chain.doFilter(request, response);
System.out.println("Filter1 after chain.doFilter");
}
public void init(FilterConfig filterConfig) throws ServletException
{
}
}
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;
public class MyFilter2 implements Filter
{
public void destroy()
{
}
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException
{
System.out.println("Filter2 before chain.doFilter");
chain.doFilter(request, response);
System.out.println("Filter2 after chain.doFilter");
}
public void init(FilterConfig filterConfig) throws ServletException
{
}
}
web.xml配置：
<filter> <filter-name>MyFilter1</filter-name> <filter-class>com.cdtax.filter.MyFilter1</filter-class> </filter> <filter> <filter-name>MyFilter2</filter-name> <filter-class>com.cdtax.filter.MyFilter2</filter-class> </filter> <filter-mapping> <filter-name>MyFilter1</filter-name> <url-pattern>/InfoServlet</url-pattern> </filter-mapping> <filter-mapping> <filter-name>MyFilter2</filter-name> <url-pattern>/InfoServlet</url-pattern> </filter-mapping>
Filter的执行顺序是跟配置中的<filter-mapping>相关联的，哪个<filter-mapping>在前，就先执行哪个Filter，如将上面的配置改为
<filter-name>MyFilter2</filter-name> <url-pattern>/InfoServlet</url-pattern> </filter-mapping> <filter-mapping> <filter-name>MyFilter1</filter-name> <url-pattern>/InfoServlet</url-pattern> </filter-mapping>
其他不动，执行结果为：
Filter2 before chain.doFilter
Filter1 before chain.doFilter
wwww
Filter1 after chain.doFilter
Filter2 after chain.doFilter
执行的结果：
Filter1 before chain.doFilter
Filter2 before chain.doFilter
wwww
Filter2 after chain.doFilter
Filter1 after chain.doFilter
6、关于FilterConfig
在web.xml中filter可以带参数：
<filter> <filter-name>CommentFilter</filter-name> <filter-class>com.cdtax.filter.CommentFilter</filter-class> <init-param> <param-name>hello</param-name> <param-value>world</param-value> </init-param> <init-param> <param-name>zhang</param-name> <param-value>li</param-value> </init-param> </filter>
在Filter中通过FilterConfig获得参数的方法：
public void init(FilterConfig filterConfig) throws ServletException
{
String value1 = filterConfig.getInitParameter("hello");
String value2 = filterConfig.getInitParameter("zhang");
System.out.println(value1 + value2);
}
通过FilterConfig可以获得ServletContext对象，也就是application，语法：
ServletContext sc = filterConfig.getServletContext();
相当于使用request：
ServletContext sc = request.getSession().getServletContext();