Filter

chain.doFilter(request,response)

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他的作用是将请求转发给过滤器链上下一个对象。这里的“下”指的是哪里 ？

值得是下一个filter，如果没有filter那就是你请求的资源。 

一般filter都是一个链,web.xml 里面配置了几个就有几个。一个一个的连在一起 

request -> filter1 -> filter2 ->filter3 -> .... -> request resource.

下面举一个例子：

input.jsp是用来提交输入的，当提交后，过滤器检测姓名和年龄，如果整常的话会提交给output.jsp,如果不正常提交给erroroutput.jsp.在此同时也有一个过滤器，它是用来检测一个页面是否设置了字符编码，如果没有则进行设置。（防止乱码问题存在）。

myfilter.java

======================

package servletbean;

import javax.swing.JOptionPane;

public class myfilter implements Filter {

public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {

response.setContentType("text/html");

response.setCharacterEncoding("GB2312");

PrintWriter out=response.getWriter();

String name="";

String age="";

int age1;

name=request.getParameter("name");

age=request.getParameter("age");

RequestDispatcher dispatch=request.getRequestDispatcher("erroroutput.jsp"); if(name==null||name==""||name==" "||age==null)

{

JOptionPane.showMessageDialog(null,"用户名和年龄输入错误！");

dispatch.forward(request, response);

return;

}

else

{

try

{

age1=Integer.parseInt(age);

}

catch(Exception e)

{

//JOptionPane.showMessageDialog(null,"年龄必须为数字！");

dispatch.forward(request,response);

return;//如果是错误页面就到erroroutput.jsp中

}

}

chain.doFilter(request, response);//这里表示是正确的，也就是说，他回去找下一个链，但是旗下面已经没有了，所以就会去跳转页面了，此时要跳转的页面就是action="output.jsp"了。

}

}

==================

encodefilter.java

===================

package servletbean;

public class encodefilter implements Filter {

public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {

if(request.getCharacterEncoding()==null)

{

System.out.println(encoding);

request.setCharacterEncoding(encoding);

}

chain.doFilter(request, response);//到下一个链

}

public void init(FilterConfig fConfig) throws ServletException {

this.config=fConfig;

encoding=fConfig.getInitParameter("encoding");

}

}

==============

input.jsp

==============

<form action="output.jsp" name="form" method="post">

<table>

<tr>

<td>name</td>

<td><input type="text" name="name" /></td>

</tr>

<tr>

<td>age</td>

<td><input type="text" name="age"/></td>

</tr>

<tr>

<td>

<input type="submit" name="ok" value="ok"/>

</td>

</tr>

</table>

</form>

================

web.xml

===============

注意：filter是有顺序的

<filter>

<description>

</description>

<display-name>

encodefilter</display-name>

<filter-name>encodefilter</filter-name>

<filter-class>servletbean.encodefilter</filter-class>

<init-param>

<param-name>encoding</param-name>

<param-value>GB2312</param-value>

</init-param>

</filter>

<filter-mapping>

<filter-name>encodefilter</filter-name>

<url-pattern>/*</url-pattern>

</filter-mapping>

<filter>

<description>

</description>

<display-name>

myfilter</display-name>

<filter-name>myfilter</filter-name>

<filter-class>servletbean.myfilter</filter-class>

</filter>

<filter-mapping>

<filter-name>myfilter</filter-name>

<url-pattern>/output.jsp</url-pattern>

</filter-mapping>