Java filter中的chain.doFilter详解

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一.chain.doFilter作用

1.一般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.

2.chain.doFilter将请求转发给过滤器链下一个filter , 如果没有filter那就是你请求的资源

二.举例

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

1.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>

2.web.xml

<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>

3.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");//获得目标编码格式    }}

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

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