Filter：JSP系统设置编码过滤器

迅速配置属于自己的过滤器。

先从JSP的编码入手，开发编码filter。

步骤一：在web.xml上添加<filter>，代码如下。

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"> 
	
	<filter>
		<filter-name>encodeFilter</filter-name>
		<filter-class>com.demo.filter.EncodeFilter</filter-class>
		<init-param>
			<param-name>encode</param-name>
			<param-value>utf-8</param-value>
		 	</init-param>
		<init-param>
			<param-name>ignore</param-name>
			<param-value>false</param-value>
		</init-param>
   	</filter>
   	<filter-mapping>
       	<filter-name>encodeFilter</filter-name>
       	<url-pattern>/*</url-pattern>			<!-- "/*"即对所有的请求进行过滤 -->
   	</filter-mapping>

上面的配置的filter，路径为com.demo.filter.EncodeFilter，命名encodeFilter，映射过滤文件为/*（即全部过滤）。另外在定义filter时，初始化了两个参数：encode、ignore，两个参数在xml里面配置然后由后台Filter类应用，前者赋值为编码方式utf-8，后者赋值false即是否开启过滤。再看看后台类的实现。

package com.demo.filter;

import java.io.IOException;
import java.util.Enumeration;

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;

/*
 * 设置传值编码:UTF-8
 */
public class EncodeFilter implements Filter {
	  
	private String encode = null;
    private boolean ignore = false;	

    public void init(FilterConfig filterConfig) throws ServletException {
        String encode = filterConfig.getInitParameter("encode");
        String ignore = filterConfig.getInitParameter("ignore");
        if (this.encode == null)
            this.encode = encode;
        if ("1".equals(ignore) || "true".equals(ignore)) {
           this.ignore = true;
        }
    }
    
	public void doFilter(ServletRequest request, ServletResponse response,FilterChain chain) throws IOException, ServletException {
		if (!ignore) {
			if (null == request.getCharacterEncoding()) {
				request.setCharacterEncoding(encode);
			}
		}
		chain.doFilter(request,response);
	}
	
	public void destroy() {
		encode = null;
		ignore = false;
	}

}

可以看到，本类EncodeFilter继承了父类Filter（抽象类），要求实现父类的三个方法：init(FilterConfig filterConfig)、doFilter(ServletRequest request, ServletResponse response,FilterChain chain)、destroy()，如EncodeFilter类的三个成员方法就是了。结合前面配置的web.xml，很快就上手了。