【Servlet】Filter过滤器的编写和配置

Servlet的Filter介绍

在Servlet作为过滤器使用时，它可以对客户的请求进行过滤处理，当它处理完成后，它会交给下一个过滤器处理，就这样，客户的请求在过滤链里一个个处理，直到请求发送到目标.如果过滤器不成功，则将试图转发到指定页面.

过滤器就像一扇门，客户端要和服务端的某个目标交互，必须通过这扇门.

典型filter应用

1 设置统一字符集 解决中文乱码问题.

request.setCharacterEncoding(“UTF-8");

post/get默认传输编码都是ISO8859-1

2 权限,比如检查会话是否有效。

如何编写Filter

开发一个自定义的Filter，需要实现Filter接口，Filter接口定义了以下的方法：

	//由Web容器调用，销毁此Filter
	destroy()
	 //由Web容器调用，初始化此Filter
	init(FilterConfig filterConfig)
	//具体过滤处理代码
	doFilter(ServletRequest request, ServletResponse response, FilterChain chain)

如何配置Filter

在web.xml文件中配置filter逻辑名 初始参数及拦截哪些pattern的请求。

<filter>
	<filter-name>EncodingFilter</filter-name>
	<filter-class>com.lessons.filter.EncodingFilter</filter-class>
	<init-param>
		<param-name>encoding</param-name>
		<param-value>utf-8</param-value>
	</init-param>//通过FilterConfig类的getInitParameter("paramName")
</filter>
<filter-mapping>
	<filter-name>EncodingFilter</filter-name>
	<url-pattern>/*</url-pattern>
</filter-mapping>

过滤器范例：设置请求编码的过滤器

package com.app.servlet;

import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;

/**
 * 设置请求编码的过滤器
 * @author user
 */
public class EncodingFilter implements Filter {
	ServletContext application;
	private String basePath;
	public void destroy() {
		// TODO Auto-generated method stub
	}
	public void doFilter(ServletRequest req, ServletResponse response,
			FilterChain chain) throws IOException, ServletException {
		// TODO Auto-generated method stub
		HttpServletRequest request = (HttpServletRequest)req;
		request.setCharacterEncoding("utf-8");

		String path = request.getContextPath();
		basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
		application.setAttribute("bashPath", basePath);
		chain.doFilter(req, response);//交给下一个过滤器
	}
	public void init(FilterConfig config) throws ServletException {
		// TODO Auto-generated method stub
		application = config.getServletContext();
	}
}

XML文件的配置

	<filter>
		<filter-name>EncodingFilter</filter-name>
		<filter-class>com.app.servlet.EncodingFilter</filter-class>
	</filter>
	<filter-mapping>
		<filter-name>EncodingFilter</filter-name>
		<url-pattern>/*</url-pattern>
	</filter-mapping>