web.xml配置详解之filter

定义

<filter>
<filter-name>encodingfilter</filter-name>
<filter-class>com.my.app.EncodingFilter</filter-class>
<init-param>
<param-name>encoding</param-name>
<param-value>UTF-8</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>encodingfilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>

作用

用于指定WEB容器的过滤器，在请求和响应对象在Servlet处理之前和之后，可以通过此过滤器对这两个对象进行处理。

备注

filter-class 中指定的过滤器类须继承 javax.servlet.Filter 具有须有以下三种方法

init(FilterConfig filterConfig)：初始化;一般情况下时读取配置文件中的init-param参数值 如 filterConfig.getInitParameter("encoding")

doFilter(...)：用于对request,response进行处理，并能过chain.doFilter(...) 交过下一个控制器

destroy()：资源销毁

示例：如编码过滤器

web.xml配置

<filter>
<filter-name>encodingfilter</filter-name>
<filter-class>com.my.app.EncodingFilter</filter-class>
<init-param>
<param-name>encoding</param-name>
<param-value>UTF-8</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>encodingfilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>

Java 代码

/**
*
*/
package com.myapp;

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;

/**
* @author louisliao
*
*/
public class EncodingFilter implements Filter {

/**
* 配置中默认的字符编码
*/
protected String encoding = null;
protected FilterConfig filterConfig;
/**
* 当没有指定默认编码时是否允许跳过过滤
*/
protected boolean ignore = true;
/**
*
*/
public EncodingFilter() {
// TODO Auto-generated constructor stub
}

/* (non-Javadoc)
* @see javax.servlet.Filter#destroy()
*/
public void destroy() {
// TODO Auto-generated method stub
this.encoding=null;
this.filterConfig=null;
}

/* (non-Javadoc)
* @see javax.servlet.Filter#doFilter(javax.servlet.ServletRequest, javax.servlet.ServletResponse, javax.servlet.FilterChain)
*/
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
// TODO Auto-generated method stub
HttpServletRequest hRequest=(HttpServletRequest)request;
HttpServletResponse hResponse=(HttpServletResponse)response;
//Conditionally select and set the character encoding to be used
if(ignore || hRequest.getCharacterEncoding()==null){
String coding=selectEncoding(hRequest);
if(coding!=null){
hRequest.setCharacterEncoding(coding);
hResponse.setCharacterEncoding(coding);
}
}
//将控制器传向下一个filter
chain.doFilter(hRequest, hResponse);

}

/* (non-Javadoc)
* @see javax.servlet.Filter#init(javax.servlet.FilterConfig)
*/
public void init(FilterConfig filterConfig) throws ServletException {
// TODO Auto-generated method stub
this.filterConfig=filterConfig;
this.encoding=filterConfig.getInitParameter("encoding");
System.out.println(this.encoding);
String value = filterConfig.getInitParameter("ignore");
if (value == null) {
this.ignore = true;
} else if (value.equalsIgnoreCase("true")) {
this.ignore = true;
} else if (value.equalsIgnoreCase("yes")) {
this.ignore = true;
} else {
this.ignore = false;
}
}
protected String selectEncoding(ServletRequest request) {
return (this.encoding);
}

}
init方法是在WEB应用启动就会调用,doFilter则是在访问filter-mapping映射到的url时会调用。