package me.gacl.web.filter;
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;
/**
* @ClassName: FilterDemo
* @Description:filter的三种典型应用：
* 、可以在filter中根据条件决定是否调用chain.doFilter(request, response)方法，
* 即是否让目标资源执行
* 、在让目标资源执行之前，可以对request\response作预处理，再让目标资源执行
* 、在目标资源执行之后，可以捕获目标资源的执行结果，从而实现一些特殊的功能
* @author: 孤傲苍狼
* @date: -- 下午::
*/ 
public class FilterDemo implements Filter {
@Override
public void init(FilterConfig filterConfig) throws ServletException {
System.out.println("----过滤器初始化----");
}
@Override
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
//对request和response进行一些预处理
request.setCharacterEncoding("UTF-");
response.setCharacterEncoding("UTF-");
response.setContentType("text/html;charset=UTF-");
System.out.println("FilterDemo执行前！！！");
chain.doFilter(request, response); //让目标资源执行，放行
System.out.println("FilterDemo执行后！！！");
}
@Override
public void destroy() {
System.out.println("----过滤器销毁----");
}
} 

<?xml version="." encoding="UTF-"?>
<web-app version="." 
xmlns="http://java.sun.com/xml/ns/javaee" 
xmlns:xsi="http://www.w.org//XMLSchema-instance" 
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee 
http://java.sun.com/xml/ns/javaee/web-app__.xsd">
<display-name></display-name> 
<welcome-file-list>
<welcome-file>index.jsp</welcome-file>
</welcome-file-list>
<!--配置过滤器-->
<filter>
<filter-name>FilterDemo</filter-name>
<filter-class>me.gacl.web.filter.FilterDemo</filter-class>
</filter>
<!--映射过滤器-->
<filter-mapping>
<filter-name>FilterDemo</filter-name>
<!--“/*”表示拦截所有的请求 -->
<url-pattern>/*</url-pattern>
</filter-mapping>
</web-app> 

package me.gacl.web.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;
public class FilterDemo implements Filter {
/* 过滤器初始化
* @see javax.servlet.Filter#init(javax.servlet.FilterConfig)
*/
@Override
public void init(FilterConfig filterConfig) throws ServletException {
System.out.println("----过滤器初始化----");
/**
* <filter>
<filter-name>FilterDemo</filter-name>
<filter-class>me.gacl.web.filter.FilterDemo</filter-class>
<!--配置FilterDemo过滤器的初始化参数-->
<init-param>
<description>配置FilterDemo过滤器的初始化参数</description>
<param-name>name</param-name>
<param-value>gacl</param-value>
</init-param>
<init-param>
<description>配置FilterDemo过滤器的初始化参数</description>
<param-name>like</param-name>
<param-value>java</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>FilterDemo</filter-name>
<!--“/*”表示拦截所有的请求 -->
<url-pattern>/*</url-pattern>
</filter-mapping>
*/
//得到过滤器的名字
String filterName = filterConfig.getFilterName();
//得到在web.xml文件中配置的初始化参数
String initParam = filterConfig.getInitParameter("name");
String initParam = filterConfig.getInitParameter("like");
//返回过滤器的所有初始化参数的名字的枚举集合。
Enumeration<String> initParameterNames = filterConfig.getInitParameterNames();
System.out.println(filterName);
System.out.println(initParam);
System.out.println(initParam);
while (initParameterNames.hasMoreElements()) {
String paramName = (String) initParameterNames.nextElement();
System.out.println(paramName);
}
}
@Override
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
System.out.println("FilterDemo执行前！！！");
chain.doFilter(request, response); //让目标资源执行，放行
System.out.println("FilterDemo执行后！！！");
}
@Override
public void destroy() {
System.out.println("----过滤器销毁----");
}
}

<filter>
<description>FilterDemo过滤器</description>
<filter-name>FilterDemo</filter-name>
<filter-class>me.gacl.web.filter.FilterDemo</filter-class>
<!--配置FilterDemo过滤器的初始化参数-->
<init-param>
<description>配置FilterDemo过滤器的初始化参数</description>
<param-name>name</param-name>
<param-value>gacl</param-value>
</init-param>
<init-param>
<description>配置FilterDemo过滤器的初始化参数</description>
<param-name>like</param-name>
<param-value>java</param-value>
</init-param>
</filter> 

<!--映射过滤器-->
<filter-mapping>
<filter-name>FilterDemo</filter-name>
<!--“/*”表示拦截所有的请求 -->
<url-pattern>/*</url-pattern>
</filter-mapping> 

<filter-mapping>
<filter-name>testFilter</filter-name>
<url-pattern>/index.jsp</url-pattern>
<dispatcher>REQUEST</dispatcher>
<dispatcher>FORWARD</dispatcher>
</filter-mapping>