前置条件需要配置webxml

<filter>        <filter-name>FilterDemo3</filter-name>        <filter-class>cn.web.servlet.filter.FilterDemo3</filter-class>        <init-param>           <param-name>encoding</param-name>           <param-value>UTF-8</param-value>        </init-param>    </filter>    <filter-mapping>        <filter-name>FilterDemo3</filter-name>        <url-pattern>/*</url-pattern>    </filter-mapping>

java代码

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 FilterDemo3 implements Filter {    private FilterConfig filterConfig;    public FilterDemo3()    {    }    @Override    public void init(FilterConfig filterConfig) throws ServletException {        this.filterConfig = filterConfig;    }    //WEB容器    @Override    public void doFilter(ServletRequest request, ServletResponse response,            FilterChain chain) throws IOException, ServletException {        //从web.xml中全部获取参数        Enumeration<String> enums  = filterConfig.getInitParameterNames();        while(enums.hasMoreElements())        {            String key = enums.nextElement();            String value = filterConfig.getInitParameter(key);            System.out.println(key+":"+value);        }        String encoding = filterConfig.getInitParameter("encoding");        response.setContentType("text/html;charset="+encoding);        chain.doFilter(request,response);    }    @Override    public void destroy() {        System.out.println("摧毁 方法: destroy()");    }}

结果：
摧毁 方法: destroy()
四月 18, 2018 9:38:57 下午 org.apache.catalina.core.StandardContext reload
信息: Reloading Context with name [/day04] has started
四月 18, 2018 9:38:57 下午 org.apache.catalina.core.StandardContext reload
信息: Reloading Context with name [/day04] is completed
encoding:UTF-8