Servlet-过滤器

一、介绍

• 一般用于解决中文乱码问题和实现访问限制。

二、项目测试
（1）编写MyServlet及过滤器FirstFilter、SecondFilter（过滤器需实现Filter接口）

public class MyServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("doGet...");
    }
}

public class FirstFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("filter01...init");
    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        System.out.println("filter01...request");
        filterChain.doFilter(servletRequest, servletResponse);
        System.out.println("filter01...response");
    }

    @Override
    public void destroy() {
        System.out.println("filter01...destroy");
    }
}

public class SecondFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("filter02...init");
    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        System.out.println("filter02...request");
        filterChain.doFilter(servletRequest, servletResponse);
        System.out.println("filter02...response");
    }

    @Override
    public void destroy() {
        System.out.println("filter02...destroy");
    }
}

（2）编写web.xml，配置Servlet及过滤器（过滤器的执行顺序按照过滤器mapping配置的先后；初始化及销毁则按照过滤器注册的先后，即过滤器声明时的顺序）

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">

    <servlet>
        <servlet-name>MyServlet</servlet-name>
        <servlet-class>com.wsh.servlet.MyServlet</servlet-class>
        <!--load-on-startup的值大于等于0时，Servlet在Tomcat启动时初始化-->
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet-mapping>
        <servlet-name>MyServlet</servlet-name>
        <url-pattern>/*</url-pattern>
    </servlet-mapping>

    <filter>
        <filter-name>FirstFilter</filter-name>
        <filter-class>com.wsh.filter.FirstFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>FirstFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

    <filter>
        <filter-name>SecondFilter</filter-name>
        <filter-class>com.wsh.filter.SecondFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>SecondFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
</web-app>

（3）配置Tomcat中的项目上下文路径

（4）运行