Filter系列教材 （一）- 过滤器 Filter 教程

步骤1:Filter概念 
步骤2:编写 FirstFilter
步骤3:配置web.xml
步骤4:访问页面
步骤5:init() 方法
步骤6:Filter启动失败

步骤 1 : Filter概念 

Filter就像一个一个哨卡，用户的请求需要经过Filter
并且可以有多个过滤器

步骤 2 : 编写 FirstFilter

开发一个简单的FirstFilter，用来打印用户访问ip地址和访问的页面

HttpServletRequest request = (HttpServletRequest) req;

doFilter()方法中的req参数的类型是ServletRequest，需要转换为HttpServletRequest类型方便调用某些方法 (参考request常见方法)

String ip = request.getRemoteAddr();

获取来路用户的ip地址

String url = request.getRequestURL().toString();

获取用户访问的页面地址

System.out.printf("%s %s 访问了 %s%n", date, ip, url);

在控制台打印出来

chain.doFilter(request, response);

过滤器放行，表示继续运行下一个过滤器，或者最终访问的某个servlet,jsp,html等等

package filter;

import java.io.IOException;

import java.text.SimpleDateFormat;

import java.util.Date;

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;

public class FirstFilter implements Filter {

    @Override

    public void destroy() {

    }

    @Override

    public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)

            throws IOException, ServletException {

        HttpServletRequest request = (HttpServletRequest) req;

        HttpServletResponse response = (HttpServletResponse) res;

        String ip = request.getRemoteAddr();

        String url = request.getRequestURL().toString();

        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

        Date d = new Date();

        String date = sdf.format(d);

        System.out.printf("%s %s 访问了 %s%n", date, ip, url);

        chain.doFilter(request, response);

    }

    @Override

    public void init(FilterConfig arg0) throws ServletException {

    }

}

步骤 3 : 配置web.xml

在web.xml中进行filter的配置，和servlet的配置很类似
 

<url-pattern>/*</url-pattern>

表示所有的访问都会过滤

如果配置成

<url-pattern>*.jsp</url-pattern>

就表示只过滤jsp

<filter>

    <filter-name>FirstFilter</filter-name>

    <filter-class>filter.FirstFilter</filter-class>

</filter>

<filter-mapping>

    <filter-name>FirstFilter</filter-name>

    <url-pattern>/*</url-pattern>

</filter-mapping>

步骤 4 : 访问页面

重启tomcat,所有用户的访问信息，都可以打印出来

步骤 5 : init() 方法

与Servlet需要配置自启动才会随着tomcat的启动而执行init()方法不一样。

Filter一定会随着tomcat的启动自启动。

package filter;

import java.io.IOException;

import java.text.SimpleDateFormat;

import java.util.Date;

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;

public class FirstFilter implements Filter {

    @Override

    public void destroy() {

    }

    @Override

    public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)

            throws IOException, ServletException {

        HttpServletRequest request = (HttpServletRequest) req;

        HttpServletResponse response = (HttpServletResponse) res;

        String ip = request.getRemoteAddr();

        String url = request.getRequestURL().toString();

        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

        Date d = new Date();

        String date = sdf.format(d);

        System.out.printf("%s %s 访问了 %s%n", date, ip, url);

        chain.doFilter(request, response);

    }

    @Override

    public void init(FilterConfig arg0) throws ServletException {

        System.out.println("First Filter init()");

    }

}

步骤 6 : Filter启动失败

Filter是web应用非常重要的一个环节，如果Filter启动失败，或者本身有编译错误，不仅这个Filter不能使用，整个web应用会启动失败，导致用户无法访问页面

在启动tomcat过程中，也会看到这样的字样： 
 

严重: Context [] startup failed due to previous errors

这常常用于提示Filter启动失败了

package filter;

import java.io.IOException;

import java.text.SimpleDateFormat;

import java.util.Date;

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;

public class FirstFilter implements Filter {

    @Override

    public void destroy() {

    }

    @Override

    public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)

            throws IOException, ServletException {

        HttpServletRequest request = (HttpServletRequest) req;

        HttpServletResponse response = (HttpServletResponse) res;

        String ip = request.getRemoteAddr();

        String url = request.getRequestURL().toString();

        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

        Date d = new Date();

        String date = sdf.format(d);

        System.out.printf("%s %s 访问了 %s%n", date, ip, url);

        chain.doFilter(request, response);

    }

    @Override

    public void init(FilterConfig arg0) throws ServletException {

        System.out.println("故意造成一个异常");

        Object o = null;

        o.toString();

    }

}

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