Servlet过滤器

 

1．Servlet过滤器

1.1 什么是过滤器

过滤器是一个程序，它先于与之相关的servlet或JSP页面运行在服务器上。过滤器可附加到一个或多个servlet或JSP页面上，并且可以检查进入这些资源的请求信息。在这之后，过滤器可以作如下的选择：

①以常规的方式调用资源（即，调用servlet或JSP页面）。

②利用修改过的请求信息调用资源。

③调用资源，但在发送响应到客户机前对其进行修改。

④阻止该资源调用，代之以转到其他的资源，返回一个特定的状态代码或生成替换输出。

1.2 Servlet过滤器的基本原理

在Servlet作为过滤器使用时，它可以对客户的请求进行处理。处理完成后，它会交给下一个过滤器处理，这样，客户的请求在过滤链里逐个处理，直到请求发送到目标为止。例如，某网站里有提交“修改的注册信息”的网页，当用户填写完修改信息并提交后，服务器在进行处理时需要做两项工作：判断客户端的会话是否有效；对提交的数据进行统一编码。这两项工作可以在由两个过滤器组成的过滤链里进行处理。当过滤器处理成功后，把提交的数据发送到最终目标；如果过滤器处理不成功，将把视图派发到指定的错误页面。

2．Servlet过滤器开发步骤

开发Servlet过滤器的步骤如下：

①编写实现Filter接口的Servlet类。

②在web.xml中配置Filter。

开发一个过滤器需要实现Filter接口，Filter接口定义了以下方法：

①destory（）由Web容器调用，初始化此Filter。

②init（FilterConfig filterConfig）由Web容器调用，初始化此Filter。

③doFilter（ServletRequest request,ServletResponse response,FilterChain chain）具体过滤处理代码。

3．一个过滤器框架实例

SimpleFilter1.java

package com.zj.sample; 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; public class SimpleFilter1 implements Filter {     @SuppressWarnings("unused")     private FilterConfig filterConfig;     public void init(FilterConfig config) throws ServletException {        this.filterConfig = config;     }     public void doFilter(ServletRequest request, ServletResponse response,            FilterChain chain) {        try {            System.out.println("Within SimpleFilter1:Filtering the Request...");            chain.doFilter(request, response);// 把处理发送到下一个过滤器            System.out .println("Within SimpleFilter1:Filtering the Response...");        } catch (IOException ioe) {            ioe.printStackTrace();        } catch (ServletException se) {            se.printStackTrace();        }     }     public void destroy() {        this.filterConfig = null;     } }

SimpleFilter2.java

package com.zj.sample; 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; public class SimpleFilter2 implements Filter {     @SuppressWarnings("unused")     private FilterConfig filterConfig;     public void init(FilterConfig config) throws ServletException {        this.filterConfig = config;     }     public void doFilter(ServletRequest request, ServletResponse response,            FilterChain chain) {        try {            System.out.println("Within SimpleFilter2:Filtering the Request...");            chain.doFilter(request, response); // 把处理发送到下一个过滤器            System.out.println("Within SimpleFilter2:Filtering the Response...");        } catch (IOException ioe) {            ioe.printStackTrace();        } catch (ServletException se) {            se.printStackTrace();        }     }     public void destroy() {        this.filterConfig = null;     } }

web.xml

<filter>     <filter-name>filter1</filter-name>     <filter-class>com.zj.sample.SimpleFilter1</filter-class> </filter> <filter-mapping>     <filter-name>filter1</filter-name>     <url-pattern>/*</url-pattern>//为所有的访问做过滤 </filter-mapping> <filter>     <filter-name>filter2</filter-name>     <filter-class>com.zj.sample.SimpleFilter2</filter-class> </filter> <filter-mapping>     <filter-name>filter2</filter-name>     <url-pattern>/*</url-pattern>//为所有的访问做过滤 </filter-mapping>

打开web容器中任意页面输出结果：（注意过滤器执行的请求/响应顺序）

Within SimpleFilter1:Filtering the Request...
Within SimpleFilter2:Filtering the Request...
Within SimpleFilter2:Filtering the Response...
Within SimpleFilter1:Filtering the Response...

4．报告过滤器

我们来试验一个简单的过滤器，只要调用相关的servlet或JSP页面，它就打印一条消息到标准输出。为实现此功能，在doFilter方法中执行过滤行为。每当调用与这个过滤器相关的servlet或JSP页面时，doFilter方法就生成一个打印输出，此输出列出请求主机和调用的URL。因为getRequestURL方法位于HttpServletRequest而不是ServletRequest中，所以把ServletRequest对象构造为HttpServletRequest类型。我们改动一下章节3的SimpleFilter1.java。

SimpleFilter1.java

package com.zj.sample; import java.io.IOException; 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; public class SimpleFilter1 implements Filter {     @SuppressWarnings("unused")     private FilterConfig filterConfig;     public void init(FilterConfig config) throws ServletException {        this.filterConfig = config;     }     public void doFilter(ServletRequest request, ServletResponse response,            FilterChain chain) {        try {            System.out.println("Within SimpleFilter1:Filtering the Request...");            HttpServletRequest req = (HttpServletRequest) request;            System.out.println(req.getRemoteHost() + " tried to access "                   + req.getRequestURL() + " on " + new Date() + ".");            chain.doFilter(request, response);            System.out.println("Within SimpleFilter1:Filtering the Response...");        } catch (IOException ioe) {            ioe.printStackTrace();        } catch (ServletException se) {            se.printStackTrace();        }     }     public void destroy() {        this.filterConfig = null;     } }

web.xml设置不变，同章节3。

测试：

输入http://localhost:8080/Test4Jsp/login.jsp

结果：

Within SimpleFilter1:Filtering the Request...
0:0:0:0:0:0:0:1 tried to access http://localhost:8080/Test4Jsp/login.jsp on Sun Mar 04 17:01:37 CST 2007.
Within SimpleFilter2:Filtering the Request...
Within SimpleFilter2:Filtering the Response...
Within SimpleFilter1:Filtering the Response...

5．访问时的过滤器（在过滤器中使用servlet初始化参数）

下面利用init设定一个正常访问时间范围，对那些不在此时间段的访问作出记录。我们改动一下章节3的SimpleFilter2.java。

SimpleFilter2.java。

package com.zj.sample; import java.io.IOException; import java.text.DateFormat; import java.util.Calendar; import java.util.GregorianCalendar; import javax.servlet.Filter; import javax.servlet.FilterChain; import javax.servlet.FilterConfig; import javax.servlet.ServletContext; import javax.servlet.ServletException; import javax.servlet.ServletRequest; import javax.servlet.ServletResponse; import javax.servlet.http.HttpServletRequest; public class SimpleFilter2 implements Filter {     @SuppressWarnings("unused")     private FilterConfig config;     private ServletContext context;     private int startTime, endTime;     private DateFormat formatter;     public void init(FilterConfig config) throws ServletException {        this.config = config;        context = config.getServletContext();        formatter = DateFormat.getDateTimeInstance(DateFormat.MEDIUM,               DateFormat.MEDIUM);        try {            startTime = Integer.parseInt(config.getInitParameter("startTime"));// web.xml            endTime = Integer.parseInt(config.getInitParameter("endTime"));// web.xml        } catch (NumberFormatException nfe) { // Malformed or null            // Default: access at or after 10 p.m. but before 6 a .m. is            // considered unusual.            startTime = 22; // 10:00 p.m.            endTime = 6; // 6: 00 a .m.        }     }     public void doFilter(ServletRequest request, ServletResponse response,            FilterChain chain) {        try {            System.out.println("Within SimpleFilter2:Filtering the Request...");            HttpServletRequest req = (HttpServletRequest) request;            GregorianCalendar calendar = new GregorianCalendar();            int currentTime = calendar.get(Calendar.HOUR_OF_DAY);            if (isUnusualTime(currentTime, startTime, endTime)) {               context.log("WARNING: " + req.getRemoteHost() + " accessed "                      + req.getRequestURL() + " on "                      + formatter.format(calendar.getTime()));               // The log file is under <CATALINA_HOME>/logs.One log per day.            }            chain.doFilter(request, response);            System.out                   .println("Within SimpleFilter2:Filtering the Response...");        } catch (IOException ioe) {            ioe.printStackTrace();        } catch (ServletException se) {            se.printStackTrace();        }     }     public void destroy() {}     // Is the current time between the start and end     // times that are marked as abnormal access times?     private boolean isUnusualTime(int currentTime, int startTime, int endTime) {        // If the start time is less than the end time (i.e.,        // they are two times on the same day), then the        // current time is considered unusual if it is        // between the start and end times.        if (startTime < endTime) {            return ((currentTime >= startTime) && (currentTime < endTime));        }        // If the start time is greater than or equal to the        // end time (i.e., the start time is on one day and        // the end time is on the next day), then the current        // time is considered unusual if it is NOT between        // the end and start times.        else {            return (!isUnusualTime(currentTime, endTime, startTime));        }     } }

web.xml设置不变。

关于Tomcat日志处理，这里补充介绍一下。config.getServletContext().log（"log message"）会将日志信息写入<CATALINA_HOME>/logs文件夹下，文件名应该为localhost_log. 2007-03-04 .txt这样的形式（按日期每天产生一个，第二天可以看见）。要得到这样一个日志文件，应该在server.xml中有：

<Logger className="org.apache.catalina.logger.FileLogger" prefix="catalina_log." suffix=".txt" timestamp="true"/>