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：

① destory （）由 Web 容器调用，初始化此 Filter 。

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

③ doFilter （ 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 >

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

 

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

结果：Within SimpleFilter1:Filtering the Request...
0:0:0:0:0:0:0:1 tried to access [url]http://localhost:8080/Test4Jsp/login.jsp[/url] 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" />

 

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