Servlet过滤器

 
1Servlet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servletJSP页面运行在服务器上。过滤器可附加到一个或多个servletJSP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servletJSP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
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。
2Servlet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:
destory()由Web容器调用,初始化此Filter
initFilterConfig filterConfig)由Web容器调用,初始化此Filter
doFilterServletRequest 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servletJSP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servletJSP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3SimpleFilter1.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:
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3SimpleFilter2.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"/>
### Servlet 过滤器的配置及使用方法 #### 1. 创建过滤器过滤器类需要实现 `javax.servlet.Filter` 接口,并重写其三个核心方法:`init`、`doFilter` 和 `destroy`。 - **`init` 方法**:在服务器启动时被调用,主要用于加载配置文件或初始化资源[^1]。 - **`doFilter` 方法**:这是过滤器的核心部分,用于拦截并处理请求和响应对象。可以在这里添加自定义逻辑,比如编码设置、权限验证等[^1]。 - **`destroy` 方法**:当服务器关闭时被调用,用于释放过滤器所占有的资源[^1]。 以下是一个简单的过滤器类示例: ```java import javax.servlet.*; import java.io.IOException; public class ExampleFilter implements Filter { public void init(FilterConfig filterConfig) throws ServletException { // 初始化操作,例如读取配置参数 String param = filterConfig.getInitParameter("exampleParam"); System.out.println("初始化参数:" + param); } public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { // 在这里可以对请求进行预处理 System.out.println("请求前的操作"); // 继续传递给下一个过滤器或目标资源 chain.doFilter(request, response); // 在这里可以对响应进行后处理 System.out.println("响应后的操作"); } public void destroy() { // 清理工作 System.out.println("销毁过滤器"); } } ``` --- #### 2. 配置过滤器 可以通过两种方式来配置过滤器:基于 XML 的配置和基于注解的配置。 ##### (1) 基于 XML 的配置 通过 `web.xml` 文件中的 `<filter>` 和 `<filter-mapping>` 标签完成过滤器的注册和映射。 - **<filter> 标签**:定义过滤器的名称及其对应的类名。 - **<filter-mapping> 标签**:指定哪些 URL 请求会被该过滤器拦截。 示例配置如下: ```xml <filter> <filter-name>ExampleFilter</filter-name> <filter-class>com.example.ExampleFilter</filter-class> <!-- 可选:定义初始化参数 --> <init-param> <param-name>exampleParam</param-name> <param-value>value</param-value> </init-param> </filter> <filter-mapping> <filter-name>ExampleFilter</filter-name> <url-pattern>/secure/*</url-pattern> </filter-mapping> ``` 在这个例子中,所有匹配 `/secure/` 路径的请求都会经过 `ExampleFilter` 处理[^4]。 ##### (2) 基于注解的配置 如果使用的 Servlet 容器支持 Servlet 3.0 或更高版本,则可以直接使用 `@WebFilter` 注解
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