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[Java教程]Java Web之Filter


Filter被称为过滤器或者拦截器,基本功能就是对调用servler过程的拦截,在servlet进行响应和处理前后实现一些特殊功能。其实,Filter过滤器就是一个实现了javax.servlet.Filter接口的类,在javax.servlet.Filter接口中定义了3个方法:

  • init(FilterConfig filterConfig) : 用来初始化过滤器,可以在init()中完成与构造方法类似的初始化功能,如果初始化代码中要使用FilterConfig对象,那么,这些初始化代码就只能在Filter的init()方法中编写,而不能在构造方法中编写。
  • doFilter(ServletRequest request, ServletResponse response, FilterChain chain) : request和response为Web服务器或Filter链中的上一个Filter对象传递过来的请求和响应对象;参数chain代表Filter链的对象,在当前Filter对象中的doFilter()方法内需要调用FilterChain对象的doFilter()方法,才能把请求交付给Filter链的下一个Filter或者目标程序去处理。
  • public void destroy() : destroy()方法在Web服务器卸载Filter之前被调用,该方法用于释放Filter对象打开的资源,例如关闭数据库和IO流。

1、实现第一个Filter程序

Index.java文件如下:

 1 package com.net; 2  3 import java.io.IOException; 4 import java.io.PrintWriter; 5  6 import javax.servlet.ServletException; 7 import javax.servlet.http.HttpServlet; 8 import javax.servlet.http.HttpServletRequest; 9 import javax.servlet.http.HttpServletResponse;10 11 public class Index extends HttpServlet {12   protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {13     PrintWriter out = response.getWriter();14     out.println("Index");15   }16 17   protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {18     doGet(request, response);19   }20 }

MyFilter.java文件如下:

 1 package com.net; 2  3 import java.io.IOException; 4 import java.io.PrintWriter; 5  6 import javax.servlet.Filter; 7 import javax.servlet.FilterChain; 8 import javax.servlet.FilterConfig; 9 import javax.servlet.ServletException;10 import javax.servlet.ServletRequest;11 import javax.servlet.ServletResponse;12 13 public class MyFilter implements Filter {14   public void init(FilterConfig fConfig) throws ServletException {15     System.out.println("MyFilter.init()");16   }17 18   public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {19     PrintWriter out = response.getWriter();20     out.println("Hello MyFilter");21     chain.doFilter(request, response);22   }23   24   public void destroy() {25     System.out.println("MyFilter.destroy()");26   }27 }

web.

 1 <filter> 2   <filter-name>MyFilter</filter-name> 3   <filter-class>com.net.MyFilter</filter-class> 4 </filter> 5 <filter-mapping> 6   <filter-name>MyFilter</filter-name> 7   <url-pattern>/Index</url-pattern> 8 </filter-mapping> 9 10 11 <servlet>12   <servlet-name>Index</servlet-name>13   <servlet-class>com.net.Index</servlet-class>14 </servlet>15 <servlet-mapping>16   <servlet-name>Index</servlet-name>17   <url-pattern>/Index</url-pattern>18 </servlet-mapping>

测试结果如下:

 

2、Filter链

Filter链示例图如下:

测试程序如下,Index.java文件和MyFilter.java文件参考实现第一个Filter程序中代码,MyFilter2.java文件和web.

MyFilter2.java文件如下:

 1 package com.net; 2  3 import java.io.IOException; 4 import java.io.PrintWriter; 5  6 import javax.servlet.Filter; 7 import javax.servlet.FilterChain; 8 import javax.servlet.FilterConfig; 9 import javax.servlet.ServletException;10 import javax.servlet.ServletRequest;11 import javax.servlet.ServletResponse;12 13 public class MyFilter2 implements Filter {14   public void init(FilterConfig fConfig) throws ServletException {15   }16 17   public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {18     PrintWriter out = response.getWriter();19     out.println("Hello MyFilter2 Start");20     chain.doFilter(request, response);21     out.println("Hello MyFilter2 End");22   }23   24   public void destroy() {25   }26 }

web.

<filter>  <filter-name>MyFilter</filter-name>  <filter-class>com.net.MyFilter</filter-class></filter><filter-mapping>  <filter-name>MyFilter</filter-name>  <url-pattern>/Index</url-pattern></filter-mapping><filter>  <filter-name>MyFilter2</filter-name>  <filter-class>com.net.MyFilter2</filter-class></filter><filter-mapping>  <filter-name>MyFilter2</filter-name>  <url-pattern>/*</url-pattern></filter-mapping><servlet>  <servlet-name>Index</servlet-name>  <servlet-class>com.net.Index</servlet-class></servlet><servlet-mapping>  <servlet-name>Index</servlet-name>  <url-pattern>/Index</url-pattern></servlet-mapping>

测试结果如下:

注意:Filter之间的调用顺序什么样的呢?这与他们在web.

请求->MyFilter -> MyFilter2 -> Index(servlet)

响应 <- MyFilter <- MyFilter2 <- Index(servlet)

 

3、Filter的生命周期

Filter的创建和销毁由web服务器负责。 web 应用程序启动时,web 服务器将创建Filter 的实例对象,并调用其init方法,完成对象的初始化功能,从而为后续的用户请求作好拦截的准备工作,filter对象只会创建一次,init方法也只会执行一次。通过init方法的参数,可获得代表当前filter配置信息的FilterConfig对象。FilterConfig接口封装了Filter程序在web.

MyFilter.java文件如下

package com.net;import java.io.IOException;import java.io.PrintWriter;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 MyFilter implements Filter {  public void init(FilterConfig fConfig) throws ServletException {    System.out.println("MyFilter.init()");        //得到过滤器的名字    String filterName = fConfig.getFilterName();    //得到在web.    String initParam = fConfig.getInitParameter("like");        System.out.println(filterName);    System.out.println(initParam);  }  public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {    PrintWriter out = response.getWriter();    out.println("Hello MyFilter");    chain.doFilter(request, response);  }    public void destroy() {    System.out.println("MyFilter.destroy()");  }}

MyFilter.java对应的web.

<filter>  <filter-name>MyFilter</filter-name>  <filter-class>com.net.MyFilter</filter-class>  <init-param>    <param-name>like</param-name>    <param-value>java</param-value>  </init-param></filter><filter-mapping>  <filter-name>MyFilter</filter-name>  <url-pattern>/Index</url-pattern></filter-mapping>