Filter被称为过滤器或者**,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>
原标题:Java Web之Filter
关键词:JAVA