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:
package com.net;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class Index extends HttpServlet {
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
PrintWriter out = response.getWriter();
out.println("Index");
}
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
doGet(request, response);
}
}
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()");
}
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()");
}
}
web.xml配置文件添加如下代码:
<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>
<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: