什么是过滤器？Filter过滤器有什么作用？

(一)什么是过滤器？有什么作用？

1.说明：浏览器访问服务器目标资源时，过滤器实现拦截，如果过滤器判断的是合理请求，那么就允许访问目标资源。

(二)使用MyFilter过滤器拦截MyServlet程序

1.MyServlet程序

package com.Test;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class MyServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html;charset=utf-8");      
        response.getWriter().print("没有Filter程序拦截，我可以在浏览器中显示，哈哈....");
    }


    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        doGet(request, response);
    }

}

2.请求MyServlet页面时，使用Filter程序拦截

package com.Test;

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 MyFilter implements Filter {

    public void init(FilterConfig fConfig) throws ServletException {
        // 1.在初始化是调用过滤器对象
    }

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {

        // 2.当拦截到用户请求时，此方法被调用
         response.setContentType("text/html;charset=utf-8");  
        response.getWriter().print("你请求的页面已经被我拦截了，哈哈....");
    }

    public void destroy() {
        // 3.销毁过滤器后，自动调用此方法，从而释放资源
    }

}

3.配置文件

  
    
  
  
    
    MyServlet
    MyServlet
    com.Test.MyServlet
  
  
    MyServlet
    /MyServlet
  

  
  
    MyFilter
    MyFilter
    com.Test.MyFilter
  
  
    MyFilter
    
    /MyServlet
  

浏览器地址栏输入：https://localhost/Project/MyServlet 结果：

(三)Filter映射

1.使用通配符“*”拦截用户的所有请求

    MyFilter
    MyFilter
    com.Test.MyFilter
  
  
    MyFilter
    
    /*
  

2.拦截不同方式的请求

在web.xml文件中，元素中的一个特殊子元素,该元素指定过滤器拦截的资源被Servlet容器调用的方式。元素的4值分别为：

1.REQUEST:如果目标资源通过RequestDispatcher的include()或forwrad()方法访问，过滤器是不会被调用。

2.INCLUDE:如果目标资源通过RequestDispatcher的include()方法访问，过滤器被调用。

3.FORWARD:如果目标资源通过RequestDispatcher的forward()方法访问,过滤器被调用。

4.ERROR:如果目标资源通过声明式异常处理机制调用的，那么该过滤器被调用。

实例演示：

1.IncludeServlet程序

package com.Test;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class IncludeServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1.使用include方法跳转页面
        response.setContentType("text/html;charset=utf-8"); 
        request.getRequestDispatcher("Test01.jsp").include(request, response);
    }


    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        doGet(request, response);
    }

}

2.IncludeFilter程序实施拦截

package com.Test;
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 IncludeFilter implements Filter {

    public void destroy() {

    }

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        //response.setContentType("text/html;charset=utf-8");  
         response.getWriter().print("IncludeServlet使用include()方法跳转的Test01.jsp，已经被我拦截，不要在请求，没用的啦");
    }

    public void init(FilterConfig fConfig) throws ServletException {

    }

}

3.Test01.jsp页面

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>

4.配置文件

  
    
  
  
    
    IncludeServlet
    IncludeServlet
    com.Test.IncludeServlet
  
  
    IncludeServlet
    /IncludeServlet
  
  
    IncludeFilter
    IncludeFilter
    com.Test.IncludeFilter
  
  

    IncludeFilter
    /Test01.jsp
    
    INCLUDE
  

输出结果：