JavaWeb21（过滤器Filter）

过滤器的作用

过滤器是用来过滤网站数据的

• 处理中文乱码
• 登陆验证
• 过滤垃圾信息等
  
  所以过滤器也需要request和response及过滤器与Servlet本质上是一样的

Filter开发步骤

1.导入所需的包

    <dependencies>
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>servlet-api</artifactId>
            <version>2.5</version>
        </dependency>

        <dependency>
            <groupId>javax.servlet.jsp</groupId>
            <artifactId>jsp-api</artifactId>
            <version>2.1</version>
        </dependency>

        <dependency>
            <groupId>taglibs</groupId>
            <artifactId>standard</artifactId>
            <version>1.1.2</version>
        </dependency>

        <dependency>
            <groupId>javax.servlet.jsp.jstl</groupId>
            <artifactId>jstl-api</artifactId>
            <version>1.2</version>
        </dependency>
    </dependencies>

2.编写过滤器（实现过滤器接口，重写方法）

CharacterEncodingFilter.java

package com.hao.filter;

import javax.servlet.*;
import java.io.IOException;

public class CharacterEncodingFilter implements Filter {
    
    
    @Override
    //初始化 web服务器启动时，就已经初始化（随时准备过滤）
    public void init(FilterConfig filterConfig) throws ServletException {
    
    
        System.out.println("过滤器已经初始化");
    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
    
    
        servletRequest.setCharacterEncoding("utf-8");
        servletResponse.setCharacterEncoding("utf-8");
        servletResponse.setContentType("text/html");
        //向下传递request,response
        filterChain.doFilter(servletRequest,servletResponse);
    }

    //销毁 Web服务器关闭的时候，过滤器会销毁
    @Override
    public void destroy() {
    
    
        System.out.println("过滤器已经销毁");
    }
}

showServlet.java

package com.servlet;

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

public class showServlet extends HttpServlet {
    
    
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        doPost(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        resp.getWriter().write("援耗");
    }
}

编写映射

<servlet>
        <servlet-name>showServlet</servlet-name>
        <servlet-class>com.servlet.showServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>showServlet</servlet-name>
        <url-pattern>/servlet/show</url-pattern>
    </servlet-mapping>

    <servlet-mapping>
        <servlet-name>showServlet</servlet-name>
        <url-pattern>/show</url-pattern>
    </servlet-mapping>

    <filter>
        <filter-name>CharacterEncodingFilter</filter-name>
        <filter-class>com.hao.filter.CharacterEncodingFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>CharacterEncodingFilter</filter-name>
<!--servlet下所有请求都会经过过滤器-->
        <url-pattern>/servlet/*</url-pattern>
    </filter-mapping>

启动服务器时发现

过滤器已经初始化紧跟在 Connected to server 说明过滤器在服务器启动时就已经初始化了

对比观察 /servlet/show 以及/show

说明过滤器会过滤 servlet下的所有请求

关闭服务器时发现过滤器已经销毁在Disconnected form server之前 说明过滤器在关闭服务器时销毁