Filter

Filter：过滤器，用来过滤器网站的数据

作用：处理中文乱码

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Filter开发步骤

　　1.导包

    <groupId>com.king</groupId>
    <artifactId>javaweb-filter</artifactId>
    <version>1.0-SNAPSHOT</version>

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

        <dependency>
            <groupId>javax.servlet.jsp</groupId>
            <artifactId>javax.servlet.jsp-api</artifactId>
            <version>2.3.3</version>
        </dependency>

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

        <!--standard标签库-->
        <dependency>
            <groupId>taglibs</groupId>
            <artifactId>standard</artifactId>
            <version>1.1.2</version>
        </dependency>

        <!--连接数据-->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.15</version>
        </dependency>

    </dependencies>

　　2.编写过滤器

package com.king.filter;

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

public class CharacterEncodingFilter implements Filter {

   //初始化，web服务器启动，就已经初始化了，随时等待过滤对象启动
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("CharacterEncodingFilter初始化");
    }

    //chain 链
    /*
    * 过滤中的所有代码，在过滤特定请求的时候都会执行
    * 必须要让过滤器继续执行，chain.doFilter(request,response);
    * */
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        request.setCharacterEncoding("utf-8");
        response.setCharacterEncoding("utf-8");
        response.setContentType("text/html;charset=utf-8");

        System.out.println("filter过滤器执行前");
        chain.doFilter(request,response);//让请求继续走，如果不写，程序到这里就被拦截停止
        System.out.println("filter过滤器执行后");
    }

    //销毁，web服务器关闭时候，过滤器销毁
    public void destroy() {
        System.out.println("CharacterEncodingFilter销毁");

    }
}

3.测试类

package com.king.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 {
        //resp.setCharacterEncoding("utf-8");
        resp.getWriter().write("你好，java");
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

4.web配置

  <servlet>
        <servlet-name>ShowServlet</servlet-name>
        <servlet-class>com.king.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.king.filter.CharacterEncodingFilter</filter-class>
    </filter>

    <filter-mapping>
        <filter-name>CharacterEncodingFilter</filter-name>
        <!--只要是/servlet的任何请求，都会经过这个过滤器-->
        <url-pattern>/servlet/*</url-pattern>
    </filter-mapping>

总结：在设置完过滤器具体功能后，要chain.daofilter之后过滤器才起作用，

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