自启动Servlet

自启动servlet也叫自动实例化servlet

特点

• 该Servlet的实例化过程不依赖于请求，而依赖于容器的启动，当Tomcat启动时就会实例化该Servlet
• 普通Servlet是在浏览器第一次向服务器发出请求时才会被创建

如何配置

通过在web.xml中的<servlet>标签中配置<load-on-startup></load-on-startup>自启动Servlet

    <servlet>
        <servlet-name>autostart</servlet-name>
        <servlet-class>www.servlet.AutoStart</servlet-class>
        <!-- 值的大小决定创建的先后顺序-->
        <load-on-startup>1</load-on-startup>
    </servlet>

 通过自启动Servlet实现文件的读取

1）在servlet标签中添加要获取的文件的路径，并设置该Servlet为自启动

    <servlet>
        <servlet-name>autostart</servlet-name>
        <servlet-class>www.servlet.AutoStart</servlet-class>
        <!--添加图片的路径-->
        <init-param>
            <param-name>path</param-name>
            <param-value>image</param-value>
        </init-param>
        <!-- 值的大小决定创建的先后顺序,该标签必须放到servlet标签的最下面-->
        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>autostart</servlet-name>
        <url-pattern>/autostart.do</url-pattern>
    </servlet-mapping>

2）在自启动Servlet的init（）方法中通过ServletConfig对象获取init-param的name值，并放到全局容器ServletContext中，

这样当Tomcat启动时init方法就会被执行，图片的路径被存放到全局容器中，所有Servlet都能拿到

import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
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 AutoStart extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doPost(req,resp);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

    }

    @Override
    public void init() throws ServletException {
        //ServletConfig只对当前servlet有效
        ServletConfig servletConfig=this.getServletConfig();
        String path=servletConfig.getInitParameter("path");
        //获取全局容器对象
        ServletContext servletContext=this.getServletContext();
        //将获取的path放到全局容器ServletContext中，所有servlet都能获取
        servletContext.setAttribute("path",path);
        System.out.println("AutoStartServlet");
    }
}

3）文件下载的Servlet

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.*;

@WebServlet("/filedown")
public class FileDown extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doPost(req,resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取ServletContext对象,继承自GenericServlet类
        ServletContext servletContext=this.getServletContext();
        //获取存放图片的目录名称
        String path=(String) servletContext.getAttribute("path");
        //将相对路径转换为绝对路径
        String realpath=servletContext.getRealPath(path+"/樱花.jpg");
        System.out.println(realpath);


        //读取要下载的文件
//        File file=new File("D:\\樱花.jpg");
        File file=new File(realpath);
        //用字节输入流存储文件
        InputStream is= new FileInputStream(file);
        //创建字节数组
        byte[] buff=new byte[is.available()];
        is.read(buff);

        //在响应添加附加信息
//        resp.addHeader("Content-Disposition", "attachment; filename="+file.getName());
        //将原来的文件名按照gbk的方式变成字节，再按iso-8859-1编码方式变成新的字符串
        resp.addHeader("Content-Disposition","attachment;filename="+
                new String(file.getName().getBytes("gbk"),"iso-8859-1"));

        //产生响应
        //创建字节输出流
        OutputStream os=resp.getOutputStream();
        //响应到客户端
        os.write(buff);
        //刷新流
        os.flush();
        //关闭流
        os.close();
    }
}