javaweb(3) Servlet

1.Servlet简介

 

 2.HelloServlet

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

public class HelloServlet extends HttpServlet {

    //由于get和post只是请求实现不同的方式，可以相互调用，业务逻辑都一样
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        PrintWriter writer = resp.getWriter();//响应流
        writer.println("HelloServlet");
    }

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

    }
}

 <servlet>
        <servlet-name>HelloServlet</servlet-name>
        <servlet-class>com.luo.servlet.HelloServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>HelloServlet</servlet-name>
        <url-pattern>/HelloServlet</url-pattern>
    </servlet-mapping>

 3.Servlet原理

　　Servlet是由web服务器调用，web服务器在收到浏览器请求之后，会：

 

 

 4.ServletContext

　　web容器在启动的时候，它会为每个web程序都创建一个对应的ServletContext对象，它代表了当前的web应用。 

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 HelloServlet extends HttpServlet {

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

        ServletContext servletContext = this.getServletContext();
        String username="十三";
        servletContext.setAttribute("username",username);
    }

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

    }
}

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 ContextServlet extends HttpServlet {

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

        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf8");
        ServletContext servletContext = this.getServletContext();
        String username = (String) servletContext.getAttribute("username");
        resp.getWriter().println("姓名："+username);
    }

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

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0"
         metadata-complete="true">

    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>com.luo.servlet.HelloServlet</servlet-class>
    </servlet>
    
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>


    <servlet>
        <servlet-name>hello2</servlet-name>
        <servlet-class>com.luo.servlet.ContextServlet</servlet-class>
    </servlet>

    <servlet-mapping>
    <servlet-name>hello2</servlet-name>
    <url-pattern>/hello2</url-pattern>
    </servlet-mapping>
</web-app>

　　2.获取初始化参数值

  <!--配置一些web应用初始化参数 -->
    <context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
    </context-param>

    resp.setContentType("text/html");
        resp.setCharacterEncoding("utf8");
        ServletContext servletContext = this.getServletContext();
        String url=servletContext.getInitParameter("url");
        resp.getWriter().println(url);

　　3.请求转发

        ServletContext servletContext = this.getServletContext();
        String username="十三";
        servletContext.setAttribute("username",username);
        servletContext.getRequestDispatcher("/hello2").forward(req,resp);  //请求转发

　　4.读取资源文件

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;
import java.io.InputStream;
import java.util.Properties;

public class Servlet02  extends HttpServlet {

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

        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf8");
        ServletContext servletContext = this.getServletContext();
        InputStream resourceAsStream = servletContext.getResourceAsStream("/WEB-INF/classes/db.properties");
        Properties properties = new Properties();
        properties.load(resourceAsStream);
        String username = properties.getProperty("username");
        String password = properties.getProperty("password");
        resp.getWriter().println(username);
        resp.getWriter().println(password);
    }

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

 5.HttpServletResponse

 

 

 　　2.常见应用

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

        //1.要获取下载文件的路径
        String realPath = "D:\\code\\javaweb_02_servlet\\response\\target\\classes\\原图.jpg";
        //2.下载的文件名是什么？
        String fileName = realPath.substring(realPath.lastIndexOf("\\") + 1);
        //3.设置想办法让浏览器能够支持下载我们需要的东西
        resp.setHeader("Content-Disposition","attachment;filename="+ URLEncoder.encode(fileName,"UTF-8"));
        //4.获取下载文件
        FileInputStream in = new FileInputStream(realPath);
        //5.创建缓冲区
        int len=0;
        byte[] buffer=new byte[1024];
        //6.获取OutPutStream对象
        ServletOutputStream out = resp.getOutputStream();
        //7.将FileOutputstream写入到buffer缓冲区,使用OutputStram将缓冲区的数据输出到客户端
        while ((len=in.read(buffer))>0){
            out.write(buffer,0,len);
        }
        in.close();
        out.close();
    }

　　3.实现重定向

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

        resp.setContentType("text/html;charset=utf-8");
        resp.sendRedirect("/image");
    }