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JavaWeb学习笔记02 简单的开始

编写一个servlet程序分成两步:1.编写普通类;2.编写HttpServlet

简单的引例

1.编写HelloServlet.java

package MyExercise;

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 {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        PrintWriter writer=resp.getWriter();
        writer.write("Hello,World!");
    }
}

作用:在响应回来的页面输出“Hello,World!”

2.在web.xml编写servlet映射

<?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">
    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>MyExercise.HelloServlet</servlet-class>
    </servlet>
<servlet-mapping>
    <servlet-name>hello</servlet-name>
    <url-pattern>/h</url-pattern>
</servlet-mapping>

</web-app>

作用:要实现网页的访问必须有servlet映射,这样能帮助服务器找到对应的实现类

tips:两个servlet-name的内容要完全一致;servlet-class是写的servlet类所在路径;url-pattern是要在浏览器访问的路径

3.配置tomcat加载war包并运行

运行结果:

servlet原理图

说明:

HttpServletRequest继承自 ServletRequest。客户端浏览器发出的请求被封装成为一个HttpServlet Request对象。对象包含了客户端请求信息包括请求的地址,请求的参数,提交的数据,上传的文件客户端的ip甚至客户端操作系统都包含在其内。

HttpServletResponse是ServletResponse的子接口,HttpServletResponse对象的主要功能用于服务器对客户端的请求进行响应。

service方法中形参接收的是HttpServletResponse接口的实例化对象,这个对象中封装了向客户端发送数据,发送响应头,发送状态码的方法。

mapping问题

1.一个servlet类可以指定多个映射路径,即可通过不同的<url-class>都可以访问同一网页,

2.含有通配符*的路径优先级问题:

如果某个url-class是/h1/*,另一个url-class是/h1/hello,

当我们在浏览器输入localhost:8080/h1/hello时候,浏览器会优先访问url-class是/h1/hello对应的servlet类(更具体的路径)

ServletContext对象的介绍和应用

ServletContext对象介绍:

web容器启动时会给每一个web程序都创建一个对应的servletcontext对象,用来管理当前所有的servlet类,作用是可以实现不同servlet类间的数据共享。

ServletContext对象应用:

1.数据共享:

测试在servlet1设置的数据能否被servlet2拿到

编写servlet类(一个set一个get):

package MyExercise;
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 Servlet1 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context=this.getServletContext();
        String name="xiaoming";
        context.setAttribute("username",name);//将数据放入context中,username相当于key,name相当于value
    }
}

package MyExercise;

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.PrintWriter;

public class Servlet2 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context=this.getServletContext();
        String name=(String) context.getAttribute("username");//object向下强制转型
        PrintWriter w=resp.getWriter();
        w.write("成功拿到了username:"+name);
    }
}

配置web.xml:

<?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">
    <servlet>
        <servlet-name>s1</servlet-name>
        <servlet-class>MyExercise.Servlet1</servlet-class>
    </servlet>
<servlet-mapping>
    <servlet-name>s1</servlet-name>
    <url-pattern>/s1</url-pattern>
</servlet-mapping>
    <servlet>
        <servlet-name>s2</servlet-name>
        <servlet-class>MyExercise.Servlet2</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>s2</servlet-name>
        <url-pattern>/s2</url-pattern>
    </servlet-mapping>

</web-app>

运行结果:

2.获取web.xml中配置的参数

web.xml中设置参数和路径映射:

<web-app>
    <context-param>
        <param-name>url</param-name>
        <param-value>复习一遍:jdbc:mysql://localhost:3306/mydb</param-value>
    </context-param>

    <servlet>
        <servlet-name>gp</servlet-name>
        <servlet-class>MyExercise.getParam</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>gp</servlet-name>
        <url-pattern>/gp</url-pattern>
    </servlet-mapping>
</web-app>

编写servlet类:

package MyExercise;

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.PrintWriter;
public class getParam extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        ServletContext context=this.getServletContext();
        String url=context.getInitParameter("url");
        PrintWriter w=resp.getWriter();
        w.write("成功获取param:"+url);
    }
}

启动tomcat获得运行结果:

3.读取生成的target文件下的Properties的内容

首先在db.properties中设置内容:

观察发现启动tomcat后其存在target->WEB-INF->classes下:

编写servlet类:

思路是需要一个文件流

public class getProperty extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context=this.getServletContext();
        InputStream is=context.getResourceAsStream("/WEB-INF/classes/db.properties");
        Properties prop=new Properties();
        prop.load(is);//下载
        String username=prop.getProperty("username");
        String pwd=prop.getProperty("password");
        PrintWriter w=resp.getWriter();
        w.write("user:"+username);
        w.write("pwd:"+pwd);

    }
}

配置web.xml并启动tomcat得到运行结果:

4.请求转发

public class Test_RequestDispatcher extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context=this.getServletContext();
        RequestDispatcher requestDispatcher=context.getRequestDispatcher("/gp");//重定向到映射路径为/gp的页面
        requestDispatcher.forward(req,resp);
    }
}

运行结果:

请求转发和重定向的区别

相同点:

都会跳转页面

不同点:

请求转发路径url不会变化

重定向url改变

重定向实例:

编写一个表单Login.jsp,我们要实现的是在请求servlet类的映射路径时跳转到此表单

<%--
  Created by IntelliJ IDEA.
  User: joyyiyi
  Date: 2023/4/5
  Time: 15:20
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<%-- 注意这里的action要和web.xml注册的保持一致-%>
<%----%>
<h1>登录页面</h1>
<form action="/lin" method="get">
    <input type="text" name="username">
    <input type="submit">
</form>

</body>
</html>

编写servlet类

public class Test_sendRedirect extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.sendRedirect("/Login.jsp");
    }
}

这里的斜杠‘/’是重点,表示当前路径,在tomcat设置了怎样的当前路径在重定向的时候也要加上

运行结果:

我输入的路径是:http://localhost:8088/sr

然后重定向跳转到页面:http://localhost:8088/Login.jsp

扩展:

通过response下载图片:


public class FileServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //1.要获取下载图片的路径
        //2.下载的文件名
        //3.让浏览器能够支持我们下载的一些东西
        //4.获取下载图片的输入流
        //5.放入缓冲区
        //6.获取outputstream对象
        //7.将fileoutputstream流写入到buffer缓冲区
        //8.关闭资源
        String path="D:\\apache_maven_projects\\javaweb-filter\\src\\main\\resources\\img.png";//这是图片的绝对路径
        String filename=path.substring(path.lastIndexOf("//")+1);
        resp.setHeader("Content-Disposition","attachment;filename="+ URLEncoder.encode(filename,"utf-8"));
        FileInputStream fis=new FileInputStream(path);
        int len=0;
        byte[] buffer=new byte[1024];
        //这里和 PrintWriter w=resp.getWriter();异曲同工
        ServletOutputStream out=resp.getOutputStream();
        while ((len=fis.read(buffer))>0){
            out.write(buffer,0,len);
        }

        fis.close();
        out.close();

    }
}

记得配置web.xml

运行结果:只有输入网页路径就自动下载请求的图片

今天记录到这,玩去了!!!!

标签: 学习 servlet java

本文转载自: https://blog.csdn.net/m0_67042480/article/details/129973384
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