JavaWeb学习(二)

Servlet

1、Servlet简介

• Servlet就是sun公司开发动态web的一门技术
• sun在这些API中提供一个接口叫做：Servlet，如果你想开发一个Servlet程序，只需要完成两个小步骤：
  • 编写一个类，实现Servlet接口，
  • 把开发好的Java类部署到web服务器中；

把实现了Servlet接口的Java程序叫做：Servlet

2、HelloServlet

• 新建一个普通的maven项目，删掉里面的src目录，在这个项目里面建立Module；这个空的工程就是Maven主工程
• 关于Maven父子工程的理解：

父项目会有：

    <modules>
        <module>servlet-01</module>
    </modules>

子项目中会有：

    <parent>
        <artifactId>javaweb-02-servlet</artifactId>
        <groupId>com.star</groupId>
        <version>1.0-SNAPSHOT</version>
    </parent>

父项目中的Java子项目可以直接使用

• Maven环境优化

  • 修改web.xml为最新的
  • 将Maven的结构搭建完成(添加Java，Resources并且标记)

• 编写一个Servlet程序
  

  • 编写一个普通类，在Java中创建包，再新建java class;
  • 实现Servlet接口，这里我们直接继承HttpServlet类;

• 因为我们没有这个servlet-api这个jar包，所以需要手动导入

  • 直接在pom.xml的dependencies标签中添加代码，会在将jar包下载到本地仓库，就可以使用了

  <!-- https://mvnrepository.com/artifact/javax.servlet/javax.servlet-api -->
      <dependency>
          <groupId>javax.servlet</groupId>
          <artifactId>javax.servlet-api</artifactId>
          <version>4.0.1</version>
          <scope>provided</scope>
      </dependency>
  

  • 或者进入Project Structure(ctrl+shift+alt+s直接进入)，点击 + 号——>java，找到Tomcat中lib文件夹下的Javax.servlet.api.jar,点击OK就可以了。

  public class HelloServlet extends HttpServlet {
  @Override
  protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
      //ServletOutputStream outputStream = resp.getOutputStream();
      //ServletInputStream inputStream = req.getInputStream();
      PrintWriter writer = resp.getWriter();
      writer.println("hello,Servlet");
      
  }
  
  @Override
  protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
      doGet(req, resp);
  }
  

}

```

• 编写Servlet的映射

为什么需要映射：我们写的是JAVA程序，但是要通过浏览器访问，而浏览器需要连接web服务器，所以我们需要再web服务中注册我们写的Servlet，还需给他一个浏览器能够访问的路径；

<!--注册Servlet-->
    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>com.kuang.servlet.HelloServlet</servlet-class>
    </servlet>
    <!--Servlet的请求路径-->
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>

• 配置Tomcat(之前已说)

注意：配置项目发布的路径就可以了

• 启动测试，OK!
  

3、Servlet原理

Servlet是由web服务器调用，web服务器再收到浏览器请求之后，会：

4、Mapping问题

• 一个Servlet可以指定一个映射路径

    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/star</url-pattern>
    </servlet-mapping>

• 一个Servlet可以指定多个映射路径

    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/star2</url-pattern>
    </servlet-mapping>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/star3</url-pattern>
    </servlet-mapping>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/star4</url-pattern>
    </servlet-mapping>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/star5</url-pattern>
    </servlet-mapping>

• 一个Servlet可以指定通用映射路径

    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/star/*</url-pattern>
    </servlet-mapping>

• 默认请求路径

    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/*</url-pattern>
    </servlet-mapping>

• 指定一些后缀或者前缀等等...

    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>*.do</url-pattern>
    </servlet-mapping>

注意：‘*’前面不能加映射的路径

• 指定优先级问题

指定了固定的映射路径优先级最高，如果找不到就走默认的处理要求；

    <!--404-->
    <servlet>
        <servlet-name>error</servlet-name>
        <servlet-class>com.star.servlet.ErrorServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>error</servlet-name>
        <url-pattern>/*</url-pattern>
    </servlet-mapping>

5、ServletContext

web容器再启动的时候，它会为每个web程序都创建一个对应的ServletContext对象，它代表的了当前的web应用；

• 共享数据

我在这个Servlet中的保存的数据，可以在另外一个Servlet中拿到；

    public class HelloServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //this.getInitParameter()   初始化参数
        //this.getServletConfig()   Servlet配置
        //this.getServletContext    Servlet上下文
        ServletContext context = this.getServletContext();

        String username="张三";//数据
        context.setAttribute("username",username);


        System.out.println("Hello");
    }
}

    public class GetServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();

        String username = (String) context.getAttribute("username");

        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        PrintWriter writer = resp.getWriter();
        writer.print("名字："+username);
    }

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

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

    <servlet>
        <servlet-name>getc</servlet-name>
        <servlet-class>com.star.servlet.GetServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>getc</servlet-name>
        <url-pattern>/getc</url-pattern>
    </servlet-mapping>

测试访问结果：

• 获取初始化参数

    <!--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>

public class ServletDemo03 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();

        //获取初始化参数
        String url = context.getInitParameter("url");
        resp.getWriter().print(url);

    }

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

    <servlet>
        <servlet-name>gp</servlet-name>
        <servlet-class>com.star.servlet.ServletDemo03</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>gp</servlet-name>
        <url-pattern>/gp</url-pattern>
    </servlet-mapping>

• 请求转发

public class ServletDemo04 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        System.out.println("进入了ServletDemo04");

        RequestDispatcher requestDispatcher = context.getRequestDispatcher("/gp");//获取请求转发的路径
        requestDispatcher.forward(req,resp);//调用forward实现请求转发
        //context.getRequestDispatcher("/gp").forward(req,resp);

    }

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

    <servlet>
        <servlet-name>sd4</servlet-name>
        <servlet-class>com.star.servlet.ServletDemo04</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>sd4</servlet-name>
        <url-pattern>/sd4</url-pattern>
    </servlet-mapping>

• 读取资源文件

Peoperties

• 在Java目录下新建Properties
• 在resources目录下新建Properties

发现：都被打包到了同一个路径下：class，我们称这个路径为classpath；

思路：需要一个文件流

username=root
password=123456

public class ServletDemo05 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");

        Properties properties = new Properties();
        properties.load(is);
        String username = properties.getProperty("username");
        String password = properties.getProperty("password");

        resp.getWriter().print(username+":"+password);

    }

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

    <servlet>
        <servlet-name>sd5</servlet-name>
        <servlet-class>com.star.servlet.ServletDemo05</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>sd5</servlet-name>
        <url-pattern>/sd5</url-pattern>
    </servlet-mapping>

访问测试结果

6、HttpServletResponse

web服务器接收到客户端的http请求，针对这个请求，分别创建一个代表请求的HttpServletRequest对象，代表响应的一个HttpServletResponse对象；

• 如果要获取客户端请求过来的参数，找HttpServletRequest
• 如果要获取客户端响应一些信息，找HttpServletResponse

1、简单分类

负责向浏览器发送数据的方法

    ServletOutputStream getOutputStream() throws IOException;
    PrintWriter getWriter() throws IOException;

负责向浏览器发送响应头的方法

    void setCharacterEncoding(String var1);

    void setContentLength(int var1);

    void setContentLengthLong(long var1);

    void setContentType(String var1);

    void setDateHeader(String var1, long var2);

    void addDateHeader(String var1, long var2);

    void setHeader(String var1, String var2);

    void addHeader(String var1, String var2);

    void setIntHeader(String var1, int var2);

    void addIntHeader(String var1, int var2);

响应的状态码

    int SC_CONTINUE = 100;
    int SC_SWITCHING_PROTOCOLS = 101;
    int SC_OK = 200;
    int SC_CREATED = 201;
    int SC_ACCEPTED = 202;
    int SC_NON_AUTHORITATIVE_INFORMATION = 203;
    int SC_NO_CONTENT = 204;
    int SC_RESET_CONTENT = 205;
    int SC_PARTIAL_CONTENT = 206;
    int SC_MULTIPLE_CHOICES = 300;
    int SC_MOVED_PERMANENTLY = 301;
    int SC_MOVED_TEMPORARILY = 302;
    int SC_FOUND = 302;
    int SC_SEE_OTHER = 303;
    int SC_NOT_MODIFIED = 304;
    int SC_USE_PROXY = 305;
    int SC_TEMPORARY_REDIRECT = 307;
    int SC_BAD_REQUEST = 400;
    int SC_UNAUTHORIZED = 401;
    int SC_PAYMENT_REQUIRED = 402;
    int SC_FORBIDDEN = 403;
    int SC_NOT_FOUND = 404;
    int SC_METHOD_NOT_ALLOWED = 405;
    int SC_NOT_ACCEPTABLE = 406;
    int SC_PROXY_AUTHENTICATION_REQUIRED = 407;
    int SC_REQUEST_TIMEOUT = 408;
    int SC_CONFLICT = 409;
    int SC_GONE = 410;
    int SC_LENGTH_REQUIRED = 411;
    int SC_PRECONDITION_FAILED = 412;
    int SC_REQUEST_ENTITY_TOO_LARGE = 413;
    int SC_REQUEST_URI_TOO_LONG = 414;
    int SC_UNSUPPORTED_MEDIA_TYPE = 415;
    int SC_REQUESTED_RANGE_NOT_SATISFIABLE = 416;
    int SC_EXPECTATION_FAILED = 417;
    int SC_INTERNAL_SERVER_ERROR = 500;
    int SC_NOT_IMPLEMENTED = 501;
    int SC_BAD_GATEWAY = 502;
    int SC_SERVICE_UNAVAILABLE = 503;
    int SC_GATEWAY_TIMEOUT = 504;
    int SC_HTTP_VERSION_NOT_SUPPORTED = 505;

2、常见应用

• 下载文件

  1. 要获取下载文件的路径
  2. 下载的文件名
  3. 设置想办法让浏览器能够支持下载我们需要的东西
  4. 获取下载文件的输入流
  5. 创建缓冲区
  6. 获取OutputStream对象
  7. 将FileOutputStream流写入到buffer缓冲区,使用Outputstream将缓冲区中的数据输出到客户端！

public class FileServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//        1. 要获取下载文件的路径
        String realPath = "E:\IdeaProjects\javaweb-02-servlet\response\src\main\resources\1.png";
        System.out.println("下载文件的路径："+realPath);
//        2. 下载的文件名
        String filename = realPath.substring(realPath.lastIndexOf("\") + 1);
        System.out.println("文件名:"+ filename);
//        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="+filename);
//        4. 获取下载文件的输入流
        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流写入到buffer缓冲区,使用Outputstream将缓冲区中的数据输出到客户端！
        while ((len=in.read(buffer))>0){
            out.write(buffer,0,len);
        }

        in.close();
        out.close();
    }

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

  <servlet>
    <servlet-name>response</servlet-name>
    <servlet-class>com.star.servlet.FileServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>response</servlet-name>
    <url-pattern>/down</url-pattern>
  </servlet-mapping>

测试访问结果

3、验证码功能

验证怎么来的？

• 前端实现
• 后端实现，需要用Java的图片类，生产一个图片

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

        //让浏览器3秒刷新一次
        resp.setHeader("refresh","3");

        //在内存中创建一个图片
        BufferedImage image = new BufferedImage(80, 20, BufferedImage.TYPE_INT_RGB);
        //得到图片
        Graphics2D g = (Graphics2D) image.getGraphics();//笔
        //设置图片的背景颜色
        g.setColor(Color.black);
        g.fillRect(0,0,80,20);
        //给图片写数据
        g.setColor(Color.RED);//换颜色
        g.setFont(new Font(null,Font.BOLD,20));//字体
        g.drawString(makenum(),0,20);//画字符串

        //告诉浏览器，这个请求用图片的方式打开
        resp.setContentType("image/jpeg");
        //网站存在缓存，不让浏览器缓存
        resp.setDateHeader("expires",-1);
        resp.setHeader("Cache-Control","no-cache");
        resp.setHeader("Pragma","no-cache");

        //把图片写给浏览器
        ImageIO.write(image,"jpg",resp.getOutputStream());

    }

    //生成随机数,一个七位数
    private String makenum(){
        Random random = new Random();
        String num = random.nextInt(9999999)+"";
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i < 7-num.length(); i++) {
            sb.append("0");
        }
        num = sb.toString() + num;
        return num;
    }

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

  <servlet>
    <servlet-name>imageServlet</servlet-name>
    <servlet-class>com.star.servlet.ImageServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>imageServlet</servlet-name>
    <url-pattern>/imageServlet</url-pattern>
  </servlet-mapping>

测试结果

-实现重定向

B，一个web资源收到客户端A请求后，B他会通知A去访问另外一个web资源C，这个过程就叫做重定向
常见场景：

• 用户登录

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

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

  <servlet>
    <servlet-name>RedirectServlet</servlet-name>
    <servlet-class>com.star.servlet.RedirectServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>RedirectServlet</servlet-name>
    <url-pattern>/rs</url-pattern>
  </servlet-mapping>

测试访问结果

请求转发和重定向的区别
相同点

• 页面都会实现跳转

不同点

• 请求转发时url不会发生变化 307

• 重定向时url会跳转 302;
  

• 简单实现登录重定向

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

        //处理请求
        String username = req.getParameter("username");
        String password = req.getParameter("password");

        System.out.println(username+":"+password);

        resp.sendRedirect("/s/success.jsp");
        req.setCharacterEncoding("utf-8");
        resp.setCharacterEncoding("utf-8");
    }

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

<html>
<body>
<h2>Hello World!</h2>

<%--这里提交的路径，需要寻找到项目的路径--%>
<%--${pageContext.request.ContextPath}--%>
<form action="${pageContext.request.contextPath}/login" method="get">
    用户名：<input type="text" name="username"><br>
    密码：<input type="password" name="password"><br>
    <input type="submit">
</form>

</body>
</html>

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>

<h1>Success</h1>

</body>
</html>

测试访问结果

7、HttpServletRequest

HttpServlet代表客户端的请求，用户通过Http协议访问服务器，HTTP请求中的所用信息会被封装带HttpServletRequest，通过这个HttpServlet的方法，获得客户端的所有信息

获得参数，请求转发

public class LoginServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        String[] hobbys = req.getParameterValues("hobbys");
        System.out.println("===================================");
        System.out.println(username);
        System.out.println(password);
        System.out.println(Arrays.toString(hobbys));
        System.out.println("===================================");

        //请求转发
        //这里的/代表当前的web应用
        req.getRequestDispatcher("/success.jsp").forward(req,resp);

        resp.setCharacterEncoding("utf-8");
    }

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

    <servlet>
        <servlet-name>LoginServlet</servlet-name>
        <servlet-class>com.star.servlet.LoginServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>LoginServlet</servlet-name>
        <url-pattern>/login</url-pattern>
    </servlet-mapping>

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>登录</title>
</head>
<body>

<h1>登录</h1>

<%--居中--%>
<div style="text-align: center">
    <%--这里表单表示的意思：以post方式提交表单，提交到我们的login请求--%>
    <form action="${pageContext.request.contextPath}/login" method="post">
        用户名：<input type="text" name="username"><br>
        密码：<input type="password" name="password"><br>
        爱好：
        <input type="checkbox" name="hobbys" value="女孩">女孩
        <input type="checkbox" name="hobbys" value="代码">代码
        <input type="checkbox" name="hobbys" value="电影">电影
        <input type="checkbox" name="hobbys" value="音乐">音乐

        <br>
        <input type="submit">
    </form>
</div>

</body>
</html>

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>

<h1>Success</h1>

</body>
</html>

测试结果访问