五、ServletContext

web容器在启动的时候，它会为每一个web程序都创建一个对应的ServletContext对象，它代表了当前的web应用。

1.共享数据

在一个Servlet中保存的数据，可以在另一个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 = "英键"; //数据
        context.setAttribute("username",username); //将一个数据保存在了ServletContext中，名字为：username。值:username
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

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");
        resp.getWriter().print("名字为:"+username);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

web.xml：

<servlet>
    <servlet-name>hello</servlet-name>
    <servlet-class>com.yingjian.servlet.HelloServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>hello</servlet-name>
    <url-pattern>/hello</url-pattern>
</servlet-mapping>
<servlet>
    <servlet-name>getcontext</servlet-name>
    <servlet-class>com.yingjian.servlet.GetServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>getcontext</servlet-name>
    <url-pattern>/getcontext</url-pattern>
</servlet-mapping>

2.获取初始化参数

web.xml：

<context-param>
    <param-name>url</param-name>
    <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
</context-param>

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String url = context.getInitParameter("url");
        resp.getWriter().print(url);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

3.请求转发

request and dispatcher

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");//转发的请求路径
        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);
    }
}

4.读取资源文件

思路：需要一个文件流

public class ServletDemo05 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/abc1.properties");
        InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/com/yingjian/servlet/abc.properties");
        Properties prop = new Properties();
        prop.load(is);
        String user = prop.getProperty("username");
        String pwd = prop.getProperty("password");
        resp.getWriter().print(user + " " + pwd);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

在java目录下新建abc.properties，在resources目录下新建abc1.properties

发现：都被打包到了同一个路径下：classes，俗称classpath（类路径）

六、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，一个代表响应的HttpServletResponse。

如果要获取客户端请求过来的参数：找HttpServletRequest

如果要给客户端响应一些信息：找HttpServletResponse

1.状态码与方法

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;

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);

2.常见应用

（1）向浏览器输出消息

（2）下载文件

步骤：

要获取下载文件的路径

设置下载的文件名

设置想办法让浏览器能够支持下载我们需要的东西

获取下载文件的输入流

创建缓冲区

获取OutputStream

将FileOutputStream流写入到buffer缓冲区

使用OutputStream讲缓冲区中的数据输出到客户端

代码示例：

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:\\IntelliJ IDEA2020.3.2\\IdeaProjects\\JavaWeb\\javaweb-02-servlet\\response\\target\\classes\\练字.png";
        System.out.println("下载文件的路径：" + realPath);
        //2. 下载的文件名是啥？
        String fileName = realPath.substring(realPath.lastIndexOf("\\") + 1);
        //3. 设置想办法让浏览器能够支持(Content-Disposition)下载我们需要的东西,中文文件名URLEncoder.encode编码，否则有可能乱码
        resp.setHeader("Content-Disposition","attachment;filename="+URLEncoder.encode(fileName,"UTF-8"));
        //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
        ServletOutputStream out = resp.getOutputStream();
        //7. 讲FileOutputStream流写入到buffer缓冲区
        while ((len=in.read(buffer))>0){
            out.write(buffer,0,len);
        }
        in.close();
        out.close();
        //8. 使用OutputStream讲缓冲区中的数据输出到客户端！
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

3.验证码功能

后端实现，需要用到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 {
        //如何让浏览器3秒自动刷新一次；
        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.white);
        g.fillRect(0,0,80,20);
        //给图片写数据
        g.setColor(Color.BLUE);
        g.setFont(new Font(null,Font.BOLD,20));
        g.drawString(makeNum(),0,20);
        //告诉浏览器，这个请求用图片的方式打开
        resp.setContentType("image/png");
        //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("Program","no-cache");
        //把图片写个浏览器
        ImageIO.write(image,"jpg",resp.getOutputStream());
    }
    //生产随机数
    private String makeNum(){
        Random random = new Random();
        String num = random.nextInt(8888888) + "";
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i < 7-num.length(); i++) {
            sb.append("0");
        }
        String s = sb.toString() + num;
        return num;
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

4.实现重定向

一个web资源收到客户端请求后，它会通知客户端去访问另外一个web资源，这个过程叫重定向。
常见场景：用户登录

void sendRedirect(String var1) throws IOException;

代码示例：

public class RedirectServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        /*
        resp.setHeader("Location","/response/red");
        resp.setStatus(302);
        */
        //重定向时候一定要注意路径问题，否则404
        resp.sendRedirect("/response/img");
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

5.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 {
        System.out.println("进入这个请求了");
        //处理请求
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        System.out.println(username+""+password);
        //重定向时候一定要注意路径问题，否则404
        resp.sendRedirect("/response/success.jsp");
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

登录页面：

<%@ page contentType="text/html; charset=UTF-8" language="java" %>
<html>
<body>
<h2>Hello World!</h2>
<%--这里提交的路径，需要找到项目的路径--%>
<%--${pageContext.request.contextPath}代表当前的项目--%>
<form action="${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>

登录成功页面：

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<h1>Success!</h1>
</body>
</html>

七、HttpServletRequest

HttpServletRequest代表客户端的请求，用户通过Http协议访问服务器，Http请求中的所有信息
步骤：获取参数，请求转发

public class LoginServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doPost(req, resp);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.setCharacterEncoding("utf-8");
        resp.setCharacterEncoding("utf-8");
        /*
        req.getParameter 获取前端传递的参数
        */
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        String[] hobby = req.getParameterValues("hobby");
        System.out.println("===============================");
        System.out.println(username);
        System.out.println(password);
        System.out.println(Arrays.toString(hobby));
        System.out.println("===============================");
        System.out.println(req.getContextPath());
        //通过请求转发    ‘/'代表当前的web应用，这里与response中的sendRedirect写的路径有所不同
        req.getRequestDispatcher("/success.jsp").forward(req,resp);
    }
}

登录页面：

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>登录</title>
</head>
<body>
<h1>登录</h1>
<div style="text-align:left;">
<%--    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="hobby" value="女孩">女孩
        <input type="checkbox" name="hobby" value="代码">代码
        <input type="checkbox" name="hobby" value="唱歌">唱歌
        <input type="checkbox" name="hobby" value="电影">电影
        <br>
        <input type="submit">
    </form>
</div>
</body>
</html>

登录成功页面：

<html>
<head>
    <title>Title</title>
</head>
<body>
<h1>登录成功!</h1>
</body>
</html>

重定向和转发的区别

相同点：页面都会实现跳转
不同点：请求转发的时候，URL不会产生变化；重定向的时候，URL会发生变化

后记

Java全栈学习路线可参考：【Java全栈学习路线】最全的Java学习路线及知识清单，Java自学方向指引，内含最全Java全栈学习技术清单~

【JavaWeb】之Servlet（下）