三十、Servlet补充

1、Servlet原理

Servlet是由Web服务器调用的，web服务器在收到浏览器请求之后，

2、Mapping问题

1. 一个Servlet可以指定一个映射路径

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

1. 一个Servlet可以指定多个映射路径

   <!--通过映射可以把多个不同的后缀映射到一个实现类上-->
   <servlet-mapping>
       <servlet-name>hello</servlet-name>
       <url-pattern>/hello2</url-pattern>
   </servlet-mapping>
   <servlet-mapping>
       <servlet-name>hello</servlet-name>
       <url-pattern>/hello3</url-pattern>
   </servlet-mapping>

1. 一个Servlet可以指定通用映射路径

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

1. 默认请求路径（不推荐使用）

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

1. 指定一些后缀或者前缀等等…

   <!--可以自定义后缀实现请求映射
       注意点，*前面不能加项目映射的路径
       /hello/*.shuai  (报错)
       -->
   <servlet-mapping>
       <servlet-name>hello</servlet-name>
       <url-pattern>*.shuai</url-pattern>
   </servlet-mapping>

1. 优先级问题
   指定了固有的映射路径优先级最高，如果找不到就会走默认的处理请求

   <!--注册404页面-->
   <servlet>
       <servlet-name>error</servlet-name>
       <servlet-class>com.shuai.Servlet.ErrorServlet</servlet-class>
   </servlet>
   <servlet-mapping>
       <servlet-name>error</servlet-name>
       <url-pattern>/*</url-pattern>
   </servlet-mapping>

3、ServletContext

web容器在启动的时候，它会为每个web程序都创建一对应的ServletContext对象，他代表了当前的web应用；

1、共享数据

我在这个Servlet中保存的数据，可以在另外一个Servlet中拿到；
存数据：

@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 = "无敌最俊朗";
        context.setAttribute("username",username);  // 将一个数据存在了ServletContext中，名字为：username,值为无敌最俊朗
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req,resp);
    }

获取数据：

 @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String username = (String) context.getAttribute("username");
        // 设置字符编码避免乱码
        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        resp.getWriter().print("名字："+ username);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
    }

web.xml

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

2、获取初始化参数

// ServletDemo3.java
@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 {
        super.doPost(req,resp);
    }

web.xml

<!--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>

3.请求转发

// ServletDemo4.java
@Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        // RequestDispatcher requestDispatcher = context.getRequestDispatcher("/gp");  // 转发的请求路径
        // requestDispatcher.forward(req,resp);  // 转发forward实现请求转发
        context.getRequestDispatcher("/gp").forward(req,resp);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
    }

web.xml

<servlet>
    <servlet-name>sd4</servlet-name>
    <servlet-class>com.shuai.servlet.ServletDemo4</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>sd4</servlet-name>
    <url-pattern>/sd4</url-pattern>
</servlet-mapping>

4、读取资源文件

Properties

• 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：classes,我们俗称这个路径为classpath:

思路：需要一个文件流

username = root
password = dfafafaf

@Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/com/shuai/servlet/aa.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 {
        super.doPost(req, resp);
    }

4、HttpServletReaponse

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HttpServletResquest对象，代表响应的一个HttpServletReaponse；

• 如果要获取客户端请求过来的参数：找HttpServletResquest
• 如果要给客户端响应一些信息：找HttpServletReaponse

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 sendError(int var1, String var2) throws IOException;
void sendError(int var1) throws IOException;
void sendRedirect(String var1) throws IOException;
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);
void setStatus(int var1);

2、下载文件

@Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        /*
        * 前四个是字符串操作
        * 后四个是IO操作*/
        // 解决乱码问题
        req.setCharacterEncoding("utf-8");
        resp.setCharacterEncoding("utf-8");
        resp.setContentType("text/html;charset=utf-8");
        // 1. 要获取下载文件的路径
        // String realPath = this.getServletContext().getRealPath("\1.jpg");
        String realPath = "G:\\JavaProjects\\javaweb-servlet-02\\Response\\target\\classes\\1.jpg";
        System.out.println("下载的文件路径："+ realPath);
        // 2. 下载的文件名是啥？
        String fileName = realPath.substring(realPath.lastIndexOf("\\") + 1);
        // String fileName = "1.jpg";
        // 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.获取下载文件的输入流
        FileInputStream in = new FileInputStream(realPath); // 把文件变成流就能用了
        // 5.创减缓冲区
        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);  // 把流写入缓冲区长度从0到len
        }
        in.close();
        out.close();
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }

3、实现重定向

B一个web资源收到客户端A请求后，B它会通知A客户端去访问另外一个web资源C，这个过程叫做重定向

常见场景：

• 用户登录

void sendRedirect(String var1) throws IOException;

测试：

@Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // resp.sendRedirect("/r/file"); // 重定向
        ServletContext context = this.getServletContext();
        context.getRequestDispatcher("/file").forward(req,resp);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }

面试题：请你聊聊重定向和转发的区别？

相同点：

• 页面都会跳转

不同点

• 请求转发的时候，url不会产生变化；
• 重定向的时候，url地址栏会发生变化；

5、HttpServletResquest

1、获取参数，请求转发

@Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // 防止请求和响应乱码
        req.setCharacterEncoding("utf-8");
        resp.setCharacterEncoding("utf-8");
        resp.setContentType("text/html;charset=utf-8");
        // 处理请求(得到前端的参数)
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        String[] hobbys = req.getParameterValues("hobbys");  // 得到数组
        System.out.println("==================");
        System.out.println(username);
        System.out.println(password);
        System.out.println(Arrays.toString(hobbys));
        System.out.println("==================");
        System.out.println(req.getContextPath());  // /r
        // 通过请求转发
        // 这里的/代表当前的web项目
        req.getRequestDispatcher("/success.jsp").forward(req,resp);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }