0x01: 什么是Servlet？

• 是sun公司開發動態web的技術
• 實現了servlet接口的Java程序

0x02: Servlet的實現類有哪些？

Servlet接口默認有兩個實現類

• HttpServlet
• GenericServlet
  HttpServlet 繼承自 GenericServlet, 一般我們自己寫類只需要繼承HttpServlet，重寫方法就可以了

public class HelloServlet extends HttpServlet {
    
    //由于get或者post只是請求實現的不同的方式，可以相互調用，業務邏輯都一樣；
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //ServletOutputStream outputStream = resp.getOutputStream();
        PrintWriter writer = resp.getWriter(); //響應流
        writer.print("Hello,Serlvet");
    }

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

0x03：Servlet的原理

瀏覽器發送http請求給web容器，web容器產生Request和Response對象，這兩個對象調用servlet接口的Service方法，Service方法再將返回的Responce信息返回給Responce對象，web容器從Responce對象讀取將其響應給客戶端。（注意，如果web容器是首次被訪問，需要先把我們的java類變成class字節碼文件）

什么是ServletContext

Web容器在啟動時，為每個web程序創建一個對應的ServletContext，它代表當前web應用

ServletContext的一些用法和特性

在同一個web應用中，不同的servlet可以共享數據

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

        String username = "黎星澄"; //數據
        context.setAttribute("username",username); //將一個數據保存在了ServletContext中，名字為：username 。值 username

    }

}

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"+username);

    }

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

獲取初始化參數

    <!--在web.xml配置一些web應用初始化參數-->
    <context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
    </context-param>

protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    ServletContext context = this.getServletContext();
    String url = context.getInitParameter("url"); //得到name為url的參數
    resp.getWriter().print(url);
}

請求轉發

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    ServletContext context = this.getServletContext();
    System.out.println("進入了ServletDemo04");
    //RequestDispatcher requestDispatcher = context.getRequestDispatcher("/xxx");           //轉發的請求路徑
    //requestDispatcher.forward(req,resp);      //調用forward實現請求轉發；
    context.getRequestDispatcher("/gp").forward(req,resp);
}
//請求轉發和重定向的區別？
請求轉發：像當于在服務器內部將請求轉給請求的資源，然后由服務器相應給客戶端
重定向：服務器將請求資源作為響應信息響應給客戶端，客戶端根據這個信息對服務器發起請求

讀取資源文件
在java目錄下新建properties
在resources目錄下新建properties
都被打包到同一路徑:classes

username=admin
password=123456

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

        InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/com/hu/servlet/test.properties"); //輸入流

        Properties prop = new Properties();                                                                               //創建一個Properties對象
        prop.load(is);                                                                                                    //將讀取到的加載到prop里
        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);
    }
}