0x01: 什麼是Servlet？

• 是sun公司開發動態web的技術
• 實現了servlet介面的Java程式

0x02: Servlet的實現類有哪些？

Servlet介面預設有兩個實現類

• 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 {
    
    //由於get或者post只是請求實現的不同的方式，可以相互呼叫，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

瀏覽器傳送http請求給web容器，web容器產生Request和Response物件，這兩個物件呼叫servlet介面的Service方法，Service方法再將返回的Responce資訊返回給Responce物件，web容器從Responce物件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位元組碼檔案）

什麼是ServletContext

Web容器在啟動時，為每個web程式建立一個對應的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可以共享資料

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

        String username = "黎星澄"; //資料
        context.setAttribute("username",username); //將一個數據儲存在了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);
    }
}

獲取初始化引數

    <!--在web.xml配置一些web應用初始化引數-->
    <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的引數
    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);      //呼叫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：像當於在伺服器內部將請求轉給請求的資源，然後由伺服器相應給客戶端
重定向：伺服器將請求資源作為響應資訊響應給客戶端，客戶端根據這個資訊對伺服器發起請求

讀取資原始檔
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();                                                                               //建立一個Properties物件
        prop.load(is);                                                                                                    //將讀取到的載入到prop裡
        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);
    }
}