Day02JavaWeb【Servlet & ServletContext】 ServletContext-作用

ServletContext-作用1 ***

• （1）什麼是ServletContext

  1:ServletContext是Servlet中自帶一個對象

  2:ServletContext對象時在第一次通路Servlet時，自動建立

  3:一個項目中隻有一個ServletContext對象

(2) 作用1：ServletContext可以實作多個Servlet之間資料的共享

getServletContext()
setAttribute(key,value)
getAttribute(key)
           

Servlet1可以往ServletContext添加鍵值對數，所有的Servlet都可以取

src\com\wzx\Demo01SetServlet.java

@WebServlet("/set")
public class Demo01SetServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            doGet(request,response);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1:擷取ServletContext
        ServletContext context =  super.getServletContext();
        //2:添加資料  key-value 集合 Map
        context.setAttribute("name","jack");
        context.setAttribute("age",22);

        System.out.println("setServlet");

    }
}
           

src\com\wzx\Demo02GetServlet.java

@WebServlet("/get")
public class Demo02GetServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            doGet(request,response);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        //1.擷取servletContext
        ServletContext context = getServletContext();
        //2.根據鍵擷取值
        String name = (String) context.getAttribute("name");
        Integer age = (Integer) context.getAttribute("age");
        System.out.println(name);
        System.out.println(age);
    }
}

           

通路測試

http://localhost:8080/web02_servletContext_war_exploded/set
http://localhost:8080/web02_servletContext_war_exploded/get
           

ServletContext-作用2

2:ServletContext可以讀取web項目中檔案的内容

• src下的檔案:

  InputStream is = Demo3FileServlet.class.getClassLoader().getResourceAsStream("a.txt");

• web目錄下檔案

  InputStream is = getServletContext().getResourceAsStream("a.txt");

src\com\wzx\Demo03FileServlet.java

@WebServlet("/file")
public class Demo03FileServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request,response);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1:java使用類加載器去讀取
        //InputStream inputStream = Demo03FileServlet.class.getClassLoader().getResourceAsStream("a.txt");
        InputStream inputStream = getServletContext().getResourceAsStream("b.txt");
        //2:将位元組流轉成字元流
        BufferedReader br = new BufferedReader(new InputStreamReader(inputStream));
        String line = br.readLine();

        response.setCharacterEncoding("utf-8");
        response.getWriter().println(line);
        System.out.println(line);
    }
}
           

通路測試

http://localhost:8080/web02_servletContext_war_exploded/file

           

ServletContext-作用3

web\WEB-INF\web.xml

3:ServletContext可以讀取整個項目的初始化資料
           

<context-param>
 		<param-name>ip</param-name>
 		<param-value>192.168.31.72</param-value>
 	</context-param>
           

src\com\wzx\Demo04ParamServlet.java

@WebServlet("/param")
public class Demo04ParamServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request,response);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1:擷取ServletContext
        ServletContext context = getServletContext();
        //2:列印
        String value = context.getInitParameter("ip");
        System.out.println(value);
    }
}

           

通路測試

http://localhost:8080/web02_servletContext_war_exploded/param

           

一個網站被多少使用者登入通路

比如 三個人 通過LoginServlet登入成功，那必須讓LoginServlet 對count值三次+1

在前面的登入案例裡面修改

（1）初始化count值

src\com\wzx\web\LoginServlet.java

@Override
    public void init() throws ServletException {
        //Servlet建立出來，ServletContext也會建立出來

        //1:擷取ServletContext
        ServletContext context = getServletContext();
        //2:添加一個count變量，預設值為0
        context.setAttribute("count",0);
    }
           

（2）每次登入成功對count值+1

src\com\wzx\web\LoginServlet.java

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1:擷取參數

        String username=request.getParameter("username");
        String password=request.getParameter("password");
        //2:處理參數
        UserService userService = new UserService();

        //2.1登入就是拿着賬号，密碼去資料庫判斷，存在就傳回true,否則就傳回false
        boolean result =  userService.login(username,password);
        response.setCharacterEncoding("utf-8");
        //2.2根傳回結果提示使用者
        if(result){

            //登入成功，對count值+1
            //a.擷取servletContext
            ServletContext context = getServletContext();
            //b.擷取裡面的count
            Integer count= (Integer) context.getAttribute("count");
            //c. count + 1
            count += 1;
            //d. 放回ServletContext
            context.setAttribute("count",count);
            //3:響應給浏覽器
            response.getWriter().println("登入成功，您是第"+count+"個登入");


        }else{
            response.getWriter().println("賬号或者密碼出錯");

        }



    }