Day02JavaWeb【Servlet & ServletContext】 ServletContext-作用

ServletContext-作用1 ***

• （1）什么是ServletContext
  1:ServletContext是Servlet中自带一个对象
  2:ServletContext对象时在第一次访问Servlet时，自动创建
  3:一个项目中只有一个ServletContext对象

(2) 作用1：ServletContext可以实现多个Servlet之间数据的共享

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

Servlet1可以往ServletContext添加键值对数，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:获取ServletContext
        ServletContext context =  super.getServletContext();
        //2:添加数据  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.获取servletContext
        ServletContext context = getServletContext();
        //2.根据键获取值
        String name = (String) context.getAttribute("name");
        Integer age = (Integer) context.getAttribute("age");
        System.out.println(name);
        System.out.println(age);
    }
}

访问测试

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

ServletContext-作用2

2:ServletContext可以读取web项目中文件的内容

• src下的文件:
  ​​​InputStream is = Demo3FileServlet.class.getClassLoader().getResourceAsStream("a.txt");​​
• web目录下文件
  ​​​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使用类加载器去读取
        //InputStream inputStream = Demo03FileServlet.class.getClassLoader().getResourceAsStream("a.txt");
        InputStream inputStream = getServletContext().getResourceAsStream("b.txt");
        //2:将字节流转成字符流
        BufferedReader br = new BufferedReader(new InputStreamReader(inputStream));
        String line = br.readLine();

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

访问测试

http://localhost:8080/web02_servletContext_war_exploded/file

ServletContext-作用3

web\WEB-INF\web.xml

3:ServletContext可以读取整个项目的初始化数据

<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:获取ServletContext
        ServletContext context = getServletContext();
        //2:打印
        String value = context.getInitParameter("ip");
        System.out.println(value);
    }
}

访问测试

http://localhost:8080/web02_servletContext_war_exploded/param

一个网站被多少用户登录访问

比如 三个人 通过LoginServlet登录成功，那必须让LoginServlet 对count值三次+1

在前面的登录案例里面修改

（1）初始化count值

src\com\wzx\web\LoginServlet.java

@Override
    public void init() throws ServletException {
        //Servlet创建出来，ServletContext也会创建出来

        //1:获取ServletContext
        ServletContext context = getServletContext();
        //2:添加一个count变量，默认值为0
        context.setAttribute("count",0);
    }

（2）每次登录成功对count值+1

src\com\wzx\web\LoginServlet.java

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1:获取参数

        String username=request.getParameter("username");
        String password=request.getParameter("password");
        //2:处理参数
        UserService userService = new UserService();

        //2.1登录就是拿着账号，密码去数据库判断，存在就返回true,否则就返回false
        boolean result =  userService.login(username,password);
        response.setCharacterEncoding("utf-8");
        //2.2根返回结果提示用户
        if(result){

            //登录成功，对count值+1
            //a.获取servletContext
            ServletContext context = getServletContext();
            //b.获取里面的count
            Integer count= (Integer) context.getAttribute("count");
            //c. count + 1
            count += 1;
            //d. 放回ServletContext
            context.setAttribute("count",count);
            //3:响应给浏览器
            response.getWriter().println("登录成功，您是第"+count+"个登录");


        }else{
            response.getWriter().println("账号或者密码出错");

        }



    }