servlet：重定向和转发 ServletContext上下文对象

1、转发和重定向区别？

• 重定向地址栏地址变化，转发不变
• 重定向发生在客户端，转发发生在服务器端，客户端不知道
• 重定向不能携带数据，转发可以
• 重定向时，客户端发送了两次请求，而转发只有一次请求（本质区别）
• 重定向：提交表单，处理成功后重定向到另一个jsp，防止表单重复提交，浏览器里url变了
• 转发：当用户登录错误时，一般会回到登录页并提示信息，这个时候用转发问题不大且方便效率高。但现在开发一般不用这个，一般用ajax异步提交，可以从后台带提示信息过去，所以现在用转发意义不大。

2、ServletContext

1. 是什么？
   Servlet上下文对象，是所有servlet对象共享的一片内存空间，我们可以把数据存在这个内存空间中，实现多个servlet对象共享数据。

2. 在上下文对象中怎么放入对象？
   1.在web.xml中配置

   <context-param>
   <param-name>testServletContext</param-name>
   <param-value>testServletContext</param-value>
    </context-param>
   
    就可以直接在servlet中访问
   
    package com.wyz.selvletContext;
   
   import java.io.IOException;
   import java.util.List;
   
   import javax.servlet.ServletContext;
   import javax.servlet.ServletException;
   import javax.servlet.http.HttpServlet;
   import javax.servlet.http.HttpServletRequest;
   import javax.servlet.http.HttpServletResponse;
   
   public class TestServletContext extends HttpServlet{
       private static final long serialVersionUID = 1L;
   
   protected void doGet(HttpServletRequest request, HttpServletResponse response)
           throws ServletException, IOException {
       doPost(request,response);
   }
   
   protected void doPost(HttpServletRequest request, HttpServletResponse response)
           throws ServletException, IOException {
       ServletContext application =this.getServletContext();
       System.out.println(application.getInitParameter("testServletContext"));
       System.out.println(application.getInitParameter("testServletContext2"));
   
   
       }
   
   public void destroy() {
   
   }
   
   public void init() throws ServletException {
   
   }
   

   }
   2.在任意一个servlet里面设置ServletContext的属性，并赋予属性值

   public class myFirstServlet extends HttpServlet {
   
   private static final long serialVersionUID = 1L;
   
   public void doPost() throws ServletException {
   
   ServletContext application=this.getServletContext();
   application.setAttribute("name2", "name2");
   
       }
   
   }
   

   得到上下文对象

       public class TestServletContext extends HttpServlet{
           private static final long serialVersionUID = 1L;
   
       protected void doGet(HttpServletRequest request, HttpServletResponse response)
           throws ServletException, IOException {
           doPost(request,response);
       }
   
       protected void doPost(HttpServletRequest request, HttpServletResponse response)
               throws ServletException, IOException {
           ServletContext application =this.getServletContext();
   
   
       application.setAttribute("name", "了就立刻搭街坊");
       System.out.println(application.getAttribute("name"));
       System.out.println(application.getAttribute("name2"));
   
       }
   
   
   
   }
   

   3.系统启动的时候启动某个servlet，在这个servlet中init方法中设置上下文对象， 这样就相当于在系统启动是初始化ServletContext。这里需要在web.xml文件中配置。

   <servlet>
   <servlet-name>myFirstServlet</servlet-name>
   <servlet-class>com.wyz.prac.myFirstServlet</servlet-class>
   <load-on-startup>0</load-on-startup>
     </servlet>
   

public class myFirstServlet extends HttpServlet {

private static final long serialVersionUID = 1L;

public void init() throws ServletException {


    System.out.println("I AM INIT METHOD");
    List<String> list=new ArrayList<String>();
    list.add("yoyo");
    list.add("toco");
    list.add("jiojio");
    ServletContext context=this.getServletContext();
    context.setAttribute("list", list);
}

}

public class TestServletContext extends HttpServlet{
    private static final long serialVersionUID = 1L;

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

    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        ServletContext application =this.getServletContext();


        ServletContext context=this.getServletContext();
        List<String> list=(List<String>)context.getAttribute("list");
        for(String str:list){
            System.out.println(str);
        }
    }



}

方法3可以用于统计用户在线人数：

a：创建InitServlet   把它设置为随系统启动就初始化（load-on-startup）
    在InitServlet类  init方法中创建一个空对象，用以存储用户的在线记录
b : 每次用户登录（loginServlet）的时候，
     我们就会调用ServletContext上下文对象，并将用户记录放入 我们准备好的对象中(哪个对象请见            InitServlet)
c: 每次用户登出(logoutServelt)的时候，我们也要调用，然后清除此登录用户信息，保持数据的正确性
d:在我们需要展示人数的servlet里面调用ServletContext上下文对象,获取所有用户信息

<!DOCTYPE html>
<html>
  <head>
<title>login.html</title>

<meta name="keywords" content="keyword1,keyword2,keyword3">
<meta name="description" content="this is my page">
<!-- 解决html5乱码问题 http-equiv="Content-Type"-->
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">

<!--<link rel="stylesheet" type="text/css" href="./styles.css">-->
  </head>

  <body>
<h1>This is my HTML page.</h1> <br>
<form action="loginServlet" name="loginForm" method="post">
    用户名：<input type="text" name="username"><br><br>
    密码：<input type="password" name="pwd"><br><br>
    <input type="submit" value="登录">
    <br><br><br><br><br><br>

</form>

  </body>
</html>



public class LoginServlet extends HttpServlet {


private static final long serialVersionUID = 1L;
public LoginServlet() {

}
public void init() throws ServletException {
    ServletContext application=this.getServletContext();

    List<String> list=new ArrayList<String>();
    application.setAttribute("userList", list);
}

}




public class TestLoginServlet extends HttpServlet {

private static final long serialVersionUID = 1L;

public TestLoginServlet() {
    super();
}


public void destroy() {

}

public void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    doPost(request,response);
}


public void doPost(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    ServletContext application=this.getServletContext();
    List<String> list=(List<String>)application.getAttribute("userList");
    String name=request.getParameter("username");
    list.add(name);
    PrintWriter out=response.getWriter();
    out.print("当前用户数："+list.size());
}

public void init() throws ServletException {

}

}