servlet 变量调用

通过web.xml定义变量并读取输出读取变量：
局部变量局部变量只能用于当前serlet
通过init-param定义变量

 <servlet>
    <servlet-name>helloworod</servlet-name>
    <servlet-class>servletStudent.helloworod</servlet-class>
    
    <init-param>
        <param-name>username</param-name>
        <param-value>sheng</param-value>
    </init-param>  

在servlet中通过this.getInitParameter("username")读取变量内容
servlet测试代码如下：

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

        response.setContentType("text/html");
        String url = request.getRequestURL().toString();
        
        System.out.println("=============================");
        System.out.println(url);
        PrintWriter out = response.getWriter();
        out.println("=======get========");
        out.println("wellcome "+this.getInitParameter("username"));
        out.println("Hello world!!! ");

运行测试结果如图：

运行结果

通过在web配置文件中通过context-param节点通过<param-name>方式定义变量名及密码。
定义全局变量方式如下：

<context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost:3386/test</param-value>
        
    </context-param>
    <context-param>
        <param-name>db_name</param-name>
        <param-value>root</param-value>
    </context-param>
    <context-param>
        <param-name>db_pass</param-name>
        <param-value>root@2017</param-value>
    </context-param>

通过this.getServletContest().getInitParameter()方式获取变量内容。

public void init(){
        String username =this.getInitParameter("username");
        System.out.println("==========init()============");
        
        System.out.println("init username:  "+username);
        
        System.out.println("this db url:"+this.getServletContext().getInitParameter("url"));
        System.out.println("this db name:"+ this.getServletContext().getInitParameter("db_name"));
        System.out.println("this db pass:"+ this.getServletContext().getInitParameter("db_pass"));
    }

运行结果如图：

获取全局变量