在Servlet程序中要想取得一个session对象，则可以通过HttpServletRequest接口完成。
HttpSessionDemoServlet

package im.weitang;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

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

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        HttpSession session = request.getSession();
        System.out.println("SESSION ID = " + session.getId());
        session.setAttribute("username", "张三");
        System.out.println("username = " + session.getAttribute("username"));
    }

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

}

修改web.xml，添加如下代码

<servlet>
    <description></description>
    <display-name>HttpSessionDemoServlet</display-name>
    <servlet-name>HttpSessionDemoServlet</servlet-name>
    <servlet-class>im.weitang.HttpSessionDemoServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>HttpSessionDemoServlet</servlet-name>
    <url-pattern>/HttpSessionDemoServlet</url-pattern>
</servlet-mapping>

程序运行后，Tomcat后台输出了：

SESSION ID = 5B9A2ED83C1C7507736B2836498F59BB
username = 张三