application 和 session 的区别

TestServlet1.java:

public class TestServlet1 extends HttpServlet {

	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		PrintWriter out = response.getWriter();
		
		ServletContext application = this.getServletContext();
		application.setAttribute("name1","Tom");
		Object o1 = application.getAttribute("name1");
		out.println(o1);
		
		HttpSession session = request.getSession();
		session.setAttribute("name2", "Cat");
		Object o2 = session.getAttribute("name2");
		out.println(o2);
	}

TestServlet2.java:

public class TestServlet2 extends HttpServlet {

	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		PrintWriter out = response.getWriter();
		
		ServletContext application = this.getServletContext();
		out.println(application.getAttribute("name1"));
		
		HttpSession session = request.getSession();
		out.println(session.getAttribute("name2"));
	}

既然我运行 TestServlet2 后两个（name1, name2）都能得到，那两者有啥区别呢？一般两者在什么情况下用呢？希望举一个具体的场景例子。