方便自己查询，嫌低级的勿喷。。。。
1.request
在servlet的doGet和doPost的参数中就有HttpServletRequest req参数，而JSP内置request对象就是HttpServletRequest接口的实例化对象，可以直接使用req。
2.response
在servlet的doGet和doPost的参数中就有HttpServletResponse resp参数，而JSP内置response对象就是HttpServletResponse接口的实例化对象，可以直接使用resp。
3.session
在servlet程序中要获取session对象，可以通过HttpServletRequest接口完成：
| No | 方法 | 描述 |
| 1 | public HttpSession getSession() | 返回当前session |
| 2 | public HttpSession getSession(boolean create) | 返回当前session，如果没有则创建一个新的session对象返回 |
package org.lxh.servletdemo ;
import java.io.* ;
import javax.servlet.* ;
import javax.servlet.http.* ;
public class HttpSessionDemoServlet extends HttpServlet {
public void doGet(HttpServletRequest req,HttpServletResponse resp) throws ServletException,IOException{
HttpSession ses = req.getSession() ;
System.out.println("SESSION ID --> " + ses.getId()) ;
ses.setAttribute("username","李兴华") ; // 设置session属性
System.out.println("username属性内容：" + ses.getAttribute("username")) ;
}
public void doPost(HttpServletRequest req,HttpServletResponse resp) throws ServletException,IOException{
this.doGet(req,resp) ;
}
}
4.application
application内置对象是ServletContext接口的实例，表示的是Servlet上下文，如果要在一个Servlet中使用此对象，直接通过GenericServlet类提供的方法即可：
| No | 方法 | 描述 |
| 1 | public ServletContext getServletContext() | 取得ServletContext对象 |
package org.lxh.servletdemo ;
import java.io.* ;
import javax.servlet.* ;
import javax.servlet.http.* ;
public class ServletContextDemoServlet extends HttpServlet {
public void doGet(HttpServletRequest req,HttpServletResponse resp) throws ServletException,IOException{
ServletContext app = super.getServletContext() ;
System.out.println("真实路径：" + app.getRealPath("/")) ;
}
public void doPost(HttpServletRequest req,HttpServletResponse resp) throws ServletException,IOException{
this.doGet(req,resp) ;
}
}
5.config
config内置对象实际上是ServletConfig接口的实例，在Servlet程序中的init()方法里找到ServletConfig接口实例：
package org.lxh.servletdemo ;
import java.io.* ;
import javax.servlet.* ;
import javax.servlet.http.* ;
public class InitParamServlet extends HttpServlet {
private String initParam = null ; // 用于保存初始化参数
public void init() throws ServletException{} public void init(ServletConfig config) throws ServletException{
this.initParam = config.getInitParameter("ref") ; // 接收的初始化参数名称暂时为ref
} public void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException,IOException{
System.out.println("初始化参数：" + this.initParam) ;
}
public void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException,IOException{
this.doGet(req,resp) ;
}
}
6.out
out对象是javax.servlet.jsp.JspWriter类的实例化对象，此类继承于java.io.Writer类，然后在HttpServletResponse resp对象中可以获取java.io.PrintWriter类的实例化对象，而java.io.PrintWriter类同样继承于java.io.Writer类，所有out对象可以通过resp.getWriter()获取到：
package org.lxh.servletdemo ;
import java.io.* ;
import javax.servlet.* ;
import javax.servlet.http.* ;
public class InputServlet extends HttpServlet{
public void doGet(HttpServletRequest req,HttpServletResponse resp)
throws ServletException,IOException{
String info = req.getParameter("info") ; // 假设参数名称为info
PrintWriter out = resp.getWriter() ;
out.println("<html>") ;
out.println("<head><title>MLDNJAVA</title></head>") ;
out.println("<body>") ;
out.println("<h1>" + info + "</h1>") ;
out.println("</body>") ;
out.println("</html>") ;
out.close() ;
}
public void doPost(HttpServletRequest req,HttpServletResponse resp)
throws ServletException,IOException{
this.doGet(req,resp) ;
}
}