HttpServlet概念与ServletContext对象

HttpServlet概念

创建类的时候继承HttpServle

实现步骤：

1）创建类继承HttpServlet类

2）覆盖doGet和doPost

3）在web.xml中进行servlet的配置

HttpServlet继承了GenericServlet，而GenericServlet实现Servlet接口， 所以我们可以继承HttpServlet来相当于创建一个Servlet

HttpServlet重写service()方法：

            1.在该方法中先将ServletRequest和ServletResponse

                强转为了HttpServletRequest和HttpServletResponse。

 2.然调用重载的service()方法，并将刚刚强转得到对象传递到重载的方法中。

 　　　　如果是GET请求，则调用doGet(HttpServletRequest request , HttpServletResponse response)

                如果是POST请求，则调用doPost(HttpServletRequest request , HttpServletResponse response)

当通过继承HttpServlet来创建一个Servlet时，只需要根据要处理的请求的类型，来重写不同的方法。

                处理get请求，则重写doGet()

                处理post请求，则重写doPost()

 ServletContext对象：

什么是 ServletContext？

ServletContext代表是一个web应用的环境（上下文）对象，

ServletContext对象一个web应用只有一个,但是一个web应用可以又多个Servlet对象！！！

ServletContext对象的生命周期？

web应用被卸载，服务器关闭

获得 ServletContext 如何获得？

ServletContext Context = getServletContext();

String getServletName() – 获取当前Servlet在web.xml中配置的名字 
String getInitParameter(String name) –获取当前Servlet指定名称的初始化参数的值 

根据相对路径获取绝对路径

package com.oracle.demo01;

import java.io.IOException;

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 Servlet01 extends HttpServlet {

	public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		//获取servletContext对象
		ServletContext context=getServletContext();
		//获取相对于服务器的相对路径获取绝对路径
		String patha=context.getRealPath("WEB-INF/classes/a.txt");
		String pathb=context.getRealPath("b.txt");
		String pathc=context.getRealPath("WEB-INF/c.txt");
		System.out.println(patha);
		System.out.println(pathb);
		System.out.println(pathc);
		
	}

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

　

ServletContext是一个域对象，用来存储数据区域就是域对象

域对象的通用的方法：

setAtrribute(String name,Object obj);（往里存键值对 键只能是String）

getAttribute(String name);（根据建取值，取到的值是object类型，需要向下转型）

removeAttribute(String name);（移出）

package com.oracle.demo01;

import java.io.IOException;

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 Servlet02 extends HttpServlet {

	public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		//获取ServletContext对象
		ServletContext context=getServletContext();
		//往ServletContext域中设置值
		context.setAttribute("name","xiahouyuan");
	}

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

　　

package com.oracle.demo01;

import java.io.IOException;

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 Servlet03 extends HttpServlet {
	public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		//获取ServletContext对象
		ServletContext context=getServletContext();
		//获取ServletContext域里面的值
		String name=(String)context.getAttribute("name");
		response.getWriter().write(name);
	}

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