[开发工具]ServletContext对象

一个web容器可以多个Servlet交互，比如我在淘宝登陆，我的登陆信息会被带到淘宝的很多页面，跳到其他页面也有登陆信息，这个操作肯定不是它自己做的，需要一个中间商，也就是ServletContext

web容器在启动的时候，它会为每个web程序都创建一个对应的ServletContext对象，它代表了当前的web应用：

• 共享数据：我在则合格Servlet中保存的数据，可以在另一个Servlet中拿到

????????

package com.godairo.servlet;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class HelloServlet extends HttpServlet {

	  @Override
	  protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

			ServletContext context = this.getServletContext();
			String username = "小可爱"; //数据
			context.setAttribute("username",username); //将一个数据保存在了ServletContext中，名字为：username，它的值为：username
			System.out.println("Hello");
	  }

	  @Override
	  protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
			doGet(req, resp);
	  }
}

package com.godairo.servlet;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class GetServlet extends HttpServlet {
	  @Override
	  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
			ServletContext context = this.getServletContext();
			String username = (String) context.getAttribute("username");

			response.setContentType("text/html");
			response.setCharacterEncoding("utf-8");
			response.getWriter().print("名字：" + username);
	  }

	  @Override
	  protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
			doGet(req, resp);
	  }
}

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
                      http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0"
         metadata-complete="true">
    
    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>com.godairo.servlet.HelloServlet</servlet-class>
    </servlet>
    
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>

    <servlet>
        <servlet-name>getusername</servlet-name>
        <servlet-class>com.godairo.servlet.GetServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>getusername</servlet-name>
        <url-pattern>/getusername</url-pattern>
    </servlet-mapping>
</web-app>

测试访问结果：先输入hello保存数据，然后输入getusername读取

• 获取初始化参数

<context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
</context-param>

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
			ServletContext context = this.getServletContext();

			String url = context.getInitParameter("url");
			response.getWriter().print(url);
	  }

• 请求转发?

????????

@Override
	  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
			ServletContext context = this.getServletContext();
			System.out.println("进入了ServletDemo04");
			//RequestDispatcher requestDispatcher = context.getRequestDispatcher("/gp");  //转发的请求路径
			//requestDispatcher.forward(request,response); //调用forward实现请求转发；
			context.getRequestDispatcher("/gp").forward(request,response);
	  }

• 读取资源文件

? ?ProPerties

• 在java目录下新建properties
• 在resources目录下新建properties

? ? ?发现：都被打包到了同一个路径下，这个丼是classes路径，我们俗称这个路径为类路径classpath

? ? ?思路：需要一个文件流。??

package com.godairo.servlet;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.InputStream;
import java.util.Properties;

public class ServletDemo05 extends HttpServlet {
	  @Override
	  protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
			InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");
			Properties properties = new Properties();
			properties.load(is);
			String username = properties.getProperty("username");
			String password = properties.getProperty("password");
			resp.getWriter().print(username+":"+password);
			
	  }

	  @Override
	  protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
			doGet(req, resp);
	  }
}

username=godairo
password=123456

?访问测试即可！

之后的共享数据会用session和request去做

获取初始化基本也没用这种方法

请求转发用request

读取资源文件会用类加载去做

这里是打基础