ServletContext对象

WEB容器在启动时，它会为每个WEB应用程序都创建一个对应的ServletContext对象，它代表当前web应用。
　　ServletConfig对象中维护了ServletContext对象的引用，开发人员在编写servlet时，可以通过ServletConfig.getServletContext方法获得ServletContext对象。
　　由于一个WEB应用中的所有Servlet共享同一个ServletContext对象，因此Servlet对象之间可以通过ServletContext对象来实现通讯。ServletContext对象通常也被称之为context域对象。

1.多个Servlet通过ServletContext对象实现数据共享

范例：ServletContextDemo1和ServletContextDemo2通过ServletContext对象实现数据共享

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 ServletContextDemo1 extends HttpServlet {
	public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String data = "国安是冠军";
        /**
         * ServletConfig对象中维护了ServletContext对象的引用，开发人员在编写servlet时，
         * 可以通过ServletConfig.getServletContext方法获得ServletContext对象。
         */
        
        ServletContext context = this.getServletConfig().getServletContext();//获得ServletContext对象
        
        context.setAttribute("data", data);  //将data存储到ServletContext对象中
    }

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

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 ServletContextDemo2 extends HttpServlet {
     


	public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
		
        ServletContext context = this.getServletContext();
        response.setContentType("text/html; charset=UTF-8");
        String data = (String) context.getAttribute("data");//从ServletContext对象中取出数据
        
        response.getWriter().print("data="+data);
    }

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

先运行ServletContextDemo1，将数据data存储到ServletContext对象中，然后运行ServletContextDemo2就可以从ServletContext对象中取出数据了，这样就实现了数据共享，如下图所示： 

2、获取WEB应用的初始化参数

　　在web.xml文件中使用<context-param>标签配置WEB应用的初始化参数，如下所示：

　获取Web应用的初始化参数，代码如下：

import java.io.IOException;

import javax.naming.Context;
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 ServletContextDemo3 extends HttpServlet {
     


	public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
		
        ServletContext  context = this.getServletContext();
      //获取整个web站点的初始化参数
        String contextInitParam = context.getInitParameter("url");
        response.getWriter().print(contextInitParam);
        
       
    }

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

3、实现请求转发