JavaWed_Servlet_ServletContext
ServletContext
2.作为域对象使用，在不同servlet之间传递数据,作用范围是整个web应用
生命周期：当web应用被加载进容器时创建代表整个web应用的ServletContext对象。当服务器关闭或web应用被移除出容器时，ServletContext对象跟着销毁。
~域：一个域就理解为一个框，这里面可以放置数据，一个域既然称作域，他就有一个可以被看见的范围，这个范围内都可以对这个域中的数据进行操作，那这样的对 象就叫做域对象。
package com.text;
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 Demo1ServletContext extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
ServletContext servletContext = this.getServletContext();
servletContext.setAttribute("tyj", "谭永坚");
}
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doGet(request, response);
}
}
package com.text;
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 Demo2ServletContext extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
ServletContext servletContext = this.getServletContext();
String str = (String) servletContext.getAttribute("tyj");
System.out.println(str);
}
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doGet(request, response);
}
}
3.在web.xml可以配置整个web应用的初始化参数，利用ServletContext去获得
<context-param>
<param-name>param1</param-name>
<param-value>pvalue1</param-value>
</context-param>
this.getServletContext().getInitParameter("param1")
this.getServletContext().getInitParameterNames()
4.在不同servlet之间进行转发
this.getServletContext().getRequestDispatcher("/servlet/Demo10Servlet").forward(request, response);
方法执行结束，service就会返回到服务器，再有服务器去调用目标servlet，其中request会重新创建，并将之前的request的数据拷贝进去。
package com.text;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
* ServletContext的作用： 3.请求的转发
*
* @author 两点水
*
*/
public class Demo3ServletContext extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
RequestDispatcher dispatcher = this.getServletContext()
.getRequestDispatcher("/servlet/Demo4ServletContext");
dispatcher.forward(request, response);
}
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doGet(request, response);
}
}
package com.text;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class Demo4ServletContext extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.getWriter().write("tyj");
}
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doGet(request, response);
}
}
5.读取资源文件
5.1由于相对路径默认相对的是java虚拟机启动的目录，所以我们直接写相对路径将会是相对于tomcat/bin目录，所以是拿不到资源的。如果写成绝对路径，当项目发布到其 他环境时，绝对路径就错了。
5.2为了解决这个问题ServletContext提供了this.getServletContext().getRealPath("/1.properties")，给进一个资源的虚拟路径，将会返回该资源在当前环境下的真 实路径。 this.getServletContext().getResourceAsStream("/1.properties")，给一个资源的虚拟路径返回到该资源真实路径的流。
5.3当在非servlet下获取资源文件时，就没有ServletContext对象用了，此时只能用类加载器
classLoader.getResourceAsStream("../../1.properties")，此方法利用类加载器直接将资源加载到内存中，有更新延迟的问题，以及如果文件太大，占用内存大。
classLoader.getResource("../1.properties").getPath()，直接返回资源的真实路径，没有更新延迟的问题。