ServletContext

Posted 2021-01-19 leagueandlegends

ServletContext:

ServletContext表示Servlet应用程序。每个Web应用程序只有一个上下文。在将一个应用程序同时部署到多个容器的分布式环境中，每台Java虚拟机上的Web应用都会有一个ServletContext对象。

通过在ServletConfig中调用getServletContext方法，可以获得ServletContext。

有了ServletContext,就可以共享从应用程序中的所有资料处访问到的信息，并且可以动态注册Web对象。前者将对象保存到ServletContext中的一个内部的Map中。保存在ServletContext中的对象被称作属性。

1：实现数据共享 setAttribute(key,value)  getAttribute(key) ，获取域对象

2：获取全局配置信息: getInitParameter() 

3：获得应用下任何资源的路径

例一：通过调用GenericServlet的 getServletContext对象得到ServletContext对象，获取域对象

先运行myservlet1.java，在运行myservlet2.java

myservlet1.java

public class myservlet extends HttpServlet {
	public void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		//通过调用GenericServlet的 getServletContext对象得到ServletContext对象
		//先访问demo1,在访问demo2
		ServletContext application=this.getServletContext();
		application.setAttribute("name", "tom");
		System.out.println(application.getClass().getName());
	}
	public void doPost(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		doGet(request,response);
	}
}

myservlet2.java

public class myservlet2 extends HttpServlet {
	public void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		String name=(String) this.getServletContext().getAttribute("name");
		
		if(name==null){
			System.out.println("你不能直接访问这个类");
		}
		System.out.println(name);
	}
	public void doPost(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		doGet(request,response);
	}
}

　　运行效果图：

例二：获取全局配置信息

获取encoding的配置信息

myservlet3.java

public class myservlet3 extends HttpServlet {
	public void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		String encoding=this.getServletContext().getInitParameter("encoding");
		System.out.println(encoding);
	}
	public void doPost(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		doGet(request,response);
	}
}

　web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" 
	xmlns="http://java.sun.com/xml/ns/javaee" 
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
	xsi:schemaLocation="http://java.sun.com/xml/ns/javaee 
	http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
  <display-name></display-name>
  <context-param>
  <param-name>encoding</param-name>
  <param-value>utf-8</param-value>
  </context-param>
  <servlet>
    <servlet-name>myservlet</servlet-name>
    <servlet-class>com.zk.myservlet.myservlet</servlet-class>
  </servlet>
  <servlet>
    <servlet-name>myservlet2</servlet-name>
    <servlet-class>com.zk.myservlet.myservlet2</servlet-class>
  </servlet>
  <servlet>
    <servlet-name>myservlet3</servlet-name>
    <servlet-class>com.zk.myservlet.myservlet3</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>myservlet</servlet-name>
    <url-pattern>/demo1</url-pattern>
  </servlet-mapping>
  <servlet-mapping>
    <servlet-name>myservlet2</servlet-name>
    <url-pattern>/demo2</url-pattern>
  </servlet-mapping>
  <servlet-mapping>
    <servlet-name>myservlet3</servlet-name>
    <url-pattern>/demo3</url-pattern>
  </servlet-mapping>	
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>
</web-app>

例三：获取资源路径

使用this.getServletContext().getRealPath获取文件的资源路径

public class myservlet1 extends HttpServlet {
	public void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		String apath=this.getServletContext().getRealPath("/WEB-INF/a.properties");//参数一定要以斜杠开头
		System.out.println(apath);
		//创建一个properties
		Properties pro=new Properties();
		pro.load(new FileInputStream(apath));
		System.out.println(pro.getProperty("akey"));
		
		String bpath=this.getServletContext().getRealPath("/WEB-INF/classes/b.properties");
		System.out.println(bpath);
		Properties pro2=new Properties();
		pro2.load(new FileInputStream(bpath));
		System.out.println(pro2.getProperty("key"));
		
		String cpath=this.getServletContext().getRealPath("/WEB-INF/classes/com/zk/myservlet/c.properties");
		System.out.println(cpath);
		Properties pro3=new Properties();
		pro3.load(new FileInputStream(cpath));
		System.out.println(pro3.get("ckey"));
	}
	public void doPost(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		doGet(request,response);
	}
}

　　运行结果图: