servlet的ServletContext接口

ServletContext

　　Servlet 上下文

每个web工程都只有一个ServletContext对象，也就是不管在哪个servlet里面，获取到的这个ServletContext对象都是同一个

web.xml

  <servlet>
  	<servlet-name>hello2</servlet-name>
  	<servlet-class>com.qf.servlet.HelloServlet2</servlet-class>

  </servlet>
  <servlet-mapping>
  	<servlet-name>hello2</servlet-name>
  	<url-pattern>/h2</url-pattern>
  </servlet-mapping>
  <servlet>
  	<servlet-name>hello</servlet-name>
  	<servlet-class>com.qf.servlet.HelloServlet3</servlet-class>
  </servlet>
  <servlet-mapping>
  	<servlet-name>hello</servlet-name>
  	<url-pattern>/h3</url-pattern>
  </servlet-mapping>

servlet类

public class HelloServlet2 extends HttpServlet {

	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		System.out.println("helloServlet2:"+getServletContext());
	}
}
-------------------------------------------------------------
public class HelloServlet3 extends HttpServlet {

	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		System.out.println("helloServlet3:"+getServletContext());
		//testServletConfig();
	}
}

浏览器先后访问url（http://localhost:8080/HelloServlet/h2、http://localhost:8080/HelloServlet/h3），查看console输出

helloServlet2:org.apache.catalina.core.ApplicationContextFacade@4b020c18
helloServlet3:org.apache.catalina.core.ApplicationContextFacade@4b020c18

　　所以两个servlet获取的是同一个ServletContext对象

ServletContext对象的获取

ServletContext context = getServletContext();

ServletContext的作用

获取全局配置参数

web.xml配置

  <context-param>
	<param-name>name</param-name>
	<param-value>wxf</param-value>
  </context-param>
  <context-param>
	<param-name>age</param-name>
	<param-value>21</param-value>
  </context-param>

servlet代码

 public class HelloServlet2 extends HttpServlet {

     @Override
     protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
         ServletContext context = getServletContext();
         Enumeration<String> names = context.getInitParameterNames();
         while (names.hasMoreElements()) {
             String name = (String) names.nextElement();
             System.out.println(name+"==="+context.getInitParameter(name));
         }
     }
 }

console输出

name===wxf
age===21

获取web工程中的资源（webContent目录下的资源）

config.properties

name=zhangsan
age=123

servlet类

 public class HelloServlet2 extends HttpServlet {

     @Override
     protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
         ServletContext context = getServletContext();

         //1.获取资源所在的绝对路径位置
         //这里得到的是项目在tomcat里面的根目录
         String path = context.getRealPath("");
         System.out.println("path==="+path);
         //这里得到的是项目在tomcat里面的根目录
         String path2 = context.getRealPath("config.properties");
         System.out.println("path2==="+path2);
         System.out.println("-------------------------");

         //2.getResourceAsStream 获取资源 流对象 相对路径
         InputStream stream = context.getResourceAsStream("config.properties");
         Properties prop = new Properties();
         prop.load(stream);
         String name = prop.getProperty("name");
         System.out.println("name==="+name);
     }
 }

console输出

path===F:\eclipse\workspace\develop_study\.metadata\.plugins\org.eclipse.wst.server.core\tmp0\wtpwebapps\HelloServlet
path2===F:\eclipse\workspace\develop_study\.metadata\.plugins\org.eclipse.wst.server.core\tmp0\wtpwebapps\HelloServlet\config.properties
-------------------------
name===zhangsan

注：

1. 通过classloader去获取web工程下的资源的方式：this.getClass().getClassLoader().getResource("../../config.properties")
2. src对应的是/WEB-INF/classes，如果想要使用ServletContext获取src目录下的文件可以使用context.getResourceAsStream("/WEB-INF/classes/config.properties")

存取数据，servlet间共享数据 域对象

        ServletContext context = getServletContext();
        context.setAttribute("name", "qf");
        System.out.println("name==="+context.getAttribute("name"));                

console输出

name===qf