定义servlet用于与其servlet容器进行通信的一组方法，例如获取文件的MIME类型，调度请求或写入日志文件。
每个Java虚拟机每个“Web应用程序”都有一个上下文。 （“Web应用程序”是安装在服务器的URL命名空间的特定子集下的servlet和内容的集合，如/ catalog，可能通过.war文件安装。）
在其部署描述符中标记为“分布式”的Web应用程序的情况下，每个虚拟机将有一个上下文实例。 在这种情况下，上下文不能用作共享全局信息的位置（因为信息不是真正的全局信息）。 使用像数据库这样的外部资源。
ServletContext对象包含在ServletConfig对象中，Web服务器在servlet初始化时提供servlet。
Defines a set of methods that a servlet uses to communicate with its servlet container, for example, to get the MIME type of a file, dispatch requests, or write to a log file.
There is one context per "web application" per Java Virtual Machine. (A "web application" is a collection of servlets and content installed under a specific subset of the server's URL namespace such as /catalog and possibly installed via a .war file.)
In the case of a web application marked "distributed" in its deployment descriptor, there will be one context instance for each virtual machine. In this situation, the context cannot be used as a location to share global information (because the information won't be truly global). Use an external resource like a database instead.
The ServletContext object is contained within the ServletConfig object, which the Web server provides the servlet when the servlet is initialized.