什么时候具体用到'单例模式'.

Ref:　　stackoverflow

　　We use single global　LogManager  to maintain a set of shared state about Loggers and log services.

• Manages a hierarchical namespace of Logger objects. All named Loggers are stored in this namespace.
• Manages a set of logging control properties. These are simple key-value pairs that can be used by Handlers and other logging objects to configure themselves.

　　Every Java application has a single instance of class Runtime that allows the application to interface with the environment in which the application is running. The current runtime can be obtained from the getRuntime method.