4->The Singleton Pattern（刀客羽朋预定）

4
The Singleton
Pattern
IN NEARLY EVERY OBJECT-oriented program, there are usually one or two resources that are created
once and shared for the duration of the entire application. For example, a database connection
in an e-commerce application is one such resource: it’s initialized when the application
launches, is used to effectuate all transactions, and is finally disconnected and destroyed when the
program ends. In your code, there’s no need to conjure a database connection each and every time;
that’s a hassle and very inefficient. Instead, your code can simply re-use the connection that’s already
been established. The challenge then is how do you refer to the connection (or to any other unique
perennial resource, such as an open file or a queue).
The Problem
How do you ensure that an instance of a particular class is exclusive (it’s always the lone instance of
that class) yet is also readily-accessible?

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