What is a singleton? Why use a singleton? A class is designed, on behalf of which represents a certain behavior (method), attributes (member variables), but in general, when we want to use this class, use the new keyword, this time will help us construct jvm an instance of the class. And we know that for new keyword and that instance, by comparison, is relatively resource-intensive. So if we want to be able to start in a new way when good jvm, or in a new one after a good example, will no longer need this kind of action, you can save a lot of resources. What classes can use a singleton? In general, we always want to stateless class can be designed as a single example, that there is nothing to this state represent? In simple terms, for the same example, if multiple threads simultaneously, and does not use additional means of thread synchronization, thread synchronization problem does not occur, we can be considered stateless, then a simple point: is not a class member variable, or its member variables are stateless, we can consider the design as a single case. Implementation Well, we already know what is a single case, why use a single example, that we are going to continue to discuss how to achieve under a single case. In general, we can put into a singleton singleton on the singleton and manage the behavior. Singleton on behalf of the behavior no matter how (here aside cloneable, reflection), jvm instance start to finish in only one category, and single cases of upper management can be understood as: no matter who is going to use this class, must observe certain rules, for example, we use a class, only 'pick up' from a designated place, so get is the same class. For example on the single management, I believe that we are most familiar with is the spring, spring all the classes into a container for later use this class have to get out of the container, thus ensuring a single case. So here is the rest of us then talk about how to achieve a singleton behavior. In general, to achieve this single embodiment, there are two ideas, private constructor, enumeration.
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