What is java singleton pattern?

java singleton pattern is a common design pattern; it is a method to ensure that a class has only one instance and provides a global access point for the entire system (providing this instance to the entire system). There are three types of singleton patterns: lazy-style singleton, hungry-style singleton, and registered singleton.

Overview of singleton mode

Singleton mode (Singleton), also called monad mode, is a commonly used design pattern. When applying this pattern, the class of a singleton object must ensure that only one instance exists. Many times, the entire system only needs to have one global object, which helps us coordinate the overall behavior of the system. For example, in a server program, the server's configuration information is stored in a file. These configuration data are uniformly read by a singleton object, and then other objects in the service process obtain the configuration information through this singleton object. Obviously , this approach simplifies configuration management in complex environments.

Specially, in computer systems, thread pools, caches, log objects, dialog boxes, printers, and graphics card driver objects are often designed as singletons. In fact, these applications all have more or less the function of resource managers. For example, each computer can have several printers, but only one Printer Spooler (singleton) to prevent two print jobs from being output to the printer at the same time. For another example, each computer can have several communication ports, and the system should centrally (single case) manage these communication ports to prevent one communication port from being called by two requests at the same time. In short, the purpose of choosing the singleton mode is to avoid inconsistent states and avoid long-term policies.

To sum up, the singleton pattern is a method to ensure that a class has only one instance and provide a global access point for the entire system.

The singleton mode has the following characteristics:

1. A singleton class can only have one instance.

2. The singleton class must create its own unique instance.

3. The singleton class must provide this instance to all other objects.

The singleton pattern ensures that a class has only one instance, instantiates itself and provides this instance to the entire system.

The singleton pattern is one of the simplest forms of design patterns. The purpose of this pattern is to make an object of a class the only instance in the system. To achieve this, you start by instantiating it on the client side. Therefore, it is necessary to use a mechanism that only allows the generation of unique instances of the object class, "blocking" all access to the object that is intended to be generated. Use factory methods to limit the instantiation process. This method should be a static method (class method), because there is no point in having an instance of the class generate another unique instance.

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