Shulou(Shulou.com)06/03 Report--

In this issue, the editor will bring you what are the advantages of the singleton model. The article is rich in content and analyzes and narrates it from a professional point of view. I hope you can get something after reading this article.

Singleton mode has the following advantages: 1, it can ensure that all objects access the unique instance; 2, because the class controls the instantiation process, the class can flexibly change the instantiation process; 3, because there is only one instance, so reduce memory overhead and system performance overhead.

Singleton pattern is one of the simplest forms of design patterns. The purpose of this pattern is to make an object of the class the only instance in the system. To achieve this, you can start by instantiating it by the client. Therefore, you need to use a mechanism that allows only unique instances of the object class to be generated to "block" access to all objects that you want to generate. Use the factory method to restrict the instantiation process. This method should be static (class method), because it makes no sense for an instance of the class to generate another unique instance.

Singleton pattern motivation

For some classes in the system, only one instance is important, for example, there can be multiple print tasks in a system, but only one working task; a system can have only one window manager or file system; and a system can have only one timing tool or ID (serial number) generator. In Windows, only one task manager can be opened. If you do not use the mechanism to unify the window object, multiple windows will pop up, and if the contents of these windows are exactly the same, they will repeat the objects and waste memory resources; if the contents of these windows are inconsistent, it means that the system has multiple states at a certain moment, which is not consistent with the reality, and will also bring misunderstandings to users, not knowing which one is the real state. So sometimes it's important to make sure that an object in the system is unique, that is, there can be only one instance of a class. [2]

How to ensure that there is only one instance of a class and that this instance is easy to access? Defining a global variable ensures that objects are accessible at any time, but it does not prevent us from instantiating multiple objects. A better solution is to make the class itself responsible for saving the only instance of it. This class ensures that no other instance is created, and it provides a method to access the instance. This is the pattern motivation of the singleton pattern.

Key points of singleton model

Obviously, there are three main points of the singleton pattern: first, there can be only one instance of a class; second, it must create the instance itself; and third, it must provide the instance to the entire system.

From the perspective of specific implementation, it is the following three points: first, the class of the singleton pattern only provides a private constructor, and the second is that the class definition contains a static private object of the class. Third, the class provides a static public function to create or obtain its own static private object.

In the object diagram below, there is a "singleton object", while "customer A", "customer B" and "customer C" are the three customer objects of the singleton object. As you can see, all customer objects share a singleton object. And from the connection line from the singleton object to itself, you can see that the singleton object holds a reference to itself.

Some resource managers are often designed in singleton mode.

In a computer system, the resources that need to be managed include software external resources, for example, each computer can have several printers, but only one Printer Spooler to prevent two print jobs from being output to the printer at the same time. Each computer can have several fax cards, but only one software should be responsible for managing fax cards to avoid two fax jobs being sent to the fax card at the same time. Each computer can have several communication ports, and the system should centrally manage these communication ports to prevent one communication port from being called by two requests at the same time.

The resources that need to be managed include the internal resources of the software. For example, most software has one or more properties files to hold the system configuration. Such a system should have an object to manage a properties file.

The internal resources of the software that need to be managed also include, for example, the components responsible for recording the number of visitors to the website, recording internal events and error messages in the software system, or checking the performance of the system. These components must be managed centrally and cannot be bullish.

These resource manager artifacts must have only one instance, which is one; they must initialize themselves, which is the second; and the third is to allow the entire system to access themselves. Therefore, they all meet the conditions of singleton pattern and are the application of singleton pattern.

These are the advantages of the singleton model shared by the editor. If you happen to have similar doubts, you might as well refer to the above analysis to understand. If you want to know more about it, you are welcome to follow the industry information channel.

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