我认为单例的使用与数据库中的多对一关系是一样的。如果代码中有许多不同的部分需要处理对象的单个实例，那么使用单例就很有意义了。

当需要管理共享资源时，可以使用单例。例如打印机假脱机程序。您的应用程序应该只有一个假脱机程序实例，以避免对相同资源的请求冲突。

或者数据库连接或者文件管理器等等。

Singleton pattern is the most pervasive pattern in the Spring containerization approach. If we look at that in terms of architectural primitives - they form a blackboard graph of objects, to which every thread can read and write. They do the dramatic act of synchronizing between multiple threads. The very reason why multiple threads need to synchronize is because there are always resources that underlie a computational program, over which contention might occur. Consider what is called a 'last seat problem'. A flight is being booked, but there are multiple ways to do it. For simplicity lets assume that the data about the flight occupancy is stored in a flat file rather than a database. Now, if there are two threads, each functionally different (i.e represented by different endpoints in the webapp) and let one of these threads A, be the thread which a prospective passenger uses to make a booking and the other one B is which a flight manager uses to close the booking - virtually closing the boarding door. Then, if these threads do not use singleton, the flight object would be detached from the real resource out-there, which we say not the actual aeroplane but the entry in the flat file. The A thread would have reference to an object, while the passenger is still fighting a dilemma whether to fly or not and then finally when he makes up his mind, the B thread would already have closed the door. But the object referenced by the A thread would still show one more seat to go. Now, cutting out the RDBMS due to our initial assumption, the system would write a ticket for the passenger and issue it to him eventhough the boarding is closed. Now, in a singleton implementation, the moment the theread B accesses the system, the universal object Flight is updated with status closed. Hence, if the passenger finally makes up his mind and clicks confirm, he would get an error right away. All this would not have been possible without the singleton. Hence, singleton allows you to stay close to the resources and avoids thread contention.

我认为如果你的应用程序有多个层次，如表示，领域和模型。Singleton是横切层的一个很好的候选者。并为系统各层提供服务。

从本质上讲，单例包装了一个服务，例如日志记录、分析，并将其提供给系统中的其他层。

是的，单例需要遵循单一责任原则。

将特定的基础设施关注点配置为单例或全局变量是非常实用的。我最喜欢的例子是依赖注入框架，它使用单例作为框架的连接点。

在这种情况下，您将依赖于基础设施来简化库的使用并避免不必要的复杂性。

管理到数据库的连接(或连接池)。

我还会使用它来检索和存储外部配置文件中的信息。