您可以在实现状态模式时使用单例(以GoF书中所示的方式)。这是因为具体的State类没有自己的状态，而是根据上下文类执行它们的操作。

你也可以让抽象工厂成为一个单例。

当你加载一个配置属性对象时，无论是从数据库还是从文件中，将它作为一个单例都会有所帮助;没有理由在服务器运行时重新读取不会改变的静态数据。

当管理对整个应用程序共享的资源的访问时，应该使用单例，并且可能存在同一个类的多个实例将是破坏性的。确保对共享资源的访问线程安全是这种模式至关重要的一个很好的例子。

当使用单例时，你应该确保你不会意外地隐藏依赖关系。理想情况下，单例对象(就像应用程序中的大多数静态变量一样)在应用程序的初始化代码执行期间设置(c#可执行文件为静态void Main()， java可执行文件为静态void Main())，然后传递给所有其他需要它的实例化类。这有助于维护可测试性。

一个实例可以在Test::Builder中找到，这个类支持几乎所有现代Perl测试模块。Builder单例存储并代理测试流程的状态和历史记录(历史测试结果，计算运行的测试次数)以及测试输出流向等内容。这些都是协调由不同作者编写的多个测试模块以在单个测试脚本中一起工作所必需的。

The history of Test::Builder's singleton is educational. Calling new() always gives you the same object. First, all the data was stored as class variables with nothing in the object itself. This worked until I wanted to test Test::Builder with itself. Then I needed two Test::Builder objects, one setup as a dummy, to capture and test its behavior and output, and one to be the real test object. At that point Test::Builder was refactored into a real object. The singleton object was stored as class data, and new() would always return it. create() was added to make a fresh object and enable testing.

Currently, users are wanting to change some behaviors of Test::Builder in their own module, but leave others alone, while the test history remains in common across all testing modules. What's happening now is the monolithic Test::Builder object is being broken down into smaller pieces (history, output, format...) with a Test::Builder instance collecting them together. Now Test::Builder no longer has to be a singleton. Its components, like history, can be. This pushes the inflexible necessity of a singleton down a level. It gives more flexibility to the user to mix-and-match pieces. The smaller singleton objects can now just store data, with their containing objects deciding how to use it. It even allows a non-Test::Builder class to play along by using the Test::Builder history and output singletons.

似乎在数据的协调和行为的灵活性之间存在着一种推拉关系，这种关系可以通过在共享数据周围使用尽可能少的行为来缓解，以确保数据的完整性。

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。

读取应该只在启动时读取的配置文件，并将它们封装在Singleton中。