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。

使用单例的方法之一是覆盖一个实例，其中必须有一个“代理”控制对资源的访问。单例在日志记录器中很好，因为它们代理访问，比如说，一个文件，这个文件只能被写入。对于像日志这样的东西，它们提供了一种方法来抽象出对日志文件之类的东西的写操作——你可以将缓存机制包装到你的单例中，等等……

也可以考虑这样一种情况，你有一个应用程序，有许多窗口/线程等，但它需要一个单一的通信点。我曾经使用它来控制我希望应用程序启动的作业。单例程序负责将作业序列化，并将它们的状态显示给程序中其他感兴趣的部分。在这种情况下，你可以把单例对象看作是在应用程序中运行的“服务器”类……HTH

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.

只读单例存储一些全局状态(用户语言、帮助文件路径、应用程序路径)是合理的。使用单例控制业务逻辑时要小心——单例几乎总是以多例告终

首先，让我们区分一下单对象和单对象。后者是前者的许多可能实现之一。而且单对象的问题与单例的问题是不同的。单对象本身并不坏，有时是做事情的唯一方法。简而言之:

单对象-我只需要程序中对象的一个实例 Singleton -创建一个带有静态字段的类。添加一个返回此字段的静态方法。在第一次调用时惰性地实例化一个字段。总是返回相同的对象。

public class Singleton {
    private static Singleton instance;

    private Singleton() {}

    public static Singleton instance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

如您所见，规范形式的“单例”模式对测试不是很友好。不过，这个问题很容易解决:只要让Singleton实现一个接口。让我们称它为“可测试单例”:)

public class Singleton implements ISingleton {
    private static Singleton instance;

    private Singleton() {}

    public static ISingleton instance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

现在我们可以模拟Singleton了，因为我们通过接口来使用它。其中一项索赔消失了。让我们看看是否可以去掉另一个声明-共享全局状态。

如果我们剥离单例模式，它的核心是惰性初始化:

public static ISingleton instance() {
    if (instance == null) {
        instance = new Singleton();
    }
    return instance;
}

这就是它存在的全部原因。这就是单对象模式。我们把它拿走，放到工厂方法中，例如:

public class SingletonFactory {
    private static ISingleton instance;

    // Knock-knock. Single Object here
    public static ISingleton simpleSingleton() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

与我们的可测试单例有什么不同?没有，因为这是单对象模式的本质——不管你是把它实现为单例、工厂方法还是服务定位器。你仍然有一些共享的全局状态。如果从多个线程访问它，这可能会成为一个问题。您将不得不使simpleSingleton()同步并处理所有多线程问题。

再说一遍:无论你选择什么方法，你都必须付出单一对象的代价。使用依赖注入容器只是将复杂性转移到框架，它将不得不处理单对象的固有问题。

回顾:

Most of people who mention Singleton mean Single Object One of the popular ways to implement it is the Singleton pattern It has its flaws that can be mitigated However, the most of Singleton's complexity roots in Single Object's complexity Regardless of how you instantiate your Single Object, it's still there, be it a Service Locator, a Factory Method or something else You can shift the complexity to a DI container which is (hopefully) well-tested Sometimes using the DI container is cumbersome - imagine injecting a LOGGER to every class