java - 为什么单例类很难测试？

Question

Effective Java 第 3 项(使用私有(private)构造函数或枚举类型实现单例属性)指出:

Making a class a singleton can make it difficult to test its clients, as it's impossible to substitute a mock implementation for a singleton unless it implements an interface that serves as its type.

出于测试目的，为什么仅实例化一个单例实例并测试其 API 是不够的？那不是客户会消费的东西吗？引用似乎暗示测试单例将涉及“模拟实现”，但为什么有必要这样做？

我看到过各种“解释”，它们或多或少是对上述引述的改写。有人可以进一步解释这一点，最好是用代码示例吗？

Answer 1

如果您的单例正在对数据库执行操作或将数据写入文件怎么办？您不希望在单元测试中发生这种情况。您可能希望模拟该对象以在内存中执行一些操作，这样您就可以验证它们而不会产生永久的副作用。单元测试应该是独立的，不应创建与数据库的连接或与外部系统执行其他可能失败的操作，然后由于不相关的原因导致单元测试失败。

使用伪 java 的示例(我是 C# 开发人员):

public class MySingleton {

    private static final MySingleton instance = new MySingleton();

    private MySingleton() { }

    public int doSomething() {
        //create connection to database, write to a file, etc..
        return something;
    }

    public static MySingleton getInstance() {
        return instance;
    }
}

public class OtherClass {

        public int myMethod() {
            //do some stuff
            int result = MySingleton.getInstance().doSomething();

            //do some other suff
            return something;
        }
}

为了测试myMethod，我们必须进行实际的数据库调用、文件操作等

@Test
public void testMyMethod() {
    OtherClass obj = new OtherClass();

    //if this fails it might be because of some external code called by 
    //MySingleton.doSomething(), not necessarily the logic inside MyMethod()

    Asserts.assertEqual(1, obj.myMethod());
}

如果 MySingleton 是这样的:

public class MyNonSingleton implements ISomeInterface {

    public MyNonSingleton() {}

    @Override
    public int doSomething() {
        //create connection to database, write to a file, etc..
        return something;
    }

}

然后您可以像这样将它作为依赖注入(inject)到 MyOtherClass 中:

public class OtherClass {

    private ISomeInterface obj;

    public OtherClass(ISomeInterface obj) {
        this.obj = obj;
    }

    public int myMethod() {
        //do some stuff
        int result = obj.doSomething();

        //do some other stuff
        return something;
    }
}

然后你可以这样测试:

@Test
public void TestMyMethod() {
    OtherClass obj = new OtherClass(new MockNonSingleton());

    //now our mock object can fake the database, filesystem etc. calls to isolate the testing to just the logic in myMethod()

    Asserts.assertEqual(1, obj.myMethod());
}