使用Java 8 Clock对类进行单元测试 [英] Unit testing a class with a Java 8 Clock

问题描述

Java 8引入了 java.time.Clock ，它可以用作许多其他 java.time 的参数对象，允许您向其中注入真实或假的时钟。例如，我知道您可以创建 Clock.fixed（）然后调用 Instant.now（时钟）并且它将返回您提供的固定 Instant 。这听起来非常适合单元测试！

Java 8 introduced java.time.Clock which can be used as an argument to many other java.time objects, allowing you to inject a real or fake clock into them. For example, I know you can create a Clock.fixed() and then call Instant.now(clock) and it will return the fixed Instant you provided. This sounds perfect for unit testing!

然而，我无法弄清楚如何最好地使用它。我有一个类，类似于以下内容：

However, I'm having trouble figuring out how best to use this. I have a class, similar to the following:

public class MyClass {
    private Clock clock = Clock.systemUTC();

    public void method1() {
        Instant now = Instant.now(clock);
        // Do something with 'now'
    }
}

现在，我想对这段代码进行单元测试。我需要能够设置 clock 来产生固定时间，以便我可以在不同时间测试 method（）。显然，我可以使用反射将时钟成员设置为特定值，但如果我不必求助于反射，那就太好了。我可以创建一个公共 setClock（）方法，但这感觉不对。我不想在方法中添加 Clock 参数，因为实际代码不应该考虑传入时钟。

Now, I want to unit test this code. I need to be able to set clock to produce fixed times so that I can test method() at different times. Clearly, I could use reflection to set the clock member to specific values, but it would be nice if I didn't have to resort to reflection. I could create a public setClock() method, but that feels wrong. I don't want to add a Clock argument to the method because the real code shouldn't be concerned with passing in a clock.

处理此问题的最佳方法是什么？这是新代码，所以我可以重新组织这个类。

What is the best approach for handling this? This is new code so I could reorganize the class.

编辑：为了澄清，我需要能够构建一个 MyClass object但是能够让那个对象看到两个不同的时钟值（好像它是一个常规的系统时钟）。因此，我无法将固定时钟传递给构造函数。

To clarify, I need to be able to construct a single MyClass object but be able to have that one object see two different clock values (as if it were a regular system clock ticking along). As such, I cannot pass a fixed clock into the constructor.

推荐答案

让我把Jon Skeet的答案和注释放入代码中：

Let me put Jon Skeet's answer and the comments into code:

正在测试的课程：

public class Foo {
    private final Clock clock;
    public Foo(Clock clock) {
        this.clock = clock;
    }

    public void someMethod() {
        Instant now = clock.instant();   // this is changed to make test easier
        System.out.println(now);   // Do something with 'now'
    }
}

单元测试：

public class FooTest() {

    private Foo foo;
    private Clock mock;

    @Before
    public void setUp() {
        mock = mock(Clock.class);
        foo = new Foo(mock);
    }

    @Test
    public void ensureDifferentValuesWhenMockIsCalled() {
        Instant first = Instant.now();                  // e.g. 12:00:00
        Instant second = first.plusSeconds(1);          // 12:00:01
        Instant thirdAndAfter = second.plusSeconds(1);  // 12:00:02

        when(mock.instant()).thenReturn(first, second, thirdAndAfter);

        foo.someMethod();   // string of first
        foo.someMethod();   // string of second
        foo.someMethod();   // string of thirdAndAfter 
        foo.someMethod();   // string of thirdAndAfter 
    }
}

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