如何对异常进行单元测试? [英] How to do unit test for Exceptions?

问题描述

如您所知，异常是在出现异常场景的情况下抛出的.那么如何模拟这些异常呢?我觉得这是挑战.对于此类代码片段:

As you know, exception is thrown at the condition of abnormal scenarios. So how to analog these exceptions? I feel it is challenge. For such code snippets:

public String getServerName() {
    try {

        InetAddress addr = InetAddress.getLocalHost();
        String hostname = addr.getHostName();
        return hostname;
    }
    catch (Exception e) {
        e.printStackTrace();
        return "";
    }
}

有人有好的想法吗?

推荐答案

其他答案已经解决了如何编写检查是否抛出异常的单元测试的一般问题.但是我认为您的问题实际上是在询问如何让代码首先抛出异常.

Other answers have addressed the general problem of how to write a unit test that checks that an exception is thrown. But I think your question is really asking about how to get the code to throw the exception in the first place.

以您的代码为例.在简单的单元测试上下文中，很难让您的 getServerName() 在内部抛出异常.问题在于，为了发生异常，代码(通常)需要在网络中断的机器上运行.安排在单元测试中发生这种情况可能是不可能的......在运行测试之前，您需要故意错误配置机器.

Take your code as an example. It would be very hard to cause your getServerName() to internally throw an exception in the context of a simple unit test. The problem is that in order for the exception to happen, the code (typically) needs to be run on a machine whose networking is broken. Arranging for that to happen in a unit test is probably impossible ... you'd need to deliberately misconfigure the machine before running the test.

那么答案是什么?

1. 在某些情况下，简单的答案就是做出务实的决定，而不是进行全面的测试.你的方法就是一个很好的例子.从代码检查中应该清楚该方法实际做什么.测试它不会证明任何事情(除了见下面的**).您所做的只是提高您的测试数量和测试覆盖率，这两者都不应该是项目目标.

1. In some cases, the simple answer is just to take the pragmatic decision and not go for total test coverage. Your method is a good example. It should be clear from code inspection what the method actually does. Testing it is not going to prove anything (except see below **). All you are doing is improve your test counts and test coverage numbers, neither of which should be project goals.

在其他情况下，将产生异常的低级代码分离出来并使其成为一个单独的类可能是明智的.然后，为了测试更高级别代码对异常的处理，您可以将类替换为将抛出所需异常的模拟类.

In other cases, it may be sensible to separate out the low-level code where the exception is being generated and make it a separate class. Then, to test the higher level code's handling of the exception, you can replace the class with a mock class that will throw the desired exceptions.

这是您给出这种治疗"的示例.(这有点做作...)

Here is your example given this "treatment". (This is a bit contrived ... )

public interface ILocalDetails {
    InetAddress getLocalHost() throws UnknownHostException;
    ...
}

<小时>

public class LocalDetails implements ILocalDetails {
    public InetAddress getLocalHost() throws UnknownHostException {
        return InetAddress.getLocalHost();
    }
}

<小时>

public class SomeClass {
    private ILocalDetails local = new LocalDetails();  // or something ...
    ...
    public String getServerName() {
        try {
            InetAddress addr = local.getLocalHost();
            return addr.getHostName();
        }
        catch (Exception e) {
            e.printStackTrace();
            return "";
        }
    }
}

现在要对此进行单元测试，您创建 ILocalDetails 接口的模拟"实现，其 getLocalHost() 方法在适当的条件下抛出您想要的异常.然后你为 SomeClass.getServerName() 创建一个单元文本，安排 SomeClass 的实例使用你的模拟"类的实例而不是普通的实例.(最后一点可以使用模拟框架完成，通过为 local 属性公开一个 setter 或使用反射 API.)

Now to unit test this, you create a "mock" implementation of the ILocalDetails interface whose getLocalHost() method throws the exception you want under the appropriate conditions. Then you create a unit text for SomeClass.getServerName(), arranging that the instance of SomeClass uses an instance of your "mock" class instead of the normal one. (The last bit could be done using a mocking framework, by exposing a setter for the local attribute or by using the reflection APIs.)

显然，您需要修改代码以使其像这样可测试.并且您可以执行的操作是有限的……例如，您现在无法创建单元测试来使真正的 LocalDetails.getLocalHost() 方法抛出异常.您需要逐案判断是否值得这样做；即单元测试的好处是否超过使类以这种方式可测试的工作(和额外的代码复杂性).(事实上​​，在这个底部有一个 static 方法是问题的很大一部分.)

Obviously, you would need to modify your code to make it testable like this. And there are limits to what you can do ... for example, you now cannot create a unit test to make the real LocalDetails.getLocalHost() method to throw an exception. You need to make a case-by-case judgement as to whether it is worth the effort of doing this; i.e. does the benefit of the unit test outweigh the work (and extra code complexity) of making the class testable in this way. (The fact that there is a static method at the bottom of this is a large part of the problem.)

** 对于这种测试，有 一个假设点.在您的示例中，原始代码捕获异常并返回空字符串这一事实 可能是一个错误......取决于方法的 API 是如何指定的......并且假设的单元测试将选择起来.但是，在这种情况下，该错误非常明显，以至于您在编写单元测试时都会发现它！假设您在发现错误时修复它们，单元测试就变得有些多余了.(你不会期望有人重新修复这个特定的错误......)

** There is a hypothetical point to this kind of testing. In your example, the fact that the original code catches an exception and returns an empty string could be a bug ... depending on how the method's API is specified ... and a hypothetical unit test would pick it up. However, in this case, the bug is so blatant that you would spot it while writing the unit test! And assuming that you fix bugs as you find them, the unit test becomes somewhat redundant. (You wouldn't expect someone to re-instate this particular bug ...)

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