单元测试是在不需要该代码单元所依赖的基础设施的情况下对代码单元(例如单个函数)进行测试。即在隔离环境中进行测试。

例如，如果您正在测试的函数连接到数据库并执行更新，那么在单元测试中您可能不想执行该更新。如果这是一个积分测试，你会这样做，但在这个例子中，它不是。

因此，单元测试将测试您正在测试的“函数”中所包含的功能，而不会产生数据库更新的副作用。

假设函数从数据库中检索了一些数字，然后执行标准偏差计算。你想测试什么?标准偏差计算正确，还是数据从数据库返回?

在单元测试中，您只想测试标准偏差的计算是否正确。在集成测试中，您希望测试标准偏差计算和数据库检索。

单元测试是一种实践，以确保您将要实现的功能或模块的行为符合预期(需求)，并确保它在边界条件和无效输入等场景下的行为。

xUnit, NUnit, mbUnit等都是帮助你编写测试的工具。

我在大学里从未学过单元测试，我花了一段时间才“学会”它。我读到它，心想“啊，对，自动化测试，我想那应该很酷”，然后我就忘记了。

It took quite a bit longer before I really figured out the point: Let's say you're working on a large system and you write a small module. It compiles, you put it through its paces, it works great, you move on to the next task. Nine months down the line and two versions later someone else makes a change to some seemingly unrelated part of the program, and it breaks the module. Worse, they test their changes, and their code works, but they don't test your module; hell, they may not even know your module exists.

现在你有了一个问题:坏代码在主干中，甚至没有人知道。最好的情况是内部测试人员在您发布之前就发现了它，但是在游戏后期修复代码的成本很高。如果没有内部测试人员发现它……嗯，这确实会非常昂贵。

The solution is unit tests. They'll catch problems when you write code - which is fine - but you could have done that by hand. The real payoff is that they'll catch problems nine months down the line when you're now working on a completely different project, but a summer intern thinks it'll look tidier if those parameters were in alphabetical order - and then the unit test you wrote way back fails, and someone throws things at the intern until he changes the parameter order back. That's the "why" of unit tests. :-)

单元测试与“只是打开一个新项目并测试这个特定的代码”的主要区别在于它是自动化的，因此是可重复的。

如果您手动测试代码，它可能会使您相信代码在当前状态下工作得很完美。但一周后，当你对它做了轻微的修改时呢?您是否愿意在代码发生任何更改时再次手工重新测试?很可能不会:-(

但是，如果您可以在任何时间运行您的测试，只需单击一下，以完全相同的方式，在几秒钟内，那么一旦有什么东西坏了，它们就会立即显示给您。如果您还将单元测试集成到您的自动化构建过程中，即使在看似完全无关的更改破坏了代码库中遥远部分的某些情况下，它们也会提醒您错误——当您甚至不会想到需要重新测试特定的功能时。

这是单元测试相对于手工测试的主要优势。但等等，还有更多:

unit tests shorten the development feedback loop dramatically: with a separate testing department it may take weeks for you to know that there is a bug in your code, by which time you have already forgotten much of the context, thus it may take you hours to find and fix the bug; OTOH with unit tests, the feedback cycle is measured in seconds, and the bug fix process is typically along the lines of an "oh sh*t, I forgot to check for that condition here" :-) unit tests effectively document (your understanding of) the behaviour of your code unit testing forces you to reevaluate your design choices, which results in simpler, cleaner design

反过来，单元测试框架使您更容易编写和运行测试。

This is my take on it. I would say unit testing is the practice of writing software tests to verify that your real software does what it is meant to. This started with jUnit in the Java world and has become a best practice in PHP as well with SimpleTest and phpUnit. It's a core practice of Extreme Programming and helps you to be sure that your software still works as intended after editing. If you have sufficient test coverage, you can do major refactoring, bug fixing or add features rapidly with much less fear of introducing other problems.

当所有单元测试都能自动运行时，这是最有效的。

单元测试通常与OO开发相关联。基本思想是创建一个脚本，为你的代码设置环境，然后练习它;您编写断言，指定您应该接收的预期输出，然后使用如上所述的框架执行您的测试脚本。

框架将针对您的代码运行所有测试，然后报告每个测试的成功或失败。phpUnit默认情况下从Linux命令行运行，尽管有HTTP接口可用于它。SimpleTest本质上是基于web的，在我看来，它更容易启动和运行。结合xDebug, phpUnit可以为您提供代码覆盖率的自动统计数据，有些人认为这非常有用。

有些团队从他们的subversion存储库编写钩子，以便在提交更改时自动运行单元测试。

将单元测试保存在与应用程序相同的存储库中是一个很好的实践。

在单元测试和TDD的哲学优势方面，这里有一些关键的“灯泡”观察，这些观察在我试探性地走上TDD启蒙之路的第一步时打动了我(没有原创或一定是新闻)……

TDD does NOT mean writing twice the amount of code. Test code is typically fairly quick and painless to write and is a key part of your design process and critically. TDD helps you to realize when to stop coding! Your tests give you confidence that you've done enough for now and can stop tweaking and move on to the next thing. The tests and the code work together to achieve better code. Your code could be bad / buggy. Your TEST could be bad / buggy. In TDD you are banking on the chances of BOTH being bad / buggy being fairly low. Often its the test that needs fixing but that's still a good outcome. TDD helps with coding constipation. You know that feeling that you have so much to do you barely know where to start? It's Friday afternoon, if you just procrastinate for a couple more hours... TDD allows you to flesh out very quickly what you think you need to do, and gets your coding moving quickly. Also, like lab rats, I think we all respond to that big green light and work harder to see it again! In a similar vein, these designer types can SEE what they're working on. They can wander off for a juice / cigarette / iphone break and return to a monitor that immediately gives them a visual cue as to where they got to. TDD gives us something similar. It's easier to see where we got to when life intervenes... I think it was Fowler who said: "Imperfect tests, run frequently, are much better than perfect tests that are never written at all". I interprete this as giving me permission to write tests where I think they'll be most useful even if the rest of my code coverage is woefully incomplete. TDD helps in all kinds of surprising ways down the line. Good unit tests can help document what something is supposed to do, they can help you migrate code from one project to another and give you an unwarranted feeling of superiority over your non-testing colleagues :)

这篇演讲很好地介绍了测试所需要的所有内容。