一个信息量更大的问题是，是什么使代码不线程安全——答案是，有四个条件必须成立……想象一下下面的代码(它是机器语言翻译)

totalRequests = totalRequests + 1
MOV EAX, [totalRequests]   // load memory for tot Requests into register
INC EAX                    // update register
MOV [totalRequests], EAX   // store updated value back to memory

The first condition is that there are memory locations that are accessible from more than one thread. Typically, these locations are global/static variables or are heap memory reachable from global/static variables. Each thread gets its own stack frame for function/method scoped local variables, so these local function/method variables, otoh, (which are on the stack) are accessible only from the one thread that owns that stack. The second condition is that there is a property (often called an invariant), which is associated with these shared memory locations, that must be true, or valid, for the program to function correctly. In the above example, the property is that “totalRequests must accurately represent the total number of times any thread has executed any part of the increment statement”. Typically, this invariant property needs to hold true (in this case, totalRequests must hold an accurate count) before an update occurs for the update to be correct. The third condition is that the invariant property does NOT hold during some part of the actual update. (It is transiently invalid or false during some portion of the processing). In this particular case, from the time totalRequests is fetched until the time the updated value is stored, totalRequests does not satisfy the invariant. The fourth and final condition that must occur for a race to happen (and for the code to therefore NOT be "thread-safe") is that another thread must be able to access the shared memory while the invariant is broken, thereby causing inconsistent or incorrect behavior.

正如其他人所指出的，线程安全意味着如果一段代码同时被多个线程使用，那么它将正常工作。

值得注意的是，这有时是有代价的，计算机时间和更复杂的编码，所以它并不总是可取的。如果一个类只能安全地在一个线程上使用，那么这样做可能会更好。

例如，Java有两个几乎相同的类:StringBuffer和StringBuilder。不同之处在于StringBuffer是线程安全的，因此StringBuffer的单个实例可以同时被多个线程使用。StringBuilder不是线程安全的，它被设计为仅由一个线程构建String的情况下(绝大多数情况下)的高性能替代品。

不要将线程安全性与决定论混淆。线程安全代码也可以是非确定性的。考虑到使用线程代码调试问题的难度，这可能是正常的情况。: -)

线程安全只是确保当一个线程修改或读取共享数据时，没有其他线程可以以改变数据的方式访问它。如果代码依赖于特定的执行顺序来确保正确性，那么除了线程安全所需的同步机制之外，还需要其他同步机制来确保这一点。

是的，是的。它意味着数据不会被多个线程同时修改。然而，您的程序可能会像预期的那样工作，并且看起来是线程安全的，即使它根本不是。

请注意，结果的不可预测性是“竞态条件”的结果，它可能导致数据以与预期顺序不同的顺序被修改。

与其认为代码或类是线程安全的，我认为将操作视为线程安全的更有帮助。如果两个操作在从任意线程上下文运行时按照指定的方式运行，那么它们就是线程安全的。在许多情况下，类将以线程安全的方式支持一些操作组合，而其他则不支持。

例如，许多像数组列表和散列集这样的集合可以保证，如果它们最初只由一个线程访问，并且在引用对任何其他线程可见后永远不会被修改，那么它们可以被任何线程组合以任意方式读取而不受干扰。

More interestingly, some hash-set collections such as the original non-generic one in .NET, may offer a guarantee that as long as no item is ever removed, and provided that only one thread ever writes to them, any thread that tries to read the collection will behave as though accessing a collection where updates might be delayed and occur in arbitrary order, but which will otherwise behave normally. If thread #1 adds X and then Y, and thread #2 looks for and sees Y and then X, it would be possible for thread #2 to see that Y exists but X doesn't; whether or not such behavior is "thread-safe" would depend upon whether thread #2 is prepared to deal with that possibility.

As a final note, some classes--especially blocking communications libraries--may have a "close" or "Dispose" method which is thread-safe with respect to all other methods, but no other methods that are thread-safe with respect to each other. If a thread performs a blocking read request and a user of the program clicks "cancel", there would be no way for a close request to be issued by the thread that's attempting to perform the read. The close/dispose request, however, may asynchronously set a flag which will cause the read request to be canceled as soon as possible. Once close is performed on any thread, the object would become useless, and all attempts at future actions would fail immediately, but being able to asynchronously terminate any attempted I/O operations is better than require that the close request be synchronized with the read (since if the read blocks forever, the synchronization request would be likewise blocked).

简单地说，如果许多线程同时执行这段代码，代码将运行良好。