我想在其他好的答案之上添加一些更多的信息。

线程安全意味着多个线程可以在同一个对象中读写数据，而不会出现内存不一致错误。在高度多线程的程序中，线程安全的程序不会对共享数据造成副作用。

看看这个SE问题，了解更多细节:

线程安全是什么意思?

线程安全程序保证内存一致性。

从oracle文档页高级并发API:

内存一致性属性:

Java™语言规范的第17章定义了内存操作(如共享变量的读写)的happens-before关系。只有当写操作发生时(在读操作之前)，一个线程的写操作的结果才保证对另一个线程的读操作可见。

synchronized和volatile结构，以及Thread.start()和Thread.join()方法，可以形成happens-before关系。

concurrent中所有类的方法及其子包将这些保证扩展到更高级别的同步。特别是:

Actions in a thread prior to placing an object into any concurrent collection happen-before actions subsequent to the access or removal of that element from the collection in another thread. Actions in a thread prior to the submission of a Runnable to an Executor happen-before its execution begins. Similarly for Callables submitted to an ExecutorService. Actions taken by the asynchronous computation represented by a Future happen-before actions subsequent to the retrieval of the result via Future.get() in another thread. Actions prior to "releasing" synchronizer methods such as Lock.unlock, Semaphore.release, and CountDownLatch.countDown happen-before actions subsequent to a successful "acquiring" method such as Lock.lock, Semaphore.acquire, Condition.await, and CountDownLatch.await on the same synchronizer object in another thread. For each pair of threads that successfully exchange objects via an Exchanger, actions prior to the exchange() in each thread happen-before those subsequent to the corresponding exchange() in another thread. Actions prior to calling CyclicBarrier.await and Phaser.awaitAdvance (as well as its variants) happen-before actions performed by the barrier action, and actions performed by the barrier action happen-before actions subsequent to a successful return from the corresponding await in other threads.

线程安全代码按照指定的方式工作，即使由不同的线程同时输入。这通常意味着，应该不间断地运行的内部数据结构或操作受到保护，不会同时进行不同的修改。

一个更容易理解的方法是，是什么使代码不是线程安全的。有两个主要问题会使线程应用程序产生不需要的行为。

Accessing shared variable without locking This variable could be modified by another thread while executing the function. You want to prevent it with a locking mechanism to be sure of the behavior of your function. General rule of thumb is to keep the lock for the shortest time possible. Deadlock caused by mutual dependency on shared variable If you have two shared variable A and B. In one function, you lock A first then later you lock B. In another function, you start locking B and after a while, you lock A. This is a potential deadlock where first function will wait for B to be unlocked when second function will wait for A to be unlocked. This issue will probably not occur in your development environment and only from time to time. To avoid it, all locks must always be in the same order.

线程安全代码是指即使有多个线程同时执行也能正常工作的代码。

http://mindprod.com/jgloss/threadsafe.html

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。

是也不是。

线程安全不仅仅是确保共享数据一次只能被一个线程访问。您必须确保对共享数据的顺序访问，同时避免竞争条件、死锁、活动锁和资源短缺。

当多个线程同时运行时，不可预知的结果并不是线程安全代码的必要条件，但这通常是一种副产品。例如，您可以使用一个共享队列、一个生产者线程和几个消费者线程来设置生产者-消费者方案，并且数据流可能完全可预测。如果你开始引入更多的消费者，你会看到更多随机的结果。