java并发编程实践 笔记（1）

线程安全 什么是线程安全(thread-safe)？ 一个类，如果能够在多线程并发访问的环境下（不管线程访问的顺序）正确表现自己的行为，并且无需外部调用添加任何同步之类的操作，这个类就是线程安全的。
这个正确性可以这么理解，就是说多线程访问的结果不会不同于单线程的访问。
线程安全的类不需要外部调用提供任何附加的同步。 无状态(stateless)的类永远是线程安全的。
什么是无状态的类？没有实例变量(field)，也没有引用其它类的field。 原子性 A race condition occurs when the correctness of a computation depends on the relative timing or interleaving of multiple threads by the runtime; in other words, when getting the right answer relies on lucky timing. 最常见的race condition是check-and-act。要防止这种race condition，我们需要原子性的操作。什么是原子性呢，就是说，你这个操作在执行的过程中，对于你操作的状态上其它的操作（包括你自己）要么全都执行完了，要么还没开始。
想说句通顺的中国话怎么这么难啊，还是给出原文吧： Operations A and B are atomic with respect to each other if, from the perspective of a thread executing A, when another thread executes B, either all of B has executed or none of it has. An atomic operation is one that is atomic with respect to all operations, including itself, that operate on the same state. 来个例子 @NotThreadSafe
public class UnsafeCountingFactorizer implements Servlet {
private long count = 0;
public long getCount() { return count; }
public void service(ServletRequest req, ServletResponse resp) {
BigInteger i = extractfromRequest(req);
BigInteger[] factors = factor(i);
++count;
encodeIntoResponse(resp, factors);
}
}
@ThreadSafe
public class CountingFactorizer implements Servlet {
private final AtomicLong count = new AtomicLong(0);
public long getCount() { return count.get(); }
public void service(ServletRequest req, ServletResponse resp) {
BigInteger i = extractfromRequest(req);
BigInteger[] factors = factor(i);
count.incrementAndGet();