1.线程安全性

一.原子性

无状态对象一定是线程安全的，假设我们希望增加一个“命中计数器”来统计所处理的请求数量，一种直观的方法是增加一个long类型的域。

@NotThreadSafe
public class UnsafeCountingFactorizer extends GenericServlet 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);
    }

    void encodeIntoResponse(ServletResponse res, BigInteger[] factors) {
    }

    BigInteger extractFromRequest(ServletRequest req) {
        return new BigInteger("7");
    }

    BigInteger[] factor(BigInteger i) {
        // Doesn't really factor
        return new BigInteger[] { i };
    }
}

UnsafeCountingFactorizer 并非线程安全的，因为递增操作++count并非原子的。它包含三个独立的操作：读取count的值，将值加1，然后将计算结果写入count。

1.竞态条件

UnsafeCountingFactorizer 中存在多个竞态条件。当某个计算的正确性取决于多个线程的交替执行时序时，那么就会发生竞态条件。最常见的竞态条件类型就是“先检查后执行”操作。
2.复合操作

@ThreadSafe
public class CountingFactorizer extends GenericServlet 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();
        encodeIntoResponse(resp, factors);
    }

    void encodeIntoResponse(ServletResponse res, BigInteger[] factors) {}
    BigInteger extractFromRequest(ServletRequest req) {return null; }
    BigInteger[] factor(BigInteger i) { return null; }
}

通过用AtoimcLong来代替long类型的计数器，能够确保所有对计数器状态的访问操作都是原子的。

二.加锁机制

假设我们希望提升Servlet的性能：将最近的计算结果缓存起来，当两个连续的请求对相同的数值进行因数分解时，可以直接使用上一次的计算结果，而无需重新计算。

@NotThreadSafe
public class UnsafeCachingFactorizer extends GenericServlet implements Servlet {
    private final AtomicReference<BigInteger> lastNumber
            = new AtomicReference<BigInteger>();
    private final AtomicReference<BigInteger[]> lastFactors
            = new AtomicReference<BigInteger[]>();

    public void service(ServletRequest req, ServletResponse resp) {
        BigInteger i = extractFromRequest(req);
        if (i.equals(lastNumber.get()))
            encodeIntoResponse(resp, lastFactors.get());
        else {
            BigInteger[] factors = factor(i);
            lastNumber.set(i);
            lastFactors.set(factors);
            encodeIntoResponse(resp, factors);
        }
    }

    void encodeIntoResponse(ServletResponse resp, BigInteger[] factors) {
    }

    BigInteger extractFromRequest(ServletRequest req) {
        return new BigInteger("7");
    }

    BigInteger[] factor(BigInteger i) {
        // Doesn't really factor
        return new BigInteger[]{i};
    }
}

这种方法并不正确。尽管这些原子引用本身都是线程安全的，但在UnsafeCachingFactorizer 中存在着竞态条件，这可能产生错误的结果。尽管对set方法每次调用都是原子的，但仍然无法同时更新lastNumber和lastFactores。在线程A获取这两个值的过程中，线程B可能修改了它们。

1.内置锁

Java提供了一种内置的锁机制来支持原子性：同步代码块。

@ThreadSafe
public class SynchronizedFactorizer extends GenericServlet implements Servlet {
    @GuardedBy("this") private BigInteger lastNumber;
    @GuardedBy("this") private BigInteger[] lastFactors;

    public synchronized void service(ServletRequest req,
                                     ServletResponse resp) {
        BigInteger i = extractFromRequest(req);
        if (i.equals(lastNumber))
            encodeIntoResponse(resp, lastFactors);
        else {
            BigInteger[] factors = factor(i);
            lastNumber = i;
            lastFactors = factors;
            encodeIntoResponse(resp, factors);
        }
    }

    void encodeIntoResponse(ServletResponse resp, BigInteger[] factors) {
    }

    BigInteger extractFromRequest(ServletRequest req) {
        return new BigInteger("7");
    }

    BigInteger[] factor(BigInteger i) {
        // Doesn't really factor
        return new BigInteger[] { i };
    }
}

2.重入

三.用锁来保护状态