compareAndSet（）无法按预期工作

Question

我编写了自己的AtomicDouble类，还有一个BankAccount类，它执行两个简单的取款和存款操作，它有一个AtomicDouble实例（余额）。我的代码的问题是，当我在deposit（）中调用addAndGet方法时，程序会陷入一个无限循环，compareAndSet（）永远不会返回真值，但当我调试它时，currentValue和atomic中的值。get（）相等，但此方法无法理解。

有趣的是，当我将if（atomic.get（）==currentValue）替换为if（atomic.compareAndSet（currentValue，nextValue））时，程序运行正常。

public class AtomicDouble extends Number {

private final AtomicReference<Double> atomic;

public AtomicDouble() {
    this(0.0);
}

public AtomicDouble(double initialValue) {
    atomic = new AtomicReference<>(initialValue);
}

public final double addAndGet(double delta) {
    while (true) {
        double currentValue = atomic.get();
        double nextValue = currentValue + delta;
        if (atomic.compareAndSet(currentValue, nextValue))
            return nextValue;
    }
}

public final double incrementAndGet() {
    return addAndGet(1);
}

public final void set(double newValue) {
    atomic.set(newValue);
}

public final double get() {
    return atomic.get();
}


public final double getAndSet(double newValue) {
    return atomic.getAndSet(newValue);
}


public float floatValue() {
    return (float) get();
}

@Override
public double doubleValue() {
    return get();
}

public int intValue() {
    return (int) get();
}

public long longValue() {
    return (long) get();
}

public String toString() {
    return Double.toString(get());
}

}

public class BankAccount {
private final AtomicDouble balance;
private String accountNumber;

public BankAccount(double balance, String accountNumber) {
    this.balance = new AtomicDouble(balance);
    this.accountNumber = accountNumber;

}

public void deposit(double number, String color) {
    System.out.println(color + "deposit  " + number + " current balance=" + balance.addAndGet(number));
}

public void withdraw(double number, String color) {
    if (this.balance.get() - number >= 0) {
        System.out.println(color + "Withdraw " + number + " current balance=" + balance.addAndGet(-number));
        return;
    }
    System.out.println(color + "Not enough balance");
}


public static void main(String[] args) {

    BankAccount bankAccount = new BankAccount(1000.0, "4234236");

    ExecutorService threadsPool = Executors.newFixedThreadPool(2);
    threadsPool.execute(new Runnable() {
        @Override
        public void run() {
            bankAccount.deposit(300.0, ThreadColor.ANSI_YELLOW);
            bankAccount.withdraw(50.0, ThreadColor.ANSI_YELLOW);
        }
    });

    threadsPool.execute(new Runnable() {
        @Override
        public void run() {
            bankAccount.deposit(203.75, ThreadColor.ANSI_BLUE);
            bankAccount.withdraw(100.0, ThreadColor.ANSI_BLUE);
        }
    });

    threadsPool.shutdown();
}

}

输出：没有输出

Answer 1

我想这是因为自动装箱。你不能有一个引用双，你有一个引用双。

操作数每次在循环中都会被“重新装箱”，因此引用永远不会相同。也就是说，当前值中的引用永远不会与原子中的引用相同。

尝试使用currentValue引用类型。

public final double addAndGet(double delta) {
    while (true) {
        Double currentValue = atomic.get();
        Double nextValue = currentValue + delta;
        if (atomic.compareAndSet(currentValue, nextValue))
            return nextValue;
    }
}

（幸运的是，双是一个不可变的类型，否则这将有一个种族危险）