java - 在减去值时无法控制 AtomicInteger 的原子行为

Question

这是我第一次在多线程中使用java.util.concurrent.atomic包进行同步。我试图通过 java.util.concurrent.atomic 处理 Kathy Sierra 书中为 OCP 提供的 AccountDanger 类的经典示例。但我无法保护 balance 变量，即 AtomicInteger 不被过度撤回。下面是我的代码:

帐户类别

package p1;

import java.util.concurrent.atomic.AtomicInteger;

public class Account {
  private AtomicInteger balance = new AtomicInteger(100);

  public int getBalance() {
    return balance.get();
  }

  public void withdraw(int amount) {
    balance.addAndGet(-amount);
    System.out.println("~~~~~~ " + balance);
  }
}

AccountDanger 类

package p1;

public class AccountDanger implements Runnable {
  private Account account = new Account();
  private int amt = 10;

  public void run() {

    for (int i = 0; i < 10; i++) {
      if (account.getBalance() >= amt) {
        System.out.println(Thread.currentThread().getName() + " is going to withdraw..");
        try {
          Thread.sleep(500);
        } catch (InterruptedException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
        }
        account.withdraw(amt);
      } else {
        System.out.println("not enough balance");
      }
      if (account.getBalance() < 0) {
        System.out.println("account is over withdrawn!!!");
      }
    }
  }

  public static void main(String[] args) throws InterruptedException {
    AccountDanger ad = new AccountDanger();
    Thread t1 = new Thread(ad, "Mark");
    Thread t2 = new Thread(ad, "Phew");
    t1.start();
    t2.start();
    t1.join();
    t2.join();
    System.out.println("final balance left is : " + ad.account.getBalance());
  }
}

我知道，我肯定有哪里错了。任何人都可以纠正我的解决方案..

Answer 1

您没有以原子方式使用 AtomicInteger(或者更确切地说，Account)。

if(account.getBalance()>=amt){
   // Other stuff
   account.withdraw(amt);

其他线程可以改变“其他东西”内部的平衡。

在Account中，您需要一个像这样的方法，使用 AtomicInteger.compareAndSet :

boolean withdrawIfBalanceEquals(int balance, int amt) {
  return this.balance.compareAndSet(balance, balance - amt);
}

仅当 AtomicInteger 的值仍等于 balance 时，才将其设置为 balance-amt，并且然后返回true。如果自从您读取其值后它已更改，compareAndSet 将返回 false。

然后，在 AccountDanger 中，您可以像这样使用它:

int balance = account.getBalance();
if (balance >= amt) {
  // Other stuff.
  account.withdrawIfBalanceEquals(balance, amt);
}

我让您决定如何最好地处理 withdrawIfBalanceEquals 返回 false 的情况；一种方法是:

while (true) {
  int balance = account.getBalance();
  if (balance >= amt) {
    // Other stuff.
    if (account.withdrawIfBalanceEquals(balance, amt)) {
      // If true, we don't have to keep looping: we withdrew the
      // balance.
      break;
    }

    // If false, keep going: you'll read the balance again,
    // check if it's still greater than balance etc.

  } else {
    System.out.println("Not enough balance");
    break;
  }
}