帐户余额未更新-线程同步Java

Question

因此，我有一个任务来创建一个程序，该程序有1个具有初始余额的银行帐户。 然后，我创建了一组卡，可以访问该帐户，存款或取款，但不能同时进行。 我创建了多个线程，然后使用这些线程访问银行帐户，提取或存入随机金额，然后退出。

我的卡类：

public class card implements Runnable {
private int id;
private account account;
private int count = 1;


card(int id, account acc1){
    this.id = id;
    this.account = acc1;

}

public int getid(){
    return this.id;
}

public int localBalance(){
    return account.getBalance();
}


public void run() {
    for(count = 0; count <= 5; count++) { //withdraw and deposit random amounts
        int transactionamount = (int)(Math.random()*10);
        if (Math.random() > 0.5) {
        System.out.printf("%-12s%-12s%-12s%-12s\n","card id:(" + getid() + ")"," ",transactionamount, account.getBalance());
        account.deposit(transactionamount);
        } else {
        System.out.printf("%-12s%-12s%-12s%-12s\n","card id:(" + getid() + ")",transactionamount," ",account.getBalance());
        account.withdraw(transactionamount);

        }
        }
        }
}

我的帐户类别：

public class account {

private int balance = 5000;


public account(int balance){
    this.balance = balance;
}

public synchronized void withdraw(int amount) {
    balance =   getBalance() - amount;
}

public synchronized void deposit(int amount) {
    balance =   getBalance() + amount;
}

public synchronized int getBalance() {
    return this.balance; 
}


public synchronized void setNewBalance(int localBalance) { //no longer in use
    balance = localBalance;
}

}

我的课程

public class Program {

public static void main(String[] args ) throws InterruptedException {
    // TODO Auto-generated method stub

    int numberofcards = 5;
    int accountbalance = 5000;


    card cardArray[] = new card[numberofcards];

    account acc1 = new account(accountbalance); //pass into account the starting balance.

    System.out.printf("\n%-12s%-12s%-12s%-12s\n","Transaction","Withdrawal","Deposit","Balance");
    System.out.printf("%-12s%-12s%-12s%-12s\n"," "," "," ",accountbalance + "\n");

    Thread[] threads = new Thread[numberofcards];

    for(int i = 1; i < numberofcards; i++ ){ 
       cardArray[i] = new card(i, acc1);
       threads[i] = new Thread(cardArray[i]);
       threads[i].start();
    }
    for(int i = 1; i < threads.length; i++ ){
       threads[i].join();
    }
    System.out.printf("\n%-12s%-12s%-12s%-12s\n","finished"," "," ","Final Balance: " + acc1.getBalance()); 


}

}

输出示例：

http://puu.sh/lvoE5/05ebcd4c74.png

如您所见，它适用于每张不同的卡，它可以回到5000，然后随机存入或提取。 我似乎无法弄清楚为什么我的代码没有不断更新金额。 任何帮助都将是惊人的，无论如何我都感到很惊讶。

编辑：摆脱了不需要的新线程.join代码并摆脱了我的run方法中的同步，但是输出仍然相同，但是，请继续指出问题，关于线程的自我教育确实很困难，所以请记住。 先感谢您

编辑2：仍然无法使用，请立即使用提现和存款。 输出附加在链接中

Answer 1

您的代码存在多个问题 ，首先是您的测试代码永远不会交织，就像我怀疑的那样。

考虑以下代码：

  for(int i = 1; i < numberofcards; i++ ){ //create amount of cards in array of cards
      cardArray[i] = new card(i, acc1);
      new Thread(cardArray[i]).start();
      new Thread(cardArray[i]).join();
  }

对于每个循环，您将创建两个线程对象（请参见new关键字） ，对于一个循环，您将启动线程，对于另一个循环，您将调用join 。 由于第二个线程未运行， 因此它只是返回而什么也不做 。

此外，由于您的run方法是同步的，所以该类中任何新的synchronized方法都不会同时运行，这毫无意义。 请参阅： 您是否应该同步运行方法？ 为什么或者为什么不？

这可能是您想要的：

Thread threads = new Threads[numberofcards];
for(int i = 1; i < numberofcards; i++ ){ 
   cardArray[i] = new card(i, acc1);
   threads[i] = new Thread(cardArray[i]);
   threads[i].start();
}
for(int i = 1; i < threads.length; i++ ){
   threads[i].join();
}

此外，您还有比赛条件：

您用$ 5000 this.cardBalance = account.getBalance()初始化每张卡，然后更新this.cardBalance 。 然后，您在account.setNewBalance(cardBalance)更新帐户，但是在设置局部变量和写入帐户期间，其他线程也已获取并更新了帐户的卡余额。 您对该帐户的修改不是原子的。

您不应该使用局部变量来表示卡中帐户的余额。 这违反了单独的实体规则，并且没有合法目的。

Answer 2

这应该起作用，注意Account类中的正确同步

帐户：

public class Account {
    private int bankBalance;

    public Account(int initialBankBalance) {
        this.bankBalance = initialBankBalance;
    }

    public synchronized int withdraw(int cardId, int amount) {
        bankBalance = bankBalance - amount;
        System.out.println("Card: " + cardId + "; withdraw: " + amount + "; after withdrawal: " + bankBalance);
        return bankBalance;
    }

    public synchronized int deposit(int cardId, int amount) {
        bankBalance = bankBalance + amount;
        System.out.println("Card: " + cardId + "; deposit: " + amount + "; after deposit: " + bankBalance);
        return bankBalance;
    }

    public synchronized int getBankBalance() {
        return bankBalance;
    }
}

卡：

public class Card implements Runnable {
    private int id;
    private Account account;

    public Card(int id, Account account) {
        this.account = account;
        this.id = id;
    }

    @Override
    public void run() {
        double depositTotal = 0;
        double withdrawTotal = 0;
        for (int i = 0; i < 5; i++) {
            if (Math.random() > 0.5) {
                int deposit = (int) (Math.random() * 10);
                depositTotal = depositTotal + deposit;
                account.deposit(id, deposit);
            } else {
                int withdraw = (int) (Math.random() * 10);
                withdrawTotal = withdrawTotal + withdraw;
                account.withdraw(id, withdraw);
            }
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        System.out.println("Card: " + id + "; withdrawal total: " + withdrawTotal);
        System.out.println("Card: " + id + "; deposit total: " + depositTotal);
    }
}

程序：

    public class Program {

    public static void main(String[] args) throws InterruptedException {
        int numCards = 2;
        int initialBankBalance = 1000;

        Account account = new Account(initialBankBalance);
        System.out.println("Starting balance: " + initialBankBalance);

        Thread[] threads = new Thread[numCards];

        for (int i = 0; i < numCards; i++) {
            Thread thread = new Thread(new Card(i+1, account));
            thread.start();
            threads[i] = thread;
        }

        for (int i = 0; i < threads.length; i++) {
            threads[i].join();
        }

        System.out.println("End balance is : " + account.getBankBalance());

    }
}

样本运行的输出：

Starting balance: 1000
Card: 1; deposit: 3; after deposit: 1003
Card: 2; withdraw: 1; after withdrawal: 1002
Card: 2; deposit: 6; after deposit: 1008
Card: 1; deposit: 3; after deposit: 1011
Card: 1; withdraw: 7; after withdrawal: 1004
Card: 2; deposit: 1; after deposit: 1005
Card: 1; deposit: 0; after deposit: 1005
Card: 2; withdraw: 7; after withdrawal: 998
Card: 1; withdraw: 2; after withdrawal: 996
Card: 2; withdraw: 7; after withdrawal: 989
Card: 2; withdrawal total: 15.0
Card: 1; withdrawal total: 9.0
Card: 2; deposit total: 7.0
Card: 1; deposit total: 6.0
End balance is : 989