Question

我是Java中的线程编程新手。要理解线程，我正在尝试编写一个简单的程序来模拟银行账户。我刚刚实施了撤销并尝试对其进行测试。输出的前几行是在下面。

T2退出前的余额：1000
T2退出后的余额：990
T1退出前的余额：1000
T1退出后的余额：980
T2退出前的余额：980
T2退出后的余额：970
T1退出前的余额：970
T1退出后的余额：960左

我的问题是为什么输出中的第3行（T1提取前的余额：1000）给出1000而不是990.如果它是正确的，它应该在第2行。我错过了一些东西。我的方法是否正确？

我的猜测是两个线程试图写入控制台并且线程T1只是没有机会在第二行写入。

class BankAccount {

    private volatile int balance;

    public BankAccount(int b){
        balance = b;
    }

    public BankAccount(){
        balance = 0;
    }


    synchronized public int getBalance(){
        return balance;
    }

    synchronized public int withdraw(int w)
    {
        int b = getBalance();
        if(w <= b){
            balance = balance-w;
            return w;
        }
        else
            return 0;
    }
}

class WithdrawAccount implements Runnable{

    private BankAccount acc;
    private int amount;

    public WithdrawAccount(){
        acc = null;
        amount = 0;
    }

    public WithdrawAccount(BankAccount acc,int amount){
        this.acc = acc;
        this.amount = amount;
    }

    public void run() {
        int w; 

        for(int i =0; i<20; i++){
            try {
                Thread.sleep(200);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

            System.out.println("Balance before "+Thread.currentThread().getName()+" withdrawl: "+acc.getBalance());
            w = acc.withdraw(amount);
            System.out.println("Balance after "+Thread.currentThread().getName()+" withdrawl: "+acc.getBalance());
            //System.out.println("amount with drawn by: "+Thread.currentThread().getName()+" "+w);

        }

    }

}

public class TestBankAccount{

    public static void main(String[] args) {

        BankAccount b = new BankAccount(1000);
        WithdrawAccount w = new WithdrawAccount(b,10);
        Thread wt1 = new Thread(w);
        wt1.setName("T1");

        Thread wt2 = new Thread(w);
        wt2.setName("T2");

        wt1.start();
        wt2.start();
    }
}

Answer 1

可能有一个线程在这些行之间调用withdraw（）：

System.out.println("Balance before "+Thread.currentThread().getName()+" withdrawl: "+acc.getBalance());
w = acc.withdraw(amount);

可能的情况：

T2调用getBalance（）;
T1调用getBalance（）;
T2出局平衡;
T2呼叫撤回（）;
T2退出后取消平衡;
T1出局平衡;
T1调用withdraw（）;
退出后T1出局平衡;

Answer 2

你没有做任何事情来同步你的run方法，所以在撤回之前和之后的printlns和撤回本身都不是原子的。你正在进行线程交错。

Answer 3

这是可能的线程交错之一：

    wt2: calls getBalance()   // retrieves 1000
    wt2: prints "Balance before T2 withdrawl: 1000"

    wt1: calls getBalance()   // retrieves 1000 also

    wt2: acc.withdraw(amount) // balance is now 990
    wt2: prints "Balance after T2 withdrawl: 990"

    wt1: acc.withdraw(amount)    // balance was 990 after wt1 withdraws. wt1 now withdraws again so balance is 980 
    wt1: prints "Balance after T2 withdrawl: 980"

Answer 4

这是一个并发问题

T1检索1000
T1将“之前”打印到系统输出1000
T2检索1000
T1撤回
T1将“after”打印到system out 990
T2将“之前”打印到系统输出1000
T2撤回
T2将“after”打印到system out 980。
...

无法按此顺序执行，但您明白了。为了让它像您想要的那样使用synchronized块。

  synchronized (acc) {
    System.out.println("Balance before " + Thread.currentThread().getName() + " withdrawl: " + acc.getBalance());
    w = acc.withdraw(amount);
    System.out.println("Balance after " + Thread.currentThread().getName() + " withdrawl: " + acc.getBalance());
  }

用Java实现银行账户

4 个答案: