Java 同步未按(我)预期工作

Question

程序员 friend 们。我正在使用非常简单的代码(或者至少看起来很简单)来测试 Java 线程功能。我有此类帐户:

public class Account {
    protected double balance;

    public synchronized void withdraw(double value) {
        this.balance = this.balance - value;
    }

    public synchronized void deposit(double value) {
        this.balance = this.balance + value;
    }

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

我有两个线程:Depositer，存款 10 美元一千次:

public class Depositer extends Thread {
    protected Account account;

    public Depositer(Account a) {
        account = a;
    }

    @Override
    public void run() {
        for(int i = 0; i < 1000; i++) {
            this.account.deposit(10);
        }
    }
}

还有 Withdrawer，提取 10 美元一千次:

public class Withdrawer extends Thread {
    protected Account account;

    public Withdrawer(Account a) {
        account = a;
    }

    @Override
    public void run() {
        for(int i = 0; i < 1000; i++) {
            this.account.withdraw(10);
        }
    }
}

此安排执行者:

public class Main {
    public static void main(String[] args) {
        Account account = new Account();
        Thread c1 = new Depositer(account);
        Thread c2 = new Withdrawer(account);

        c2.start();
        c1.start();

        System.out.println(account.getBalance());
    }
}

由于方法是同步的，我只是期望最后余额始终为0，但有时不会发生这种情况。我真的不明白为什么。有人能看出我的错在哪里吗？

Answer 1

c2.start() 和 c1.start() 异步运行该进程。您的主线程需要等待这两个线程完成才能打印结果。

调用

try {
    c2.join();
    c1.join();
} catch (InterruptedException e) {
    e.printStackTrace();
}

在调用 println 之前。

参见join .

Waits for this thread to die.