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Head First Java中多个线程同时取款，为什么会出现余额不足的问题？
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head first java中的线程问题
问题：
在以下这段代码中，我们创建了两个线程来分别执行ryanandmonicajob类的run方法。每个线程对bankaccount对象进行取款操作，并在取款前检查账户余额是否大于等于取款金额。然而，输出结果却令人惊讶。
代码：
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class bankaccount { private int balance = 100; public int getbalance() { return balance; } public void withdraw(int amount) { balance = balance - amount; } } public class ryanandmonicajob implements runnable { private bankaccount account = new bankaccount(); public static void main(string[] args) { ryanandmonicajob thejob = new ryanandmonicajob(); thread one = new thread(thejob); thread two = new thread(thejob); one.setname("ryan"); two.setname("monica"); one.start(); two.start(); } public void run() { for (int x = 0; x < 10; x++) { makewithdrawal(10); if (account.getbalance() < 0) { system.out.println("overdrawn!"); } } } private void makewithdrawal(int amount) { string currentthread = thread.currentthread().getname(); if (account.getbalance() >= amount) { system.out.println(currentthread + " is about to withdraw"); try { system.out.println(currentthread + " is going to sleep"); thread.sleep(500); } catch (interruptedexception ex) { ex.printstacktrace(); } system.out.println(currentthread + " woke up."); account.withdraw(amount); system.out.println(currentthread + " completes the withdrawal"); } else { system.out.println("sorry, not enough for " + currentthread); } } }输出：
ryan is about to withdraw ryan: sorry, not enough for ryan monica: is about to withdraw ...
问题原因：
当ryan线程执行makewithdrawal方法时，它先输出"ryan is about to withdraw"，然后进入sleep状态。然而，这并不是像你想象的那样交出控制权给另一个线程。相反，在这个sleep时间内，monica线程仍然继续执行，并且已经检测到账户余额不足，所以输出"ryan: sorry, not enough for ryan"。
一旦ryan线程从sleep状态中苏醒，它继续执行makewithdrawal方法。此时，账户余额已经发生了变化，并且不再足以支持取款操作。因此，它输出"monica is about to withdraw"。
解决方法：
为了避免这种不同步，我们可以在makewithdrawal方法中使用synchronized块来确保账户对象的访问是同步的。
private void makeWithdrawal(int amount) { synchronized (account) { String currentThread = Thread.currentThread().getName(); if (account.getBalance() >= amount) { System.out.println(currentThread + " is about to withdraw"); try { System.out.println(currentThread + " is going to sleep"); Thread.sleep(500); } catch (InterruptedException ex) { ex.printStackTrace(); } System.out.println(currentThread + " woke up."); account.withdraw(amount); System.out.println(currentThread + " completes the withdrawal"); } else { System.out.println("Sorry, not enough for " + currentThread); } } }