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：
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 = 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);
}
}
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