- public class ATM {
- Account acc;
-
- //作为演示,省略了密码验证
- public boolean login(String name) {
- if (acc != null)
- throw new IllegalArgumentException("Already logged in!");
- acc = new Account(name);
- return true;
- }
-
- public void deposit(float amt) {
- acc.deposit(amt);
- }
-
- public void withdraw(float amt) throws InsufficientBalanceException {
- acc.withdraw(amt);
- }
-
- public float getBalance() {
- return acc.getBalance();
- }
-
- public void logout () {
- acc = null;
- }
-
- }
下面是ATMTester,在ATMTester中首先生成了10个ATM实例,然后启动10个线程,同时登录John的账户,先查询余额,然后,再提取余额的80%,然后再存入等额的款(以维持最终的余额的不变)。按照我们的预想,应该不会发生金额不足的问题。首先看代码:
- public class ATMTester {
- private static final int NUM_OF_ATM = 10;
- public static void main(String[] args) {
- ATMTester tester = new ATMTester();
-
- final Thread thread[] = new Thread[NUM_OF_ATM];
- final ATM atm[] = new ATM[NUM_OF_ATM];
- for (int i=0; i<NUM_OF_ATM; i++) {
- atm[i] = new ATM();
- thread[i] = new Thread(tester.new Runner(atm[i]));
- thread[i].start();
- }
-
- }
-
- class Runner implements Runnable {
- ATM atm;
-
- Runner(ATM atm) {
- this.atm = atm;
- }
-
- public void run() {
- atm.login("John");
- //查询余额
- float bal = atm.getBalance();
- try {
- Thread.sleep(1); //模拟人从查询到取款之间的间隔
- } catch (InterruptedException e) {
- // ignore it
- }
-
- try {
- System.out.println("Your balance is:" + bal);
- System.out.println("withdraw:" + bal * 0.8f);
- atm.withdraw(bal * 0.8f);
- System.out.println("deposit:" + bal * 0.8f);
- atm.deposit(bal * 0.8f);
- } catch (InsufficientBalanceException e1) {
- System.out.println("余额不足!");
- } finally {
- atm.logout();
- }
- }
- }
- }
运行ATMTester,结果如下(每次运行结果都有所差异):
Your balance is:1000.0
withdraw:800.0
deposit:800.0
Your balance is:1000.0
Your balance is:1000.0
withdraw:800.0
withdraw:800.0
余额不足!
Your balance is:200.0
Your balance is:200.0
Your balance is:200.0
余额不足!
Your balance is:200.0
Your balance is:200.0
Your balance is:200.0
Your balance is:200.0
withdraw:160.0
withdraw:160.0
withdraw:160.0
withdraw:160.0
withdraw:160.0
withdraw:160.0
withdraw:160.0
deposit:160.0
余额不足!
余额不足!
余额不足!
余额不足!
余额不足!
余额不足!
为什么会出现这样的情况?因为我们这儿是多个ATM同时对同一账户进行操作,比如一个ATM查询出了余额为1000,第二个ATM也查询出了余额1000,然后两者都期望提取出800,那么只有第1个用户能够成功提出,因为在第1个提出800后,账户真实的余额就只有200了,而第二个用户仍认为余额为 1000。这个问题是由于多个ATM同时对同一个账户进行操作所不可避免产生的后果。要解决这个问题,就必须限制同一个账户在某一时刻,只能由一个ATM 进行操作。如何才能做到这一点?直接通过synchronized关键字可以吗?非常遗憾!因为我们现在需要对整个Account的多个方法进行同步,这是跨越多个方法的,而synchronized仅能对方法或者代码块进行同步。在下一篇我们将通过编写一个锁对象达到这个目的。