Java同步代码块解决银行取钱的安全问题实例分析

synchronized(obj)
{
  //此处代码块就是同步代码块。
}

public class Account
{
   // 封装账户编号、账户余额的两个成员变量
   private String accountNo;
   private double balance;
   public Account(){}
   // 构造器
   public Account(String accountNo , double balance)
   {
      this.accountNo = accountNo;
      this.balance = balance;
   }
   // 此处省略了accountNo和balance的setter和getter方法
   // accountNo的setter和getter方法
   public void setAccountNo(String accountNo)
   {
      this.accountNo = accountNo;
   }
   public String getAccountNo()
   {
      return this.accountNo;
   }
   // balance的setter和getter方法
   public void setBalance(double balance)
   {
      this.balance = balance;
   }
   public double getBalance()
   {
      return this.balance;
   }
   // 下面两个方法根据accountNo来重写hashCode()和equals()方法
   public int hashCode()
   {
      return accountNo.hashCode();
   }
   public boolean equals(Object obj)
   {
      if(this == obj)
        return true;
      if (obj !=null
        && obj.getClass() == Account.class)
      {
        Account target = (Account)obj;
        return target.getAccountNo().equals(accountNo);
      }
      return false;
   }
}

public class DrawThread extends Thread {
   // 模拟用户账户
   private Account account;
   // 当前取钱线程所希望取的钱数
   private double drawAmount;
   public DrawThread(String name, Account account, double drawAmount) {
      super(name);
      this.account = account;
      this.drawAmount = drawAmount;
   }
   // 当多条线程修改同一个共享数据时，将涉及数据安全问题。
   public void run() {
      synchronized (account) {
        // 账户余额大于取钱数目
        if (account.getBalance() >= drawAmount) {
           // 吐出钞票
           System.out.println(getName() + "取钱成功！吐出钞票:" + drawAmount);
           try {
              Thread.sleep(1);
           } catch (InterruptedException ex) {
              ex.printStackTrace();
           }
           // 修改余额
           account.setBalance(account.getBalance() - drawAmount);
           System.out.println("\t余额为: " + account.getBalance());
        } else {
           System.out.println(getName() + "取钱失败！余额不足！");
        }
      }
   }
}

public class DrawTest
{
   public static void main(String[] args)
   {
      // 创建一个账户
      Account acct = new Account("1234567" , 1000);
      // 模拟两个线程对同一个账户取钱
      new DrawThread("甲" , acct , 800).start();
      new DrawThread("乙" , acct , 800).start();
   }
}