使用atomic实现银行存取款

一、应用场景

   在通常情况下，我们在用多线程处理一个问题时，当多个线程要同时处理一个共享的数据时，往往需要通过synchronized的进行同步，在线程比较多的情况会导致锁竞争太厉害，大部分的时间都花费在了线程切换之间，而对实际的业务缺操作的相对比较少。

下面我们用java.util.concurrent.atomic来实现一个类似的功能，以银行存取款为例子：

public class AtomicAccount {

	
	AtomicLong balance;
	
	public AtomicAccount(long money) {
		balance = new AtomicLong(money);
		System.out.println("Total Money:"+balance);
	}
	
	public void deposit(long money) {
		balance.addAndGet(money);
	}
	
	public void withdraw(long money,int delay) {
		long oldvalue = balance.get();
		if(oldvalue>=money) {
			try {
				Thread.sleep(delay);
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
[color=red]			//如果balance和oldvalue相同，则balance=oldvalue - money;
			if(balance.compareAndSet(oldvalue, oldvalue - money)) {[/color]
				System.out.println(Thread.currentThread().getName()+" withdraw " + money +" successfull!" + "balance="+balance);
			} else {
				System.out.println(Thread.currentThread().getName()+" withdraw " + money +" failed!" + "balance="+balance);
			}
		} else {
			System.out.println(Thread.currentThread().getName()+" withdraw " + money +" balance is not enough,withdraw failed!" + "balance="+balance);
		}
	}
}

 public class AtomicAccountTest extends Thread{

	AtomicAccount account;
	
	int delay;
	
	public AtomicAccountTest(AtomicAccount account , int delay) {
		this.account = account;
		this.delay = delay;
	}
	
	public void run() {
		account.withdraw(100, delay);
	}
	
	public static void main(String[] args) {
		
		AtomicAccount account = new AtomicAccount(100);
		AtomicAccountTest thread_1 = new AtomicAccountTest(account,1000);
		AtomicAccountTest thread_2 = new AtomicAccountTest(account,0);
		
		thread_1.start();
		thread_2.start();
	}
}

运行结果：

Total Money:100
Thread-1 withdraw 100 successfull!balance=0
Thread-0 withdraw 100 failed!balance=0


二、说明：

//如果balance和oldvalue相同，则balance=oldvalue - money;
执行到if(balance.compareAndSet(oldvalue, oldvalue - money)) 的时候，会判断balance与oldvalue是否相同，如果相同，说明没有程序篡改，则balance=oldvalue - money