使用atomic實現銀行存取款

[size=xx-large]一、應用場景[/size]

[size=medium] 在通常情況下，我們在用多線程處理一個問題時，當多個線程要同時處理一個共享的數據時，往往需要通過synchronized的進行同步，在線程比較多的情況會導致鎖競爭太厲害，大部分的時間都花費在了線程切換之間，而對實際的業務缺操作的相對比較少。

下面我們用java.util.concurrent.atomic來實現一個類似的功能，以銀行存取款爲例子：[/size]

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，則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();
	}
}

運行結果：

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


[size=xx-large]二、說明：[/size]

[size=medium]//如果balance和oldvalue相同，則balance=oldvalue - money;
執行到if(balance.compareAndSet(oldvalue, oldvalue - money)) 的時候，會判斷balance與oldvalue是否相同，如果相同，說明沒有程序篡改，則balance=oldvalue - money[/size]