JAVA多线程小例:使用多线程实现银行转帐(未同步与同步)

使用多线程模拟实现银行转帐功能，先看看未使用同步的代码：

 

 

package com.test.current;

public class UnsynchBankTest {
	public static final int NACCOUNTS = 100;//帐户数
	public static final double INITIAL_BALANCE = 1000;//初始金额
	public static void main(String[] args) {
		Bank b = new Bank(NACCOUNTS, INITIAL_BALANCE);
		int i;
		for (i = 0; i < NACCOUNTS; i++) {
			TransferRunnable r = new TransferRunnable(b, i, INITIAL_BALANCE);
			Thread t = new Thread(r);
			t.start();
		}
	}

}
class Bank{
	private final double[] accounts;
	//initialBalance 初始金额
	public Bank(int n,double initialBalance) {
		accounts = new double[n];
		for (int i = 0; i < accounts.length; i++) {
			accounts[i] = initialBalance;
		}
	}
	//转帐
	public void transfer(int from ,int to ,double amount){
		if(accounts[from] < amount){ //金额不够
			return;
		}
		System.out.print(Thread.currentThread());
		accounts[from] -= amount;
		System.out.printf(" %10.2f from %d to %d",amount,from,to);
		accounts[to] += amount;
		System.out.printf(" Total balance %10.2f%n",getTotalBalance());
	}
	
	public double getTotalBalance(){
		double sum = 0;
		for (double a : accounts) {
			sum += a;
		}
		return sum;
	}
	
	public int size(){
		return accounts.length;
	}
}

class TransferRunnable implements Runnable{
	private Bank bank;
	private int fromAccount;
	private double maxAmount;//最大转帐金额
	private int DELAY =10;

	public TransferRunnable(Bank b, int from, double max) {
		bank = b;
		fromAccount = from;
		maxAmount = max;
	}
	public void run() {
		int toAccount = (int)(bank.size() * Math.random());
		double amount = maxAmount * Math.random();
		bank.transfer(fromAccount, toAccount, amount);
		try {
			Thread.sleep((int)(DELAY * Math.random()));
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
	
}

 最后打印的结果

 

 

由于 transfer方法中的accounts[to] += amount;操作是非原子的

指令可能会按下面的步骤执行

1,将accounts[to]载入寄存器

2,增加amount

3,将结果写回accounts[to]

 

假设某个线程执行了第1，2步后被中断，另一个线程对同一个位置的数组执行了1，2，3步，这时之前被中断的线程被唤醒了，执行了第3步，那么前一个线程的所做的更新就被抹去了。结果导致总金额不对。

 

二，使用ReentrantLock锁来保持同步

 

//转帐
	public void transfer(int from ,int to ,double amount){
		bankLock.lock();
		try{
			if(accounts[from] < amount){ //金额不够
				return;
			}
			System.out.print(Thread.currentThread());
			accounts[from] -= amount;
			System.out.printf(" %10.2f from %d to %d",amount,from,to);
			accounts[to] += amount;
			System.out.printf(" Total balance %10.2f%n",getTotalBalance());
		}finally{
			bankLock.unlock();
		}
	}//转帐

使用	synchronized 也可以实现同步

public synchronized void transfer(int from ,int to ,double amount){
		if(accounts[from] < amount){ //金额不够
			return;
		}
		System.out.print(Thread.currentThread());
		accounts[from] -= amount;
		System.out.printf(" %10.2f from %d to %d",amount,from,to);
		accounts[to] += amount;
		System.out.printf(" Total balance %10.2f%n",getTotalBalance());
	}