Java多线程——线程同步

Java多线程——线程同步 1.不安全的买票

//不安全的买票
	//线程不安全，有负数或者多人买到同一张票
public class UnsafeBuyTicket {
	public static void main(String[] args) {
		BuyTicket buyTicket = new BuyTicket();

		new Thread(buyTicket,"苦逼的我").start();
		new Thread(buyTicket,"牛逼的你们").start();
		new Thread(buyTicket,"可恶的黄牛党").start();
	}
}

class BuyTicket implements Runnable{
	//票数
	private int num = 10;
	private boolean flag = true;//判断是否停止线程    Thread的stop已经废弃不建议使用

	@Override
	public void run() {
		//买票
		while (flag){
			buy();
		}
	}

	public void buy(){
		//判断是否有票
		if(num<=0){
			flag = false;
			return ;
		}

		try {
			//模拟延时（放大问题的发生性）
			Thread.sleep(100);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		//买票
		System.out.println(Thread.currentThread().getName()+"买到了第"+num--+"票");

	}
}

解决方法：通过关键字synchronized来设置同步锁 在buy()方法前面添加synchronized关键字来锁定这个方法

public synchronized void buy(){
	//判断是否有票
	if(num<=0){
		flag = false;
		return ;
	}

	try {
		//模拟延时（放大问题的发生性）
		Thread.sleep(100);
	} catch (InterruptedException e) {
		e.printStackTrace();
	}
	//买票
	System.out.println(Thread.currentThread().getName()+"买到了第"+num--+"票");

}

如上图添加了synchronized关键字后就避免了多人抢到同一张票的情况，也避免了在没有票的情况下继续出票的情况 2.不安全的取钱

//不安全的取钱
	//两个人去银行取钱，账户
public class UnsafeBank {
	public static void main(String[] args) {
		Account account = new Account(100, "结婚基金");
		Drawing you = new Drawing(account, 50, "你");
		Drawing grilFriend = new Drawing(account, 100, "grilFriend");

		grilFriend.start();
		you.start();
	}
}

//账户
class Account{
	int money;//余额
	String name;//卡名称
	public Account(int money, String name) {
		this.money = money;
		this.name = name;
	}

}


//银行：模拟取钱
class  Drawing extends Thread{
	Account account;//账户
	int drawingMoney;//取多少钱
	int nowMoney;//现在手里有多少钱

	public Drawing(Account account,int drawingMoney,String name){
		super(name);
		this.account = account;
		this.drawingMoney = drawingMoney;
	}

	@Override
	public void run() {
		//判断账户里面有没有钱
		if(account.money-drawingMoney<0){
			System.out.println(Thread.currentThread().getName()+"钱不够了，取不了");
			return;
		}
		//sleep 放大问题的发生性
		try {
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}

		//卡内余额 = 余额 - 你取的钱
		account.money = account.money - drawingMoney;
		//你手上的钱
		nowMoney = nowMoney + drawingMoney;

		System.out.println(account.name +"余额为："+account.money);
		//hread.currentThread().getName() = this.getName()；
		System.out.println(this.getName()+"手里的钱"+nowMoney);
	}
}

上述的问题就是你去50万，你老婆去100万，结果银行还倒贴了50万

解决方法：给银行账户添加锁 synchronized(account)

@Override
public void run() {
	synchronized (account){
		//判断账户里面有没有钱
		if(account.money-drawingMoney<0){
			System.out.println(Thread.currentThread().getName()+"钱不够了，取不了");
			return;
		}
		//sleep 放大问题的发生性
		try {
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}

		//卡内余额 = 余额 - 你取的钱
		account.money = account.money - drawingMoney;
		//你手上的钱
		nowMoney = nowMoney + drawingMoney;

		System.out.println(account.name +"余额为："+account.money);
		//hread.currentThread().getName() = this.getName()；
		System.out.println(this.getName()+"手里的钱"+nowMoney);
	}
}

只要有一个人取完后另一个人就会因为余额不足取不了，保证了线程的安全