三大不安全案例

1、车站买票

package syn;
​
//不安全的买票
//线程不安全，有负数
public class UnsafeBuyTicket {public static void main(String[] args) {BuyTicket buyTicket = new BuyTicket();new Thread(buyTicket,"xiaoming").start();new Thread(buyTicket,"xiaozhang").start();new Thread(buyTicket,"xiaowang").start();
​}
​
}
​
class BuyTicket implements Runnable{//票private int ticketNums = 10;boolean flag = true;@Overridepublic void run() {//买票while (flag){buy();}}private void buy(){//判断是否有票if(ticketNums<=0){flag=false;return;}//模拟延时try {Thread.sleep(100);} catch (InterruptedException e) {throw new RuntimeException(e);}//买票System.out.println(Thread.currentThread().getName()+"拿到第"+ticketNums--+"张票");}
}

线程不安全，出现负数

假设只剩下最后一张票，三个人同时去买票，由于每个线程在自己的工作内存交互，内存控制不当会造成数据不一致（互不影响），因此一个人拿了最后一张票，一个人拿了第0张票，一个人拿了-1张票。如下图：

2、银行取钱

package syn;
​
//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) {//账户Account account = new Account(100,"基金");
​Drawing you = new Drawing(account,50,"你");Drawing girl = new Drawing(account,100,"girl");
​you.start();girl.start();
​}
}
​
//账户
class Account{int money;//余额String name;//卡名public Account(int money,String name){this.money=money;this.name=name;}
}
//银行；模拟取款
class Drawing extends Thread{Account account;//账户//取了多少钱int drawingMoney;//现在手里有多少钱int nowMoney;
​public Drawing(Account account,int drawingMoney,String name){super(name);this.account=account;this.drawingMoney=drawingMoney;}//取钱@Overridepublic void run() {//判断有没有钱if (account.money-drawingMoney<0){System.out.println(Thread.currentThread().getName()+"钱不够，取不了");return;}//sleep可以放大问题发生性try {Thread.sleep(1000);} catch (InterruptedException e) {throw new RuntimeException(e);}//卡内余额 = 余额 - 你取的钱account.money = account.money - drawingMoney;//你手里的钱nowMoney = nowMoney + drawingMoney;System.out.println(account.name+"余额为："+account.money);System.out.println(this.getName()+"手里的钱："+nowMoney);
​}
}

3、不安全集合

package syn;
​
import java.util.ArrayList;
import java.util.List;
​
//线程不安全的集合
public class UnsafeList {public static void main(String[] args) {List<String> list = new ArrayList<String>();for (int i = 0; i < 10000; i++) {new Thread(()->{list.add(Thread.currentThread().getName());}).start();}try {Thread.sleep(3000);} catch (InterruptedException e) {throw new RuntimeException(e);}System.out.println(list.size());}
}

小于10000的原因：可能同一时刻两个线程添加到了集合里面同一个位置。