线程不安全的三个案例
1. 不安全的买票
代码:
package multithreading.UnSafe;
// 不安全的买票
// 线程不安全,有负数
public class UnsafeBuyTicket {
public static void main(String[] args) {
BuyTicket station = new BuyTicket();
new Thread(station,"苦逼的我").start();
new Thread(station,"牛逼的你们").start();
new Thread(station,"可恶的黄牛党").start();
}
}
class BuyTicket implements Runnable{
// 票
private int ticketNums = 10;
boolean flag = true; // 外部停止方式
@Override
public void run() {
// 买票
while(flag){
try {
buy();
} catch (Exception e) {
e.printStackTrace();
}
}
}
private void buy(){
// 判断是否有票
if (ticketNums <= 0){
flag = false;
return;
}
// 模拟延时,放大问题的发生性
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
// 买票
System.out.println(Thread.currentThread().getName()+"拿到"+ticketNums--);
}
}
运行结果:
2. 不安全的取钱
代码:
package multithreading.UnSafe;
// 不安全的取钱
// 两人去银行取钱,账户
public class UnsafeBank {
public static void main(String[] args) {
// 账户
Account account = new Account(100,"结婚基金");
Drawing you = new Drawing(account,50,"你");
Drawing girlFriend = new Drawing(account,100,"girlFriend");
you.start();
girlFriend.start();
}
}
// 账户
class Account{
int money; // 金额
String name; // 卡名
public Account(int money,String name){
this.money = money;
this.name = name;
}
}
// 银行
class Drawing extends Thread{ // 不涉及多个对象
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
@Override
public 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;
}
// 模拟延时,使两个线程同时抢占资源
// sleep放大问题的发生性
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
// 卡内金额 = 金额 - 你取的钱
account.money = account.money - drawingMoney;
// 你手里的钱
nowMoney = nowMoney + drawingMoney;
System.out.println(account.name+"金额为:"+account.money);
// Thread.currentThread().getName() = this.getName()
System.out.println(this.getName()+"你手里的钱:"+nowMoney);
}
}
运行结果:
3. 不安全的买票
代码:
package multithreading.UnSafe;
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();
}
System.out.println(list.size());
}
}
运行结果:
- 得到的新线程小于10000条,因为线程同步运行时,数据会覆盖
- 引入线程休眠后可得到接近10000的结果,但仍然存在误差,不安全