锁

锁

• synchronized默认的锁this(用在方法修饰符后面)

• 如果要锁其他的内容要用synchronized块

• 锁的对象是变化的量

package syn;
//数组
import java.util.ArrayList;
​
public class UnsafeList {
    public static void main(String[] args) throws InterruptedException {
        ArrayList<String> list = new ArrayList<String>();
        for (int i = 0; i < 10000; i++) {
            new Thread(()->{
                synchronized (list) {
                    list.add(Thread.currentThread().getName());
                }
            }).start();
        }
        Thread.sleep(3000);
        System.out.println(list.size());
    }
}
​

package syn;
//银行同时取钱，钱不够但是都取出了钱
public class UnsafeBank {
​
​
    public static void main(String[] args) {
​
    Account account = new Account(10000000,"第一桶金");
    Drawing you = new Drawing(account,50,"你");
    Drawing girlFriends = new Drawing(account,1000000,"你的对象");
    you.start();
    girlFriends.start();
    }
​
}
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;
    //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;
    }
    //synchronized默认锁的本身,this
    @Override
    public void run() {
        synchronized(account){
            //判断有没有钱
            if (account.money - drawingMoney < 0){
                System.out.println(Thread.currentThread().getName()+"钱不够");
                return;
            }
            //sleep可以放大问题的发生性
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            account.money = account.money - drawingMoney;
            nowMoney = nowMoney + drawingMoney;
            System.out.println(account.name +"余额为："+account.money);
            System.out.println(this.getName()+"手里的钱:"+nowMoney);
        }
    }
}

package syn;
​
//不安全的买票
/*
线程不安全:
    1.有负数
 */
​
public class UnsafeBuyTicket {
​
    public static void main(String[] args) {
​
        BuyTicket buyTicket = new BuyTicket();
​
        new Thread(buyTicket,"小红").start();
        new Thread(buyTicket,"小华").start();
        new Thread(buyTicket,"黄牛").start();
    }
​
}
class BuyTicket implements Runnable{
​
    private int ticketNums = 10;
    boolean flag = true;
    @Override
    public void run() {
        //买票
        while (flag){
            try {
                buy();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
    //synchronized 同步方法
    private synchronized void buy() throws InterruptedException {
        //判断是否有票
        if (ticketNums <= 0) {
            flag = false;
            return;
        }
        Thread.sleep(100);
        System.out.println(Thread.currentThread().getName()+"拿到"+ticketNums--);
    }
}
​