javaSE21/10/6

多线程

线程状态

守护线程

• 线程分为用户线程和守护线程
• 虚拟机必须确保用户线程执行完毕
• 虚拟机不用等待守护线程执行完毕，如后台记录操作日志，监控内存，垃圾回收等待
• thread.setDaemon(true) 设置守护线程

线程同步

并发：同一个对象被多个线程同时操作

队列和锁

线程不安全案例

• 买票不安全

//线程不安全，出现0和负数
public class UnsafeBuyTicket {
    public static void main(String[] args) {
        BuyTicket b = new BuyTicket();
        new Thread(b,"我们").start();
        new Thread(b,"你们").start();
        new Thread(b,"黄牛").start();
    }


}
class BuyTicket implements Runnable{
    private int ticketNums = 10;
    boolean flag = true;
    @Override
    public void run() {
        //买票
        while (flag){
            buy();
        }
    }
    public void buy(){
        //判断是否有票
        if (ticketNums<=0){
            flag = false;
            System.out.println("您手慢了，票卖完了");
            return;
        }
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName()+"抢到第"+ticketNums--+"张票");

    }
}

• 取钱不安全

public class UnsafeBank {
    public static void main(String[] args) {
        Account account = new Account(2000);
        Drawing drawing = new Drawing(account,2000,"李华");
        Drawing drawing1 = new Drawing(account,2000,"李华妈妈");
        drawing.start();
        drawing1.start();
    }
}
class Account{
    int money;//余额
    public Account(int money) {
        this.money = money;
    }
}
class Drawing extends Thread{
    Account account;//实例化一个账户
    int drawMoney;//取了多少钱
    public Drawing(Account account,int drawMoney,String name){
        super(name);//父类的名，即Thread的名
        this.account=account;
        this.drawMoney=drawMoney;
    }
    //取钱
    @Override
    public void run() {
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        if (account.money<drawMoney){
            System.out.println(Thread.currentThread().getName()+"您的余额不足!");
            return;
        }
        account.money=account.money-drawMoney;
        System.out.println(Thread.currentThread().getName()+"您取了"+drawMoney);
        System.out.println(Thread.currentThread().getName()+"，您的账户余额为"+account.money);
    }
}

线程同步

• synchronized关键字，分为同步方法和同步块
• synchronize方法控制对“对象”的访问，每个对象对应一把锁，该方法必须获得调用该方法的对象的锁才能执行，否则线程会阻塞，方法一旦执行，就独占该锁，知道该方法返回才释放锁，后面被阻塞的线程才能获得这个锁，继续执行
• 锁的对象是变化的量，需要增删改的对象，同步方法锁的是类本身，同步块锁的是任何对象
• 解决买票不安全的问题

public synchronized void buy(){
        //判断是否有票
        if (ticketNums<=0){
            flag = false;
            System.out.println("您手慢了，票卖完了");
            return;
        }
        try {
            Thread.sleep(10);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName()+"抢到第"+ticketNums--+"张票");

    }

• 解决银行取钱不安全的问题

public void run() {
        synchronized(account){
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            if (account.money<drawMoney){
                System.out.println(Thread.currentThread().getName()+"您的余额不足!");
                return;
            }
            account.money=account.money-drawMoney;
            System.out.println(Thread.currentThread().getName()+"您取了"+drawMoney);
            System.out.println(Thread.currentThread().getName()+"，您的账户余额为"+account.money);
        }

    }