多线程.线程同步

同步方法：synchronized

同步块：synchronized(Obj){}

并发：多个线程操作同一个资源

队列和锁

每个对象都有一个锁，sleep不会释放锁

• 由于同一进程的多个线程共享同一块存储空间，在带来方便发同时，也带来了访问冲突问题，为了保证数据在方法中被访问时的准确性，在访问时加入锁机制synchronized，当一个线程获取对象的排它锁，独占资源，其他线程必须等待，使用后释放锁即可。存在以下问题：

• 一个线程持有锁会导致其他所以需要此锁的线程挂起

• 在多线程竞争下，加锁，释放锁，会导致比较多的上下文切换和调度延时，引起性能问题

• 如果一个优先级高的线程等待一个优先级低的线程释放锁会导致优先级倒置，引起性能问题

package oop.syn;

// 不安全的买票
public class UnsafeBuyTicket {
    public static void main(String[] args) {
        BuyTicket station = new BuyTicket();

        new Thread(station,"我").start();
        new Thread(station,"你").start();
        new Thread(station,"他").start();
    }
}

class BuyTicket implements Runnable{
    //票
    int ticketNums = 10;
    boolean flag = true;//外部停止方式
    @Override
    public void run() {
        //买票
        while(flag){
            try {
                buy();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
    //synchronized 同步方法，锁的是this
    private synchronized void buy() throws InterruptedException {
        //判断是否有票
        if (ticketNums<=0){
            flag = false;
            return;
        }
        //模拟延时
        Thread.sleep(100);
        //买票
        System.out.println(Thread.currentThread().getName()+"拿到"+ticketNums--);
    }
}

package oop.syn;

//不安全的取钱
//两个人去银行取钱
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 he = new Drawing(account,100,"他");

        you.start();
        he.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;
    //现在有多少钱
    int nowMoney;

    public Drawing(Account account,int drawingMoney,String name){
        super(name);
        this.account = account;
        this.drawingMoney = drawingMoney;
    }
    //取钱
    //synchronized 默认锁的是this.
    @Override
    public  void run() {

        //锁的对象就是变化的量，需要增删改的对象
        synchronized (account){ //判断有没有钱
            if (account.money -drawingMoney<0){
                System.out.println(Thread.currentThread().getName()+"钱不够，取不了");
                return;
            }

            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            //卡内余额 = 余额-你的钱
            account.money = account.money-drawingMoney;
            //你手里的钱
            nowMoney = nowMoney+drawingMoney;

            System.out.println(account.name+"余额为："+account.money);
            //Thread.currentThread().getName() = this.getName()  [class Drawing extends Thread】
            System.out.println(this.getName()+"手里的钱："+nowMoney);}
    }
}

package oop.syn;

import java.util.ArrayList;
import java.util.List;

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

搜索

复制