关于三种线程不安全的情况,用synchronized解决的办法

很多时候我们都会谈到线程不安全的情况.由于cpu的执行速度太快,出现了几个线程争夺一个资源的情况,进而引发一系列的问题.
比如说,买票不安全,银行取钱不安全,甚至面试会问的ArrayList在线程中安不安全,答案肯定是否定的,毕竟会出现数据覆盖的问题.
下面列举三种不安全的代码
买票不安全,几个人买到了同一张票:

package com.qiu.syn;
//不安全的买票
//线程不安全,有拿到重复的票
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,"黄牛").start();
    }

}
class BuyTicket implements Runnable{
private int TicketNums= 10;
boolean flag =true;//标志位,外部停止方式
    @Override
    public void run() {
        //买票
        while(flag){
            try {
                buy();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

​    }
​    private void buy() throws InterruptedException {
​        //判断是否又有票
​        if (TicketNums<1){
​            flag =false;
​            return;
​        }
​        Thread.sleep(100);
​        //买票
​        System.out.println(Thread.currentThread().getName()+"拿到了"+TicketNums--);
​    }
}

银行取钱不安全:比如总数100,你取50,她取100,结果余额会变成-50.

代码:

package com.qiu.syn;
//不安全的取钱
//两个人去银行里面取钱
public class UnSafeBank {
    public static void main(String[] args) {
        //账户
        Account account = new Account(100,"装修的钱");
        Drawing me = new Drawing(account,50,"我");
        Drawing you = new Drawing(account,100,"你");
        me.start();
        you.start();
    }
}
//账户
class Account{
    int money;
    String name;

​    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;
​    //现在手里还有多少钱
​    int nowMoney;
​    public Drawing(Account account,int drawingMoney,String name){
​       super(name);//调用父类的名字,就是当前线程的名字
​        this.account=account;
​        this.drawingMoney=drawingMoney;

​    }

​    @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");
​        }
​        try {//放大问题的发生性
​            Thread.sleep(1000);
​        } catch (InterruptedException e) {
​            e.printStackTrace();
​        }
​        //卡内余额
​        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()
​        System.out.println(this.getName()+"手里的钱"+nowMoney);
​    }
}

ArrayList线程不安全:

package com.qiu.syn;

import java.util.*;

//线程不安全的集合
//原因是因为两个数同一时间添加到了同一个位置,相比上一个位置的数被覆盖掉了,所以就少了很多数
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(1/10);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(list.size());
        }
    }

这个时候我们就会办法解决问题了,所以引进了Synchronized锁还有Lock锁,这里重点讲解一下Synchronized锁
利用上面的两种方法后,我们可以改进上面的代码

1. private synchronized void buy()

package com.qiu.syn;
//不安全的买票
//线程不安全,有拿到重复的票
public class UnSafeBuyTicket {
    public static void main(String[] args) {
        BuyTicket station = new BuyTicket();
        new Thread(station,"你").start();
        new Thread(station,"我").start();
        new Thread(station,"黄牛").start();
    }

}
class BuyTicket implements Runnable{
private int TicketNums= 10;
boolean flag =true;//标志位,外部停止方式
    @Override
    public void run() {
        //买票
        while(flag){
            try {
                buy();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

    }
    //synchronized 同步方法锁的是用这个方法的对象
    private synchronized void buy() throws InterruptedException {
        //判断是否又有票
        if (TicketNums<1){
            flag =false;
            return;
        }
        Thread.sleep(1000);
        //买票
        System.out.println(Thread.currentThread().getName()+"拿到了"+TicketNums--);
    }
}

2.同步块

package com.qiu.syn;
//不安全的取钱
//两个人去银行里面取钱
public class UnSafeBank {
    public static void main(String[] args) {
        //账户
        Account account = new Account(100,"装修的钱");
        Drawing me = new Drawing(account,50,"我");
        Drawing you = new Drawing(account,100,"你");
        me.start();
        you.start();
    }
}
//账户
class Account{
    int money;
    String name;

    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;
    //现在手里还有多少钱
    int nowMoney;
    public Drawing(Account account,int drawingMoney,String name){
        super(name);//调用父类的名字,就是当前线程的名字
        this.account=account;
        this.drawingMoney=drawingMoney;

    }

    @Override
    //synchronized默认锁的是this.
    //锁的对象是变化的量
    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,取不了");
                return;
            }
            try {//放大问题的发生性
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            //卡内余额
            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()
            System.out.println(this.getName()+"手里的钱"+nowMoney);
        }
    }
}

3.在变化的参数中加synchronized

package com.qiu.syn;

import java.util.*;

//线程不安全的集合
//原因是因为两个数同一时间添加到了同一个位置,相比上一个位置的数被覆盖掉了,所以就少了很多数
public class UnSafeList {
    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());
        }
    }

凡事有利也有弊,synchronized锁也不一定是完美的