线程同步机制

　　并发：同一个对象被多个线程同时操作

 　　线程同步：

 　　线程不安全案例1：

//不安全的买票    可能出现负数票，或者抢到了同一张票

public class UnsafeBuyTicket implements Runnable{

    private int ticketNums=10; //票
    boolean flag=true; //外部停止方式
    @Override
    public void run() {

        while (flag){
            buy();
        }

    }

    private void buy(){
        if(ticketNums<0){
            flag=false;
            return;
        }
        //模拟延时
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName()+"拿到了第"+ticketNums--+"票");
    }

    public static void main(String[] args) {

        UnsafeBuyTicket unsafeBuyTicket=new UnsafeBuyTicket();

        new Thread(unsafeBuyTicket,"学生").start();
        new Thread(unsafeBuyTicket,"老师").start();
        new Thread(unsafeBuyTicket,"黄牛党").start();
    }
}

　　线程不安全案例2：

//不安全的取钱
//两个人去银行取钱
public class UnsafeBank {

    public static void main(String[] args) {

        Account account=new Account(100,"结婚基金");

        Drawing you=new Drawing(account,50,"你");
        Drawing girlFriend=new Drawing(account,100,"grilFriend");

        you.start();
        girlFriend.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
    int drawingMoney; //取了多少钱
    int nowMoney;   //现在手里有多少钱

    public Drawing(Account account, int drawingMoney, String name) {

        super(name);
        this.account = account;
        this.drawingMoney = drawingMoney;
    }

    //取钱
    @Override
    public void run() {
        if(account.money-drawingMoney<0){
            System.out.println(Thread.currentThread().getName()+"钱不够了，取不了了");
        }
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        //卡内余额=余额-你取的钱
        account.money=account.money-drawingMoney;
        //你手里的钱
        nowMoney=nowMoney+drawingMoney;

        System.out.println(account.name+"余额为："+account.money);
        System.out.println(this.getName()+"手里的钱："+nowMoney);
    }
}

　　线程不安全案例3

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

public class UnsafeList {

    public static void main(String[] args) {

        List<String> list=new ArrayList<>();
        for(int i=0;i<10000;i++){

            new Thread(()->{
               list.add(Thread.currentThread().getName());
            }).start();
        }
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(list.size());
    }
}

同步方法

同步方法弊端

 　　解决方案一：在方法中使用  synchronized锁住方法  对应上面第一个案例

//不安全的买票
public class UnsafeBuyTicket implements Runnable{

    private int ticketNums=10; //票
    boolean flag=true; //外部停止方式
    @Override
    public void run() {

        while (flag){
            buy();
            //模拟延时
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

    }

    private synchronized void buy(){
        if(ticketNums<1){
            flag=false;
            return;
        }

        System.out.println(Thread.currentThread().getName()+"拿到了第"+ticketNums--+"票");
    }

    public static void main(String[] args) {

        UnsafeBuyTicket unsafeBuyTicket=new UnsafeBuyTicket();

        new Thread(unsafeBuyTicket,"学生").start();
        new Thread(unsafeBuyTicket,"老师").start();
        new Thread(unsafeBuyTicket,"黄牛党").start();
    }
}

　　同步块  锁的对象就是变化的量，需要增删改的对象

//不安全的取钱
//两个人去银行取钱
public class UnsafeBank {

    public static void main(String[] args) {

        Account account=new Account(100,"结婚基金");

        Drawing you=new Drawing(account,50,"你");
        Drawing girlFriend=new Drawing(account,100,"grilFriend");

        you.start();
        girlFriend.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
    int drawingMoney; //取了多少钱
    int nowMoney;   //现在手里有多少钱

    public Drawing(Account account, int drawingMoney, String name) {

        super(name);
        this.account = account;
        this.drawingMoney = drawingMoney;
    }

    //取钱
    @Override
    public void run() {

        synchronized(account){   
            if(account.money-drawingMoney<0){
                System.out.println(Thread.currentThread().getName()+"钱不够了，取不了了");
　　　　　　　　　 return;
            }
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            //卡内余额=余额-你取的钱
            account.money=account.money-drawingMoney;
            //你手里的钱
            nowMoney=nowMoney+drawingMoney;

            System.out.println(account.name+"余额为："+account.money);
            System.out.println(this.getName()+"手里的钱："+nowMoney);
        }
    }

}