[开发工具]同步方法及同步代码块

synchronized方法和synchronized块

synchronized方法必须获得对象的锁才能执行，否则线程阻塞，方法一旦执行，就独占此锁，直到方法返回才释放锁，后面被阻塞的线程才能获得这个锁，继续执行

synchronized会影响效率

同步块synchronized(Obj){}，Obj是同步监听器，第一个线程访问，所定Obj，完后解锁Obj，第二个才能访问

同步方法弊端：方法里面需要修改的内容才需要锁，锁的太多，浪费资源

package com.wuming.syn.synch;
//不安全的买票
public class UnSafeBuyTicket {
    public static void main(String[] args) {
        BuyTicket station = new BuyTicket();
        new Thread(station,"苦逼的我").start();
        new Thread(station,"牛逼的你们").start();
        new Thread(station,"可恶的黄牛党").start();
    }
}

class BuyTicket implements Runnable{
    //票
    private int ticketNums=10;
    boolean flag=true;//停止方式
    /**
     * When an object implementing interface <code>Runnable</code> is used
     * to create a thread, starting the thread causes the object's
     * <code>run</code> method to be called in that separately executing
     * thread.
     * <p>
     * The general contract of the method <code>run</code> is that it may
     * take any action whatsoever.
     *
     * @see Thread#run()
     */
    @Override
    public void run() {
        //买票
        while(flag){
            try {
                buy();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
    //synchronized同步方法，锁的是this BuyTicket
    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--);
        //加了synchronized，安全
    /*    苦逼的我拿到10
                可恶的黄牛党拿到9
        可恶的黄牛党拿到8
                牛逼的你们拿到7
        可恶的黄牛党拿到6
                苦逼的我拿到5
        苦逼的我拿到4
                可恶的黄牛党拿到3
        牛逼的你们拿到2
                可恶的黄牛党拿到1*/
    }
}

===============

package com.wuming.syn;
//不安全的取钱
//两个人去银行取钱，账户
public class UnSafeBank {
    public static void main(String[] args) {
        //账户
        Account account = new Account(100,"结婚基金");
        Drawing you = new Drawing(account,50,"你");
        Drawing girlFriend = new Drawing(account,100,"girlFriend");
        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
    //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);//线程name
        this.account = account;
        this.drawingMoney = drawingMoney;
        this.nowMoney = nowMoney;
    }
//取钱
    //synchronized默认锁的是本身
    @Override
    public void run() {
        //锁的对象是变化的量，增删改;改成this就是指银行，线程不安全如下
   /*     结婚基金余额为：-50
        结婚基金余额为：-50
        你手里的钱50
                girlFriend手里的钱100*/
        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;
            }
            //sleep可以放大问题的发生性
            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);
            //Thread.currentThread().getName()=this.getName();
            System.out.println(this.getName()+"手里的钱"+nowMoney);
        }
        //使用同步块 synchronized (account)，线程安全
    /*    结婚基金余额为：50
        你手里的钱50
        girlFriend钱不够，取不了*/


    }
}

===============

package com.wuming.syn.synch;

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

//线程不安全的集合
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){//线程安全
                    list.add(Thread.currentThread().getName());
                }
            }).start();
        }
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(list.size());//每次运行都不够10000，线程不安全，两个线程同时操作同一个位置，
        //两个数组添加到同一个位置，就会覆盖掉，元素就会少

    }
}

10000