1.为什么使用synchronized和java.util.concurrent.locks.Lock?

1>线程不安全，因为线程是随机的,它可以随机访问你可见的共享变量。如果共享变量被修改，对另一个线程操作来说，它的数据就不准确

2>数据不安全，数据被共享

即共享变量、可见性和原子性

3>指令重排序(as-if-serial)

 

   加锁，是只一个线程可以访问，其他线程等待访问的线程结束，后面的线程由cpu随机分配访问。

*线程状态

线程状态：创建->就绪->运行->死亡

线程从就绪状态被唤醒，运行时，有其他线程正在运行且访问的是它要访问的内容，那么，它会阻塞。

阻塞->就绪

休眠状态：即等待（阻塞）状态

运行时，如果cpu随机分配线程抢占了当前线程的资源，该线程进入就绪状态。

public class Account {    private long money = 0;//余额        public Account(){}        public long cunQian(int num){
   //存钱        money  = money + num;        return num;    }        public long quQian(int num){
   //取钱         money = money - num;         return num;    }        public long getMoney(){        return money;    }}

import java.util.Random;public class Thread02 extends Thread{    private Account acc = null;    public Thread02(Account acc){        this.acc = acc;    }    @Override    public void run() {        long cunQian = acc.cunQian(new Random().nextInt(1000));        System.out.println("存了:"+cunQian+";账户内有："+acc.getMoney());        long quQian = acc.quQian(new Random().nextInt(1000));        System.out.println("取了:"+quQian+";账户内有："+acc.getMoney());    }}

无锁

public class Thread02Main {    public static void main(String[] args) {        Account acc = new Account();        Thread02 t1 = new Thread02(acc);        Thread02 t2 = new Thread02(acc);        Thread02 t3 = new Thread02(acc);        t1.start();        t2.start();        t3.start();            }}

 

*无序且数据错误（效果和有锁比较）

 

 

有锁

import java.util.Random;public class Thread02 extends Thread{    private Account acc = null;    public Thread02(Account acc){        this.acc = acc;    }    @Override    public void run() {        synchronized (acc) {            long cunQian = acc.cunQian(new Random().nextInt(1000));            System.out.println("存了:"+cunQian+";账户内有："+acc.getMoney());            long quQian = acc.quQian(new Random().nextInt(1000));            System.out.println("取了:"+quQian+";账户内有："+acc.getMoney());        }    }}

public class Account {    private long money = 0;        public Account(){}        public synchronized long cunQian(int num){
   //存钱        money  = money + num;        return num;    }        public synchronized long quQian(int num){
   //取钱         money = money - num;         return num;    }        public long getMoney(){        return money;    }} *先存后取，账户数据正确。负数是因为存取钱是随机数。