1. Why synchronized and java.util.concurrent.locks.Lock?
1> thread-safe, because the thread is random, it can be seen random access your shared variables. If the shared variable is modified, another thread operation, its data is not accurate
2> insecure data, the data is shared
That is shared variables, visibility and atomicity
3> command reordering (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。
* Thread State
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("I took:" + quQian + "; the account has:" + acc.getMoney ()); } }
no lock
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(); } }
* Disorderly and data errors (compare the effectiveness and locked)
locked
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。