线程同步
并发
- 并发:同一个对象被多个线程同时操作:多人同时抢1张票、多人同时在一个账号里取钱
//并发问题举例1:
//不安全的买票:会出现负数、0、不同的人抢到同一张表
package www.bh.c.threadtest;
public class BuyTickets implements Runnable{
public static void main(String[] args) {
BuyTickets buyTickets = new BuyTickets();
new Thread(buyTickets,"你").start();
new Thread(buyTickets,"我").start();
new Thread(buyTickets,"他").start();
}
private int tickets=10;
boolean flag=true;
@Override
public void run() {
while (flag){
try {
buy();
} catch (Exception e) {
e.printStackTrace();
}
}
}
public void buy(){
//判断是否有票
if (tickets<=0){
flag=false;
return;
}
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName()+"拿到了第"+tickets--+"张票");
}
}
//并发问题举例2:
//不安全的取钱:账户余额会出现负数
package www.bh.c.threadtest;
public class GetMoney {
public static void main(String[] args) {
Account account = new Account(100,"学费");
Drawing you = new Drawing(account, 50, "you");
Drawing yousister = new Drawing(account, 100, "sister");
yousister.start();
you.start();
}
}
//账户
class Account{
int money;//余额
String name;//账号
public Account(int money, String name) {
this.money = money;
this.name = name;
}
}
//银行取钱
class Drawing extends Thread{
Account account;//账户
//取得额度
int drawingMoney;
//取到手的钱
int nowMoney;
public Drawing(Account account,int drawingMoney,String name){
super(name);
this.account=account;
this.drawingMoney=drawingMoney;
}
@Override
public void run() {
//判断余额是否足够
if (account.money-drawingMoney<0){
System.out.println(Thread.currentThread().getName()+"账户余额不足");
return;
}
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
//卡内余额=余额-取得额度
account.money=account.money-drawingMoney;
//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()+"手里的总额"+nowMoney);
}
}
//并发问题举例3:
//不安全的集合:添加不全
package www.bh.c.threadtest;
import java.util.ArrayList;
import java.util.List;
public class ListTest {
public static void main(String[] args) {
List<String> list = new ArrayList<>();
for (int i = 0; i < 1000; i++) {
new Thread(()-> {
list.add(Thread.currentThread().getName());}).start();
}
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(list.size());//<999
}
}
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处理多线程问题时,多个线程访问一个对象,并且某些线程还想修改这个对象,这时候我们就需要线程同步;线程同步其实是一种等待机制,多个需要同时访问此对象的线程进入这个对象的等待池形成队列,等待前面线程使用完毕,下一个线程再使用
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形成条件:队列+锁
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锁机制:synchronized:当一个线程获得对象的排它锁,独占资源,其他线程必须等待,使用后释放锁即可,锁机制存在以下问题:
- 一个线程持有锁会导致其他所有此锁的线程挂起
- 在多线程竞争下,加锁,释放锁会导致比较多的上下文切换和调度延时,引起性能问题
- 如果一个优先级高的线程等待一个优先级低的线程释放锁,会导致优先级倒置,引起性能问题
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同步方法:synchronized关键字(默认锁的是this)
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synchronized方法控制对“对象”的访问,每个对象对应一把锁,每个synchronized都必须获得调用该方法的对象的锁才能执行,否则线程会阻塞,方法一旦执行,就独占该锁,知道该方法返回才释放锁,后面被阻塞的线程才能获得这个锁,继续执行
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缺陷:将一个大的方法申明为synchronized将会影响效率,所以方法里面需要修改的内容才需要锁,锁的太多,浪费资源
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用法一:synchronized方法:public synchronized void method(int args){}
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用法二:synchronized块:synchronized(Obj){}
- Obj称之为同步监视器,推荐使用共享资源作为同步监视器
- 同步监视器的执行过程:
- 1.第一个线程访问,锁定同步监视器,执行其中代码
- 2.第二个线程访问,发现同步监视器被锁定,无法访问
- 3.第一个线程访问完毕,解锁同步监视器
- 4.第二个线程访问,发现同步监视器没有锁,锁定同步监视器,执行其中代码
//并发问题举例1: //不安全的买票:会出现负数、0、不同的人抢到同一张表 //解决方案:synchronized 同步方法 package www.bh.c.threadtest; public class BuyTickets implements Runnable{ public static void main(String[] args) { BuyTickets buyTickets = new BuyTickets(); new Thread(buyTickets,"你").start(); new Thread(buyTickets,"我").start(); new Thread(buyTickets,"他").start(); } private int tickets=10; boolean flag=true; @Override public void run() { while (flag){ try { buy(); } catch (Exception e) { e.printStackTrace(); } } } public synchronized void buy(){ //synchronized 同步方法 //判断是否有票 if (tickets<=0){ flag=false; return; } try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println(Thread.currentThread().getName()+"拿到了第"+tickets--+"张票"); } }
//并发问题举例2: //不安全的取钱:账户余额会出现负数 //解决方案:synchronized 同步块 package www.bh.c.threadtest; public class GetMoney { public static void main(String[] args) { Account account = new Account(100,"学费"); Drawing you = new Drawing(account, 50, "you"); Drawing yousister = new Drawing(account, 100, "sister"); yousister.start(); you.start(); } } //账户 class Account{ int money;//余额 String name;//账号 public Account(int money, String name) { this.money = money; this.name = name; } } //银行取钱 class Drawing extends Thread{ Account account;//账户 //取得额度 int drawingMoney; //取到手的钱 int nowMoney; public Drawing(Account account,int drawingMoney,String name){ super(name); this.account=account; this.drawingMoney=drawingMoney; } @Override public void run() { //同步块,锁的对象是变化的量,需要增删改的量 synchronized (account){ //判断余额是否足够 if (account.money-drawingMoney<0){ System.out.println(Thread.currentThread().getName()+"账户余额不足"); return; } try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } //卡内余额=余额-取得额度 account.money=account.money-drawingMoney; //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()+"手里的总额"+nowMoney); } } }
//并发问题举例3: //不安全的集合:添加不全 //解决方案1:synchronized 同步块 package www.bh.c.threadtest; import java.util.ArrayList; import java.util.List; public class ListTest { public static void main(String[] args) { List<String> list = new ArrayList<>(); for (int i = 0; i < 1000; i++) { new Thread(()-> { synchronized (list){ list.add(Thread.currentThread().getName());} }).start(); } try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println(list.size()); } }
//解决方案2:JUC安全类型的集合 package www.bh.c.threadtest; import java.util.concurrent.CopyOnWriteArrayList; public class JucListTest { public static void main(String[] args) { CopyOnWriteArrayList copy=new CopyOnWriteArrayList<String>(); for (int i = 0; i < 1000; i++) { new Thread(()->{ copy.add(Thread.currentThread().getName()); }).start(); } try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println(copy.size());//1000 } }
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