同步方法：我們只需要針對方法提出一套機制，這套機制就是synchronized關鍵字，它包括兩種用法：synchronized方法和synchronized塊

同步方法：public synchronized void method(int args){}

缺陷：若將一個大的方法申明為synchronized將會影響效率

同步塊：synchronized(Obj){}

Obj稱之為同步監視器。

監視的物件為需要進行增、刪、改、查的物件

1.加synchronized的買票安全

public class UnsafeBuyTicket {
  public static void main(String[] args) {
​
    BuyTicket buyTicket=new BuyTicket();
    new Thread(buyTicket,"小明").start();
    new Thread(buyTicket,"小紅").start();
    new Thread(buyTicket,"黃牛黨").start();
   }
}
​
class BuyTicket implements Runnable{
  private int ticketNums =10;
  boolean flag=true;
  @Override
  public void run() {
    //買票
    while (flag){
      try {
        buy();
       } catch (InterruptedException e) {
        e.printStackTrace();
       }
     }
   }
  private synchronized void buy() throws InterruptedException{
    //判斷是否有票
    if(ticketNums<=0){
      return;
     }
    //買票
     Thread.sleep(1000);
    
    System.out.println(Thread.currentThread().getName()+"拿到"+ticketNums--);
   }
}
 

2.銀行賬戶加synchronized(){}，需要進行修改的物件

public class UnsafeBank {
  public static void main(String[] args) {
  Account account=new Account(100,"結婚基金");
​
  Drawing you=new Drawing(account,50,"你");
  Drawing girlFriend=new Drawing(account,100,"girlFriend");
​
  you.start();
  girlFriend.start();
​
   }
}
//賬戶
class Account{
  int money;//餘額
  String name;//卡明
​
  public Account(int money, String name) {
    this.money = money;
    this.name = name;
   }
}
​
//銀行
class Drawing extends Thread{
  Account account;//賬戶
  int drawingMoney;//取多少錢
  int nowMoney;//還剩多少錢
​
  public Drawing(Account account,int drawingMoney,String name){
    super(name);
    this.account=account;
    this.drawingMoney=drawingMoney;
   }
​
  //取錢的方法
  @Override
  public void run() {
    
    synchronized(account){
      
     //判斷是否還有錢
    if (account.money-drawingMoney<0){
      System.out.println(Thread.currentThread().getName()+"錢不夠，錢取不了");
      return;
     }
    try {
      Thread.sleep(1000);
     } catch (InterruptedException e) {
      e.printStackTrace();
     }
​
    //卡內餘額=餘額-你取的錢
    account.money=account.money-drawingMoney;
    //你手裡的錢
    nowMoney=nowMoney+drawingMoney;
​
    System.out.println(account.name+"餘額為："+account.money);
    System.out.println(this.getName()+"手裡的錢"+nowMoney);
   }
    
  }
    
}
​
 

3.加synchronized(){}

public class UnsafeList {
  public static void main(String[] args) throws InterruptedException {
    List<String> list = new ArrayList<String>();
    for (int i = 0; i < 10000; i++) {
      new Thread(()->{
        synchronized(list){
           list.add(Thread.currentThread().getName());
         }
        
       }).start();
     }
    Thread.sleep(1000);
    System.out.println(list.size());
   }
}