synchronized的使用场景可以归结为3种：
① 修饰静态方法，给当前类对象加锁，进入同步方法时需要获得类对象的锁
② 修饰实例方法，给当前实例变量加锁，进入同步方法时需要获得当前实例的锁
③ 修饰同步方法块，指定加锁对象（可以是实例对象，也可以是类变量），对给定对象加锁，进入同步方法块时需要获得加锁对象的锁

1、静态方法

Class BankAccount{
      private static int accountNum; // 一共有多少个银行账号
      
      public static synchronized void setAccountNum(){
                accountNum = accountNum + 1;
      }
}

2、成员方法

public class BankAccount{
        private double balance;
        private static Logger logger = LoggerFactory.getLogger(BankAccount.class);

        public synchronized void deposite(double moneyToAdd){
                String threadName = Thread.currentThread().getName();
                logger.info(threadName + "--当前银行余额为：" + this.balance);
                balance = balance + moneyToAdd;
                logger.info(threadName + "--存后银行余额为：" + this.balance);
        }
}

3、同步代码块，块对象是实例对象

public class BankAccount{
        private double balance;
        private static Logger logger = LoggerFactory.getLogger(BankAccount.class);

        public void deposite(double moneyToAdd){
                String threadName = Thread.currentThread().getName();
                logger.info(threadName + "--当前银行余额为：" + this.balance);
                synchronized(this){
                      balance = balance + moneyToAdd;
                }
                logger.info(threadName + "--存后银行余额为：" + this.balance);
        }
}

4、同步代码块，块对象是类对象

Class BankAccount{
      private static int accountNum; // 一共有多少个银行账号
      
      public synchronized void setAccountNum(){
          synchronized(BankAccount.class){
                 accountNum = accountNum + 1;
          }  
      }
}