java 쓰레드 - 동기화 문제(Accout) 예제

import java.util.Scanner;

import javax.lang.model.util.ElementScanner6;

class Account {

int balance = 200;

public void withdraw(int money) {

    if (balance >= money) {
        String threadName = Thread.currentThread().getName(); // 쓰레드 이름 받아오기
        try {
            System.out.println(threadName +  "::슬립전:" + balance);
            Thread.sleep(1000);

        } catch (Exception e) {

        }
        System.out.println(threadName +  "::빼기전:" + balance);
        balance -= money;
        System.out.println(threadName +  "::빼기후:" + balance);
    }

} // withdraw

}

class A implements Runnable {

Account acc = new Account();

public void run() {
    String threadName = Thread.currentThread().getName(); // 쓰레드 이름 받아오기

    System.out.println(threadName +  "::balance:" + acc.balance);

    while (acc.balance > 0) {

        // 100, 200, 300중의 한 값을 임으로 선택해서 출금(withdraw)

        int money = 200;//(int) (Math.random() * 3 + 1) * 100;
        acc.withdraw(money);
        System.out.println(threadName +  "::balance:" + acc.balance);

    }

} // run()

}

public class ThreadSyncTest2 {
public static void main(String[] args) {
Runnable r = new A();

    Thread t1 = new Thread(r,"첫번째");
    Thread t2 = new Thread(r,"두번째");

    t1.start();
    t2.start();

}

}

출력

두번째::balance:200
첫번째::balance:200
두번째::슬립전:200
첫번째::슬립전:200
첫번째::빼기전:200
첫번째::빼기후:0
첫번째::balance:0
두번째::빼기전:0
두번째::빼기후:-200
두번째::balance:-200