Java: 简单模拟多线程访问同样变量导致的问题

简单模拟多线程访问同样变量导致的问题，需要使用Transaction控制，或加锁。

package practice;

public class RyanAndMonicaJob implements Runnable...{
    private BankAccount account = new BankAccount();
    public static void main(String[] args)...{
        RyanAndMonicaJob runner = new RyanAndMonicaJob();
        Thread one = new Thread(runner);
        Thread two = new Thread(runner);
        one.setName("Ryan");
        two.setName("Monica");
        one.start();
        two.start();
    }
    
    public void run()...{
        for(int i=0; i< 10; i++)...{
            withdraw(30);
        }
        if(account.getBalance()<0)...{
            System.out.println("the account is overdrawed!");
        }
    }
    private void withdraw(int amount)...{
        if(account.getBalance()>amount)...{
            try...{
                System.out.println(Thread.currentThread().getName() +" is going to sleep.");
                Thread.sleep(500);
            }catch(Exception e)...{
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + " woke up");
            account.withdraw(amount);
            System.out.println("after " + Thread.currentThread().getName() + " withdraw, the account is: " + account.getBalance());
        }else...{
            System.out.println("sorry " + Thread.currentThread().getName() + " the money is : " + account.getBalance());
        }
    }
}

package practice;

public class BankAccount ...{
    int balance = 200;
    public int getBalance()...{
        return balance;
    }
    public void withdraw()...{
        balance = balance - 150;
    }
}

修正其中的问题方法一：把其中的方法作为一个Transaction

package practice;

public class RyanAndMonicaJob implements Runnable...{
    private BankAccount account = new BankAccount();
    public static void main(String[] args)...{
        RyanAndMonicaJob runner = new RyanAndMonicaJob();
        Thread one = new Thread(runner);
        Thread two = new Thread(runner);
        one.setName("Ryan");
        two.setName("Monica");
        one.start();
        two.start();
    }
    
    public void run()...{
        for(int i=0; i< 10; i++)...{
            withdraw(30);
        }
        if(account.getBalance()<0)...{
            System.out.println("the account is overdrawed!");
        }
    }
    private synchronized void withdraw(int amount)...{
        if(account.getBalance()>amount)...{
            try...{
                System.out.println(Thread.currentThread().getName() +" is going to sleep.");
                Thread.sleep(500);
            }catch(Exception e)...{
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + " woke up");
            account.withdraw(amount);
            System.out.println("after " + Thread.currentThread().getName() + " withdraw, the account is: " + account.getBalance());
        }else...{
            System.out.println("sorry " + Thread.currentThread().getName() + " the money is : " + account.getBalance());
        }
    }
}

其中只增加了一个关键字：synchronized 同步方法非常简单，这样方法就作为了原子来执行。

但是，如果只有这个方法访问account，那么不会出现问题，如果还有方法需要访问account，那么需要使用Object key 才有效了，否则即便这个方法封锁了自己，仍然可能获得错误的account而执行错误的操作。

如下的例子导致了“Lost Update”

package practice;

public class TestSync implements Runnable...{
    private int balance;
    
    public void run()...{
        for(int i=0;i<5000;i++)...{
            increaseBalance();
        }
    }
    
    private void increaseBalance()...{
        System.out.println(Thread.currentThread().getName() + ++balance);
    }
}

package practice;

public class TestSyncTest ...{
    public static void main(String[] args)...{
        TestSync runner = new TestSync();
        Thread one = new Thread(runner);
        Thread two = new Thread(runner);
        one.setName("A");
        two.setName("B");
        one.start();
        two.start();
    }
}

在执行过程中是很有出错概率的。解决方法还是一样的：在该increaseBalance()上加上synchronized关键字。
出现的适用范围也是一样，只有歌操作balance的的方法是有效的，但是如果还有其它线程用不同方法操作balance就会遇到问题。