java - 反射(reflection):为什么会有setAccessible()这样的方法？

Question

只是奇怪，为什么发明Java的人要写setAccessible(boolean flag)这样的方法，这使得访问修饰符(特别是private)变得毫无用处，无法保护字段、方法和构造函数免受攻击被达到？请看下面的简单示例:

public class BankAccount
{
    private double balance = 100.0;

    public boolean withdrawCash(double cash)
    {
        if(cash <= balance)
        {
            balance -= cash;
            System.out.println("You have withdrawn " + cash + " dollars! The new balance is: " + balance);
            return true;
        }
        else System.out.println("Sorry, your balance (" + balance + ") is less than what you have requested (" + cash + ")!");
        return false;
    }
}

import java.lang.reflect.Field;

public class Test
{
    public static void main(String[] args) throws Exception
    {
        BankAccount myAccount = new BankAccount();
        myAccount.withdrawCash(150);

        Field f = BankAccount.class.getDeclaredFields()[0];
        f.setAccessible(true);
        f.set(myAccount, 1000000); // I am a millionaire now ;)

        myAccount.withdrawCash(500000);
    }
}

输出:

Sorry, your balance (100.0) is less than what you have requested
(150.0)! You have withdrawn 500000.0 dollars! The new balance is: 500000.0

Answer 1

因为某些代码是可信代码——也就是说，如果本地应用程序想要这样做，也许这没什么大不了的。对于不受信任的代码，虽然 - 即小程序、Web 启动应用程序、RMI stub 或任何其他下载的代码 - 有一个 SecurityManager，它(通常基于策略文件)有机会说“对不起，查理”并拒绝 setAccessible() 请求。