通过反射访问main方法中的局部变量 [英] Accessing local variables in the main method via reflection

问题描述

只是玩Java反射，我想我大部分时间都在掌握它。我从这个问题/答案中了解在大多数情况下，我只限于静态变量。如果我有一个类的实例，我可以访问非静态变量，这是有道理的，我得到那么多。

Just having a play around with Java reflection and I think I'm getting the hang of it for the most part. I understand from this question/answer that, for the most part, I'm limited to static variables. If I have an instance of the class though, I can access non-static variables, which does make sense, I get that much.

说我有以下两个类：

public class A
{
    private static int _staticInt;

    public static void main(String[] args)
    {
        B instanceOfB = new B();
    }
}

public class B
{
    private int _nonStaticInt;
    public Game() {}
}

我理解如何访问 _staticInt ，这不是问题。
我的理解是我可以以相同的方式获得 _nonStaticInt 的字段（即 Field f = B.class.getDeclaredField（_ nonStaticInt）; ）。从其他研究（javadocs，trail等）我收集到我需要一个 B 的实例，以获得 _nonStaticInt 。

I understand how to access _staticInt, that's not an issue. My understanding is that I can get the Field for _nonStaticInt in the same way (i.e. Field f = B.class.getDeclaredField("_nonStaticInt");). From other research (javadocs, trails, etc) I have gathered that I need an instance of B in order to get the value of _nonStaticInt.

所以我的问题; 由于 main 是静态的，可以访问 instanceOfB 以访问 _nonStaticInt 的值？我不认为这是可能的，但我认为在放弃这个想法之前，最好先咨询比我更了解的人。

So my question; Since main is static, is it possible to access instanceOfB in order to access the value of _nonStaticInt? I don't think it is possible, but I thought it's always best to consult people that are more knowledgable than myself before giving up on the idea.

推荐答案

由于main是静态的，是否可以访问instanceOfB以访问_nonStaticInt的值？

Since main is static, is it possible to access instanceOfB in order to access the value of _nonStaticInt?

不。使用Java Reflection API无法访问的局部变量（是否在静态方法中）。反射仅适用于类型级别，而不是字节代码级别 ² 。

"No." Local variables (being in a static method or not) cannot be accessed with the Java Reflection API. Reflection only works at the type level, not the byte-code level².

对链接问题的所述理解是正确的; 非静态（实例）字段的反射访问逻辑需要实例。也就是说，问题不在于反映B类型，问题在于获取 B实例（分配给局部变量）以反映。

The stated understanding of the linked question is correct; reflection access of a non-static (instance) field logically requires an instance. That is, the issue then isn't about reflecting on the B type, the issue is about obtaining the B instance (which is assigned to a local variable) to reflect upon.

要做到这一点，B实例必须以某种方式流血 - 例如分配给静态字段或作为参数传递给main ¹ 中的方法/构造函数 - 以便稍后可以将其与反射一起用作访问实例成员的对象。

To do this the B instance has to be "bled" somehow - e.g. assigned to a static field or passed as an argument to a method/constructor from main¹ - so that it can be used with reflection later as the object who's instance members are to be accessed.

最干净的方法可能是将B实例传递到适当的上下文（或DI），也许是IoC的助手......也许更改类型以避免完全使用反射。

The cleanest approach would probably be to pass the B instance down through the appropriate context (or "DI"), perhaps with the aide of IoC .. and maybe changing the type to avoid the use of reflection entirely.

¹ 另一种流血B实例的方法是附加一个调试器并检查/使用执行框架的主要方法中的局部变量 - 但这听起来像是试图用俱乐部拍打苍蝇。

¹ Another possible way to "bleed" the B instance is to attach a debugger and inspect/use the local variable within the main methods executing frame - but this sounds like trying to swat a fly with a club.

² 即使是像BCEL / ASM这样的工具也不会在main方法的执行期间立即提供帮助。相反，它将用于解构方法，添加所需的钩子/代码以流血或使用创建的实例，然后构造一个修改的方法来执行。

² Even tooling like BCEL/ASM wouldn't immediately help during the execution of the main method. Rather it would be used to deconstruct the method, add in the required hooks/code to "bleed" or use the instance created, and then construct a modified method to execute.

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