使用 Java 反射更改私有静态最终字段 [英] Change private static final field using Java reflection

问题描述

我有一个带有 private static final 字段的类，不幸的是，我需要在运行时更改它.

I have a class with a private static final field that, unfortunately, I need to change it at run-time.

使用反射我得到这个错误:java.lang.IllegalAccessException: Can not set static final boolean field

Using reflection I get this error: java.lang.IllegalAccessException: Can not set static final boolean field

有什么办法可以改变这个值吗?

Is there any way to change the value?

Field hack = WarpTransform2D.class.getDeclaredField("USE_HACK");
hack.setAccessible(true);
hack.set(null, true);

推荐答案

假设没有 SecurityManager 阻止您这样做，您可以使用 setAccessible 绕过 private 并重置修饰符去掉final，实际上修改了一个private static final 字段.

Assuming no SecurityManager is preventing you from doing this, you can use setAccessible to get around private and resetting the modifier to get rid of final, and actually modify a private static final field.

这是一个例子:

import java.lang.reflect.*;

public class EverythingIsTrue {
   static void setFinalStatic(Field field, Object newValue) throws Exception {
      field.setAccessible(true);

      Field modifiersField = Field.class.getDeclaredField("modifiers");
      modifiersField.setAccessible(true);
      modifiersField.setInt(field, field.getModifiers() & ~Modifier.FINAL);

      field.set(null, newValue);
   }
   public static void main(String args[]) throws Exception {      
      setFinalStatic(Boolean.class.getField("FALSE"), true);

      System.out.format("Everything is %s", false); // "Everything is true"
   }
}

假设没有抛出SecurityException，上面的代码打印Everything is true".

Assuming no SecurityException is thrown, the above code prints "Everything is true".

这里实际做的事情如下:

What's actually done here is as follows:

• main 中的原始 boolean 值 true 和 false 被自动装箱为引用类型 Boolean 常数"Boolean.TRUE 和 Boolean.FALSE
• 反射用于更改public static final Boolean.FALSE 引用Boolean.TRUE
引用的Boolean
• 因此，随后每当 false 自动装箱为 Boolean.FALSE 时，它引用的 Boolean 与所引用的相同通过 Boolean.TRUE
• 所有 假" 现在都是 真"

• The primitive boolean values true and false in main are autoboxed to reference type Boolean "constants" Boolean.TRUE and Boolean.FALSE
• Reflection is used to change the public static final Boolean.FALSE to refer to the Boolean referred to by Boolean.TRUE
• As a result, subsequently whenever a false is autoboxed to Boolean.FALSE, it refers to the same Boolean as the one refered to by Boolean.TRUE
• Everything that was "false" now is "true"

• 使用反射更改 static final File.separatorChar 用于单元测试
• 如何将 setAccessible 限制为仅合法"使用?
  • 有搞乱Integer的缓存、改变String等的例子
  • Using reflection to change static final File.separatorChar for unit testing
  • How to limit setAccessible to only "legitimate" uses?
    • Has examples of messing with Integer's cache, mutating a String, etc

    每当您执行此类操作时都应格外小心.它可能不起作用，因为 SecurityManager 可能存在，但即使没有，根据使用模式，它可能会也可能不会起作用.

    Extreme care should be taken whenever you do something like this. It may not work because a SecurityManager may be present, but even if it doesn't, depending on usage pattern, it may or may not work.

    JLS 17.5.3 Final 的后续修改字段

    在某些情况下，例如反序列化，系统将需要在构造后更改对象的final 字段.final 字段可以通过反射和其他依赖于实现的方式来改变.唯一具有合理语义的模式是构造一个对象，然后更新对象的 final 字段.对象不应该对其他线程可见，也不应该读取 final 字段，直到对象的 final 字段的所有更新完成.final 字段的冻结发生在设置了 final 字段的构造函数的末尾，以及在每次修改 final 之后立即发生场通过反射或其他特殊机制.

    In some cases, such as deserialization, the system will need to change the final fields of an object after construction. final fields can be changed via reflection and other implementation dependent means. The only pattern in which this has reasonable semantics is one in which an object is constructed and then the final fields of the object are updated. The object should not be made visible to other threads, nor should the final fields be read, until all updates to the final fields of the object are complete. Freezes of a final field occur both at the end of the constructor in which the final field is set, and immediately after each modification of a final field via reflection or other special mechanism.

    即便如此，仍有许多并发症.如果在字段声明中将 final 字段初始化为编译时常量，则可能不会观察到对 final 字段的更改，因为使用了该 final 字段在编译时替换为编译时常量.

    Even then, there are a number of complications. If a final field is initialized to a compile-time constant in the field declaration, changes to the final field may not be observed, since uses of that final field are replaced at compile time with the compile-time constant.

    另一个问题是规范允许对 final 字段进行积极优化.在一个线程中，允许对 final 字段的读取重新排序，并在构造函数中没有发生对 final 字段的修改.

    Another problem is that the specification allows aggressive optimization of final fields. Within a thread, it is permissible to reorder reads of a final field with those modifications of a final field that do not take place in the constructor.

    另见

    • JLS 15.28 常量表达式
      • 这种技术不太可能与原始的 private static final boolean 一起使用，因为它可以作为编译时常量内联，因此new"价值可能无法观察

      See also

      • JLS 15.28 Constant Expression
        • It's unlikely that this technique works with a primitive private static final boolean, because it's inlineable as a compile-time constant and thus the "new" value may not be observable

        本质上，

        field.getModifiers() & ~Modifier.FINAL
        

        关闭与 field.getModifiers() 中的 Modifier.FINAL 对应的位.& 是按位与，而 ~ 是按位补码.

        turns off the bit corresponding to Modifier.FINAL from field.getModifiers(). & is the bitwise-and, and ~ is the bitwise-complement.

        • 维基百科/按位运算

        还是解决不了?，像我一样陷入抑郁了吗?你的代码是这样的吗?

        Still not being able to solve this?, have fallen onto depression like I did for it? Does your code looks like this?

        public class A {
            private final String myVar = "Some Value";
        }
        

        阅读对此答案的评论，特别是@Pshemo 的评论，它提醒我常量表达式 的处理方式不同，因此不可能修改它.因此，您需要将代码更改为如下所示:

        Reading the comments on this answer, specially the one by @Pshemo, it reminded me that Constant Expressions are handled different so it will be impossible to modify it. Hence you will need to change your code to look like this:

        public class A {
            private final String myVar;
        
            private A() {
                myVar = "Some Value";
            }
        }
        

        如果你不是班级的主人......我觉得你是！

        有关此行为原因的更多详细信息阅读此内容?

        For more details about why this behavior read this?

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