通过反射更改最终变量，为什么静态和非静态最终变量之间存在差异 [英] changing final variables through reflection, why difference between static and non-static final variable

问题描述

请参考以下代码. 运行代码时，我可以更改最终的非静态变量的值.但是，如果我尝试更改最终静态变量的值，则会抛出java.lang.IllegalAccessException.

我的问题是，为什么在非静态最终变量也不会抛出异常，反之亦然?为什么会有所不同?

import java.lang.reflect.Field;
import java.util.Random;

public class FinalReflection {

    final static int stmark =  computeRandom();
    final int inmark = computeRandom();

    public static void main(String[] args) throws SecurityException, NoSuchFieldException, IllegalArgumentException, IllegalAccessException {
        FinalReflection obj = new FinalReflection();
        System.out.println(FinalReflection.stmark);
        System.out.println(obj.inmark);
        Field staticFinalField  = FinalReflection.class.getDeclaredField("stmark");
        Field instanceFinalField  = FinalReflection.class.getDeclaredField("inmark");
        staticFinalField.setAccessible(true);
        instanceFinalField.setAccessible(true);

        instanceFinalField.set(obj, 100);
        System.out.println(obj.inmark);

        staticFinalField.set(FinalReflection.class, 101);
        System.out.println(FinalReflection.stmark);

    }

    private static int computeRandom() {
        return new Random().nextInt(5);
    }
}

解决方案

FinalReflectionobj = new FinalReflection();
System.out.println(FinalReflection.stmark);
System.out.println(obj.inmark);
Field staticFinalField  = FinalReflection.class.getDeclaredField("stmark");
Field instanceFinalField  = FinalReflection.class.getDeclaredField("inmark");
staticFinalField.setAccessible(true);
instanceFinalField.setAccessible(true);

//EXTRA CODE
//Modify the final using reflection
Field modifiersField = Field.class.getDeclaredField("modifiers");
modifiersField.setAccessible(true);
modifiersField.setInt(staticFinalField, staticFinalField.getModifiers() & ~Modifier.FINAL);


instanceFinalField.set(obj, 100);
System.out.println(obj.inmark);
staticFinalField.set(FinalReflection.class, 101);
System.out.println(FinalReflection.stmark);

该解决方案并非没有缺点，它可能在所有情况下均不起作用:

如果在字段声明中将final字段初始化为编译时常量，则对final字段的更改可能不可见，因为使用编译时替换了最终字段的用法，时间常数.

另一个问题是，该规范允许对final字段进行积极的优化.在线程内，允许对final字段的读取进行重新排序，而对final字段的修改不会在构造函数中发生. 更多也在类似的问题中对此进行了解释./p>

Please refer to the below code. When I run the code, I am able to change the value of a final non-static variable. But if I try to change the value of a final static variable then it throws java.lang.IllegalAccessException.

My question is why doesn't it throw an exception in case of non-static final variable also or vice versa. Why the difference?

import java.lang.reflect.Field;
import java.util.Random;

public class FinalReflection {

    final static int stmark =  computeRandom();
    final int inmark = computeRandom();

    public static void main(String[] args) throws SecurityException, NoSuchFieldException, IllegalArgumentException, IllegalAccessException {
        FinalReflection obj = new FinalReflection();
        System.out.println(FinalReflection.stmark);
        System.out.println(obj.inmark);
        Field staticFinalField  = FinalReflection.class.getDeclaredField("stmark");
        Field instanceFinalField  = FinalReflection.class.getDeclaredField("inmark");
        staticFinalField.setAccessible(true);
        instanceFinalField.setAccessible(true);

        instanceFinalField.set(obj, 100);
        System.out.println(obj.inmark);

        staticFinalField.set(FinalReflection.class, 101);
        System.out.println(FinalReflection.stmark);

    }

    private static int computeRandom() {
        return new Random().nextInt(5);
    }
}

解决方案

FinalReflectionobj = new FinalReflection();
System.out.println(FinalReflection.stmark);
System.out.println(obj.inmark);
Field staticFinalField  = FinalReflection.class.getDeclaredField("stmark");
Field instanceFinalField  = FinalReflection.class.getDeclaredField("inmark");
staticFinalField.setAccessible(true);
instanceFinalField.setAccessible(true);

//EXTRA CODE
//Modify the final using reflection
Field modifiersField = Field.class.getDeclaredField("modifiers");
modifiersField.setAccessible(true);
modifiersField.setInt(staticFinalField, staticFinalField.getModifiers() & ~Modifier.FINAL);


instanceFinalField.set(obj, 100);
System.out.println(obj.inmark);
staticFinalField.set(FinalReflection.class, 101);
System.out.println(FinalReflection.stmark);

This solution does not come without some downsides, it may not work in all cases:

In case a final field is initialized to a compile-time constant in the field declaration, changes to the final field may not be visible, since uses of that final field are replaced at compile time with the compile-time constant.

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. More on this is also explained in this similar question.

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