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);
}
}
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.