Change private static final field using Java reflection

Question

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

Using reflection I get this error: java.lan

Answer 1

The accepted answer worked for me until deployed on JDK 1.8u91. Then I realized it failed at field.set(null, newValue); line when I had read the value via reflection before calling of setFinalStatic method.

Probably the read caused somehow different setup of Java reflection internals (namely sun.reflect.UnsafeQualifiedStaticObjectFieldAccessorImpl in failing case instead of sun.reflect.UnsafeStaticObjectFieldAccessorImpl in success case) but I didn't elaborate it further.

Since I needed to temporarily set new value based on old value and later set old value back, I changed signature little bit to provide computation function externally and also return old value:

public static  T assignFinalField(Object object, Class clazz, String fieldName, UnaryOperator newValueFunction) {
    Field f = null, ff = null;
    try {
        f = clazz.getDeclaredField(fieldName);
        final int oldM = f.getModifiers();
        final int newM = oldM & ~Modifier.FINAL;
        ff = Field.class.getDeclaredField("modifiers");
        ff.setAccessible(true);
        ff.setInt(f,newM);
        f.setAccessible(true);

        T result = (T)f.get(object);
        T newValue = newValueFunction.apply(result);

        f.set(object,newValue);
        ff.setInt(f,oldM);

        return result;
    } ...

However for general case this would not be sufficient.