Java reflection - access protected field

Two issues you may be having issues with - the field might not be accessible normally (private), and it's not in the class you are looking at, but somewhere up the hierarchy.

Something like this would work even with those issues:

public class SomeExample {

  public static void main(String[] args) throws Exception{
    Object myObj = new SomeDerivedClass(1234);
    Class myClass = myObj.getClass();
    Field myField = getField(myClass, "value");
    myField.setAccessible(true); //required if field is not normally accessible
    System.out.println("value: " + myField.get(myObj));
  }

  private static Field getField(Class clazz, String fieldName)
        throws NoSuchFieldException {
    try {
      return clazz.getDeclaredField(fieldName);
    } catch (NoSuchFieldException e) {
      Class superClass = clazz.getSuperclass();
      if (superClass == null) {
        throw e;
      } else {
        return getField(superClass, fieldName);
      }
    }
  }
}

class SomeBaseClass {
  private Integer value;

  SomeBaseClass(Integer value) {
    this.value = value;
  }
}

class SomeDerivedClass extends SomeBaseClass {
  SomeDerivedClass(Integer value) {
    super(value);
  }
}

Use FieldUtils.writeField(object, "fieldname", value, true) or readField(object, "fieldname", true) from Apache Commons lang3.

field = myclass.getDeclaredField("myname");
field.setAccessible(true);
field.set(myinstance, newvalue);

Use reflection to access the members of the class instance, make them accessible and set their respective values. Of course you'd have to know the name of each member you want to change, but I guess that won't be a problem.

public class ReflectionUtil {
    public static Field getField(Class clazz, String fieldName) throws NoSuchFieldException {
        try {
            return clazz.getDeclaredField(fieldName);
        } catch (NoSuchFieldException e) {
            Class superClass = clazz.getSuperclass();
            if (superClass == null) {
                throw e;
            } else {
                return getField(superClass, fieldName);
            }
        }
    }
    public static void makeAccessible(Field field) {
        if (!Modifier.isPublic(field.getModifiers()) ||
            !Modifier.isPublic(field.getDeclaringClass().getModifiers()))
        {
            field.setAccessible(true);
        }
    }
}

public class Application {
    public static void main(String[] args) throws Exception {
        KalaGameState obj = new KalaGameState();
        Field field = ReflectionUtil.getField(obj.getClass(), 'turn');
        ReflectionUtil.makeAccessible(field);
        field.setInt(obj, 666);
        System.out.println("turn is " + field.get(obj));
    }
}

If using Spring, ReflectionTestUtils provides some handy tools that help out here with minimal effort.

For example, to get a protected field value that is known to be an int:

int theIntValue = (int)ReflectionTestUtils.getField(theClass, "theProtectedIntField");

Or alternatively to set this field's value:

ReflectionTestUtils.setField(theClass, "theProtectedIntField", theIntValue);