Java反射：从超类继承的类中调用GetDeclaredFields（）方法

Question

I have a bunch of sensors, which should be inherited from a superclass.

In my superclass, as well as in the sensor itself, there are static final attributes and possible value ranges of my attributes, which defines my sensor.

Now I want to source out a method, which returns a HashMap of attributes and its values ranges in the superclass.

Because of the fact, that the sensor never changes its parameters, I want to use this getAttributes() method as a class method for optional calling it in my main[].

My problem is: How can I get the attributes in a static way (I used reflections at the moment) So well.

A UML diagram of what I said:

my main method looks like:

public static void main(String[] args){
    HashMap<String, List<String>> a = SensorA.getConfigurationAttributes();
}

and my abstract component class:

public static HashMap<String, List<String>> getConfigurationAttributes(boolean getMandatoryOnly){
    Field[] classAttributes = this.getClass().getDeclaredFields();  // error: cannot use this in static context
    // ... other code, working with the attributes.
}   

Of course, now, Java says: "cannot use this in a static context." But how can I correct this error?

On the one hand, remove the static tag would solve the problem, but on the other hand I have to instantiate every sensor before getting my attribute information.

Answer 1

You're using the keyword "this" which references an instance of that object inside a static method. That's incorrect.

In order to achieve the same task, use this code instead :

Field[] fields = Class.forName("my.package.MyClass").getDeclaredFields();

Answer 2

You can do that statically if you pass along the class for which you want to retrieve those fields as an argument to your method.

Else you can only do it for the class you are in, which kind of defeats the purpose since you already know all the declared fields.

Static methods are not inherited, so you cannot invoke it magically on the sub class (for this you indeed need to remove the static modifier).

Thus, do something like this to maybe achieve what you want:

public static HashMap<String, List<String>> getConfigurationAttributes(boolean getMandatoryOnly, Class c) 
{
     Field[] classAttributes = c.getDeclaredFields();
     // ... other code, working with the attributes.
} 

Edit: And now the obligatory warning about reflection: Using reflection like this is not a very good or elegant solution. Reflection is costly, works around a lot of language features and not a very clean way of implementing things. Please avoid reflection where ever you can, in most cases there is simple, elegant and easier solutions.