get static initialization block to run in a java without loading the class

Question

I have a few classes as shown here

public class TrueFalseQuestion implements Question{
    static{
        QuestionFactory.registerType("TrueFalse", "Question");
    }
    public TrueFalseQuestion(){}
}

...

public class QuestionFactory {

 static final HashMap<String, String > map =  new HashMap<String,String>();

 public static void registerType(String questionName, String ques ) {
     map.put(questionName, ques);
     }
 }



public class FactoryTester {
    public static void main(String[] args) {
        System.out.println(QuestionFactory.map.size());
        // This prints 0. I want it to print 1
    }
}

How can I change TrueFalseQuestion class so that the static method is always run so that I get 1 instead of 0 when I run my main method? I do not want any change in the main method.

I am actually trying to implement the factory patterns where the subclasses register with the factory but i have simplified the code for this question.

Answer 1

The static initializer for the class can't be executed if the class is never loaded.

So you either need to load all the correct classes (which will be hard, since you don't know them all at compile time) or get rid of the requirement for the static initializer.

One way to do the latter is to use the ServiceLoader.

With the ServiceLoader you simply put a file in META-INF/services/package.Question and list all implementations. You can have multiple such files, one per .jar file. This way you can easily ship additional Question implementations separate from your main program.

In the QuestionFactory you can then simply use ServiceLodaer.load(Question.class) to get a ServiceLoader, which implements Iterable<Question> and can be used like this:

for (Question q : ServiceLoader.load(Question.class)) {
    System.out.println(q);
}