CODEWITHSUNDEEP
javagenerics
Glory to Russia
Glory to Russia

Reputation: 18712

How to get the class object of a Java generic?

I have following class:

public abstract class AbstractService<PersistenceClass extends IPersistence,
    RequestClass, ResponseClass> {

    public String processRequest(final String aRequestAsText)
    {
        [...]

        final RequestClass request = mapper.readValue(aRequestAsText, 
            RequestClass);

        [...]
    }

}

The mapper.readValue invokation is normally implemented as 

mapper.readValue(aRequestAsText, Request.class)

How can I code it, if the class RequestClass is a generic?

Update 1 (06.09.2013): This code seems to work:

public class GenericTypeBindigsFinder implements IGenericTypeBindigsFinder {

    @SuppressWarnings("rawtypes")
    @Override
    public List<Class> getGenericTypes(final Class aClass) {
        final List<Class> result = new LinkedList<Class>();

        final ParameterizedType gen = (ParameterizedType) aClass.getGenericSuperclass();

        final Type [] types = gen.getActualTypeArguments();

        for (int i = 0; i < types.length; i++) {
            if (types[i] instanceof Class)
            {
                final Class clazz = (Class)types[i];
                result.add(clazz);
            }
        }

        return result;
    }

}

You can find the corresponding unit test here.

Upvotes: 0

Views: 181

Answers (2)

rec
rec

Reputation: 10895

So you want to access the generic types bound in AbstractService. You can do it via reflection. See the example code here: 

import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.lang.reflect.TypeVariable;

public class GenericClass extends GenericSuper<String, Integer> {
  public GenericClass() {
    ParameterizedType gen = (ParameterizedType) getClass().getGenericSuperclass();
    TypeVariable<?> typeVars[] = getClass().getSuperclass().getTypeParameters();
    Type [] types = gen.getActualTypeArguments();
    for (int i = 0; i < typeVars.length; i++) {
      System.out.println("Generic parameter " + typeVars[i] + 
                         " is bound to " + types[i]);
    }
  }

  public static void main(String[] args) {
    new GenericClass();
  }
}

class GenericSuper<X, Y> {};

Output is: 

Generic parameter X is bound to class java.lang.String
Generic parameter Y is bound to class java.lang.Integer

You can cast types[i] to Class.

For a much more sophisticated solution, see getTypeArguments(...).

Upvotes: 1

ragklaat
ragklaat

Reputation: 946

Try to pass object or an interface preferably to the method, then you can get the class of it with the getClass() method or maybe instanceof.

Upvotes: 2

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