How can I determine the type of a generic field in Java?

Question

I have been trying to determine the type of a field in a class. I've seen all the introspection methods but haven't quite figured out how to do it. This is going to be used to generate xml/json from a java class. I've looked at a number of the questions here but haven't found exactly what I need.

Example:

class Person {
    public final String name;
    public final List<Person> children;
}

When I marshall this object, I need to know that the chidren field is a list of objects of type Person, so I can marshall it properly.

I had tried

for (Field field : Person.class.getDeclaredFields()) {
    System.out.format("Type: %s%n", field.getType());
}

But this will only tell me that it's a List, not a List of Persons

Thanks

Answer 1

Have a look at Obtaining Field Types from the Java Tutorial Trail: The Reflection API.

Basically, what you need to do is to get all java.lang.reflect.Field of your class and call Field#getType() on each of them (check edit below). To get all object fields including public, protected, package and private access fields, simply use Class.getDeclaredFields(). Something like this:

for (Field field : Person.class.getDeclaredFields()) {
    System.out.format("Type: %s%n", field.getType());
    System.out.format("GenericType: %s%n", field.getGenericType());
}

EDIT: As pointed out by wowest in a comment, you actually need to call Field#getGenericType(), check if the returned Type is a ParameterizedType and then grab the parameters accordingly. Use ParameterizedType#getRawType() and ParameterizedType#getActualTypeArgument() to get the raw type and an array of the types argument of a ParameterizedType respectively. The following code demonstrates this:

for (Field field : Person.class.getDeclaredFields()) {
    System.out.print("Field: " + field.getName() + " - ");
    Type type = field.getGenericType();
    if (type instanceof ParameterizedType) {
        ParameterizedType pType = (ParameterizedType)type;
        System.out.print("Raw type: " + pType.getRawType() + " - ");
        System.out.println("Type args: " + pType.getActualTypeArguments()[0]);
    } else {
        System.out.println("Type: " + field.getType());
    }
}

And would output:

Field: name - Type: class java.lang.String
Field: children - Raw type: interface java.util.List - Type args: class foo.Person