GenericsUtils.java

/*
 * Made by Wannes 'W' De Smet
 * (c) 2011 Wannes De Smet
 * All rights reserved.
 * 
 */
package net.wgr.core;

import java.lang.reflect.*;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * @author wannes
 * @created Jul 3, 2011
 */
public class GenericsUtils {

    /**
     * Get the underlying class for a type, or null if the type is a variable
     * type.
     *
     * @param type the type
     * @return the underlying class
     * @author Ian Robertson,
     * http://www.artima.com/weblogs/viewpost.jsp?thread=208860
     */
    public static Class<?> getClass(Type type) {
        if (type instanceof Class) {
            return (Class) type;
        } else if (type instanceof ParameterizedType) {
            return getClass(((ParameterizedType) type).getRawType());
        } else if (type instanceof GenericArrayType) {
            Type componentType = ((GenericArrayType) type).getGenericComponentType();
            Class<?> componentClass = getClass(componentType);
            if (componentClass != null) {
                return Array.newInstance(componentClass, 0).getClass();
            } else {
                return null;
            }
        } else {
            return null;
        }
    }

    /**
     * Get the actual type arguments a child class has used to extend a generic
     * base class.
     *
     * @param baseClass  the base class
     * @param childClass the child class
     * @return a list of the raw classes for the actual type arguments.
     * @author Ian Robertson,
     * http://www.artima.com/weblogs/viewpost.jsp?thread=208860
     */
    public static <T> List<Class<?>> getTypeArguments(
            Class<T> baseClass, Class<? extends T> childClass) {
        Map<Type, Type> resolvedTypes = new HashMap<Type, Type>();
        Type type = childClass;
        // start walking up the inheritance hierarchy until we hit baseClass
        while (!getClass(type).equals(baseClass)) {
            if (type instanceof Class) {
                // there is no useful information for us in raw types, so just keep going.
                type = ((Class) type).getGenericSuperclass();
            } else {
                ParameterizedType parameterizedType = (ParameterizedType) type;
                Class<?> rawType = (Class) parameterizedType.getRawType();

                Type[] actualTypeArguments = parameterizedType.getActualTypeArguments();
                TypeVariable<?>[] typeParameters = rawType.getTypeParameters();
                for (int i = 0; i < actualTypeArguments.length; i++) {
                    resolvedTypes.put(typeParameters[i], actualTypeArguments[i]);
                }

                if (!rawType.equals(baseClass)) {
                    type = rawType.getGenericSuperclass();
                }
            }
        }

        // finally, for each actual type argument provided to baseClass, determine (if possible)
        // the raw class for that type argument.
        Type[] actualTypeArguments;
        if (type instanceof Class) {
            actualTypeArguments = ((Class) type).getTypeParameters();
        } else {
            actualTypeArguments = ((ParameterizedType) type).getActualTypeArguments();
        }
        List<Class<?>> typeArgumentsAsClasses = new ArrayList<Class<?>>();
        // resolve types by chasing down type variables.
        for (Type baseType : actualTypeArguments) {
            while (resolvedTypes.containsKey(baseType)) {
                baseType = resolvedTypes.get(baseType);
            }
            typeArgumentsAsClasses.add(getClass(baseType));
        }
        return typeArgumentsAsClasses;
    }
}