fullstack/pregelix/pregelix-api/src/main/java/edu/uci/ics/pregelix/api/util/ReflectionUtils.java

/*
 * Copyright 2009-2010 by The Regents of the University of California
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * you may obtain a copy of the License from
 * 
 *     http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package edu.uci.ics.pregelix.api.util;

import java.lang.reflect.Array;
import java.lang.reflect.Field;
import java.lang.reflect.GenericArrayType;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.lang.reflect.TypeVariable;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * Helper methods to get type arguments to generic classes. Courtesy of Ian
 * Robertson (overstock.com). Make sure to use with abstract generic classes,
 * not interfaces.
 */
public class ReflectionUtils {
    /**
     * Do not instantiate.
     */
    private ReflectionUtils() {
    }

    /**
     * Get the underlying class for a type, or null if the type is a variable
     * type.
     * 
     * @param type
     *            the type
     * @return the underlying class
     */
    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 <T>
     *            Type to evaluate.
     * @param baseClass
     *            the base class
     * @param childClass
     *            the child class
     * @return a list of the raw classes for the actual type arguments.
     */
    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;
    }

    /**
     * Try to directly set a (possibly private) field on an Object.
     * 
     * @param target
     *            Target to set the field on.
     * @param fieldname
     *            Name of field.
     * @param value
     *            Value to set on target.
     */
    public static void setField(Object target, String fieldname, Object value) throws NoSuchFieldException,
            IllegalAccessException {
        Field field = findDeclaredField(target.getClass(), fieldname);
        field.setAccessible(true);
        field.set(target, value);
    }

    /**
     * Find a declared field in a class or one of its super classes
     * 
     * @param inClass
     *            Class to search for declared field.
     * @param fieldname
     *            Field name to search for
     * @return Field or will throw.
     * @throws NoSuchFieldException
     *             When field not found.
     */
    private static Field findDeclaredField(Class<?> inClass, String fieldname) throws NoSuchFieldException {
        while (!Object.class.equals(inClass)) {
            for (Field field : inClass.getDeclaredFields()) {
                if (field.getName().equalsIgnoreCase(fieldname)) {
                    return field;
                }
            }
            inClass = inClass.getSuperclass();
        }
        throw new NoSuchFieldException();
    }
}