Java Reflection Generic Type from Class getGenericType(Class type, Class clazz)

Description

Obtain the concrete type used by an implementation of an interface that uses a generic type.

License

Apache License

Parameter

Parameter	Description
type	The interface that defines a generic type
clazz	The class that implements the interface with a concrete type
T	The interface type

Return

concrete type used by the implementation

Declaration

private static <T> Object getGenericType(Class<T> type, Class<? extends T> clazz) 

Method Source Code

//package com.java2s;
/*/*from w w  w  . j ava2s. c om*/
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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 at
 *
 *      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.
 */

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

public class Main {
    /**
     * Obtain the concrete type used by an implementation of an interface that
     * uses a generic type.
     *
     * @param type  The interface that defines a generic type
     * @param clazz The class that implements the interface with a concrete type
     * @param <T>   The interface type
     *
     * @return concrete type used by the implementation
     */
    private static <T> Object getGenericType(Class<T> type, Class<? extends T> clazz) {

        // Look to see if this class implements the generic interface

        // Get all the interfaces
        Type[] interfaces = clazz.getGenericInterfaces();
        for (Type iface : interfaces) {
            // Only need to check interfaces that use generics
            if (iface instanceof ParameterizedType) {
                ParameterizedType pi = (ParameterizedType) iface;
                // Look for the generic interface
                if (pi.getRawType() instanceof Class) {
                    if (type.isAssignableFrom((Class<?>) pi.getRawType())) {
                        return getTypeParameter(clazz, pi.getActualTypeArguments()[0]);
                    }
                }
            }
        }

        // Interface not found on this class. Look at the superclass.
        @SuppressWarnings("unchecked")
        Class<? extends T> superClazz = (Class<? extends T>) clazz.getSuperclass();

        Object result = getGenericType(type, superClazz);
        if (result instanceof Class<?>) {
            // Superclass implements interface and defines explicit type for
            // generic
            return result;
        } else if (result instanceof Integer) {
            // Superclass implements interface and defines unknown type for
            // generic
            // Map that unknown type to the generic types defined in this class
            ParameterizedType superClassType = (ParameterizedType) clazz.getGenericSuperclass();
            return getTypeParameter(clazz, superClassType.getActualTypeArguments()[((Integer) result).intValue()]);
        } else {
            // Error will be logged further up the call stack
            return null;
        }
    }

    /**
     * For a generic parameter, return either the Class used or if the type
     * is unknown, the index for the type in definition of the class
     *
     * @param clazz defining class
     * @param argType the type argument of interest
     *
     * @return An instance of {@link Class} representing the type used by the
     *         type parameter or an instance of {@link Integer} representing
     *         the index for the type in the definition of the defining class
     */
    private static Object getTypeParameter(Class<?> clazz, Type argType) {
        if (argType instanceof Class<?>) {
            return argType;
        }
        TypeVariable<?>[] tvs = clazz.getTypeParameters();
        for (int i = 0; i < tvs.length; i++) {
            if (tvs[i].equals(argType)) {
                return Integer.valueOf(i);
            }
        }
        return null;
    }
}

Related

1. getGenericType(Class clazz)
2. getGenericType(Class clazz)
3. getGenericType(Class clazz, int index)
4. getGenericType(Class target)
5. getGenericType(Class type)
6. getGenericType(Object o, Class declaringClass, int idx)
7. getGenericType(Type type, Class rawType, int index)
8. getGenericTypeArgument(Class clazz, int index)
9. getGenericTypeArgumentFromGenericSuperType(Type genericSuperclass, Class baseRequested)