commons/src/main/java/org/apache/tapestry5/services/GenericsResolver.java

// 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 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.

package org.apache.tapestry5.services;

import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.lang.reflect.Type;
import java.util.Iterator;
import java.util.ServiceLoader;

import org.apache.tapestry5.internal.services.GenericsResolverImpl;

/**
 * <p>Methods related to the use of Java 5+ generics.
 * Instances should be obtained through {@link GenericsResolver.Provider#getInstance()}.</p>
 * 
 * <p>
 * If you have exceptions or bad results with classes using Generics, such as exceptions
 * or missing BeanModel properties,
 * you should try adding the <code>genericsresolver-guava</code> Tapestry subproject to our classpath.
 * </p>
 * 
 * @since 5.5.0
 */
@SuppressWarnings("unchecked")
public interface GenericsResolver
{
    /**
     * Analyzes the method in the context of containingClass and returns the Class that is represented by
     * the method's generic return type. Any parameter information in the generic return type is lost. If you want
     * to preserve the type parameters of the return type consider using
     * {@link #extractActualType(java.lang.reflect.Type, java.lang.reflect.Method)}.
     *
     * @param containingClass class which either contains or inherited the method
     * @param method          method from which to extract the return type
     * @return the class represented by the methods generic return type, resolved based on the context .
     * @see #resolve(java.lang.reflect.Type,java.lang.reflect.Type)
     * @see #asClass(java.lang.reflect.Type)
     */
    Class<?> extractGenericReturnType(Class<?> containingClass, Method method);

    /**
     * Analyzes the field in the context of containingClass and returns the Class that is represented by
     * the field's generic type. Any parameter information in the generic type is lost, if you want
     * to preserve the type parameters of the return type consider using
     * #getTypeVariableIndex(java.lang.reflect.TypeVariable).
     *
     * @param containingClass class which either contains or inherited the field
     * @param field           field from which to extract the type
     * @return the class represented by the field's generic type, resolved based on the containingClass.
     * @see #extractActualType(java.lang.reflect.Type, java.lang.reflect.Field)
     * @see #resolve(java.lang.reflect.Type,java.lang.reflect.Type)
     * @see #asClass(java.lang.reflect.Type)
     */
    Class extractGenericFieldType(Class containingClass, Field field);

    /**
     * Analyzes the method in the context of containingClass and returns the Class that is represented by
     * the method's generic return type. Any parameter information in the generic return type is lost.
     *
     * @param containingType Type which is/represents the class that either contains or inherited the method
     * @param method         method from which to extract the generic return type
     * @return the generic type represented by the methods generic return type, resolved based on the containingType.
     * @see #resolve(java.lang.reflect.Type,java.lang.reflect.Type)
     */
    Type extractActualType(Type containingType, Method method);

    /**
     * Analyzes the method in the context of containingClass and returns the Class that is represented by
     * the method's generic return type. Any parameter information in the generic return type is lost.
     *
     * @param containingType Type which is/represents the class that either contains or inherited the field
     * @param field          field from which to extract the generic return type
     * @return the generic type represented by the methods generic return type, resolved based on the containingType.
     * @see #resolve(java.lang.reflect.Type,java.lang.reflect.Type)
     */
    Type extractActualType(Type containingType, Field field);

    /**
     * Resolves the type parameter based on the context of the containingType.
     *
     * {@link java.lang.reflect.TypeVariable} will be unwrapped to the type argument resolved form the class
     * hierarchy. This may be something other than a simple Class if the type argument is a ParameterizedType for
     * instance (e.g. {@code List<E>; List<Map<Long, String>>}, E would be returned as a ParameterizedType with the raw
     * type Map and type arguments Long and String.
     *
     *
     * @param type
     *          the generic type (ParameterizedType, GenericArrayType, WildcardType, TypeVariable) to be resolved
     * @param containingType
     *          the type which his
     * @return
     *          the type resolved to the best of our ability.
     */
    Type resolve(final Type type, final Type containingType);
    
    /**
     * Convenience class for getting a {@link GenericsResolver} instance.
     */
    final static public class Provider 
    {

        final private static GenericsResolver instance;
        
        static 
        {
            
            ServiceLoader<GenericsResolver> serviceLoader = ServiceLoader.load(GenericsResolver.class);
            Iterator<GenericsResolver> iterator = serviceLoader.iterator();
            if (iterator.hasNext()) 
            {
                instance = iterator.next();
            }
            else 
            {
                instance = new GenericsResolverImpl();
            }
        }
        
        /**
         * Returns a cached {@linkplain GenericsResolver} instance. 
         * If {@link ServiceLoader} finds one instance, it returns the first one found. If not,
         * it returns {@link GenericsResolverImpl}.
         * @return a {@link GenericsResolver} instance.
         */
        public static GenericsResolver getInstance() 
        {
            return instance;
        }
        
    }
    
    /**
     * Get the class represented by the reflected type.
     * This method is lossy; You cannot recover the type information from the class that is returned.
     *
     * {@code TypeVariable} the first bound is returned. If your type variable extends multiple interfaces that information
     * is lost.
     *
     * {@code WildcardType} the first lower bound is returned. If the wildcard is defined with upper bounds
     * then {@code Object} is returned.
     *
     * @param actualType
     *           a Class, ParameterizedType, GenericArrayType
     * @return the un-parameterized class associated with the type.
     */
    Class asClass(Type actualType);
    
}