testing/multiton/src/main/java/org/apache/axiom/testing/multiton/Multiton.java

/*
 * 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.
 */
package org.apache.axiom.testing.multiton;

import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.lang.reflect.ParameterizedType;
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.ServiceLoader;

/**
 * Base class for multitons. A multiton is a class that has a fixed set of instances. The base class
 * also provides a simple mechanism that allows to extend the behavior of a multiton using adapters.
 * <p>
 * The set of instances of a multiton is determined by:
 * <ul>
 * <li>The values of all public static final fields in the multiton class that have types that are
 * assignment compatible with the multiton class.
 * <li>The array elements returned by methods in the multiton class annotated with
 * {@link Instances}. These methods must be private static, have no parameters and return an array
 * with an component type assignment compatible with the multiton class.
 * </ul>
 */
@SuppressWarnings({"rawtypes", "unchecked"})
public abstract class Multiton {
    private static final Map<Class<?>,List<?>> instancesMap = new HashMap<>();
    private static final Map<Class,List<AdapterFactory>> adapterFactoryMap = new HashMap<>();
    private final Adapters adapters = new Adapters();
    
    static {
        for (Iterator<AdapterFactory> it = ServiceLoader.load(AdapterFactory.class, Multiton.class.getClassLoader()).iterator(); it.hasNext(); ) {
            AdapterFactory adapterFactory = it.next();
            // TODO: only works in the basic case where the factory directly implements AdapterFactory as the first interface
            Class clazz = ((Class<?>)((ParameterizedType)adapterFactory.getClass().getGenericInterfaces()[0]).getActualTypeArguments()[0]).asSubclass(Multiton.class);
            List<AdapterFactory> adapterFactories = adapterFactoryMap.get(clazz);
            if (adapterFactories == null) {
                adapterFactories = new ArrayList<>();
                adapterFactoryMap.put(clazz, adapterFactories);
            }
            adapterFactories.add(adapterFactory);
        }
    }

    /**
     * Get all instances of the given multiton. See the Javadoc of the {@link Multiton} class for
     * information about how the instances are determined.
     * 
     * @param multitonClass the multiton class
     * @return the list of instances
     * @throws MultitonInstantiationException if an error occurred
     */
    public static synchronized <T extends Multiton> List<T> getInstances(Class<T> multitonClass) {
        List<T> instances = (List<T>)instancesMap.get(multitonClass);
        if (instances == null) {
            instances = new ArrayList<>();
            for (Field field : multitonClass.getDeclaredFields()) {
                int mod = field.getModifiers();
                if (Modifier.isPublic(mod) && Modifier.isStatic(mod) && Modifier.isFinal(mod)
                        && multitonClass.isAssignableFrom(field.getType())) {
                    try {
                        instances.add(multitonClass.cast(field.get(null)));
                    } catch (IllegalAccessException ex) {
                        throw new MultitonInstantiationException(ex);
                    }
                }
            }
            for (Method method : multitonClass.getDeclaredMethods()) {
                if (method.getAnnotation(Instances.class) != null) {
                    int mod = method.getModifiers();
                    if (!Modifier.isPrivate(mod) || !Modifier.isStatic(mod)) {
                        throw new MultitonInstantiationException("Methods annotated with @Instances must be private static");
                    }
                    if (method.getParameterTypes().length > 0) {
                        throw new MultitonInstantiationException("Methods annotated with @Instances must not take any parameters");
                    }
                    Class<?> returnType = method.getReturnType();
                    if (!returnType.isArray() || !multitonClass.isAssignableFrom(returnType.getComponentType())) {
                        throw new MultitonInstantiationException("Invalid return type for method annotated with @Instances");
                    }
                    method.setAccessible(true);
                    try {
                        for (Object instance : (Object[])method.invoke(null)) {
                            instances.add(multitonClass.cast(instance));
                        }
                    } catch (IllegalAccessException ex) {
                        throw new MultitonInstantiationException(ex);
                    } catch (InvocationTargetException ex) {
                        throw new MultitonInstantiationException(ex.getCause());
                    }
                }
            }
            instances = Collections.unmodifiableList(instances);
            instancesMap.put(multitonClass, instances);
        }
        return instances;
    }
    
    public final <T> T getAdapter(Class<T> type) {
        synchronized (adapters) {
            if (!adapters.initialized()) {
                Class<?> clazz = getClass();
                while (clazz != Multiton.class) {
                    List<AdapterFactory> adapterFactories = adapterFactoryMap.get(clazz);
                    if (adapterFactories != null) {
                        for (AdapterFactory adapterFactory : adapterFactories) {
                            adapterFactory.createAdapters(this, adapters);
                        }
                    }
                    clazz = clazz.getSuperclass();
                }
            }
            return adapters.get(type);
        }
    }
}