DefaultConstructorAllocator.java

/*
 * Copyright (C) 2011 Google Inc.
 *
 * 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 com.smartandroid.sa.json;

import java.lang.reflect.Constructor;

/**
 * Use the default constructor on the class to instantiate an object.
 * 
 * @author Joel Leitch
 */
final class DefaultConstructorAllocator {
	private static final Constructor<Null> NULL_CONSTRUCTOR = createNullConstructor();

	private final Cache<Class<?>, Constructor<?>> constructorCache;

	public DefaultConstructorAllocator() {
		this(200);
	}

	public DefaultConstructorAllocator(int cacheSize) {
		constructorCache = new LruCache<Class<?>, Constructor<?>>(cacheSize);
	}

	// for testing purpose
	final boolean isInCache(Class<?> cacheKey) {
		return constructorCache.getElement(cacheKey) != null;
	}

	private static final Constructor<Null> createNullConstructor() {
		try {
			return getNoArgsConstructor(Null.class);
		} catch (Exception e) {
			return null;
		}
	}

	public <T> T newInstance(Class<T> c) throws Exception {
		Constructor<T> constructor = findConstructor(c);
		return (constructor != null) ? constructor.newInstance() : null;
	}

	@SuppressWarnings("unchecked")
	private <T> Constructor<T> findConstructor(Class<T> c) {
		Constructor<T> cachedElement = (Constructor<T>) constructorCache
				.getElement(c);
		if (cachedElement != null) {
			if (cachedElement == NULL_CONSTRUCTOR) {
				return null;
			} else {
				return cachedElement;
			}
		}

		Constructor<T> noArgsConstructor = getNoArgsConstructor(c);
		if (noArgsConstructor != null) {
			constructorCache.addElement(c, noArgsConstructor);
		} else {
			constructorCache.addElement(c, NULL_CONSTRUCTOR);
		}
		return noArgsConstructor;
	}

	private static <T> Constructor<T> getNoArgsConstructor(Class<T> c) {
		try {
			Constructor<T> declaredConstructor = c.getDeclaredConstructor();
			declaredConstructor.setAccessible(true);
			return declaredConstructor;
		} catch (Exception e) {
			return null;
		}
	}

	// placeholder class for Null constructor
	private static final class Null {
	}
}