/*
 * Copyright 2002-2018 the original author or authors.
 *
 * 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
 *
 *      https://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.springframework.beans.factory.config;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.beans.SimpleTypeConverter;
import org.springframework.beans.TypeConverter;
import org.springframework.beans.factory.BeanClassLoaderAware;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.BeanFactoryAware;
import org.springframework.beans.factory.DisposableBean;
import org.springframework.beans.factory.FactoryBean;
import org.springframework.beans.factory.FactoryBeanNotInitializedException;
import org.springframework.beans.factory.InitializingBean;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;
import org.springframework.util.ObjectUtils;
import org.springframework.util.ReflectionUtils;

Simple template superclass for FactoryBean implementations that creates a singleton or a prototype object, depending on a flag. 
If the "singleton" flag is true (the default), this class will create the object that it creates exactly once on initialization and subsequently return said singleton instance on all calls to the getObject() method. 
Else, this class will create a new instance every time the getObject() method is invoked. Subclasses are responsible for implementing the abstract createInstance() template method to actually create the object(s) to expose. 
Author:
Juergen Hoeller, Keith Donald
Type parameters:
<T> – the bean type
See Also:
setSingleton
createInstance()
Since:
1.0.2/**
 * Simple template superclass for {@link FactoryBean} implementations that
 * creates a singleton or a prototype object, depending on a flag.
 *
 * <p>If the "singleton" flag is {@code true} (the default),
 * this class will create the object that it creates exactly once
 * on initialization and subsequently return said singleton instance
 * on all calls to the {@link #getObject()} method.
 *
 * <p>Else, this class will create a new instance every time the
 * {@link #getObject()} method is invoked. Subclasses are responsible
 * for implementing the abstract {@link #createInstance()} template
 * method to actually create the object(s) to expose.
 *
 * @author Juergen Hoeller
 * @author Keith Donald
 * @since 1.0.2
 * @param <T> the bean type
 * @see #setSingleton
 * @see #createInstance()
 */
public abstract class AbstractFactoryBean<T>
		implements FactoryBean<T>, BeanClassLoaderAware, BeanFactoryAware, InitializingBean, DisposableBean {

	
Logger available to subclasses. /** Logger available to subclasses. */
	protected final Log logger = LogFactory.getLog(getClass());

	private boolean singleton = true;

	@Nullable
	private ClassLoader beanClassLoader = ClassUtils.getDefaultClassLoader();

	@Nullable
	private BeanFactory beanFactory;

	private boolean initialized = false;

	@Nullable
	private T singletonInstance;

	@Nullable
	private T earlySingletonInstance;


	
Set if a singleton should be created, or a new object on each request otherwise. Default is true (a singleton). /**
	 * Set if a singleton should be created, or a new object on each request
	 * otherwise. Default is {@code true} (a singleton).
	 */
	public void setSingleton(boolean singleton) {
		this.singleton = singleton;
	}

	@Override
	public boolean isSingleton() {
		return this.singleton;
	}

	@Override
	public void setBeanClassLoader(ClassLoader classLoader) {
		this.beanClassLoader = classLoader;
	}

	@Override
	public void setBeanFactory(@Nullable BeanFactory beanFactory) {
		this.beanFactory = beanFactory;
	}

	
Return the BeanFactory that this bean runs in.
/**
	 * Return the BeanFactory that this bean runs in.
	 */
	@Nullable
	protected BeanFactory getBeanFactory() {
		return this.beanFactory;
	}

	
Obtain a bean type converter from the BeanFactory that this bean
runs in. This is typically a fresh instance for each call,
since TypeConverters are usually not thread-safe.
Falls back to a SimpleTypeConverter when not running in a BeanFactory.
See Also:
getTypeConverter.getTypeConverter()
SimpleTypeConverter/**
	 * Obtain a bean type converter from the BeanFactory that this bean
	 * runs in. This is typically a fresh instance for each call,
	 * since TypeConverters are usually <i>not</i> thread-safe.
	 * <p>Falls back to a SimpleTypeConverter when not running in a BeanFactory.
	 * @see ConfigurableBeanFactory#getTypeConverter()
	 * @see org.springframework.beans.SimpleTypeConverter
	 */
	protected TypeConverter getBeanTypeConverter() {
		BeanFactory beanFactory = getBeanFactory();
		if (beanFactory instanceof ConfigurableBeanFactory) {
			return ((ConfigurableBeanFactory) beanFactory).getTypeConverter();
		}
		else {
			return new SimpleTypeConverter();
		}
	}

	
Eagerly create the singleton instance, if necessary.
/**
	 * Eagerly create the singleton instance, if necessary.
	 */
	@Override
	public void afterPropertiesSet() throws Exception {
		if (isSingleton()) {
			this.initialized = true;
			this.singletonInstance = createInstance();
			this.earlySingletonInstance = null;
		}
	}


	
Expose the singleton instance or create a new prototype instance.
See Also:
createInstance()
getEarlySingletonInterfaces()/**
	 * Expose the singleton instance or create a new prototype instance.
	 * @see #createInstance()
	 * @see #getEarlySingletonInterfaces()
	 */
	@Override
	public final T getObject() throws Exception {
		if (isSingleton()) {
			return (this.initialized ? this.singletonInstance : getEarlySingletonInstance());
		}
		else {
			return createInstance();
		}
	}

	
Determine an 'early singleton' instance, exposed in case of a
circular reference. Not called in a non-circular scenario.
/**
	 * Determine an 'early singleton' instance, exposed in case of a
	 * circular reference. Not called in a non-circular scenario.
	 */
	@SuppressWarnings("unchecked")
	private T getEarlySingletonInstance() throws Exception {
		Class<?>[] ifcs = getEarlySingletonInterfaces();
		if (ifcs == null) {
			throw new FactoryBeanNotInitializedException(
					getClass().getName() + " does not support circular references");
		}
		if (this.earlySingletonInstance == null) {
			this.earlySingletonInstance = (T) Proxy.newProxyInstance(
					this.beanClassLoader, ifcs, new EarlySingletonInvocationHandler());
		}
		return this.earlySingletonInstance;
	}

	
Expose the singleton instance (for access through the 'early singleton' proxy).
Throws:
IllegalStateException – if the singleton instance is not initialized
Returns:
the singleton instance that this FactoryBean holds/**
	 * Expose the singleton instance (for access through the 'early singleton' proxy).
	 * @return the singleton instance that this FactoryBean holds
	 * @throws IllegalStateException if the singleton instance is not initialized
	 */
	@Nullable
	private T getSingletonInstance() throws IllegalStateException {
		Assert.state(this.initialized, "Singleton instance not initialized yet");
		return this.singletonInstance;
	}

	
Destroy the singleton instance, if any.
See Also:
destroyInstance(Object)/**
	 * Destroy the singleton instance, if any.
	 * @see #destroyInstance(Object)
	 */
	@Override
	public void destroy() throws Exception {
		if (isSingleton()) {
			destroyInstance(this.singletonInstance);
		}
	}


	
This abstract method declaration mirrors the method in the FactoryBean
interface, for a consistent offering of abstract template methods.
See Also:
getObjectType.getObjectType()/**
	 * This abstract method declaration mirrors the method in the FactoryBean
	 * interface, for a consistent offering of abstract template methods.
	 * @see org.springframework.beans.factory.FactoryBean#getObjectType()
	 */
	@Override
	@Nullable
	public abstract Class<?> getObjectType();

	
Template method that subclasses must override to construct
the object returned by this factory.
Invoked on initialization of this FactoryBean in case of a singleton; else, on each getObject() call. 
Throws:
Exception – if an exception occurred during object creation
See Also:
getObject()
Returns:
the object returned by this factory/**
	 * Template method that subclasses must override to construct
	 * the object returned by this factory.
	 * <p>Invoked on initialization of this FactoryBean in case of
	 * a singleton; else, on each {@link #getObject()} call.
	 * @return the object returned by this factory
	 * @throws Exception if an exception occurred during object creation
	 * @see #getObject()
	 */
	protected abstract T createInstance() throws Exception;

	
Return an array of interfaces that a singleton object exposed by this
FactoryBean is supposed to implement, for use with an 'early singleton
proxy' that will be exposed in case of a circular reference.
The default implementation returns this FactoryBean's object type, provided that it is an interface, or null otherwise. The latter indicates that early singleton access is not supported by this FactoryBean. This will lead to a FactoryBeanNotInitializedException getting thrown. 
See Also:
FactoryBeanNotInitializedException
Returns:
the interfaces to use for 'early singletons', or null to indicate a FactoryBeanNotInitializedException/**
	 * Return an array of interfaces that a singleton object exposed by this
	 * FactoryBean is supposed to implement, for use with an 'early singleton
	 * proxy' that will be exposed in case of a circular reference.
	 * <p>The default implementation returns this FactoryBean's object type,
	 * provided that it is an interface, or {@code null} otherwise. The latter
	 * indicates that early singleton access is not supported by this FactoryBean.
	 * This will lead to a FactoryBeanNotInitializedException getting thrown.
	 * @return the interfaces to use for 'early singletons',
	 * or {@code null} to indicate a FactoryBeanNotInitializedException
	 * @see org.springframework.beans.factory.FactoryBeanNotInitializedException
	 */
	@Nullable
	protected Class<?>[] getEarlySingletonInterfaces() {
		Class<?> type = getObjectType();
		return (type != null && type.isInterface() ? new Class<?>[] {type} : null);
	}

	
Callback for destroying a singleton instance. Subclasses may
override this to destroy the previously created instance.
The default implementation is empty.
Params:
instance – the singleton instance, as returned by createInstance()
Throws:
Exception – in case of shutdown errors
See Also:
createInstance()/**
	 * Callback for destroying a singleton instance. Subclasses may
	 * override this to destroy the previously created instance.
	 * <p>The default implementation is empty.
	 * @param instance the singleton instance, as returned by
	 * {@link #createInstance()}
	 * @throws Exception in case of shutdown errors
	 * @see #createInstance()
	 */
	protected void destroyInstance(@Nullable T instance) throws Exception {
	}


	
Reflective InvocationHandler for lazy access to the actual singleton object.
/**
	 * Reflective InvocationHandler for lazy access to the actual singleton object.
	 */
	private class EarlySingletonInvocationHandler implements InvocationHandler {

		@Override
		public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
			if (ReflectionUtils.isEqualsMethod(method)) {
				// Only consider equal when proxies are identical.
				return (proxy == args[0]);
			}
			else if (ReflectionUtils.isHashCodeMethod(method)) {
				// Use hashCode of reference proxy.
				return System.identityHashCode(proxy);
			}
			else if (!initialized && ReflectionUtils.isToStringMethod(method)) {
				return "Early singleton proxy for interfaces " +
						ObjectUtils.nullSafeToString(getEarlySingletonInterfaces());
			}
			try {
				return method.invoke(getSingletonInstance(), args);
			}
			catch (InvocationTargetException ex) {
				throw ex.getTargetException();
			}
		}
	}

}