/*
 * 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.aop.target;

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

import org.springframework.aop.TargetSource;
import org.springframework.lang.Nullable;

TargetSource implementation that will lazily create a user-managed object. 
Creation of the lazy target object is controlled by the user by implementing the createObject() method. This TargetSource will invoke this method the first time the proxy is accessed. 
Useful when you need to pass a reference to some dependency to an object
but you don't actually want the dependency to be created until it is first used.
A typical scenario for this is a connection to a remote resource.
Author:
Rob Harrop, Juergen Hoeller
See Also:
isInitialized()
createObject()
Since:
1.2.4/**
 * {@link org.springframework.aop.TargetSource} implementation that will
 * lazily create a user-managed object.
 *
 * <p>Creation of the lazy target object is controlled by the user by implementing
 * the {@link #createObject()} method. This {@code TargetSource} will invoke
 * this method the first time the proxy is accessed.
 *
 * <p>Useful when you need to pass a reference to some dependency to an object
 * but you don't actually want the dependency to be created until it is first used.
 * A typical scenario for this is a connection to a remote resource.
 *
 * @author Rob Harrop
 * @author Juergen Hoeller
 * @since 1.2.4
 * @see #isInitialized()
 * @see #createObject()
 */
public abstract class AbstractLazyCreationTargetSource implements TargetSource {

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

	
The lazily initialized target object. /** The lazily initialized target object. */
	private Object lazyTarget;


	
Return whether the lazy target object of this TargetSource
has already been fetched.
/**
	 * Return whether the lazy target object of this TargetSource
	 * has already been fetched.
	 */
	public synchronized boolean isInitialized() {
		return (this.lazyTarget != null);
	}

	
This default implementation returns null if the target is null (it is hasn't yet been initialized), or the target class if the target has already been initialized. 
Subclasses may wish to override this method in order to provide a meaningful value when the target is still null. 
See Also:
isInitialized()/**
	 * This default implementation returns {@code null} if the
	 * target is {@code null} (it is hasn't yet been initialized),
	 * or the target class if the target has already been initialized.
	 * <p>Subclasses may wish to override this method in order to provide
	 * a meaningful value when the target is still {@code null}.
	 * @see #isInitialized()
	 */
	@Override
	@Nullable
	public synchronized Class<?> getTargetClass() {
		return (this.lazyTarget != null ? this.lazyTarget.getClass() : null);
	}

	@Override
	public boolean isStatic() {
		return false;
	}

	
Returns the lazy-initialized target object,
creating it on-the-fly if it doesn't exist already.
See Also:
createObject()/**
	 * Returns the lazy-initialized target object,
	 * creating it on-the-fly if it doesn't exist already.
	 * @see #createObject()
	 */
	@Override
	public synchronized Object getTarget() throws Exception {
		if (this.lazyTarget == null) {
			logger.debug("Initializing lazy target object");
			this.lazyTarget = createObject();
		}
		return this.lazyTarget;
	}

	@Override
	public void releaseTarget(Object target) throws Exception {
		// nothing to do
	}


	
Subclasses should implement this method to return the lazy initialized object.
Called the first time the proxy is invoked.
Throws:
Exception – if creation failed
Returns:
the created object/**
	 * Subclasses should implement this method to return the lazy initialized object.
	 * Called the first time the proxy is invoked.
	 * @return the created object
	 * @throws Exception if creation failed
	 */
	protected abstract Object createObject() throws Exception;

}