/*
 * 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.web.context;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;

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

import org.springframework.context.ApplicationContextInitializer;
import org.springframework.lang.Nullable;
import org.springframework.web.WebApplicationInitializer;

Convenient base class for WebApplicationInitializer implementations that register a ContextLoaderListener in the servlet context. 
The only method required to be implemented by subclasses is createRootApplicationContext(), which gets invoked from registerContextLoaderListener(ServletContext). 
Author:
Arjen Poutsma, Chris Beams, Juergen Hoeller
Since:
3.2/**
 * Convenient base class for {@link WebApplicationInitializer} implementations
 * that register a {@link ContextLoaderListener} in the servlet context.
 *
 * <p>The only method required to be implemented by subclasses is
 * {@link #createRootApplicationContext()}, which gets invoked from
 * {@link #registerContextLoaderListener(ServletContext)}.
 *
 * @author Arjen Poutsma
 * @author Chris Beams
 * @author Juergen Hoeller
 * @since 3.2
 */
public abstract class AbstractContextLoaderInitializer implements WebApplicationInitializer {

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


	@Override
	public void onStartup(ServletContext servletContext) throws ServletException {
		registerContextLoaderListener(servletContext);
	}

	
Register a ContextLoaderListener against the given servlet context. The ContextLoaderListener is initialized with the application context returned from the createRootApplicationContext() template method. 
Params:
servletContext – the servlet context to register the listener against/**
	 * Register a {@link ContextLoaderListener} against the given servlet context. The
	 * {@code ContextLoaderListener} is initialized with the application context returned
	 * from the {@link #createRootApplicationContext()} template method.
	 * @param servletContext the servlet context to register the listener against
	 */
	protected void registerContextLoaderListener(ServletContext servletContext) {
		WebApplicationContext rootAppContext = createRootApplicationContext();
		if (rootAppContext != null) {
			ContextLoaderListener listener = new ContextLoaderListener(rootAppContext);
			listener.setContextInitializers(getRootApplicationContextInitializers());
			servletContext.addListener(listener);
		}
		else {
			logger.debug("No ContextLoaderListener registered, as " +
					"createRootApplicationContext() did not return an application context");
		}
	}

	
Create the "root" application context to be provided to the ContextLoaderListener. 
The returned context is delegated to ContextLoaderListener(WebApplicationContext) and will be established as the parent context for any DispatcherServlet application contexts. As such, it typically contains middle-tier services, data sources, etc. 
See Also:
AbstractDispatcherServletInitializer
Returns:
the root application context, or null if a root context is not desired/**
	 * Create the "<strong>root</strong>" application context to be provided to the
	 * {@code ContextLoaderListener}.
	 * <p>The returned context is delegated to
	 * {@link ContextLoaderListener#ContextLoaderListener(WebApplicationContext)} and will
	 * be established as the parent context for any {@code DispatcherServlet} application
	 * contexts. As such, it typically contains middle-tier services, data sources, etc.
	 * @return the root application context, or {@code null} if a root context is not
	 * desired
	 * @see org.springframework.web.servlet.support.AbstractDispatcherServletInitializer
	 */
	@Nullable
	protected abstract WebApplicationContext createRootApplicationContext();

	
Specify application context initializers to be applied to the root application context that the ContextLoaderListener is being created with. 
See Also:
createRootApplicationContext()
ContextLoaderListener.setContextInitializers
Since:
4.2/**
	 * Specify application context initializers to be applied to the root application
	 * context that the {@code ContextLoaderListener} is being created with.
	 * @since 4.2
	 * @see #createRootApplicationContext()
	 * @see ContextLoaderListener#setContextInitializers
	 */
	@Nullable
	protected ApplicationContextInitializer<?>[] getRootApplicationContextInitializers() {
		return null;
	}

}