/*
 * 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.server.adapter;

import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.ServletException;
import javax.servlet.ServletRegistration;

import org.springframework.context.ApplicationContext;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.http.server.reactive.HttpHandler;
import org.springframework.http.server.reactive.ServletHttpHandlerAdapter;
import org.springframework.util.Assert;
import org.springframework.web.WebApplicationInitializer;

Base class for a WebApplicationInitializer that installs a Spring Reactive Web Application on a Servlet container. 
Spring configuration is loaded and given to WebHttpHandlerBuilder which scans the context looking for specific beans and creates a reactive HttpHandler. The resulting handler is installed as a Servlet through the ServletHttpHandlerAdapter. 
Author:
Rossen Stoyanchev
Since:
5.0.2/**
 * Base class for a {@link org.springframework.web.WebApplicationInitializer}
 * that installs a Spring Reactive Web Application on a Servlet container.
 *
 * <p>Spring configuration is loaded and given to
 * {@link WebHttpHandlerBuilder#applicationContext WebHttpHandlerBuilder}
 * which scans the context looking for specific beans and creates a reactive
 * {@link HttpHandler}. The resulting handler is installed as a Servlet through
 * the {@link ServletHttpHandlerAdapter}.
 *
 * @author Rossen Stoyanchev
 * @since 5.0.2
 */
public abstract class AbstractReactiveWebInitializer implements WebApplicationInitializer {

	
The default servlet name to use. See getServletName. /**
	 * The default servlet name to use. See {@link #getServletName}.
	 */
	public static final String DEFAULT_SERVLET_NAME = "http-handler-adapter";


	@Override
	public void onStartup(ServletContext servletContext) throws ServletException {
		String servletName = getServletName();
		Assert.hasLength(servletName, "getServletName() must not return null or empty");

		ApplicationContext applicationContext = createApplicationContext();
		Assert.notNull(applicationContext, "createApplicationContext() must not return null");

		refreshApplicationContext(applicationContext);
		registerCloseListener(servletContext, applicationContext);

		HttpHandler httpHandler = WebHttpHandlerBuilder.applicationContext(applicationContext).build();
		ServletHttpHandlerAdapter servlet = new ServletHttpHandlerAdapter(httpHandler);

		ServletRegistration.Dynamic registration = servletContext.addServlet(servletName, servlet);
		if (registration == null) {
			throw new IllegalStateException("Failed to register servlet with name '" + servletName + "'. " +
					"Check if there is another servlet registered under the same name.");
		}

		registration.setLoadOnStartup(1);
		registration.addMapping(getServletMapping());
		registration.setAsyncSupported(true);
	}

	
Return the name to use to register the ServletHttpHandlerAdapter. 
By default this is DEFAULT_SERVLET_NAME. /**
	 * Return the name to use to register the {@link ServletHttpHandlerAdapter}.
	 * <p>By default this is {@link #DEFAULT_SERVLET_NAME}.
	 */
	protected String getServletName() {
		return DEFAULT_SERVLET_NAME;
	}

	
Return the Spring configuration that contains application beans including the ones detected by WebHttpHandlerBuilder.applicationContext. /**
	 * Return the Spring configuration that contains application beans including
	 * the ones detected by {@link WebHttpHandlerBuilder#applicationContext}.
	 */
	protected ApplicationContext createApplicationContext() {
		AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();
		Class<?>[] configClasses = getConfigClasses();
		Assert.notEmpty(configClasses, "No Spring configuration provided through getConfigClasses()");
		context.register(configClasses);
		return context;
	}

	
Specify @Configuration and/or @Component classes that make up the application configuration. The config classes are given to createApplicationContext(). /**
	 * Specify {@link org.springframework.context.annotation.Configuration @Configuration}
	 * and/or {@link org.springframework.stereotype.Component @Component}
	 * classes that make up the application configuration. The config classes
	 * are given to {@linkplain #createApplicationContext()}.
	 */
	protected abstract Class<?>[] getConfigClasses();

	
Refresh the given application context, if necessary.
/**
	 * Refresh the given application context, if necessary.
	 */
	protected void refreshApplicationContext(ApplicationContext context) {
		if (context instanceof ConfigurableApplicationContext) {
			ConfigurableApplicationContext cac = (ConfigurableApplicationContext) context;
			if (!cac.isActive()) {
				cac.refresh();
			}
		}
	}

	
Register a ServletContextListener that closes the given application context when the servlet context is destroyed. 
Params:
servletContext – the servlet context to listen to
applicationContext – the application context that is to be closed when servletContext is destroyed/**
	 * Register a {@link ServletContextListener} that closes the given
	 * application context when the servlet context is destroyed.
	 * @param servletContext the servlet context to listen to
	 * @param applicationContext the application context that is to be
	 * closed when {@code servletContext} is destroyed
	 */
	protected void registerCloseListener(ServletContext servletContext, ApplicationContext applicationContext) {
		if (applicationContext instanceof ConfigurableApplicationContext) {
			ConfigurableApplicationContext cac = (ConfigurableApplicationContext) applicationContext;
			ServletContextDestroyedListener listener = new ServletContextDestroyedListener(cac);
			servletContext.addListener(listener);
		}
	}

	
Return the Servlet mapping to use. Only the default Servlet mapping '/'
and path-based Servlet mappings such as '/api/*' are supported.
By default this is set to '/'.
/**
	 * Return the Servlet mapping to use. Only the default Servlet mapping '/'
	 * and path-based Servlet mappings such as '/api/*' are supported.
	 * <p>By default this is set to '/'.
	 */
	protected String getServletMapping() {
		return "/";
	}


	private static class ServletContextDestroyedListener implements ServletContextListener {

		private final ConfigurableApplicationContext applicationContext;

		public ServletContextDestroyedListener(ConfigurableApplicationContext applicationContext) {
			this.applicationContext = applicationContext;
		}

		@Override
		public void contextInitialized(ServletContextEvent sce) {
		}

		@Override
		public void contextDestroyed(ServletContextEvent sce) {
			this.applicationContext.close();
		}
	}

}