https://spring.io/projects/spring-boot: Spring Boot (Pivotal Software, Inc.) 
/*
 * Copyright 2012-2020 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.boot.web.servlet.support;

import java.sql.Driver;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.Collections;

import javax.servlet.Filter;
import javax.servlet.Servlet;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletException;

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

import org.springframework.boot.SpringApplication;
import org.springframework.boot.builder.ParentContextApplicationContextInitializer;
import org.springframework.boot.builder.SpringApplicationBuilder;
import org.springframework.boot.context.event.ApplicationEnvironmentPreparedEvent;
import org.springframework.boot.web.servlet.ServletContextInitializer;
import org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationListener;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.Ordered;
import org.springframework.core.annotation.MergedAnnotations;
import org.springframework.core.annotation.MergedAnnotations.SearchStrategy;
import org.springframework.core.env.ConfigurableEnvironment;
import org.springframework.util.Assert;
import org.springframework.web.WebApplicationInitializer;
import org.springframework.web.context.ConfigurableWebEnvironment;
import org.springframework.web.context.ContextLoaderListener;
import org.springframework.web.context.WebApplicationContext;


An opinionated WebApplicationInitializer to run a SpringApplication from a traditional WAR deployment. Binds Servlet, Filter and ServletContextInitializer beans from the application context to the server. 
 To configure the application either override the configure(SpringApplicationBuilder) method (calling SpringApplicationBuilder.sources(Class<?>...)) or make the initializer itself a @Configuration. If you are using SpringBootServletInitializer in combination with other WebApplicationInitializers you might also want to add an @Ordered annotation to configure a specific startup order. 

Note that a WebApplicationInitializer is only needed if you are building a war file and
deploying it. If you prefer to run an embedded web server then you won't need this at
all.

Author:Dave Syer, Phillip Webb, Andy Wilkinson
See Also:
Since:2.0.0
/**
 * An opinionated {@link WebApplicationInitializer} to run a {@link SpringApplication}
 * from a traditional WAR deployment. Binds {@link Servlet}, {@link Filter} and
 * {@link ServletContextInitializer} beans from the application context to the server.
 * <p>
 * To configure the application either override the
 * {@link #configure(SpringApplicationBuilder)} method (calling
 * {@link SpringApplicationBuilder#sources(Class...)}) or make the initializer itself a
 * {@code @Configuration}. If you are using {@link SpringBootServletInitializer} in
 * combination with other {@link WebApplicationInitializer WebApplicationInitializers} you
 * might also want to add an {@code @Ordered} annotation to configure a specific startup
 * order.
 * <p>
 * Note that a WebApplicationInitializer is only needed if you are building a war file and
 * deploying it. If you prefer to run an embedded web server then you won't need this at
 * all.
 *
 * @author Dave Syer
 * @author Phillip Webb
 * @author Andy Wilkinson
 * @since 2.0.0
 * @see #configure(SpringApplicationBuilder)
 */
public abstract class SpringBootServletInitializer implements WebApplicationInitializer {

	protected Log logger; // Don't initialize early

	private boolean registerErrorPageFilter = true;

	
Set if the ErrorPageFilter should be registered. Set to false if error page mappings should be handled via the server and not Spring Boot. 
Params:
/**
	 * Set if the {@link ErrorPageFilter} should be registered. Set to {@code false} if
	 * error page mappings should be handled via the server and not Spring Boot.
	 * @param registerErrorPageFilter if the {@link ErrorPageFilter} should be registered.
	 */
	protected final void setRegisterErrorPageFilter(boolean registerErrorPageFilter) {
		this.registerErrorPageFilter = registerErrorPageFilter;
	}

	@Override
	public void onStartup(ServletContext servletContext) throws ServletException {
		// Logger initialization is deferred in case an ordered
		// LogServletContextInitializer is being used
		this.logger = LogFactory.getLog(getClass());
		WebApplicationContext rootApplicationContext = createRootApplicationContext(servletContext);
		if (rootApplicationContext != null) {
			servletContext.addListener(new SpringBootContextLoaderListener(rootApplicationContext, servletContext));
		}
		else {
			this.logger.debug("No ContextLoaderListener registered, as createRootApplicationContext() did not "
					+ "return an application context");
		}
	}

	
Deregisters the JDBC drivers that were registered by the application represented by the given servletContext. The default implementation deregisters every Driver that was loaded by the web application's class
loader. 
Params:
  • servletContext – the web application's servlet context
Since:2.3.0
/**
	 * Deregisters the JDBC drivers that were registered by the application represented by
	 * the given {@code servletContext}. The default implementation
	 * {@link DriverManager#deregisterDriver(Driver) deregisters} every {@link Driver}
	 * that was loaded by the {@link ServletContext#getClassLoader web application's class
	 * loader}.
	 * @param servletContext the web application's servlet context
	 * @since 2.3.0
	 */
	protected void deregisterJdbcDrivers(ServletContext servletContext) {
		for (Driver driver : Collections.list(DriverManager.getDrivers())) {
			if (driver.getClass().getClassLoader() == servletContext.getClassLoader()) {
				try {
					DriverManager.deregisterDriver(driver);
				}
				catch (SQLException ex) {
					// Continue
				}
			}
		}
	}

	protected WebApplicationContext createRootApplicationContext(ServletContext servletContext) {
		SpringApplicationBuilder builder = createSpringApplicationBuilder();
		builder.main(getClass());
		ApplicationContext parent = getExistingRootWebApplicationContext(servletContext);
		if (parent != null) {
			this.logger.info("Root context already created (using as parent).");
			servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, null);
			builder.initializers(new ParentContextApplicationContextInitializer(parent));
		}
		builder.initializers(new ServletContextApplicationContextInitializer(servletContext));
		builder.contextClass(AnnotationConfigServletWebServerApplicationContext.class);
		builder = configure(builder);
		builder.listeners(new WebEnvironmentPropertySourceInitializer(servletContext));
		SpringApplication application = builder.build();
		if (application.getAllSources().isEmpty()
				&& MergedAnnotations.from(getClass(), SearchStrategy.TYPE_HIERARCHY).isPresent(Configuration.class)) {
			application.addPrimarySources(Collections.singleton(getClass()));
		}
		Assert.state(!application.getAllSources().isEmpty(),
				"No SpringApplication sources have been defined. Either override the "
						+ "configure method or add an @Configuration annotation");
		// Ensure error pages are registered
		if (this.registerErrorPageFilter) {
			application.addPrimarySources(Collections.singleton(ErrorPageFilterConfiguration.class));
		}
		application.setRegisterShutdownHook(false);
		return run(application);
	}

	
Returns the SpringApplicationBuilder that is used to configure and create the SpringApplication. The default implementation returns a new SpringApplicationBuilder in its default state. 
Returns:the SpringApplicationBuilder.
Since:1.3.0
/**
	 * Returns the {@code SpringApplicationBuilder} that is used to configure and create
	 * the {@link SpringApplication}. The default implementation returns a new
	 * {@code SpringApplicationBuilder} in its default state.
	 * @return the {@code SpringApplicationBuilder}.
	 * @since 1.3.0
	 */
	protected SpringApplicationBuilder createSpringApplicationBuilder() {
		return new SpringApplicationBuilder();
	}

	
Called to run a fully configured SpringApplication. 
Params:
  • application – the application to run
Returns:the WebApplicationContext
/**
	 * Called to run a fully configured {@link SpringApplication}.
	 * @param application the application to run
	 * @return the {@link WebApplicationContext}
	 */
	protected WebApplicationContext run(SpringApplication application) {
		return (WebApplicationContext) application.run();
	}

	private ApplicationContext getExistingRootWebApplicationContext(ServletContext servletContext) {
		Object context = servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
		if (context instanceof ApplicationContext) {
			return (ApplicationContext) context;
		}
		return null;
	}

	
Configure the application. Normally all you would need to do is to add sources
(e.g. config classes) because other settings have sensible defaults. You might
choose (for instance) to add default command line arguments, or set an active
Spring profile.

Params:
  • builder – a builder for the application context
See Also:
Returns:the application builder
/**
	 * Configure the application. Normally all you would need to do is to add sources
	 * (e.g. config classes) because other settings have sensible defaults. You might
	 * choose (for instance) to add default command line arguments, or set an active
	 * Spring profile.
	 * @param builder a builder for the application context
	 * @return the application builder
	 * @see SpringApplicationBuilder
	 */
	protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
		return builder;
	}

	
ApplicationListener to trigger initPropertySources.initPropertySources(ServletContext, ServletConfig). 
/**
	 * {@link ApplicationListener} to trigger
	 * {@link ConfigurableWebEnvironment#initPropertySources(ServletContext, javax.servlet.ServletConfig)}.
	 */
	private static final class WebEnvironmentPropertySourceInitializer
			implements ApplicationListener<ApplicationEnvironmentPreparedEvent>, Ordered {

		private final ServletContext servletContext;

		private WebEnvironmentPropertySourceInitializer(ServletContext servletContext) {
			this.servletContext = servletContext;
		}

		@Override
		public void onApplicationEvent(ApplicationEnvironmentPreparedEvent event) {
			ConfigurableEnvironment environment = event.getEnvironment();
			if (environment instanceof ConfigurableWebEnvironment) {
				((ConfigurableWebEnvironment) environment).initPropertySources(this.servletContext, null);
			}
		}

		@Override
		public int getOrder() {
			return Ordered.HIGHEST_PRECEDENCE;
		}

	}

	
ContextLoaderListener for the initialized context. 
/**
	 * {@link ContextLoaderListener} for the initialized context.
	 */
	private class SpringBootContextLoaderListener extends ContextLoaderListener {

		private final ServletContext servletContext;

		SpringBootContextLoaderListener(WebApplicationContext applicationContext, ServletContext servletContext) {
			super(applicationContext);
			this.servletContext = servletContext;
		}

		@Override
		public void contextInitialized(ServletContextEvent event) {
			// no-op because the application context is already initialized
		}

		@Override
		public void contextDestroyed(ServletContextEvent event) {
			try {
				super.contextDestroyed(event);
			}
			finally {
				// Use original context so that the classloader can be accessed
				deregisterJdbcDrivers(this.servletContext);
			}
		}

	}

}