Add AbstractReactiveWebInitializer

This commit introduces a new AbstractReactiveWebInitializer in
spring-web that relies on WebHttpHandlerBuilder to detect request
processing infrastructure beans from an ApplicationContext.

This eliminates the need to create a DispatcherHandler, since it is
expected to be a Spring bean, and as a result the initializers in
spring-webflux have been deprecated.

Issue: SPR-16144

spring-web/src/main/java/org/springframework/web/server/adapter/AbstractReactiveWebInitializer.java

@@ -0,0 +1,96 @@
+/*
+ * Copyright 2002-2017 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
+ *
+ *      http://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.ServletException;
+import javax.servlet.ServletRegistration;
+
+import org.springframework.context.ApplicationContext;
+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 {@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 {@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 empty or null");
+
+		ApplicationContext applicationContext = createApplicationContext();
+		Assert.notNull(applicationContext, "createApplicationContext() must not return null.");
+
+		HttpHandler httpHandler = WebHttpHandlerBuilder.applicationContext(applicationContext).build();
+		ServletHttpHandlerAdapter servlet = new ServletHttpHandlerAdapter(httpHandler);
+
+		ServletRegistration.Dynamic registration = servletContext.addServlet(servletName, servlet);
+		Assert.notNull(registration, "Failed to register servlet '" + servletName + "'.");
+
+		registration.setLoadOnStartup(1);
+		registration.addMapping("/");
+		registration.setAsyncSupported(true);
+	}
+
+	/**
+	 * 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 {@link WebHttpHandlerBuilder#applicationContext}.
+	 */
+	protected ApplicationContext createApplicationContext() {
+		AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();
+		Class<?>[] configClasses = getConfigClasses();
+		Assert.notEmpty(configClasses, "No Spring configuration provided.");
+		return context;
+	}
+
+	/**
+	 * 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();
+
+}