Improve performance with BackgroundPreinitializer

Add a BackgroundPreinitializer to trigger early initialization in a
background thread of time consuming tasks. By moving certain
initialization tasks to background thread and triggering them early
we can improve the critical path when the application starts. For
example, Tomcat's MBeanFactory class parses several XML files when
first loaded. If we trigger the load in a background thread it completes
before Tomcat actually needs to use it.

The initial set of initializers included with this commit are:

- Tomcat MBeanFactory
- javax.validation
- Spring's AllEncompassingFormHttpMessageConverter

See gh-4252

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/BackgroundPreinitializer.java

@@ -0,0 +1,111 @@
+/*
+ * Copyright 2012-2015 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.boot.autoconfigure;
+
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+import javax.validation.Validation;
+
+import org.apache.catalina.mbeans.MBeanFactory;
+
+import org.springframework.boot.context.event.ApplicationStartedEvent;
+import org.springframework.context.ApplicationListener;
+import org.springframework.http.converter.support.AllEncompassingFormHttpMessageConverter;
+
+/**
+ * {@link ApplicationListener} to trigger early initialization in a background thread of
+ * time consuming tasks.
+ *
+ * @author Phillip Webb
+ * @since 1.3.0
+ */
+public class BackgroundPreinitializer
+		implements ApplicationListener<ApplicationStartedEvent> {
+
+	@Override
+	public void onApplicationEvent(ApplicationStartedEvent event) {
+		ExecutorService executor = Executors.newSingleThreadExecutor();
+		submit(executor, new MessageConverterInitializer());
+		submit(executor, new MBeanFactoryInitializer());
+		submit(executor, new ValidationInitializer());
+		executor.shutdown();
+	}
+
+	private void submit(ExecutorService executor, Runnable runnable) {
+		executor.submit(new FailSafeRunnable(runnable));
+	}
+
+	/**
+	 * Wrapper to ignore any thrown exceptions.
+	 */
+	private static class FailSafeRunnable implements Runnable {
+
+		private final Runnable delegate;
+
+		FailSafeRunnable(Runnable delegate) {
+			this.delegate = delegate;
+		}
+
+		@Override
+		public void run() {
+			try {
+				this.delegate.run();
+			}
+			catch (Throwable ex) {
+				// Ignore
+			}
+		}
+
+	}
+
+	/**
+	 * Early initializer for Spring MessageConverters.
+	 */
+	private static class MessageConverterInitializer implements Runnable {
+
+		@Override
+		public void run() {
+			new AllEncompassingFormHttpMessageConverter();
+		}
+
+	}
+
+	/**
+	 * Early initializer to load Tomcat MBean XML.
+	 */
+	private static class MBeanFactoryInitializer implements Runnable {
+
+		@Override
+		public void run() {
+			new MBeanFactory();
+		}
+
+	}
+
+	/**
+	 * Early initializer for javax.validation.
+	 */
+	private static class ValidationInitializer implements Runnable {
+
+		@Override
+		public void run() {
+			Validation.byDefaultProvider().configure();
+		}
+
+	}
+}

spring-boot-autoconfigure/src/main/resources/META-INF/spring.factories

@@ -2,6 +2,10 @@
 org.springframework.context.ApplicationContextInitializer=\
 org.springframework.boot.autoconfigure.logging.AutoConfigurationReportLoggingInitializer
 
+# Application Listeners
+org.springframework.context.ApplicationListener=\
+org.springframework.boot.autoconfigure.BackgroundPreinitializer
+
 # Auto Configure
 org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
 org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\