Disable Log4J2's shutdown hook by default

Log4J2 enables its shutdown hook by default. When the JVM is exiting,
this creates a race between logging that happens during the
application context being closed and Log4J2 being shut down such that
the logging is lost.

This commit updates SpringBootConfigurationFactory so that it
produces a custom sub-class of DefaultConfiguration that disables the
shutdown hook by default. In addition to solving the problem described
above, this also aligns the Log4J2 logging system with the
logging.register-shutdown-hook property which defaults to false.

Closes gh-11360

Author: wilkinsona

spring-boot/src/main/java/org/springframework/boot/logging/log4j2/SpringBootConfigurationFactory.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2016 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -27,12 +27,18 @@
 import org.springframework.boot.logging.LoggingSystem;
 
 /**
- * Spring Boot {@link ConfigurationFactory} that prevents logger warnings from being
- * printed when the application first starts. This factory is ordered last and is
- * triggered by a {@code log4j2.springboot} classpath resource (which is bundled in this
- * jar). If the {@link Log4J2LoggingSystem} is active, a {@link DefaultConfiguration} is
- * returned with the expectation that the system will later re-initialize Log4J2 with the
- * correct configuration file.
+ * Spring Boot {@link ConfigurationFactory} that customizes Log4J2's default configuration
+ * to:
+ *
+ * <ol>
+ * <li>Prevent logger warnings from being printed when the application first starts.
+ * <li>Disable its shutdown hook
+ * </ol>
+ *
+ * This factory is ordered last and is triggered by a {@code log4j2.springboot} classpath
+ * resource (which is bundled in this jar). If the {@link Log4J2LoggingSystem} is active,
+ * a custom {@link DefaultConfiguration} is returned with the expectation that the system
+ * will later re-initialize Log4J2 with the correct configuration file.
  *
  * @author Phillip Webb
  * @since 1.5.0
@@ -53,10 +59,18 @@ public Configuration getConfiguration(LoggerContext loggerContext,
 			ConfigurationSource source) {
 		if (source != null && source != ConfigurationSource.NULL_SOURCE) {
 			if (LoggingSystem.get(loggerContext.getClass().getClassLoader()) != null) {
-				return new DefaultConfiguration();
+				return new SpringBootConfiguration();
 			}
 		}
 		return null;
 	}
 
+	private static final class SpringBootConfiguration extends DefaultConfiguration {
+
+		private SpringBootConfiguration() {
+			this.isShutdownHookEnabled = false;
+		}
+
+	}
+
 }

spring-boot/src/test/java/org/springframework/boot/logging/log4j2/SpringBootConfigurationFactoryTests.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2012-2018 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
+ *
+ *      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.logging.log4j2;
+
+import java.io.ByteArrayInputStream;
+import java.io.IOException;
+
+import org.apache.logging.log4j.core.LoggerContext;
+import org.apache.logging.log4j.core.config.ConfigurationSource;
+import org.junit.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link SpringBootConfigurationFactory}.
+ *
+ * @author Andy Wilkinson
+ */
+public class SpringBootConfigurationFactoryTests {
+
+	@Test
+	public void producesConfigurationWithShutdownHookDisabled() throws IOException {
+		ConfigurationSource source = new ConfigurationSource(
+				new ByteArrayInputStream(new byte[0]));
+		assertThat(new SpringBootConfigurationFactory()
+				.getConfiguration(new LoggerContext(""), source).isShutdownHookEnabled())
+						.isFalse();
+	}
+
+}