Prevent restarts from switching off Log4J2-based logging

During a restart, the Restarter runs all registered shutdown hooks. This
breaks Log4J2 as it leaves it in a shutdown state that leaves logging
switched off such that no output it produced when the application starts
up again.

This commit introduces a new RestartListener abstraction.
RestartListeners are notified prior to the application being restarted.
A Log4J2-specific implementation is provided that prepares Log4J2 for
restart by removing any shutdown callbacks from its shutdown callback
registry. This prevents the restart from shutting down Log4J2, ensuring
that it still functions when the application restarts.

Closes gh-4279

spring-boot-devtools/pom.xml

@@ -30,6 +30,16 @@
 			<artifactId>spring-boot-autoconfigure</artifactId>
 		</dependency>
 		<!-- Optional -->
+		<dependency>
+			<groupId>javax.servlet</groupId>
+			<artifactId>javax.servlet-api</artifactId>
+			<optional>true</optional>
+		</dependency>
+		<dependency>
+			<groupId>org.apache.logging.log4j</groupId>
+			<artifactId>log4j-core</artifactId>
+			<optional>true</optional>
+		</dependency>
 		<dependency>
 			<groupId>org.springframework</groupId>
 			<artifactId>spring-web</artifactId>
@@ -45,11 +55,6 @@
 			<artifactId>spring-security-web</artifactId>
 			<optional>true</optional>
 		</dependency>
-		<dependency>
-			<groupId>javax.servlet</groupId>
-			<artifactId>javax.servlet-api</artifactId>
-			<optional>true</optional>
-		</dependency>
 		<!-- Annotation processing -->
 		<dependency>
 			<groupId>org.springframework.boot</groupId>

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/log4j2/Log4J2RestartListener.java

@@ -0,0 +1,62 @@
+/*
+ * 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.devtools.log4j2;
+
+import java.lang.reflect.Field;
+import java.util.Collection;
+
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.core.util.Cancellable;
+import org.apache.logging.log4j.core.util.ShutdownCallbackRegistry;
+import org.apache.logging.log4j.spi.LoggerContextFactory;
+
+import org.springframework.boot.devtools.restart.RestartListener;
+import org.springframework.util.ClassUtils;
+import org.springframework.util.ReflectionUtils;
+
+/**
+ * {@link RestartListener} that prepares Log4J2 for an application restart.
+ *
+ * @author Andy Wilkinson
+ */
+public class Log4J2RestartListener implements RestartListener {
+
+	@Override
+	public void beforeRestart() {
+		if (ClassUtils.isPresent("org.apache.logging.log4j.LogManager",
+				getClass().getClassLoader())) {
+			prepareLog4J2ForRestart();
+		}
+	}
+
+	private void prepareLog4J2ForRestart() {
+		LoggerContextFactory factory = LogManager.getFactory();
+		Field field = ReflectionUtils.findField(factory.getClass(),
+				"shutdownCallbackRegistry");
+		ReflectionUtils.makeAccessible(field);
+		ShutdownCallbackRegistry shutdownCallbackRegistry = (ShutdownCallbackRegistry) ReflectionUtils
+				.getField(field, factory);
+		Field hooksField = ReflectionUtils.findField(shutdownCallbackRegistry.getClass(),
+				"hooks");
+		ReflectionUtils.makeAccessible(hooksField);
+		@SuppressWarnings("unchecked")
+		Collection<Cancellable> state = (Collection<Cancellable>) ReflectionUtils
+				.getField(hooksField, shutdownCallbackRegistry);
+		state.clear();
+	}
+
+}

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/RestartApplicationListener.java

@@ -16,12 +16,15 @@
 
 package org.springframework.boot.devtools.restart;
 
+import java.util.List;
+
 import org.springframework.boot.context.event.ApplicationFailedEvent;
 import org.springframework.boot.context.event.ApplicationReadyEvent;
 import org.springframework.boot.context.event.ApplicationStartedEvent;
 import org.springframework.context.ApplicationEvent;
 import org.springframework.context.ApplicationListener;
 import org.springframework.core.Ordered;
+import org.springframework.core.io.support.SpringFactoriesLoader;
 
 /**
  * {@link ApplicationListener} to initialize the {@link Restarter}.
@@ -56,7 +59,11 @@ public class RestartApplicationListener
 			String[] args = event.getArgs();
 			DefaultRestartInitializer initializer = new DefaultRestartInitializer();
 			boolean restartOnInitialize = !AgentReloader.isActive();
-			Restarter.initialize(args, false, initializer, restartOnInitialize);
+			List<RestartListener> restartListeners = SpringFactoriesLoader
+					.loadFactories(RestartListener.class, getClass().getClassLoader());
+			Restarter.initialize(args, false, initializer, restartOnInitialize,
+					restartListeners
+							.toArray(new RestartListener[restartListeners.size()]));
 		}
 		else {
 			Restarter.disable();

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/RestartListener.java

@@ -0,0 +1,31 @@
+/*
+ * 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.devtools.restart;
+
+/**
+ * Listener that is notified of application restarts.
+ *
+ * @author Andy Wilkinson
+ */
+public interface RestartListener {
+
+	/**
+	 * Called before an application restart.
+	 */
+	void beforeRestart();
+
+}

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/Restarter.java

@@ -108,6 +108,8 @@ public class Restarter {
 
 	private final BlockingDeque<LeakSafeThread> leakSafeThreads = new LinkedBlockingDeque<LeakSafeThread>();
 
+	private final RestartListener[] listeners;
+
 	private boolean finished = false;
 
 	private Lock stopLock = new ReentrantLock();
@@ -118,10 +120,11 @@ public class Restarter {
 	 * @param args the application arguments
 	 * @param forceReferenceCleanup if soft/weak reference cleanup should be forced
 	 * @param initializer the restart initializer
+	 * @param listeners listeners to be notified of restarts
 	 * @see #initialize(String[])
 	 */
 	protected Restarter(Thread thread, String[] args, boolean forceReferenceCleanup,
-			RestartInitializer initializer) {
+			RestartInitializer initializer, RestartListener... listeners) {
 		Assert.notNull(thread, "Thread must not be null");
 		Assert.notNull(args, "Args must not be null");
 		Assert.notNull(initializer, "Initializer must not be null");
@@ -134,6 +137,7 @@ public class Restarter {
 		this.args = args;
 		this.exceptionHandler = thread.getUncaughtExceptionHandler();
 		this.leakSafeThreads.add(new LeakSafeThread());
+		this.listeners = listeners;
 	}
 
 	private String getMainClassName(Thread thread) {
@@ -246,6 +250,7 @@ public class Restarter {
 
 			@Override
 			public Void call() throws Exception {
+				Restarter.this.beforeRestart();
 				Restarter.this.stop();
 				Restarter.this.start(failureHandler);
 				return null;
@@ -324,6 +329,12 @@ public class Restarter {
 		System.runFinalization();
 	}
 
+	private void beforeRestart() {
+		for (RestartListener listener : this.listeners) {
+			listener.beforeRestart();
+		}
+	}
+
 	@SuppressWarnings("rawtypes")
 	private void triggerShutdownHooks() throws Exception {
 		Class<?> hooksClass = Class.forName("java.lang.ApplicationShutdownHooks");
@@ -512,13 +523,15 @@ public class Restarter {
 	 * @param initializer the restart initializer
 	 * @param restartOnInitialize if the restarter should be restarted immediately when
 	 * the {@link RestartInitializer} returns non {@code null} results
+	 * @param listeners listeners to be notified of restarts
 	 */
 	public static void initialize(String[] args, boolean forceReferenceCleanup,
-			RestartInitializer initializer, boolean restartOnInitialize) {
+			RestartInitializer initializer, boolean restartOnInitialize,
+			RestartListener... listeners) {
 		if (instance == null) {
 			synchronized (Restarter.class) {
 				instance = new Restarter(Thread.currentThread(), args,
-						forceReferenceCleanup, initializer);
+						forceReferenceCleanup, initializer, listeners);
 			}
 			instance.initialize(restartOnInitialize);
 		}

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

@@ -15,3 +15,7 @@ org.springframework.boot.devtools.autoconfigure.RemoteDevToolsAutoConfiguration
 org.springframework.boot.env.EnvironmentPostProcessor=\
 org.springframework.boot.devtools.env.DevToolsHomePropertiesPostProcessor,\
 org.springframework.boot.devtools.env.DevToolsPropertyDefaultsPostProcessor
+
+# Restart Listeners
+org.springframework.boot.devtools.restart.RestartListener=\
+org.springframework.boot.devtools.log4j2.Log4J2RestartListener

spring-boot-devtools/src/test/java/org/springframework/boot/devtools/restart/RestarterTests.java

@@ -96,6 +96,7 @@ public class RestarterTests {
 		String output = this.out.toString();
 		assertThat(StringUtils.countOccurrencesOf(output, "Tick 0"), greaterThan(1));
 		assertThat(StringUtils.countOccurrencesOf(output, "Tick 1"), greaterThan(1));
+		assertThat(TestRestartListener.restarts, greaterThan(1));
 	}
 
 	@Test
@@ -214,7 +215,8 @@ public class RestarterTests {
 		}
 
 		public static void main(String... args) {
-			Restarter.initialize(args, false, new MockRestartInitializer());
+			Restarter.initialize(args, false, new MockRestartInitializer(), true,
+					new TestRestartListener());
 			AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(
 					SampleApplication.class);
 			context.registerShutdownHook();
@@ -276,4 +278,15 @@ public class RestarterTests {
 
 	}
 
+	private static class TestRestartListener implements RestartListener {
+
+		private static int restarts;
+
+		@Override
+		public void beforeRestart() {
+			restarts++;
+		}
+
+	}
+
 }