Only clean up logging system when root application context is closed

Previously, LoggingApplicationListener would clean up the logging
system in response to any application context with which it was
registered being closed. This caused problems when a child context was
closed. Specifically, closing the child context would cause any
SLF4J-based logging systems to unregister the JUL bridge handler
preventing an JUL logging being bridged into Logback or Log4J2.

This commit updates LoggingApplicationListener so that the logging
system is only cleaned up when a root application context is
closed.

Closes gh-4651

spring-boot/src/main/java/org/springframework/boot/logging/LoggingApplicationListener.java

@@ -145,7 +145,8 @@ public class LoggingApplicationListener implements SmartApplicationListener {
 			onApplicationEnvironmentPreparedEvent(
 					(ApplicationEnvironmentPreparedEvent) event);
 		}
-		else if (event instanceof ContextClosedEvent) {
+		else if (event instanceof ContextClosedEvent && ((ContextClosedEvent) event)
+				.getApplicationContext().getParent() == null) {
 			onContextClosedEvent();
 		}
 	}

spring-boot/src/test/java/org/springframework/boot/logging/LoggingApplicationListenerTests.java

@@ -39,8 +39,10 @@ import org.springframework.boot.test.EnvironmentTestUtils;
 import org.springframework.boot.test.OutputCapture;
 import org.springframework.context.event.ContextClosedEvent;
 import org.springframework.context.support.GenericApplicationContext;
+import org.springframework.test.util.ReflectionTestUtils;
 
 import static org.hamcrest.Matchers.containsString;
+import static org.hamcrest.Matchers.is;
 import static org.hamcrest.Matchers.not;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertThat;
@@ -86,6 +88,7 @@ public class LoggingApplicationListenerTests {
 		System.clearProperty("LOG_FILE");
 		System.clearProperty("LOG_PATH");
 		System.clearProperty("PID");
+		System.clearProperty(LoggingSystem.SYSTEM_PROPERTY);
 		if (this.context != null) {
 			this.context.close();
 		}
@@ -302,6 +305,37 @@ public class LoggingApplicationListenerTests {
 		assertFalse(bridgeHandlerInstalled());
 	}
 
+	@Test
+	public void closingContextCleansUpLoggingSystem() {
+		System.setProperty(LoggingSystem.SYSTEM_PROPERTY,
+				TestCleanupLoggingSystem.class.getName());
+		this.initializer.onApplicationEvent(
+				new ApplicationStartedEvent(this.springApplication, new String[0]));
+		TestCleanupLoggingSystem loggingSystem = (TestCleanupLoggingSystem) ReflectionTestUtils
+				.getField(this.initializer, "loggingSystem");
+		assertThat(loggingSystem.cleanedUp, is(false));
+		this.initializer.onApplicationEvent(new ContextClosedEvent(this.context));
+		assertThat(loggingSystem.cleanedUp, is(true));
+	}
+
+	@Test
+	public void closingChildContextDoesNotCleanUpLoggingSystem() {
+		System.setProperty(LoggingSystem.SYSTEM_PROPERTY,
+				TestCleanupLoggingSystem.class.getName());
+		this.initializer.onApplicationEvent(
+				new ApplicationStartedEvent(this.springApplication, new String[0]));
+		TestCleanupLoggingSystem loggingSystem = (TestCleanupLoggingSystem) ReflectionTestUtils
+				.getField(this.initializer, "loggingSystem");
+		assertThat(loggingSystem.cleanedUp, is(false));
+		GenericApplicationContext childContext = new GenericApplicationContext();
+		childContext.setParent(this.context);
+		this.initializer.onApplicationEvent(new ContextClosedEvent(childContext));
+		assertThat(loggingSystem.cleanedUp, is(false));
+		this.initializer.onApplicationEvent(new ContextClosedEvent(this.context));
+		assertThat(loggingSystem.cleanedUp, is(true));
+		childContext.close();
+	}
+
 	private boolean bridgeHandlerInstalled() {
 		Logger rootLogger = LogManager.getLogManager().getLogger("");
 		Handler[] handlers = rootLogger.getHandlers();
@@ -312,4 +346,34 @@ public class LoggingApplicationListenerTests {
 		}
 		return false;
 	}
+
+	static final class TestCleanupLoggingSystem extends LoggingSystem {
+
+		private boolean cleanedUp = false;
+
+		public TestCleanupLoggingSystem(ClassLoader classLoader) {
+
+		}
+
+		@Override
+		public void beforeInitialize() {
+
+		}
+
+		@Override
+		public void initialize(String configLocation, LogFile logFile) {
+
+		}
+
+		@Override
+		public void setLogLevel(String loggerName, LogLevel level) {
+
+		}
+
+		@Override
+		public void cleanUp() {
+			this.cleanedUp = true;
+		}
+
+	}
 }