Apply Log4J2LoggingSystem.FILTER to main config

Update Log4J2LoggingSystem so that the FILTER is applied to the main
configuration and not to the root logger. Prior to this commit calls
to `logger.isErrorEnabled()` would not consider the filter and hence
would always return `true`. This caused `SpringApplication` to silently
swallow exceptions.

Fixes gh-5271

spring-boot/src/main/java/org/springframework/boot/logging/log4j2/Log4J2LoggingSystem.java

@@ -130,13 +130,13 @@ public class Log4J2LoggingSystem extends Slf4JLoggingSystem {
 	@Override
 	public void beforeInitialize() {
 		super.beforeInitialize();
-		getRootLoggerConfig().addFilter(FILTER);
+		getLoggerContext().getConfiguration().addFilter(FILTER);
 	}
 
 	@Override
 	public void initialize(LoggingInitializationContext initializationContext,
 			String configLocation, LogFile logFile) {
-		getRootLoggerConfig().removeFilter(FILTER);
+		getLoggerContext().getConfiguration().removeFilter(FILTER);
 		super.initialize(initializationContext, configLocation, logFile);
 	}
 
@@ -204,10 +204,6 @@ public class Log4J2LoggingSystem extends Slf4JLoggingSystem {
 		return new ShutdownHandler();
 	}
 
-	private LoggerConfig getRootLoggerConfig() {
-		return getLoggerContext().getConfiguration().getLoggerConfig("");
-	}
-
 	private LoggerConfig getLoggerConfig(String name) {
 		name = (StringUtils.hasText(name) ? name : LogManager.ROOT_LOGGER_NAME);
 		return getLoggerContext().getConfiguration().getLoggers().get(name);

spring-boot/src/test/java/org/springframework/boot/logging/log4j2/Log4J2LoggingSystemTests.java

@@ -209,6 +209,13 @@ public class Log4J2LoggingSystemTests extends AbstractLoggingSystemTests {
 		assertThat(fileContents, is(expectedOutput));
 	}
 
+	@Test
+	public void beforeInitializeFilterDisablesErrorLogging() throws Exception {
+		this.loggingSystem.beforeInitialize();
+		assertFalse(this.logger.isErrorEnabled());
+		this.loggingSystem.initialize(null, null, getLogFile(null, tmpDir()));
+	}
+
 	@Test
 	public void customExceptionConversionWord() throws Exception {
 		System.setProperty("LOG_EXCEPTION_CONVERSION_WORD", "%ex");