Reset StatusLogger fallback listener stream on initialization

Update `Log4J2LoggingSystem` so that the `StatusLogger` fallback
listener has its print stream reset on each initialization. This
allows output capture to work with the status listener.

Fixes gh-43578

Co-authored-by: Phillip Webb <phil.webb@broadcom.com>

spring-boot-project/spring-boot-tools/spring-boot-test-support/src/main/java/org/springframework/boot/testsupport/logging/ConfigureClasspathToPreferLog4j2.java

@@ -35,8 +35,8 @@ import org.springframework.boot.testsupport.classpath.ClassPathOverrides;
 @Target(ElementType.TYPE)
 @Documented
 @ClassPathExclusions("log4j-to-slf4j-*.jar")
-@ClassPathOverrides({ "org.apache.logging.log4j:log4j-core:2.19.0",
+@ClassPathOverrides({ "org.apache.logging.log4j:log4j-core:2.24.3",
-		"org.apache.logging.log4j:log4j-slf4j-impl:2.19.0" })
+		"org.apache.logging.log4j:log4j-slf4j-impl:2.24.3" })
 public @interface ConfigureClasspathToPreferLog4j2 {
 
 }

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2024 the original author or authors.
+ * Copyright 2012-2025 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.
@@ -47,6 +47,7 @@ import org.apache.logging.log4j.core.net.ssl.SslConfigurationFactory;
 import org.apache.logging.log4j.core.util.AuthorizationProvider;
 import org.apache.logging.log4j.core.util.NameUtil;
 import org.apache.logging.log4j.jul.Log4jBridgeHandler;
+import org.apache.logging.log4j.status.StatusLogger;
 import org.apache.logging.log4j.util.PropertiesUtil;
 
 import org.springframework.boot.context.properties.bind.BindResult;
@@ -213,6 +214,7 @@ public class Log4J2LoggingSystem extends AbstractLoggingSystem {
 		if (isAlreadyInitialized(loggerContext)) {
 			return;
 		}
+		resetFallbackListenerStream(StatusLogger.getLogger());
 		Environment environment = initializationContext.getEnvironment();
 		if (environment != null) {
 			getLoggerContext().putObject(ENVIRONMENT_KEY, environment);
@@ -224,6 +226,21 @@ public class Log4J2LoggingSystem extends AbstractLoggingSystem {
 		markAsInitialized(loggerContext);
 	}
 
+	/**
+	 * Reset the stream used by the fallback listener to the current system out. This
+	 * allows the fallback lister to work with any captured output streams in a similar
+	 * way to the {@code follow} attribute of the {@code literal Console} appender.
+	 * @param statusLogger the status logger to update
+	 */
+	private void resetFallbackListenerStream(StatusLogger statusLogger) {
+		try {
+			statusLogger.getFallbackListener().setStream(System.out);
+		}
+		catch (NoSuchMethodError ex) {
+			// Ignore for older versions of Log4J
+		}
+	}
+
 	@Override
 	protected void loadDefaults(LoggingInitializationContext initializationContext, LogFile logFile) {
 		String location = getPackagedConfigFile((logFile != null) ? "log4j2-file.xml" : "log4j2.xml");

spring-boot-tests/spring-boot-smoke-tests/spring-boot-smoke-test-structured-logging-log4j2/src/main/java/smoketest/structuredlogging/log4j2/DuplicateJsonMembersCustomizer.java

@@ -0,0 +1,32 @@
+/*
+ * Copyright 2012-2025 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
+ *
+ *      https://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 smoketest.structuredlogging.log4j2;
+
+import java.util.Objects;
+
+import org.springframework.boot.json.JsonWriter.Members;
+import org.springframework.boot.logging.structured.StructuredLoggingJsonMembersCustomizer;
+
+public class DuplicateJsonMembersCustomizer implements StructuredLoggingJsonMembersCustomizer<Object> {
+
+	@Override
+	public void customize(Members<Object> members) {
+		members.add("test").as(Objects::toString);
+		members.add("test").as(Objects::toString);
+	}
+
+}

spring-boot-tests/spring-boot-smoke-tests/spring-boot-smoke-test-structured-logging-log4j2/src/main/resources/application.properties

@@ -2,3 +2,6 @@ logging.structured.format.console=ecs
 #---
 spring.config.activate.on-profile=custom
 logging.structured.format.console=smoketest.structuredlogging.log4j2.CustomStructuredLogFormatter
+#---
+spring.config.activate.on-profile=on-error
+logging.structured.json.customizer=smoketest.structuredlogging.log4j2.DuplicateJsonMembersCustomizer

spring-boot-tests/spring-boot-smoke-tests/spring-boot-smoke-test-structured-logging-log4j2/src/test/java/smoketest/structuredlogging/log4j2/SampleLog4j2StructuredLoggingApplicationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2024 the original author or authors.
+ * Copyright 2012-2025 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.
@@ -62,4 +62,10 @@ class SampleLog4j2StructuredLoggingApplicationTests {
 		assertThat(output).contains("epoch=").contains("msg=\"Starting SampleLog4j2StructuredLoggingApplication");
 	}
 
+	@Test
+	void shouldCaptureCustomizerError(CapturedOutput output) {
+		SampleLog4j2StructuredLoggingApplication.main(new String[] { "--spring.profiles.active=on-error" });
+		assertThat(output).contains("The name 'test' has already been written");
+	}
+
 }