Add runtime hints for Log4j Core 2

This change introduces runtime hints for Log4j Core 2 to support its
integration with Spring Boot native images.

Starting with Log4j 2.25.x, Log4j includes built-in GraalVM reachability
metadata, allowing native image generation without requiring additional
manual configuration.

This contribution complements that by adding Spring Boot–specific
metadata:

* Registers default Spring Boot configuration files.
* Registers classes referenced via `ClassUtils.isPresent(...)` checks.

See gh-46410

Signed-off-by: Piotr P. Karwasz <piotr@github.copernik.eu>

core/spring-boot/build.gradle

@@ -71,6 +71,9 @@ dependencies {
 	testImplementation("org.hibernate.validator:hibernate-validator")
 	testImplementation("org.jboss.logging:jboss-logging")
 	testImplementation("org.springframework.data:spring-data-r2dbc")
+
+	// Used in Log4J2RuntimeHintsTests
+	testRuntimeOnly("com.fasterxml.jackson.dataformat:jackson-dataformat-yaml")
 }
 
 def syncJavaTemplates = tasks.register("syncJavaTemplates", Sync) {

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

@@ -90,44 +90,50 @@ public class Log4J2LoggingSystem extends AbstractLoggingSystem {
 
 	private static final String OPTIONAL_PREFIX = "optional:";
 
-	private static final String LOG4J_BRIDGE_HANDLER = "org.apache.logging.log4j.jul.Log4jBridgeHandler";
+	/**
+	 * JUL handler that routes messages to the Log4j API (optional dependency).
+	 */
+	static final String LOG4J_BRIDGE_HANDLER = "org.apache.logging.log4j.jul.Log4jBridgeHandler";
 
-	private static final String LOG4J_LOG_MANAGER = "org.apache.logging.log4j.jul.LogManager";
+	/**
+	 * JUL LogManager that routes messages to the Log4j API as the backend.
+	 */
+	static final String LOG4J_LOG_MANAGER = "org.apache.logging.log4j.jul.LogManager";
 
 	/**
 	 * JSON tree parser used by Log4j 2 (optional dependency).
 	 */
-	private static final String JSON_TREE_PARSER_V2 = "com.fasterxml.jackson.databind.ObjectMapper";
+	static final String JSON_TREE_PARSER_V2 = "com.fasterxml.jackson.databind.ObjectMapper";
 
 	/**
 	 * JSON tree parser embedded in Log4j 3.
 	 */
-	private static final String JSON_TREE_PARSER_V3 = "org.apache.logging.log4j.kit.json.JsonReader";
+	static final String JSON_TREE_PARSER_V3 = "org.apache.logging.log4j.kit.json.JsonReader";
 
 	/**
 	 * Configuration factory for properties files (Log4j 2).
 	 */
-	private static final String PROPS_CONFIGURATION_FACTORY_V2 = "org.apache.logging.log4j.core.config.properties.PropertiesConfigurationFactory";
+	static final String PROPS_CONFIGURATION_FACTORY_V2 = "org.apache.logging.log4j.core.config.properties.PropertiesConfigurationFactory";
 
 	/**
 	 * Configuration factory for properties files (Log4j 3, optional dependency).
 	 */
-	private static final String PROPS_CONFIGURATION_FACTORY_V3 = "org.apache.logging.log4j.config.properties.JavaPropsConfigurationFactory";
+	static final String PROPS_CONFIGURATION_FACTORY_V3 = "org.apache.logging.log4j.config.properties.JavaPropsConfigurationFactory";
 
 	/**
 	 * YAML tree parser used by Log4j 2 (optional dependency).
 	 */
-	private static final String YAML_TREE_PARSER_V2 = "com.fasterxml.jackson.dataformat.yaml.YAMLMapper";
+	static final String YAML_TREE_PARSER_V2 = "com.fasterxml.jackson.dataformat.yaml.YAMLMapper";
 
 	/**
 	 * Configuration factory for YAML files (Log4j 2, embedded).
 	 */
-	private static final String YAML_CONFIGURATION_FACTORY_V2 = "org.apache.logging.log4j.core.config.yaml.YamlConfigurationFactory";
+	static final String YAML_CONFIGURATION_FACTORY_V2 = "org.apache.logging.log4j.core.config.yaml.YamlConfigurationFactory";
 
 	/**
 	 * Configuration factory for YAML files (Log4j 3, optional dependency).
 	 */
-	private static final String YAML_CONFIGURATION_FACTORY_V3 = "org.apache.logging.log4j.config.yaml.YamlConfigurationFactory";
+	static final String YAML_CONFIGURATION_FACTORY_V3 = "org.apache.logging.log4j.config.yaml.YamlConfigurationFactory";
 
 	private static final SpringEnvironmentPropertySource propertySource = new SpringEnvironmentPropertySource();
 
@@ -616,8 +622,10 @@ public class Log4J2LoggingSystem extends AbstractLoggingSystem {
 	@Order(0)
 	public static class Factory implements LoggingSystemFactory {
 
-		private static final boolean PRESENT = ClassUtils
-			.isPresent("org.apache.logging.log4j.core.impl.Log4jContextFactory", Factory.class.getClassLoader());
+		static final String LOG4J_CORE_CONTEXT_FACTORY = "org.apache.logging.log4j.core.impl.Log4jContextFactory";
+
+		private static final boolean PRESENT = ClassUtils.isPresent(LOG4J_CORE_CONTEXT_FACTORY,
+				Factory.class.getClassLoader());
 
 		@Override
 		public @Nullable LoggingSystem getLoggingSystem(ClassLoader classLoader) {

core/spring-boot/src/main/java/org/springframework/boot/logging/log4j2/Log4J2RuntimeHints.java

@@ -0,0 +1,67 @@
+/*
+ * Copyright 2025-present 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 org.springframework.boot.logging.log4j2;
+
+import org.jspecify.annotations.Nullable;
+
+import org.springframework.aot.hint.RuntimeHints;
+import org.springframework.aot.hint.RuntimeHintsRegistrar;
+import org.springframework.util.ClassUtils;
+
+/**
+ * {@link RuntimeHintsRegistrar} implementation for {@link Log4J2LoggingSystem}.
+ *
+ * @author Piotr P. Karwasz
+ */
+class Log4J2RuntimeHints implements RuntimeHintsRegistrar {
+
+	@Override
+	public void registerHints(RuntimeHints hints, @Nullable ClassLoader classLoader) {
+		if (!ClassUtils.isPresent(Log4J2LoggingSystem.Factory.LOG4J_CORE_CONTEXT_FACTORY, classLoader)) {
+			return;
+		}
+		registerTypeForReachability(hints, classLoader, Log4J2LoggingSystem.Factory.LOG4J_CORE_CONTEXT_FACTORY);
+		// Register default Log4j2 configuration files
+		hints.resources().registerPattern("org/springframework/boot/logging/log4j2/log4j2.xml");
+		hints.resources().registerPattern("org/springframework/boot/logging/log4j2/log4j2-file.xml");
+		hints.resources().registerPattern("log4j2.springboot");
+		// Declares the types that Log4j2LoggingSystem checks for existence reflectively.
+		registerTypeForReachability(hints, classLoader, Log4J2LoggingSystem.JSON_TREE_PARSER_V2);
+		registerTypeForReachability(hints, classLoader, Log4J2LoggingSystem.JSON_TREE_PARSER_V3);
+		registerTypeForReachability(hints, classLoader, Log4J2LoggingSystem.PROPS_CONFIGURATION_FACTORY_V2);
+		registerTypeForReachability(hints, classLoader, Log4J2LoggingSystem.PROPS_CONFIGURATION_FACTORY_V3);
+		registerTypeForReachability(hints, classLoader, Log4J2LoggingSystem.YAML_TREE_PARSER_V2);
+		registerTypeForReachability(hints, classLoader, Log4J2LoggingSystem.YAML_CONFIGURATION_FACTORY_V2);
+		registerTypeForReachability(hints, classLoader, Log4J2LoggingSystem.YAML_CONFIGURATION_FACTORY_V3);
+		// Register JUL to Log4j 2 bridge handler
+		registerTypeForReachability(hints, classLoader, Log4J2LoggingSystem.LOG4J_BRIDGE_HANDLER);
+		registerTypeForReachability(hints, classLoader, Log4J2LoggingSystem.LOG4J_LOG_MANAGER);
+		// Don't need to register the custom Log4j 2 plugins,
+		// since they will be registered by the Log4j 2 `GraalvmPluginProcessor`.
+	}
+
+	/**
+	 * Registers the type to prevent GraalVM from removing it during the native build.
+	 * @param hints the runtime hints to register with
+	 * @param classLoader the class loader to use for type resolution
+	 * @param typeName the name of the type to register
+	 */
+	private void registerTypeForReachability(RuntimeHints hints, @Nullable ClassLoader classLoader, String typeName) {
+		hints.reflection().registerTypeIfPresent(classLoader, typeName);
+	}
+
+}

core/spring-boot/src/main/resources/META-INF/spring/aot.factories

@@ -6,6 +6,7 @@ org.springframework.boot.context.config.ConfigDataLocationRuntimeHints,\
 org.springframework.boot.context.config.ConfigDataPropertiesRuntimeHints,\
 org.springframework.boot.env.PropertySourceRuntimeHints,\
 org.springframework.boot.logging.java.JavaLoggingSystemRuntimeHints,\
+org.springframework.boot.logging.log4j2.Log4J2RuntimeHints,\
 org.springframework.boot.logging.logback.LogbackRuntimeHints,\
 org.springframework.boot.logging.structured.ElasticCommonSchemaProperties$ElasticCommonSchemaPropertiesRuntimeHints,\
 org.springframework.boot.logging.structured.GraylogExtendedLogFormatProperties$GraylogExtendedLogFormatPropertiesRuntimeHints,\

core/spring-boot/src/test/java/org/springframework/boot/logging/log4j2/Log4J2RuntimeHintsTests.java

@@ -0,0 +1,73 @@
+/*
+ * Copyright 2012-present 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 org.springframework.boot.logging.log4j2;
+
+import org.apache.logging.log4j.core.impl.Log4jContextFactory;
+import org.apache.logging.log4j.jul.Log4jBridgeHandler;
+import org.apache.logging.log4j.jul.LogManager;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.aot.hint.ReflectionHints;
+import org.springframework.aot.hint.RuntimeHints;
+import org.springframework.aot.hint.TypeReference;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link Log4J2RuntimeHints}.
+ *
+ * @author Piotr P. Karwasz
+ */
+class Log4J2RuntimeHintsTests {
+
+	@Test
+	void registersHintsForTypesCheckedByLog4J2LoggingSystem() {
+		ReflectionHints reflection = registerHints();
+		// Once Log4j Core is reachable, GraalVM will automatically
+		// add reachability metadata embedded in the Log4j Core jar and extensions.
+		assertThat(reflection.getTypeHint(Log4jContextFactory.class)).isNotNull();
+		assertThat(reflection.getTypeHint(Log4jBridgeHandler.class)).isNotNull();
+		assertThat(reflection.getTypeHint(LogManager.class)).isNotNull();
+	}
+
+	/**
+	 *
+	 */
+	@Test
+	void registersHintsForConfigurationFileParsers() {
+		ReflectionHints reflection = registerHints();
+		// JSON
+		assertThat(reflection.getTypeHint(TypeReference.of("com.fasterxml.jackson.databind.ObjectMapper"))).isNotNull();
+		// YAML
+		assertThat(reflection.getTypeHint(TypeReference.of("com.fasterxml.jackson.dataformat.yaml.YAMLMapper")))
+			.isNotNull();
+	}
+
+	@Test
+	void doesNotRegisterHintsWhenLog4jCoreIsNotAvailable() {
+		RuntimeHints hints = new RuntimeHints();
+		new Log4J2RuntimeHints().registerHints(hints, ClassLoader.getPlatformClassLoader());
+		assertThat(hints.reflection().typeHints()).isEmpty();
+	}
+
+	private ReflectionHints registerHints() {
+		RuntimeHints hints = new RuntimeHints();
+		new Log4J2RuntimeHints().registerHints(hints, getClass().getClassLoader());
+		return hints.reflection();
+	}
+
+}