Add API to list supported configuration file extensions

log4j-1.2-api/src/main/java/org/apache/log4j/config/Log4j1ConfigurationFactory.java

@@ -18,6 +18,8 @@
 
 import java.io.IOException;
 import java.io.InputStream;
+import java.util.Collections;
+import java.util.List;
 import org.apache.logging.log4j.core.LoggerContext;
 import org.apache.logging.log4j.core.config.Configuration;
 import org.apache.logging.log4j.core.config.ConfigurationException;
@@ -53,4 +55,9 @@ public Configuration getConfiguration(final LoggerContext loggerContext, final C
     protected String[] getSupportedTypes() {
         return SUFFIXES;
     }
+
+    @Override
+    public List<String> getSupportedExtensions() {
+        return Collections.emptyList();
+    }
 }

log4j-1.2-api/src/main/java/org/apache/log4j/config/PropertiesConfigurationFactory.java

@@ -16,6 +16,8 @@
  */
 package org.apache.log4j.config;
 
+import java.util.Collections;
+import java.util.List;
 import org.apache.logging.log4j.core.LoggerContext;
 import org.apache.logging.log4j.core.config.Configuration;
 import org.apache.logging.log4j.core.config.ConfigurationFactory;
@@ -52,6 +54,11 @@ protected String[] getSupportedTypes() {
         return new String[] {FILE_EXTENSION};
     }
 
+    @Override
+    public List<String> getSupportedExtensions() {
+        return Collections.emptyList();
+    }
+
     @Override
     public Configuration getConfiguration(final LoggerContext loggerContext, final ConfigurationSource source) {
         final int interval = PropertiesUtil.getProperties().getIntegerProperty(Log4j1Configuration.MONITOR_INTERVAL, 0);

log4j-1.2-api/src/main/java/org/apache/log4j/config/package-info.java

@@ -18,7 +18,7 @@
  * Log4j 1.x compatibility layer.
  */
 @Export
-@Version("2.20.1")
+@Version("2.21.0")
 @Open("org.apache.logging.log4j.core")
 package org.apache.log4j.config;
 

log4j-1.2-api/src/main/java/org/apache/log4j/xml/XmlConfigurationFactory.java

@@ -16,6 +16,8 @@
  */
 package org.apache.log4j.xml;
 
+import java.util.Collections;
+import java.util.List;
 import org.apache.log4j.config.Log4j1Configuration;
 import org.apache.logging.log4j.core.LoggerContext;
 import org.apache.logging.log4j.core.config.Configuration;
@@ -56,6 +58,11 @@ protected String[] getSupportedTypes() {
         return new String[] {FILE_EXTENSION};
     }
 
+    @Override
+    public List<String> getSupportedExtensions() {
+        return Collections.emptyList();
+    }
+
     @Override
     public Configuration getConfiguration(final LoggerContext loggerContext, final ConfigurationSource source) {
         final int interval = PropertiesUtil.getProperties().getIntegerProperty(Log4j1Configuration.MONITOR_INTERVAL, 0);

log4j-1.2-api/src/main/java/org/apache/log4j/xml/package-info.java

@@ -18,7 +18,7 @@
  * Log4j 1.x compatibility layer.
  */
 @Export
-@Version("2.20.2")
+@Version("2.21.0")
 package org.apache.log4j.xml;
 
 import org.osgi.annotation.bundle.Export;

log4j-1.2-api/src/test/java/org/apache/log4j/BasicConfigurationFactory.java

@@ -17,6 +17,8 @@
 package org.apache.log4j;
 
 import java.net.URI;
+import java.util.Collections;
+import java.util.List;
 import org.apache.logging.log4j.Level;
 import org.apache.logging.log4j.core.LoggerContext;
 import org.apache.logging.log4j.core.config.AbstractConfiguration;
@@ -35,6 +37,11 @@ public String[] getSupportedTypes() {
         return new String[] {"*"};
     }
 
+    @Override
+    public List<String> getSupportedExtensions() {
+        return Collections.emptyList();
+    }
+
     @Override
     public Configuration getConfiguration(final LoggerContext loggerContext, final ConfigurationSource source) {
         return new BasicConfiguration(loggerContext);

log4j-core-test/src/main/java/org/apache/logging/log4j/core/test/BasicConfigurationFactory.java

@@ -17,6 +17,8 @@
 package org.apache.logging.log4j.core.test;
 
 import java.net.URI;
+import java.util.Collections;
+import java.util.List;
 import org.apache.logging.log4j.Level;
 import org.apache.logging.log4j.core.LoggerContext;
 import org.apache.logging.log4j.core.config.AbstractConfiguration;
@@ -41,6 +43,11 @@ public String[] getSupportedTypes() {
         return null;
     }
 
+    @Override
+    public List<String> getSupportedExtensions() {
+        return Collections.emptyList();
+    }
+
     @Override
     public Configuration getConfiguration(final LoggerContext loggerContext, final ConfigurationSource source) {
         return null;

log4j-core-test/src/main/java/org/apache/logging/log4j/core/test/package-info.java

@@ -15,7 +15,7 @@
  * limitations under the license.
  */
 @Export
-@Version("2.25.0")
+@Version("2.26.0")
 @BaselineIgnore("2.25.0")
 package org.apache.logging.log4j.core.test;
 

log4j-core-test/src/test/java/org/apache/logging/log4j/core/config/ConfigurationFactoryTest.java

@@ -19,6 +19,7 @@
 import static org.apache.logging.log4j.util.Unbox.box;
 import static org.junit.jupiter.api.Assertions.assertAll;
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertInstanceOf;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.junit.jupiter.api.Assertions.assertThrows;
@@ -36,6 +37,10 @@
 import org.apache.logging.log4j.core.Logger;
 import org.apache.logging.log4j.core.LoggerContext;
 import org.apache.logging.log4j.core.appender.ConsoleAppender;
+import org.apache.logging.log4j.core.config.json.JsonConfigurationFactory;
+import org.apache.logging.log4j.core.config.properties.PropertiesConfigurationFactory;
+import org.apache.logging.log4j.core.config.xml.XmlConfigurationFactory;
+import org.apache.logging.log4j.core.config.yaml.YamlConfigurationFactory;
 import org.apache.logging.log4j.core.filter.ThreadContextMapFilter;
 import org.apache.logging.log4j.core.test.junit.LoggerContextSource;
 import org.apache.logging.log4j.test.junit.TempLoggingDir;
@@ -130,4 +135,55 @@ void properties(final LoggerContext context) throws IOException {
         final Path logFile = loggingPath.resolve("test-properties.log");
         checkFileLogger(context, logFile);
     }
+
+    @Test
+    void getSupportedExtensions() {
+        final List<String> extensions = ConfigurationFactory.getActiveFileExtensions();
+        assertNotNull(extensions);
+        assertFalse(extensions.isEmpty());
+        assertTrue(extensions.contains("xml"));
+        assertTrue(extensions.contains("properties"));
+        final long distinctCount = extensions.stream().distinct().count();
+        assertEquals(extensions.size(), distinctCount);
+    }
+
+    @Test
+    void xmlConfigurationFactoryExtensions() {
+        final XmlConfigurationFactory factory = new XmlConfigurationFactory();
+        final List<String> extensions = factory.getSupportedExtensions();
+        assertEquals(1, extensions.size());
+        assertEquals("xml", extensions.get(0));
+    }
+
+    @Test
+    void propertiesConfigurationFactoryExtensions() {
+        final PropertiesConfigurationFactory factory = new PropertiesConfigurationFactory();
+        final List<String> extensions = factory.getSupportedExtensions();
+        assertEquals(1, extensions.size());
+        assertEquals("properties", extensions.get(0));
+    }
+
+    @Test
+    @Tag("json")
+    void jsonConfigurationFactoryExtensions() {
+        final JsonConfigurationFactory factory = new JsonConfigurationFactory();
+        final List<String> extensions = factory.getSupportedExtensions();
+        assertTrue(extensions.size() == 0 || extensions.size() == 2);
+        if (!extensions.isEmpty()) {
+            assertTrue(extensions.contains("json"));
+            assertTrue(extensions.contains("jsn"));
+        }
+    }
+
+    @Test
+    @Tag("yaml")
+    void yamlConfigurationFactoryExtensions() {
+        final YamlConfigurationFactory factory = new YamlConfigurationFactory();
+        final List<String> extensions = factory.getSupportedExtensions();
+        assertTrue(extensions.size() == 0 || extensions.size() == 2);
+        if (!extensions.isEmpty()) {
+            assertTrue(extensions.contains("yml"));
+            assertTrue(extensions.contains("yaml"));
+        }
+    }
 }

log4j-core-test/src/test/java/org/apache/logging/log4j/core/config/builder/CustomConfigurationFactory.java

@@ -17,6 +17,8 @@
 package org.apache.logging.log4j.core.config.builder;
 
 import java.net.URI;
+import java.util.Collections;
+import java.util.List;
 import org.apache.logging.log4j.Level;
 import org.apache.logging.log4j.core.Filter;
 import org.apache.logging.log4j.core.LoggerContext;
@@ -86,4 +88,9 @@ public Configuration getConfiguration(
     protected String[] getSupportedTypes() {
         return new String[] {"*"};
     }
+
+    @Override
+    public List<String> getSupportedExtensions() {
+        return Collections.emptyList();
+    }
 }

log4j-core/src/main/java/org/apache/logging/log4j/core/config/ConfigurationFactory.java

@@ -27,6 +27,7 @@
 import java.util.Map;
 import java.util.concurrent.locks.Lock;
 import java.util.concurrent.locks.ReentrantLock;
+import java.util.stream.Collectors;
 import org.apache.logging.log4j.Level;
 import org.apache.logging.log4j.Logger;
 import org.apache.logging.log4j.core.LoggerContext;
@@ -263,6 +264,14 @@ public static void removeConfigurationFactory(final ConfigurationFactory factory
 
     protected abstract String[] getSupportedTypes();
 
+    /**
+     * Returns the file extensions supported by this configuration factory.
+     *
+     * @return list of supported file extensions (e.g., ["xml", "json"])
+     * @since 2.25.0
+     */
+    public abstract List<String> getSupportedExtensions();
+
     protected String getTestPrefix() {
         return TEST_PREFIX;
     }
@@ -553,6 +562,16 @@ public String[] getSupportedTypes() {
             return null;
         }
 
+        @Override
+        public List<String> getSupportedExtensions() {
+            return getFactories().stream()
+                    .filter(factory -> factory != this)
+                    .filter(ConfigurationFactory::isActive)
+                    .flatMap(factory -> factory.getSupportedExtensions().stream())
+                    .distinct()
+                    .collect(Collectors.toList());
+        }
+
         @Override
         public Configuration getConfiguration(final LoggerContext loggerContext, final ConfigurationSource source) {
             if (source != null) {
@@ -617,4 +636,19 @@ private String[] parseConfigLocations(final String configLocations) {
     static List<ConfigurationFactory> getFactories() {
         return factories;
     }
+
+    /**
+     * Returns all configuration file extensions currently supported at runtime.
+     * This aggregates extensions from all active configuration factories.
+     *
+     * @return list of supported file extensions
+     * @since 2.25.0
+     */
+    public static List<String> getActiveFileExtensions() {
+        return getFactories().stream()
+                .filter(ConfigurationFactory::isActive)
+                .flatMap(factory -> factory.getSupportedExtensions().stream())
+                .distinct()
+                .collect(Collectors.toList());
+    }
 }

log4j-core/src/main/java/org/apache/logging/log4j/core/config/json/JsonConfigurationFactory.java

@@ -16,6 +16,9 @@
  */
 package org.apache.logging.log4j.core.config.json;
 
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
 import org.apache.logging.log4j.core.LoggerContext;
 import org.apache.logging.log4j.core.config.Configuration;
 import org.apache.logging.log4j.core.config.ConfigurationFactory;
@@ -71,4 +74,22 @@ public Configuration getConfiguration(final LoggerContext loggerContext, final C
     public String[] getSupportedTypes() {
         return SUFFIXES;
     }
+
+    /**
+     * Returns the file extensions supported by this JSON configuration factory.
+     *
+     * <p>This method returns JSON file extensions that this factory can process.
+     * The factory requires Jackson dependencies to be available on the classpath.
+     * If Jackson libraries are missing, this method returns an empty list.</p>
+     *
+     * @return a list containing "json" and "jsn" extensions if Jackson dependencies
+     *         are available and the factory is active, empty list otherwise
+     */
+    @Override
+    public List<String> getSupportedExtensions() {
+        if (!isActive()) {
+            return Collections.emptyList();
+        }
+        return Arrays.asList("json", "jsn");
+    }
 }

log4j-core/src/main/java/org/apache/logging/log4j/core/config/json/package-info.java

@@ -18,7 +18,7 @@
  * Classes and interfaces supporting configuration of Log4j 2 with JSON.
  */
 @Export
-@Version("2.20.1")
+@Version("2.21.0")
 package org.apache.logging.log4j.core.config.json;
 
 import org.osgi.annotation.bundle.Export;

log4j-core/src/main/java/org/apache/logging/log4j/core/config/properties/PropertiesConfigurationFactory.java

@@ -18,6 +18,9 @@
 
 import java.io.IOException;
 import java.io.InputStream;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
 import java.util.Properties;
 import org.apache.logging.log4j.core.LoggerContext;
 import org.apache.logging.log4j.core.config.ConfigurationException;
@@ -40,6 +43,24 @@ protected String[] getSupportedTypes() {
         return new String[] {".properties"};
     }
 
+    /**
+     * Returns the file extensions supported by this Properties configuration factory.
+     *
+     * <p>This method returns Properties file extensions that this factory can process.
+     * The factory supports standard Java properties files used for Log4j 2 configuration.
+     * No external dependencies are required for properties file support.</p>
+     *
+     * @return a list containing "properties" extension if the factory is active,
+     *         empty list if inactive
+     */
+    @Override
+    public List<String> getSupportedExtensions() {
+        if (!isActive()) {
+            return Collections.emptyList();
+        }
+        return Arrays.asList("properties");
+    }
+
     @Override
     public PropertiesConfiguration getConfiguration(
             final LoggerContext loggerContext, final ConfigurationSource source) {

log4j-core/src/main/java/org/apache/logging/log4j/core/config/properties/package-info.java

@@ -18,7 +18,7 @@
  * Configuration using Properties files.
  */
 @Export
-@Version("2.20.1")
+@Version("2.21.0")
 package org.apache.logging.log4j.core.config.properties;
 
 import org.osgi.annotation.bundle.Export;