apache · jhl221123 · Jul 9, 2025
diff --git a/log4j-1.2-api/src/main/java/org/apache/log4j/Category.java b/log4j-1.2-api/src/main/java/org/apache/log4j/Category.java
@@ -35,13 +35,15 @@
 import org.apache.log4j.spi.HierarchyEventListener;
 import org.apache.log4j.spi.LoggerRepository;
 import org.apache.log4j.spi.LoggingEvent;
+import org.apache.logging.log4j.core.config.ConfigurationFactory;
 import org.apache.logging.log4j.message.LocalizedMessage;
 import org.apache.logging.log4j.message.MapMessage;
 import org.apache.logging.log4j.message.Message;
 import org.apache.logging.log4j.message.ObjectMessage;
 import org.apache.logging.log4j.message.SimpleMessage;
 import org.apache.logging.log4j.spi.ExtendedLogger;
 import org.apache.logging.log4j.spi.LoggerContext;
+import org.apache.logging.log4j.util.PropertiesUtil;
 import org.apache.logging.log4j.util.StackLocatorUtil;
 import org.apache.logging.log4j.util.Strings;
 
@@ -52,6 +54,10 @@ public class Category implements AppenderAttachable {
 
     private static final String FQCN = Category.class.getName();
 
+    private static boolean isFullCompatibilityEnabled() {
+        return PropertiesUtil.getProperties().getBooleanProperty(ConfigurationFactory.LOG4J1_EXPERIMENTAL);
+    }
+
     /**
      * Tests if the named category exists (in the default hierarchy).
      *
@@ -192,6 +198,9 @@ protected Category(final String name) {
      */
     @Override
     public void addAppender(final Appender appender) {
+        if (!isFullCompatibilityEnabled()) {
+            return;
+        }
         if (appender != null) {
             if (LogManager.isLog4jCorePresent()) {
                 CategoryUtil.addAppender(logger, AppenderAdapter.adapt(appender));
@@ -572,6 +581,9 @@ void maybeLog(
      */
     @Override
     public void removeAllAppenders() {
+        if (!isFullCompatibilityEnabled()) {
+            return;
+        }
         if (aai != null) {
             final Vector appenders = new Vector();
             for (final Enumeration iter = aai.getAllAppenders(); iter != null && iter.hasMoreElements(); ) {
@@ -593,6 +605,9 @@ public void removeAllAppenders() {
      */
     @Override
     public void removeAppender(final Appender appender) {
+        if (!isFullCompatibilityEnabled()) {
+            return;
+        }
         if (appender == null || aai == null) {
             return;
         }
@@ -611,6 +626,9 @@ public void removeAppender(final Appender appender) {
      */
     @Override
     public void removeAppender(final String name) {
+        if (!isFullCompatibilityEnabled()) {
+            return;
+        }
         if (name == null || aai == null) {
             return;
         }
@@ -622,6 +640,9 @@ public void removeAppender(final String name) {
     }
 
     public void setAdditivity(final boolean additivity) {
+        if (!isFullCompatibilityEnabled()) {
+            return;
+        }
         if (LogManager.isLog4jCorePresent()) {
             CategoryUtil.setAdditivity(logger, additivity);
         }
@@ -635,6 +656,9 @@ final void setHierarchy(final LoggerRepository repository) {
     }
 
     public void setLevel(final Level level) {
+        if (!isFullCompatibilityEnabled()) {
+            return;
+        }
         setLevel(level != null ? level.getVersion2Level() : null);
     }
 
@@ -645,10 +669,16 @@ private void setLevel(final org.apache.logging.log4j.Level level) {
     }
 
     public void setPriority(final Priority priority) {
+        if (!isFullCompatibilityEnabled()) {
+            return;
+        }
         setLevel(priority != null ? priority.getVersion2Level() : null);
     }
 
     public void setResourceBundle(final ResourceBundle bundle) {
+        if (!isFullCompatibilityEnabled()) {
+            return;
+        }
         this.bundle = bundle;
     }
 

diff --git a/log4j-1.2-api/src/main/java/org/apache/log4j/LogManager.java b/log4j-1.2-api/src/main/java/org/apache/log4j/LogManager.java
@@ -26,7 +26,9 @@
 import org.apache.log4j.spi.NOPLoggerRepository;
 import org.apache.log4j.spi.RepositorySelector;
 import org.apache.log4j.spi.RootLogger;
+import org.apache.logging.log4j.core.config.ConfigurationFactory;
 import org.apache.logging.log4j.spi.LoggerContext;
+import org.apache.logging.log4j.util.PropertiesUtil;
 import org.apache.logging.log4j.util.StackLocatorUtil;
 
 /**
@@ -64,6 +66,10 @@ public final class LogManager {
 
     private static final boolean LOG4J_CORE_PRESENT;
 
+    private static boolean isFullCompatibilityEnabled() {
+        return PropertiesUtil.getProperties().getBooleanProperty(ConfigurationFactory.LOG4J1_EXPERIMENTAL);
+    }
+
     static {
         LOG4J_CORE_PRESENT = checkLog4jCore();
         // By default, we use a DefaultRepositorySelector which always returns 'hierarchy'.
@@ -201,6 +207,9 @@ static void reconfigure(final ClassLoader classLoader) {
     }
 
     public static void resetConfiguration() {
+        if (!isFullCompatibilityEnabled()) {
+            return;
+        }
         resetConfiguration(StackLocatorUtil.getCallerClassLoader(2));
     }
 
@@ -225,6 +234,9 @@ public static void setRepositorySelector(final RepositorySelector selector, fina
      * Shuts down the current configuration.
      */
     public static void shutdown() {
+        if (!isFullCompatibilityEnabled()) {
+            return;
+        }
         shutdown(StackLocatorUtil.getCallerClassLoader(2));
     }
 

diff --git a/log4j-1.2-api/src/test/java/org/apache/log4j/CategoryTest.java b/log4j-1.2-api/src/test/java/org/apache/log4j/CategoryTest.java
@@ -16,6 +16,7 @@
  */
 package org.apache.log4j;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertInstanceOf;
@@ -27,11 +28,14 @@
 import java.util.Collections;
 import java.util.Enumeration;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
+import java.util.ResourceBundle;
 import java.util.function.Consumer;
 import org.apache.log4j.bridge.AppenderAdapter;
 import org.apache.log4j.bridge.AppenderWrapper;
 import org.apache.log4j.spi.LoggingEvent;
+import org.apache.log4j.varia.NullAppender;
 import org.apache.logging.log4j.core.Layout;
 import org.apache.logging.log4j.core.LogEvent;
 import org.apache.logging.log4j.core.LoggerContext;
@@ -42,6 +46,7 @@
 import org.apache.logging.log4j.message.Message;
 import org.apache.logging.log4j.message.ObjectMessage;
 import org.apache.logging.log4j.message.SimpleMessage;
+import org.apache.logging.log4j.test.junit.SetTestProperty;
 import org.apache.logging.log4j.util.Constants;
 import org.apache.logging.log4j.util.Strings;
 import org.junit.jupiter.api.AfterAll;
@@ -52,6 +57,7 @@
 /**
  * Tests of Category.
  */
+@SetTestProperty(key = "log4j1.compatibility", value = "true")
 class CategoryTest {
 
     static ConfigurationFactory cf = new BasicConfigurationFactory();
@@ -409,6 +415,72 @@ void testGetAppender() {
         }
     }
 
+    @Test
+    @SetTestProperty(key = "log4j1.compatibility", value = "false")
+    void testSetLevelCompatibilityDisabled() {
+        final Category category = Category.getInstance("TestCategory");
+        final Level initialLevel = category.getEffectiveLevel();
+        category.setLevel(Level.FATAL);
+        assertEquals(initialLevel, category.getEffectiveLevel(), "level should be unchanged when compatibility is off");
+    }
+
+    @Test
+    @SetTestProperty(key = "log4j1.compatibility", value = "false")
+    void testSetAdditivityCompatibilityDisabled() {
+        final Category category = Category.getInstance("TestCategory");
+        final boolean initialAdditivity = category.getAdditivity();
+        category.setAdditivity(!initialAdditivity);
+        assertEquals(
+                initialAdditivity,
+                category.getAdditivity(),
+                "additivity should be unchanged when compatibility is off");
+    }
+
+    @Test
+    @SetTestProperty(key = "log4j1.compatibility", value = "false")
+    void testAddAppenderCompatibilityDisabled() {
+        final Category category = Category.getInstance("TestCategory");
+        category.addAppender(new NullAppender());
+        assertFalse(
+                category.getAllAppenders().hasMoreElements(), "no appenders should be added when compatibility is off");
+    }
+
+    @Test
+    @SetTestProperty(key = "log4j1.compatibility", value = "false")
+    void testSetPriorityCompatibilityDisabled() {
+        final Category category = Category.getInstance("TestCategory");
+        final Priority initialPriority = category.getPriority();
+        category.setPriority(Level.FATAL);
+        assertEquals(initialPriority, category.getPriority(), "priority should be unchanged when compatibility is off");
+    }
+
+    @Test
+    @SetTestProperty(key = "log4j1.compatibility", value = "false")
+    void testSetResourceBundleCompatibilityDisabled() {
+        final Category category = Category.getInstance("TestCategory");
+        final ResourceBundle bundle = ResourceBundle.getBundle("L7D", new Locale("en", "US"));
+        category.setResourceBundle(bundle);
+        assertNull(category.getResourceBundle(), "resource bundle should not be set when compatibility is off");
+    }
+
+    @Test
+    @SetTestProperty(key = "log4j1.compatibility", value = "false")
+    void testRemoveAppenderCompatibilityDisabled() {
+        final Category category = Category.getInstance("TestCategory");
+        assertDoesNotThrow(
+                () -> category.removeAppender("TestAppender"),
+                "removeAppender should be a no-op and not throw exceptions when compatibility is off");
+    }
+
+    @Test
+    @SetTestProperty(key = "log4j1.compatibility", value = "false")
+    void testRemoveAllAppendersCompatibilityDisabled() {
+        final Category category = Category.getInstance("TestCategory");
+        assertDoesNotThrow(
+                category::removeAllAppenders,
+                "removeAllAppenders should be a no-op and not throw exceptions when compatibility is off");
+    }
+
     /**
      * Derived category to check method signature of forcedLog.
      */

diff --git a/log4j-1.2-api/src/test/java/org/apache/log4j/LogManagerTest.java b/log4j-1.2-api/src/test/java/org/apache/log4j/LogManagerTest.java
@@ -16,17 +16,20 @@
  */
 package org.apache.log4j;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import java.util.Collections;
 import java.util.Enumeration;
 import java.util.List;
 import java.util.stream.Collectors;
+import org.apache.logging.log4j.test.junit.SetTestProperty;
 import org.junit.jupiter.api.Test;
 
 /**
  * Tests {@link LogManager}.
  */
+@SetTestProperty(key = "log4j1.compatibility", value = "true")
 class LogManagerTest {
 
     private static final String SIMPLE_NAME = LogManagerTest.class.getSimpleName();
@@ -47,4 +50,20 @@ void testGetCurrentLoggers() {
         assertTrue(names.contains(SIMPLE_NAME + ".foo"));
         assertTrue(names.contains(SIMPLE_NAME + ".foo.bar"));
     }
+
+    @Test
+    @SetTestProperty(key = "log4j1.compatibility", value = "false")
+    void testResetConfigurationCompatibilityDisabled() {
+        assertDoesNotThrow(
+                () -> LogManager.resetConfiguration(),
+                "resetConfiguration should be a no-op and not throw exceptions when compatibility is off");
+    }
+
+    @Test
+    @SetTestProperty(key = "log4j1.compatibility", value = "false")
+    void testShutdownCompatibilityDisabled() {
+        assertDoesNotThrow(
+                () -> LogManager.shutdown(),
+                "shutdown should be a no-op and not throw exceptions when compatibility is off");
+    }
 }
diff --git a/log4j-1.2-api/src/test/java/org/apache/log4j/LoggerTest.java b/log4j-1.2-api/src/test/java/org/apache/log4j/LoggerTest.java
@@ -38,6 +38,7 @@
 import org.apache.logging.log4j.core.config.Property;
 import org.apache.logging.log4j.core.layout.PatternLayout;
 import org.apache.logging.log4j.core.test.appender.ListAppender;
+import org.apache.logging.log4j.test.junit.SetTestProperty;
 import org.junit.jupiter.api.AfterAll;
 import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.BeforeEach;
@@ -46,6 +47,7 @@
 /**
  * Used for internal unit testing the Logger class.
  */
+@SetTestProperty(key = "log4j1.compatibility", value = "true")
 class LoggerTest {
 
     Appender a1;

diff --git a/src/changelog/.2.x.x/3667_respect_log4j1_compatibility_flag.xml b/src/changelog/.2.x.x/3667_respect_log4j1_compatibility_flag.xml
@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<entry xmlns="https://logging.apache.org/xml/ns"
+       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+       xsi:schemaLocation="
+           https://logging.apache.org/xml/ns
+           https://logging.apache.org/xml/ns/log4j-changelog-0.xsd"
+       type="fixed">
+    <issue id="3667" link="https://github.com/apache/logging-log4j2/issues/3667"/>
+    <description format="asciidoc">
+        Programmatic configuration changes using the Log4j 1.x API now respect the `log4j1.compatibility` flag.
+    </description>
+</entry>