Fix Android-related issues in Log4j Core (#3071)

This fixes three issues encountered in the [`log4j-samples-android`](https://github.com/apache/logging-log4j-samples/tree/main/log4j-samples-android) test project:

1. Disables the `jvmrunargs` lookup on Android and fixes it on the other platforms. Previously, the lookup always returned `null`.
2. Switches the default context selector to `BasicContextSelector` on Android. `StackLocator` is broken on Android: it cannot use our JDK 8 code (missing `sun.reflect` classes), but also it cannot use our JDK 11+ code (missing multi-release JAR support). This causes `ClassLoaderContextSelector` to use two different logger contexts for the same classloader.
3. Fixes a `ParserConfigurationException` caused by the lack of XInclude capabilities in Android's XML parser. The fix to [LOG4J2-3531](https://issues.apache.org/jira/browse/LOG4J2-3531) didn't cover all the cases.

Closes #3056.
Part of #2832.

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

@@ -218,14 +218,16 @@ private static void setFeature(
      */
     private static void enableXInclude(final DocumentBuilderFactory factory) {
         try {
-            // Alternative: We set if a system property on the command line is set, for example:
-            // -DLog4j.XInclude=true
             factory.setXIncludeAware(true);
             // LOG4J2-3531: Xerces only checks if the feature is supported when creating a factory. To reproduce:
             // -Dorg.apache.xerces.xni.parser.XMLParserConfiguration=org.apache.xerces.parsers.XML11NonValidatingConfiguration
-            factory.newDocumentBuilder();
-        } catch (final UnsupportedOperationException | ParserConfigurationException e) {
-            factory.setXIncludeAware(false);
+            try {
+                factory.newDocumentBuilder();
+            } catch (final ParserConfigurationException e) {
+                factory.setXIncludeAware(false);
+                LOGGER.warn("The DocumentBuilderFactory [{}] does not support XInclude: {}", factory, e);
+            }
+        } catch (final UnsupportedOperationException e) {
             LOGGER.warn("The DocumentBuilderFactory [{}] does not support XInclude: {}", factory, e);
         } catch (final AbstractMethodError | NoSuchMethodError err) {
             LOGGER.warn(

log4j-core/src/main/java/org/apache/logging/log4j/core/impl/CoreDefaultBundle.java

@@ -40,13 +40,15 @@
 import org.apache.logging.log4j.core.lookup.RuntimeStrSubstitutor;
 import org.apache.logging.log4j.core.lookup.StrLookup;
 import org.apache.logging.log4j.core.lookup.StrSubstitutor;
+import org.apache.logging.log4j.core.selector.BasicContextSelector;
 import org.apache.logging.log4j.core.selector.ClassLoaderContextSelector;
 import org.apache.logging.log4j.core.selector.ContextSelector;
 import org.apache.logging.log4j.core.time.Clock;
 import org.apache.logging.log4j.core.time.NanoClock;
 import org.apache.logging.log4j.core.time.internal.DummyNanoClock;
 import org.apache.logging.log4j.core.util.DefaultShutdownCallbackRegistry;
 import org.apache.logging.log4j.core.util.ShutdownCallbackRegistry;
+import org.apache.logging.log4j.core.util.internal.SystemUtils;
 import org.apache.logging.log4j.kit.env.PropertyEnvironment;
 import org.apache.logging.log4j.kit.recycler.RecyclerFactory;
 import org.apache.logging.log4j.kit.recycler.RecyclerFactoryProvider;
@@ -157,7 +159,9 @@ public RecyclerFactory defaultRecyclerFactory(
     @SingletonFactory
     @ConditionalOnMissingBinding
     public ContextSelector defaultContextSelector(final ConfigurableInstanceFactory instanceFactory) {
-        return new ClassLoaderContextSelector(instanceFactory);
+        return SystemUtils.isOsAndroid()
+                ? new BasicContextSelector(instanceFactory)
+                : new ClassLoaderContextSelector(instanceFactory);
     }
 
     @SingletonFactory

log4j-core/src/main/java/org/apache/logging/log4j/core/lookup/JmxRuntimeInputArgumentsLookup.java

@@ -17,11 +17,13 @@
 package org.apache.logging.log4j.core.lookup;
 
 import java.lang.management.ManagementFactory;
-import java.util.List;
 import java.util.Map;
+import org.apache.logging.log4j.Logger;
 import org.apache.logging.log4j.core.LogEvent;
+import org.apache.logging.log4j.core.util.internal.SystemUtils;
 import org.apache.logging.log4j.plugins.Plugin;
 import org.apache.logging.log4j.plugins.PluginFactory;
+import org.apache.logging.log4j.status.StatusLogger;
 import org.apache.logging.log4j.util.Lazy;
 
 /**
@@ -34,23 +36,17 @@
 @Plugin("jvmrunargs")
 public class JmxRuntimeInputArgumentsLookup extends MapLookup {
 
+    private static final Logger LOGGER = StatusLogger.getLogger();
+
     private static final Lazy<JmxRuntimeInputArgumentsLookup> INSTANCE = Lazy.lazy(() -> {
-        final List<String> argsList = ManagementFactory.getRuntimeMXBean().getInputArguments();
-        return new JmxRuntimeInputArgumentsLookup(MapLookup.toMap(argsList));
+        return new JmxRuntimeInputArgumentsLookup(getMapFromJmx());
     });
 
     @PluginFactory
     public static JmxRuntimeInputArgumentsLookup getInstance() {
         return INSTANCE.get();
     }
 
-    /**
-     * Constructor when used directly as a plugin.
-     */
-    public JmxRuntimeInputArgumentsLookup() {
-        super();
-    }
-
     public JmxRuntimeInputArgumentsLookup(final Map<String, String> map) {
         super(map);
     }
@@ -63,4 +59,15 @@ public String lookup(final LogEvent ignored, final String key) {
         final Map<String, String> map = getMap();
         return map == null ? null : map.get(key);
     }
+
+    private static Map<String, String> getMapFromJmx() {
+        if (!SystemUtils.isOsAndroid()) {
+            try {
+                return MapLookup.toMap(ManagementFactory.getRuntimeMXBean().getInputArguments());
+            } catch (LinkageError e) {
+                LOGGER.warn("Failed to get JMX arguments from JVM.", e);
+            }
+        }
+        return Map.of();
+    }
 }

log4j-core/src/main/java/org/apache/logging/log4j/core/util/internal/SystemUtils.java

@@ -0,0 +1,40 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to you 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
+ *
+ *      http://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.apache.logging.log4j.core.util.internal;
+
+import org.apache.logging.log4j.Logger;
+import org.apache.logging.log4j.status.StatusLogger;
+
+public final class SystemUtils {
+
+    private static final Logger LOGGER = StatusLogger.getLogger();
+
+    private static String getJavaVendor() {
+        try {
+            return System.getProperty("java.vendor");
+        } catch (final SecurityException e) {
+            LOGGER.warn("Unable to determine Java vendor.", e);
+        }
+        return "Unknown";
+    }
+
+    public static boolean isOsAndroid() {
+        return getJavaVendor().contains("Android");
+    }
+
+    private SystemUtils() {}
+}

src/changelog/.3.x.x/3056_android_support.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<entry xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+       xmlns="https://logging.apache.org/xml/ns"
+       xsi:schemaLocation="https://logging.apache.org/xml/ns https://logging.apache.org/xml/ns/log4j-changelog-0.xsd"
+       type="fixed">
+  <issue id="3056" link="https://github.com/apache/logging-log4j2/issues/3056"/>
+  <description format="asciidoc">Fix Android-related issues in Log4j Core.</description>
+</entry>

src/site/antora/modules/ROOT/pages/faq.adoc

@@ -214,3 +214,55 @@ When you are using Log4j with ProGuard/R8 enabled, you need to add the following
 ----
 -keep,allowoptimization class org.apache.logging.log4j.** { *; }
 ----
+
+[#android]
+== Can I use Log4j with Android?
+
+Of course, you can!
+Since version `2.25.0` both the Log4j API and our three Log4j API implementations are tested for compatibility with the Android platform.
+
+If you use
+xref:manual/api.adoc[Log4j API]
+in an Android project, you have four choices for the Log4j API implementation:
+
+[#android-log4j-core]
+Log4j Core::
++
+Our
+xref:manual/implementation.adoc[reference Log4j API implementation]
+works on Android out-of-the-box.
+However, due to the limitations of the Android platform, the following features will not work:
++
+* The
+xref:manual/configuration.adoc#xinclude[XInclude feature]
+for XML configuration files will not work if you are using the standard Android XML parser.
+You might need to add the
+https://xerces.apache.org/[Xerces parser]
+to use the feature.
+* Due to the lack of Android support for multi-release JARs, some location-based features like the no-arg
+{log4j2-url}/javadoc/log4j-api/org/apache/logging/log4j/LogManager.html#getLogger()[`LogManager.getLogger()`]
+method or
+xref:manual/systemproperties.adoc#log4j.loggerContext.selector[`ClassLoaderContextSelector`]
+(default on JRE) are not available.
+You should use `BasicContextSelector` (default on Android) or `BasicAsyncLoggerContextSelector` instead.
+
+[#android-jul]
+JUL::
+[#android-logback]
+Logback::
++
+Both our
+xref:manual/installation.adoc#impl-jul[Log4j API-to-JUL]
+and
+xref:manual/installation.adoc#impl-logback[Log4j API-to-SLF4J]
+bridges are tested for compatibility with Android.
+
+[#android-native]
+Log4j API-to-Android logging API bridge::
++
+If you wish to bridge Log4j API to
+https://developer.android.com/reference/android/util/Log[Android's native logging API]
+directly, you can use the **third-party** `com.celeral:log4j2-android` artifact.
+See the
+https://github.com/Celeral/log4j2-android[`log4j2-android` project website]
+for more information.

src/site/antora/modules/ROOT/partials/manual/systemproperties/properties-logger-context.adoc

@@ -26,7 +26,11 @@
 | link:../javadoc/log4j-core/org/apache/logging/log4j/core/selector/ContextSelector.html[`Class<? extends ContextSelector>`]
 
 | Default value
-| link:../javadoc/log4j-core/org/apache/logging/log4j/core/selector/ClassLoaderContextSelector.html[`ClassLoaderContextSelector`]
+|
+link:../javadoc/log4j-core/org/apache/logging/log4j/core/selector/ClassLoaderContextSelector.html[`ClassLoaderContextSelector`]
+
+(on Android)
+link:../javadoc/log4j-core/org/apache/logging/log4j/core/selector/BasicContextSelector.html[`BasicContextSelector`]
 |===
 
 Specifies the fully qualified class name of the