Fix-ups due to #2230 review

Author: ppkarwasz

log4j-core/src/main/java/org/apache/logging/log4j/core/async/AsyncLoggerConfig.java

@@ -108,11 +108,9 @@ protected AsyncLoggerConfig(
     @Override
     public void initialize() {
         final Configuration configuration = getConfiguration();
-        DisruptorConfiguration disruptorConfig = configuration.getExtension(DisruptorConfiguration.class);
-        if (disruptorConfig == null) {
-            disruptorConfig = DisruptorConfiguration.newBuilder().build();
-            configuration.addExtension(disruptorConfig);
-        }
+        final DisruptorConfiguration disruptorConfig = configuration.addExtensionIfAbsent(
+                DisruptorConfiguration.class,
+                () -> DisruptorConfiguration.newBuilder().build());
         delegate = disruptorConfig.getAsyncLoggerConfigDelegate();
         delegate.setLogEventFactory(getLogEventFactory());
         super.initialize();

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

@@ -91,8 +91,6 @@
 @ServiceConsumer(value = ScriptManagerFactory.class, cardinality = Cardinality.SINGLE, resolution = Resolution.OPTIONAL)
 public abstract class AbstractConfiguration extends AbstractFilterable implements Configuration {
 
-    private static final ConfigurationExtension[] EMPTY_EXTENSIONS = new ConfigurationExtension[0];
-
     /**
      * The instance factory for this configuration. This may be a child factory to a LoggerContext
      * in most cases, though this might be a root level factory for null configurations.
@@ -159,7 +157,7 @@ public abstract class AbstractConfiguration extends AbstractFilterable implement
     private final WeakReference<LoggerContext> loggerContext;
     private final PropertyEnvironment contextProperties;
     private final Lock configLock = new ReentrantLock();
-    private ConfigurationExtension[] extensions = EMPTY_EXTENSIONS;
+    private final List<ConfigurationExtension> extensions = new CopyOnWriteArrayList<>();
 
     /**
      * Constructor.
@@ -1154,19 +1152,28 @@ public void setNanoClock(final NanoClock nanoClock) {
     }
 
     @Override
-    public void addExtension(ConfigurationExtension extension) {
-        Objects.requireNonNull(extension);
-        extensions = Arrays.copyOf(extensions, extensions.length + 1);
-        extensions[extensions.length - 1] = extension;
+    public <T extends ConfigurationExtension> T addExtensionIfAbsent(
+            final Class<T> extensionType, final Supplier<? extends T> supplier) {
+        for (final ConfigurationExtension extension : extensions) {
+            if (extensionType.isInstance(extension)) {
+                return extensionType.cast(extension);
+            }
+        }
+        return addExtension(supplier.get());
+    }
+
+    private <T extends ConfigurationExtension> T addExtension(final T extension) {
+        extensions.add(Objects.requireNonNull(extension));
+        return extension;
     }
 
     @Override
-    public <T extends ConfigurationExtension> T getExtension(Class<T> extensionType) {
+    public <T extends ConfigurationExtension> T getExtension(final Class<T> extensionType) {
         T result = null;
         for (final ConfigurationExtension extension : extensions) {
             if (extensionType.isInstance(extension)) {
                 if (result == null) {
-                    result = (T) extension;
+                    result = extensionType.cast(extension);
                 } else {
                     LOGGER.warn(
                             "Multiple configuration elements found for type {}. Only the first will be used.",

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

@@ -238,17 +238,25 @@ default RecyclerFactory getRecyclerFactory() {
     }
 
     /**
-     * Registers a new configuration extension.
-     *
-     * @param extension a configuration extension.
+     * Registers a new configuration extension, if it doesn't exist.
+     * <p>
+     *     To preventing polluting the main configuration element,
+     *     each JAR that wishes to extend the {@link Configuration} should use a single child element.
+     * </p>
+     * @param extensionType the concrete type of the extension,
+     * @param supplier a factory to create a new extension element,
+     * @return the current extension if present or a newly generated one.
      * @since 3.0
+     * @see #getExtension(Class)
      */
-    void addExtension(ConfigurationExtension extension);
+    <T extends ConfigurationExtension> T addExtensionIfAbsent(Class<T> extensionType, Supplier<? extends T> supplier);
 
     /**
      * Returns an extension of the given type.
-     *
-     * @param extensionType a type of extension,
+     * <p>
+     *     Only the first extension of the given type is returned.
+     * </p>
+     * @param extensionType a concrete type of the extension,
      * @return an extension the matches the given type.
      * @since 3.0
      */

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

@@ -17,6 +17,6 @@
 package org.apache.logging.log4j.core.config;
 
 /**
- * A {@link ConfigurationExtension} is used to add new child elements to a {@link Configuration}.
+ * Marker interface used by new child elements of a {@link Configuration}.
  */
 public interface ConfigurationExtension {}