Changes the priority of the attributes/elements

If an element of type "middleware" is present, the corresponding
attribute of `<SplunkHttp>` is ignored.

I replaced a manual search of the `Node` element with `@PluginElement`
parameters, to eliminate errors in the logs (and simplify the code).

Author: Piotr P. Karwasz

src/main/java/com/splunk/logging/HttpEventCollectorLog4jAppender.java

@@ -25,14 +25,11 @@
 import org.apache.logging.log4j.core.Layout;
 import org.apache.logging.log4j.core.LogEvent;
 import org.apache.logging.log4j.core.appender.AbstractAppender;
-import org.apache.logging.log4j.core.config.Node;
 import org.apache.logging.log4j.core.config.Property;
 import org.apache.logging.log4j.core.config.plugins.Plugin;
 import org.apache.logging.log4j.core.config.plugins.PluginAttribute;
 import org.apache.logging.log4j.core.config.plugins.PluginElement;
 import org.apache.logging.log4j.core.config.plugins.PluginFactory;
-import org.apache.logging.log4j.core.config.plugins.PluginNode;
-import org.apache.logging.log4j.core.config.plugins.util.PluginType;
 import org.apache.logging.log4j.core.layout.PatternLayout;
 
 import com.splunk.logging.HttpEventCollectorMiddleware.HttpSenderMiddleware;
@@ -147,9 +144,12 @@ public static HttpEventCollectorLog4jAppender createAppender(
             @PluginAttribute("batch_interval") final String batchInterval,
             @PluginAttribute("retries_on_error") final String retriesOnError,
             @PluginAttribute("send_mode") final String sendMode,
+            @PluginElement(MIDDLEWARE_TYPE) HttpSenderMiddleware middleware,
             @PluginAttribute(MIDDLEWARE_TYPE) final String middlewareClassName,
             @PluginAttribute("disableCertificateValidation") final String disableCertificateValidation,
+            @PluginElement(BODY_SERIALIZER_TYPE) EventBodySerializer eventBodySerializer,
             @PluginAttribute(BODY_SERIALIZER_TYPE) final String eventBodySerializerClassName,
+            @PluginElement(HEADER_SERIALIZER_TYPE) EventHeaderSerializer eventHeaderSerializer,
             @PluginAttribute(HEADER_SERIALIZER_TYPE) final String eventHeaderSerializerClassName,
             @PluginAttribute(value = "includeLoggerName", defaultBoolean = true) final boolean includeLoggerName,
             @PluginAttribute(value = "includeThreadName", defaultBoolean = true) final boolean includeThreadName,
@@ -161,8 +161,7 @@ public static HttpEventCollectorLog4jAppender createAppender(
             @PluginAttribute(value = "read_timeout", defaultLong = HttpEventCollectorSender.TimeoutSettings.DEFAULT_READ_TIMEOUT) final long readTimeout,
             @PluginAttribute(value = "write_timeout", defaultLong = HttpEventCollectorSender.TimeoutSettings.DEFAULT_WRITE_TIMEOUT) final long writeTimeout,
             @PluginElement("Layout") Layout<? extends Serializable> layout,
-            @PluginElement("Filter") final Filter filter,
-            @PluginNode final Node node
+            @PluginElement("Filter") final Filter filter
     )
     {
         // The raw endpoint presumes that a single post is a single event.
@@ -206,45 +205,24 @@ public static HttpEventCollectorLog4jAppender createAppender(
                     .build();
         }
 
-        // Retrieve the additional elements through Log4j plugin mechanism first
-        HttpSenderMiddleware middleware = null;
-        EventBodySerializer eventBodySerializer = null;
-        EventHeaderSerializer eventHeaderSerializer = null;
-        for (final Node child : node.getChildren()) {
-            final PluginType< ? > pluginType = child.getType();
-            final Object component = child.getObject();
-            switch (pluginType.getElementName()) {
-                case MIDDLEWARE_TYPE :
-                    middleware = (HttpSenderMiddleware) component;
-                    break;
-                case BODY_SERIALIZER_TYPE :
-                     eventBodySerializer = (EventBodySerializer) component;
-                     break;
-                case HEADER_SERIALIZER_TYPE :
-                    eventHeaderSerializer = (EventHeaderSerializer) component;
-                    break;
-                default:
-           }
-        }
-
         // Fallback on instantiating classes
-        if (middlewareClassName != null && !middlewareClassName.isEmpty()) {
+        if (middleware == null && middlewareClassName != null && !middlewareClassName.isEmpty()) {
             try {
                 middleware = (HttpSenderMiddleware) Class.forName(middlewareClassName).newInstance();
             } catch (Exception e) {
                 LOGGER.warn("The middleware {} could not be instantiated.", middlewareClassName, e);
             }
         }
 
-        if (eventBodySerializerClassName != null && !eventBodySerializerClassName.isEmpty()) {
+        if (eventBodySerializer == null && eventBodySerializerClassName != null && !eventBodySerializerClassName.isEmpty()) {
             try {
                 eventBodySerializer = (EventBodySerializer) Class.forName(eventBodySerializerClassName).newInstance();
             } catch (final Exception e) {
                 LOGGER.warn("The event body serializer {} could not be instantiated.", eventBodySerializerClassName, e);
             }
         }
 
-        if (eventHeaderSerializerClassName != null && !eventHeaderSerializerClassName.isEmpty()) {
+        if (eventHeaderSerializer == null && eventHeaderSerializerClassName != null && !eventHeaderSerializerClassName.isEmpty()) {
             try {
                 eventHeaderSerializer = (EventHeaderSerializer) Class.forName(eventHeaderSerializerClassName).newInstance();
             } catch (final Exception e) {