Add java docs

Signed-off-by: Louis Chu <[email protected]>

flint-core/src/main/java/org/opensearch/flint/core/logging/CustomJsonLayout.java

@@ -18,6 +18,13 @@
 import java.util.HashMap;
 import java.util.Map;
 
+/**
+ * CustomJsonLayout is a plugin for formatting log events as JSON strings.
+ *
+ * <p>The layout is designed to be used with OpenSearch Flint logging. It extracts environment-specific information,
+ * such as the cluster name, from the environment variable "FLINT_CLUSTER_NAME" and splits it into domain name and client ID.</p>
+ *
+ */
 @Plugin(name = "CustomJsonLayout", category = "Core", elementType = Layout.ELEMENT_TYPE, printObject = true)
 public class CustomJsonLayout extends AbstractStringLayout {
     private static final ObjectMapper OBJECT_MAPPER = new ObjectMapper();
@@ -41,11 +48,24 @@ protected CustomJsonLayout(Charset charset) {
         super(charset);
     }
 
+    /**
+     * Plugin factory method to create an instance of CustomJsonLayout.
+     *
+     * @param charset The charset for encoding the log event. If not specified, defaults to UTF-8.
+     * @return A new instance of CustomJsonLayout with the specified charset.
+     */
     @PluginFactory
     public static CustomJsonLayout createLayout(@PluginAttribute(value = "charset", defaultString = "UTF-8")  Charset charset) {
         return new CustomJsonLayout(charset);
     }
 
+    /**
+     * Converts the log event to a JSON string.
+     * If the log event's message does not follow the expected format, it returns the formatted message directly.
+     *
+     * @param event the log event to format.
+     * @return A string representation of the log event in JSON format.
+     */
     @Override
     public String toSerializable(LogEvent event) {
         if (!(event.getMessage() instanceof OperationMessage)) {

flint-core/src/main/scala/org/opensearch/flint/core/http/handler/HttpStatusCodeResultPredicate.java

@@ -18,6 +18,7 @@
  * @param <T> result type (supposed to be HttpResponse for OS client)
  */
 public class HttpStatusCodeResultPredicate<T> implements CheckedPredicate<T> {
+
   private static final Logger LOG = Logger.getLogger(HttpStatusCodeResultPredicate.class.getName());
 
   /**