Add javadocs

ts-segment-uploader/src/main/java/com/pinterest/kafka/tieredstorage/uploader/dlq/DeadLetterQueueHandler.java

@@ -11,6 +11,15 @@
 
 import static com.pinterest.kafka.tieredstorage.uploader.SegmentUploaderConfiguration.TS_SEGMENT_UPLOADER_PREFIX;
 
+/**
+ * Abstract class for handling failed uploads after retry exhaustion by sending them to a dead letter queue.
+ *
+ * The handler is responsible for sending failed uploads to the dead letter queue and for polling the dead letter queue
+ * for failed uploads to retry. The handler can be specified in the uploader configuration
+ * (see {@link #HANDLER_CLASS_CONFIG_KEY}). The configuration expects the FQDN of the handler class.
+ *
+ * Note that the handler must have a public constructor so that it can be instantiated by reflection.
+ */
 public abstract class DeadLetterQueueHandler {
 
     private static final Logger LOG = LogManager.getLogger(DeadLetterQueueHandler.class);
@@ -24,12 +33,36 @@ public DeadLetterQueueHandler(SegmentUploaderConfiguration config) {
         validateConfig();
     }
 
+    /**
+     * Validate the configuration.
+     */
     protected abstract void validateConfig();
 
+    /**
+     * Send the failed upload to the dead letter queue. The handler should return a Future that completes successfully
+     * if the upload was successfully sent to the dead letter queue, and completes exceptionally / fails if the upload
+     * could not be sent to the dead letter queue.
+     *
+     * @param uploadTask the failed upload task
+     * @param throwable the throwable that caused the upload to fail
+     * @return a Future that completes successfully if the upload was successfully sent to the dead letter queue, and
+     * completes exceptionally / fails if the upload could not be sent to the dead letter queue
+     */
     public abstract Future<Boolean> send(DirectoryTreeWatcher.UploadTask uploadTask, Throwable throwable);
 
+    /**
+     * Poll the dead letter queue for failed uploads to retry.
+     *
+     * @return a collection of failed uploads to retry
+     */
     public abstract Collection<DirectoryTreeWatcher.UploadTask> poll();
 
+    /**
+     * Create a DeadLetterQueueHandler from the configuration.
+     *
+     * @param config the configuration
+     * @return the DeadLetterQueueHandler
+     */
     public static DeadLetterQueueHandler createHandler(SegmentUploaderConfiguration config) {
         String className = config.getProperty(HANDLER_CLASS_CONFIG_KEY);
         if (className == null) {