[ISSUE #3375] Lazy init GetMessageResult only when find message

store/src/main/java/org/apache/rocketmq/store/DefaultMessageStore.java

@@ -537,7 +537,8 @@ public class DefaultMessageStore implements MessageStore {
         long minOffset = 0;
         long maxOffset = 0;
 
-        GetMessageResult getResult = new GetMessageResult();
+        // lazy init when find msg.
+        GetMessageResult getResult = null;
 
         final long maxOffsetPy = this.commitLog.getMaxOffset();
 
@@ -574,6 +575,9 @@ public class DefaultMessageStore implements MessageStore {
                         int i = 0;
                         final int maxFilterMessageCount = Math.max(16000, maxMsgNums * ConsumeQueue.CQ_STORE_UNIT_SIZE);
                         final boolean diskFallRecorded = this.messageStoreConfig.isDiskFallRecorded();
+
+                        getResult = new GetMessageResult(maxMsgNums);
+
                         ConsumeQueueExt.CqExtUnit cqExtUnit = new ConsumeQueueExt.CqExtUnit();
                         for (; i < bufferConsumeQueue.getSize() && i < maxFilterMessageCount; i += ConsumeQueue.CQ_STORE_UNIT_SIZE) {
                             long offsetPy = bufferConsumeQueue.getByteBuffer().getLong();
@@ -677,6 +681,11 @@ public class DefaultMessageStore implements MessageStore {
         long elapsedTime = this.getSystemClock().now() - beginTime;
         this.storeStatsService.setGetMessageEntireTimeMax(elapsedTime);
 
+        // lazy init no data found.
+        if (getResult == null) {
+            getResult = new GetMessageResult(0);
+        }
+
         getResult.setStatus(status);
         getResult.setNextBeginOffset(nextBeginOffset);
         getResult.setMaxOffset(maxOffset);

store/src/main/java/org/apache/rocketmq/store/GetMessageResult.java

@@ -23,10 +23,8 @@ import org.apache.rocketmq.store.stats.BrokerStatsManager;
 
 public class GetMessageResult {
 
-    private final List<SelectMappedBufferResult> messageMapedList =
-        new ArrayList<SelectMappedBufferResult>(100);
-
-    private final List<ByteBuffer> messageBufferList = new ArrayList<ByteBuffer>(100);
+    private final List<SelectMappedBufferResult> messageMapedList;
+    private final List<ByteBuffer> messageBufferList;
 
     private GetMessageStatus status;
     private long nextBeginOffset;
@@ -40,6 +38,13 @@ public class GetMessageResult {
     private int msgCount4Commercial = 0;
 
     public GetMessageResult() {
+        messageMapedList = new ArrayList<>(100);
+        messageBufferList = new ArrayList<>(100);
+    }
+
+    public GetMessageResult(int resultSize) {
+        messageMapedList = new ArrayList<>(resultSize);
+        messageBufferList = new ArrayList<>(resultSize);
     }
 
     public GetMessageStatus getStatus() {