From 36c2b2cb5194b9c20a207b47bed0dbe58ac68a76 Mon Sep 17 00:00:00 2001
From: KUAN-HAO HUANG <101171023+rich7420@users.noreply.github.com>
Date: Tue, 25 Aug 2026 18:16:30 +0800
Subject: [PATCH] HDDS-16277. Reuse CharsetEncoder/Decoder in StringCodec

StringCodecBase allocated a fresh CharsetEncoder/CharsetDecoder on every
encode/decode call. Cache them per thread (ThreadLocal) since the codec
instances are singletons and the coders are stateful and not thread-safe.
The encoder is reset() before each reuse; the single-arg
CharsetDecoder.decode resets internally, so behavior is unchanged.
---
 .../org/apache/hadoop/hdds/utils/db/StringCodecBase.java | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/hadoop-hdds/common/src/main/java/org/apache/hadoop/hdds/utils/db/StringCodecBase.java b/hadoop-hdds/common/src/main/java/org/apache/hadoop/hdds/utils/db/StringCodecBase.java
index f64f19318311..2ff7fe250859 100644
--- a/hadoop-hdds/common/src/main/java/org/apache/hadoop/hdds/utils/db/StringCodecBase.java
+++ b/hadoop-hdds/common/src/main/java/org/apache/hadoop/hdds/utils/db/StringCodecBase.java
@@ -42,6 +42,9 @@ abstract class StringCodecBase implements Codec<String> {
   private final Charset charset;
   private final boolean fixedLength;
   private final int maxBytesPerChar;
+  // CharsetEncoder/CharsetDecoder are stateful and not thread-safe: reuse one per thread.
+  private final ThreadLocal<CharsetEncoder> threadLocalEncoder = ThreadLocal.withInitial(this::newEncoder);
+  private final ThreadLocal<CharsetDecoder> threadLocalDecoder = ThreadLocal.withInitial(this::newDecoder);
 
   StringCodecBase(Charset charset) {
     this.charset = charset;
@@ -97,7 +100,7 @@ private int getSerializedSizeUpperBound(String s) {
   private <E extends Exception> PutToByteBuffer<E> encode(
       String string, Integer serializedSize, Function<String, E> newE) {
     return buffer -> {
-      final CoderResult result = newEncoder().encode(
+      final CoderResult result = threadLocalEncoder.get().reset().encode(
           CharBuffer.wrap(string), buffer, true);
       if (result.isError()) {
         throw newE.apply("Failed to encode with " + charset + ": " + result
@@ -114,7 +117,7 @@ private <E extends Exception> PutToByteBuffer<E> encode(
 
   String decodeNoFallback(ByteBuffer buffer) throws CodecException {
     try {
-      return newDecoder().decode(buffer.asReadOnlyBuffer()).toString();
+      return threadLocalDecoder.get().decode(buffer.asReadOnlyBuffer()).toString();
     } catch (Exception e) {
       throw new CodecException("Failed to decode " + buffer, e);
     }
@@ -123,7 +126,7 @@ String decodeNoFallback(ByteBuffer buffer) throws CodecException {
   String decodeWithFallback(ByteBuffer buffer) {
     Runnable error = null;
     try {
-      return newDecoder().decode(buffer.asReadOnlyBuffer()).toString();
+      return threadLocalDecoder.get().decode(buffer.asReadOnlyBuffer()).toString();
     } catch (Exception e) {
       error = () -> LOG.warn("Failed to decode buffer with {}, buffer = (hex) {}",
           charset, StringUtils.bytes2Hex(buffer, 20), e);