refactor: use static initialization for GsonSingleton. (#757)

Replace lazy initialization with static initialization to ensure thread safety. The previous implementation had a race condition where multiple threads could create separate Gson instances.

Author: overcat

CHANGELOG.md

@@ -8,6 +8,7 @@
 - fix: add overflow check in `TimeBounds.expiresAfter()` to prevent integer overflow when timeout is too large.
 - fix: add validation for `ManageDataOperation` value length to ensure it does not exceed 64 bytes.
 - fix: use `StandardCharsets.UTF_8` explicitly when converting byte arrays to strings to ensure consistent behavior across different platforms.
+- refactor: use static initialization for `GsonSingleton` to ensure thread safety.
 
 ## 2.2.1
 

src/main/java/org/stellar/sdk/responses/gson/GsonSingleton.java

@@ -26,58 +26,54 @@
  * <p>Usually, you shouldn't need to use this class directly.
  */
 public class GsonSingleton {
-  private static Gson instance = null;
+  private static final Gson INSTANCE = createInstance();
 
-  protected GsonSingleton() {}
+  private GsonSingleton() {}
 
   public static Gson getInstance() {
-    if (instance == null) {
-      TypeToken<Page<AccountResponse>> accountPageType = new TypeToken<Page<AccountResponse>>() {};
-      TypeToken<Page<AssetResponse>> assetPageType = new TypeToken<Page<AssetResponse>>() {};
-      TypeToken<Page<EffectResponse>> effectPageType = new TypeToken<Page<EffectResponse>>() {};
-      TypeToken<Page<LedgerResponse>> ledgerPageType = new TypeToken<Page<LedgerResponse>>() {};
-      TypeToken<Page<LiquidityPoolResponse>> liquidityPoolPageType =
-          new TypeToken<Page<LiquidityPoolResponse>>() {};
-      TypeToken<Page<OfferResponse>> offerPageType = new TypeToken<Page<OfferResponse>>() {};
-      TypeToken<Page<OperationResponse>> operationPageType =
-          new TypeToken<Page<OperationResponse>>() {};
-      TypeToken<Page<PathResponse>> pathPageType = new TypeToken<Page<PathResponse>>() {};
-      TypeToken<Page<TradeResponse>> tradePageType = new TypeToken<Page<TradeResponse>>() {};
-      TypeToken<Page<TradeAggregationResponse>> tradeAggregationPageType =
-          new TypeToken<Page<TradeAggregationResponse>>() {};
-      TypeToken<Page<TransactionResponse>> transactionPageType =
-          new TypeToken<Page<TransactionResponse>>() {};
-      TypeToken<Page<ClaimableBalanceResponse>> claimableBalancePageType =
-          new TypeToken<Page<ClaimableBalanceResponse>>() {};
+    return INSTANCE;
+  }
+
+  private static Gson createInstance() {
+    TypeToken<Page<AccountResponse>> accountPageType = new TypeToken<Page<AccountResponse>>() {};
+    TypeToken<Page<AssetResponse>> assetPageType = new TypeToken<Page<AssetResponse>>() {};
+    TypeToken<Page<EffectResponse>> effectPageType = new TypeToken<Page<EffectResponse>>() {};
+    TypeToken<Page<LedgerResponse>> ledgerPageType = new TypeToken<Page<LedgerResponse>>() {};
+    TypeToken<Page<LiquidityPoolResponse>> liquidityPoolPageType =
+        new TypeToken<Page<LiquidityPoolResponse>>() {};
+    TypeToken<Page<OfferResponse>> offerPageType = new TypeToken<Page<OfferResponse>>() {};
+    TypeToken<Page<OperationResponse>> operationPageType =
+        new TypeToken<Page<OperationResponse>>() {};
+    TypeToken<Page<PathResponse>> pathPageType = new TypeToken<Page<PathResponse>>() {};
+    TypeToken<Page<TradeResponse>> tradePageType = new TypeToken<Page<TradeResponse>>() {};
+    TypeToken<Page<TradeAggregationResponse>> tradeAggregationPageType =
+        new TypeToken<Page<TradeAggregationResponse>>() {};
+    TypeToken<Page<TransactionResponse>> transactionPageType =
+        new TypeToken<Page<TransactionResponse>>() {};
+    TypeToken<Page<ClaimableBalanceResponse>> claimableBalancePageType =
+        new TypeToken<Page<ClaimableBalanceResponse>>() {};
 
-      instance =
-          new GsonBuilder()
-              .registerTypeAdapter(Asset.class, new AssetDeserializer())
-              .registerTypeAdapter(Predicate.class, new PredicateDeserializer())
-              .registerTypeAdapter(OperationResponse.class, new OperationDeserializer())
-              .registerTypeAdapter(EffectResponse.class, new EffectDeserializer())
-              .registerTypeAdapter(
-                  accountPageType.getType(), new PageDeserializer<>(accountPageType))
-              .registerTypeAdapter(assetPageType.getType(), new PageDeserializer<>(assetPageType))
-              .registerTypeAdapter(effectPageType.getType(), new PageDeserializer<>(effectPageType))
-              .registerTypeAdapter(ledgerPageType.getType(), new PageDeserializer<>(ledgerPageType))
-              .registerTypeAdapter(
-                  liquidityPoolPageType.getType(), new PageDeserializer<>(liquidityPoolPageType))
-              .registerTypeAdapter(offerPageType.getType(), new PageDeserializer<>(offerPageType))
-              .registerTypeAdapter(
-                  operationPageType.getType(), new PageDeserializer<>(operationPageType))
-              .registerTypeAdapter(pathPageType.getType(), new PageDeserializer<>(pathPageType))
-              .registerTypeAdapter(tradePageType.getType(), new PageDeserializer<>(tradePageType))
-              .registerTypeAdapter(
-                  tradeAggregationPageType.getType(),
-                  new PageDeserializer<>(tradeAggregationPageType))
-              .registerTypeAdapter(
-                  transactionPageType.getType(), new PageDeserializer<>(transactionPageType))
-              .registerTypeAdapter(
-                  claimableBalancePageType.getType(),
-                  new PageDeserializer<>(claimableBalancePageType))
-              .create();
-    }
-    return instance;
+    return new GsonBuilder()
+        .registerTypeAdapter(Asset.class, new AssetDeserializer())
+        .registerTypeAdapter(Predicate.class, new PredicateDeserializer())
+        .registerTypeAdapter(OperationResponse.class, new OperationDeserializer())
+        .registerTypeAdapter(EffectResponse.class, new EffectDeserializer())
+        .registerTypeAdapter(accountPageType.getType(), new PageDeserializer<>(accountPageType))
+        .registerTypeAdapter(assetPageType.getType(), new PageDeserializer<>(assetPageType))
+        .registerTypeAdapter(effectPageType.getType(), new PageDeserializer<>(effectPageType))
+        .registerTypeAdapter(ledgerPageType.getType(), new PageDeserializer<>(ledgerPageType))
+        .registerTypeAdapter(
+            liquidityPoolPageType.getType(), new PageDeserializer<>(liquidityPoolPageType))
+        .registerTypeAdapter(offerPageType.getType(), new PageDeserializer<>(offerPageType))
+        .registerTypeAdapter(operationPageType.getType(), new PageDeserializer<>(operationPageType))
+        .registerTypeAdapter(pathPageType.getType(), new PageDeserializer<>(pathPageType))
+        .registerTypeAdapter(tradePageType.getType(), new PageDeserializer<>(tradePageType))
+        .registerTypeAdapter(
+            tradeAggregationPageType.getType(), new PageDeserializer<>(tradeAggregationPageType))
+        .registerTypeAdapter(
+            transactionPageType.getType(), new PageDeserializer<>(transactionPageType))
+        .registerTypeAdapter(
+            claimableBalancePageType.getType(), new PageDeserializer<>(claimableBalancePageType))
+        .create();
   }
 }