fix sonar bug

Author: lvs007

src/main/java/org/tron/core/services/RpcApiService.java

@@ -108,6 +108,7 @@
 public class RpcApiService implements Service {
 
   public static final String CONTRACT_VALIDATE_EXCEPTION = "ContractValidateException: {}";
+  public static final String CONTRACT_VALIDATE_ERROR = "contract validate error : ";
   private int port = Args.getInstance().getRpcPort();
   private Server apiServer;
 
@@ -205,7 +206,7 @@ private void callContract(Contract.TriggerSmartContract request,
       trxExtBuilder.setResult(retBuilder);
     } catch (ContractValidateException | VMIllegalException e) {
       retBuilder.setResult(false).setCode(response_code.CONTRACT_VALIDATE_ERROR)
-          .setMessage(ByteString.copyFromUtf8("contract validate error : " + e.getMessage()));
+          .setMessage(ByteString.copyFromUtf8(CONTRACT_VALIDATE_ERROR + e.getMessage()));
       trxExtBuilder.setResult(retBuilder);
       logger.warn(CONTRACT_VALIDATE_EXCEPTION, e.getMessage());
     } catch (RuntimeException e) {
@@ -731,7 +732,7 @@ private void createTransactionExtention(Message request, ContractType contractTy
         retBuilder.setResult(true).setCode(response_code.SUCCESS);
       } catch (ContractValidateException e) {
         retBuilder.setResult(false).setCode(response_code.CONTRACT_VALIDATE_ERROR)
-            .setMessage(ByteString.copyFromUtf8("contract validate error : " + e.getMessage()));
+            .setMessage(ByteString.copyFromUtf8(CONTRACT_VALIDATE_ERROR + e.getMessage()));
         logger.debug(CONTRACT_VALIDATE_EXCEPTION, e.getMessage());
       } catch (Exception e) {
         retBuilder.setResult(false).setCode(response_code.OTHER_ERROR)