Some refactoring for JdbcTransaction

src/main/java/com/scalar/db/transaction/jdbc/JdbcTransaction.java

@@ -37,18 +37,21 @@
 public class JdbcTransaction implements DistributedTransaction {
   private static final Logger LOGGER = LoggerFactory.getLogger(JdbcTransaction.class);
 
+  private final String txId;
   private final JdbcService jdbcService;
   private final Connection connection;
   private final RdbEngine rdbEngine;
   private Optional<String> namespace;
   private Optional<String> tableName;
 
   JdbcTransaction(
+      String txId,
       JdbcService jdbcService,
       Connection connection,
       RdbEngine rdbEngine,
       Optional<String> namespace,
       Optional<String> tableName) {
+    this.txId = txId;
     this.jdbcService = jdbcService;
     this.connection = connection;
     this.rdbEngine = rdbEngine;
@@ -58,7 +61,7 @@ public class JdbcTransaction implements DistributedTransaction {
 
   @Override
   public String getId() {
-    throw new UnsupportedOperationException("doesn't support this operation");
+    return txId;
   }
 
   @Override

src/main/java/com/scalar/db/transaction/jdbc/JdbcTransactionManager.java

@@ -16,6 +16,7 @@
 import com.scalar.db.storage.jdbc.query.QueryBuilder;
 import java.sql.SQLException;
 import java.util.Optional;
+import java.util.UUID;
 import javax.annotation.concurrent.ThreadSafe;
 import org.apache.commons.dbcp2.BasicDataSource;
 import org.slf4j.Logger;
@@ -81,19 +82,20 @@ public Optional<String> getTable() {
 
   @Override
   public JdbcTransaction start() throws TransactionException {
+    String txId = UUID.randomUUID().toString();
+    return start(txId);
+  }
+
+  @Override
+  public JdbcTransaction start(String txId) throws TransactionException {
     try {
       return new JdbcTransaction(
-          jdbcService, dataSource.getConnection(), rdbEngine, namespace, tableName);
+          txId, jdbcService, dataSource.getConnection(), rdbEngine, namespace, tableName);
     } catch (SQLException e) {
       throw new TransactionException("failed to start the transaction", e);
     }
   }
 
-  @Override
-  public JdbcTransaction start(String txId) throws TransactionException {
-    throw new UnsupportedOperationException("doesn't support starting transaction with txId");
-  }
-
   @Deprecated
   @Override
   public JdbcTransaction start(Isolation isolation) throws TransactionException {
@@ -103,7 +105,7 @@ public JdbcTransaction start(Isolation isolation) throws TransactionException {
   @Deprecated
   @Override
   public JdbcTransaction start(String txId, Isolation isolation) throws TransactionException {
-    throw new UnsupportedOperationException("doesn't support starting transaction with txId");
+    return start(txId);
   }
 
   @Deprecated
@@ -123,19 +125,19 @@ public JdbcTransaction start(SerializableStrategy strategy) throws TransactionEx
   @Override
   public JdbcTransaction start(String txId, SerializableStrategy strategy)
       throws TransactionException {
-    throw new UnsupportedOperationException("doesn't support starting transaction with txId");
+    return start(txId);
   }
 
   @Deprecated
   @Override
   public JdbcTransaction start(String txId, Isolation isolation, SerializableStrategy strategy)
       throws TransactionException {
-    throw new UnsupportedOperationException("doesn't support starting transaction with txId");
+    return start(txId);
   }
 
   @Override
   public TransactionState getState(String txId) {
-    throw new UnsupportedOperationException("doesn't support this operation");
+    return TransactionState.UNKNOWN; // always returns UNKNOWN
   }
 
   @Override