Error when reading BLOB field from Oracle usin Toplink

Question

We experience a very annoying problem when trying to read a BLOB
field from Oracle 8.1.6.2.0 using TOPLink 3.6.3. I have attached the
exception stack trace that is reported to the console. As far as I can
judge a fault at oracle.sql.LobPlsqlUtil.plsql_length() happens first and
then at TOPLink.Private.DatabaseAccess.DatabasePlatform.convertObject().
The exception is permanently repeating that is very critical for us.
ServerSession(929808)--Connection(5625701)--SELECT LOBBODY, ID, LABEL, FK_OBJECT_ID, FK_OBJECTTYPE FROM NOTE WHERE (ID = 80020)
INTERNAL EXCEPTION STACK:
java.lang.NullPointerException
at oracle.sql.LobPlsqlUtil.plsql_length(LobPlsqlUtil.java:936)
at oracle.sql.LobPlsqlUtil.plsql_length(LobPlsqlUtil.java:102)
at oracle.jdbc.dbaccess.DBAccess.lobLength(DBAccess.java:709)
at oracle.sql.LobDBAccessImpl.length(LobDBAccessImpl.java:58)
at oracle.sql.BLOB.length(BLOB.java:71)
at TOPLink.Private.Helper.ConversionManager.convertObjectToByteArray(ConversionManager.java:309)
at TOPLink.Private.Helper.ConversionManager.convertObject(ConversionManager.java:166)
at TOPLink.Private.DatabaseAccess.DatabasePlatform.convertObject(DatabasePlatform.java:594)
at TOPLink.Public.Mappings.SerializedObjectMapping.getAttributeValue(SerializedObjectMapping.java:43)
at TOPLink.Public.Mappings.DirectToFieldMapping.valueFromRow(DirectToFieldMapping.java:490)
at TOPLink.Public.Mappings.DatabaseMapping.readFromRowIntoObject(DatabaseMapping.java:808)
at TOPLink.Private.Descriptors.ObjectBuilder.buildAttributesIntoObject(ObjectBuilder.java:173)
at TOPLink.Private.Descriptors.ObjectBuilder.buildObject(ObjectBuilder.java:325)
at TOPLink.Private.Descriptors.ObjectBuilder.buildObjectsInto(ObjectBuilder.java:373)
at TOPLink.Public.QueryFramework.ReadAllQuery.execute(ReadAllQuery.java:366)
at TOPLink.Public.QueryFramework.DatabaseQuery.execute(DatabaseQuery.java:406)
I have started the application with Oracle JDBC logging on and found that the problem may originate in a possible lack of syncronization in the pooled connection implementation:
DRVR FUNC OracleConnection.isClosed() returned false
DRVR OPER OracleConnection.close()
DRVR FUNC OracleConnection.prepareCall(sql)
DRVR DBG1 SQL: "begin ? := dbms_lob.getLength (?); end;"
DRVR FUNC DBError.throwSqlException(errNum=73, obj=null)
DRVR FUNC DBError.findMessage(errNum=73, obj=null)
DRVR FUNC DBError.throwSqlException(reason="Logical handle no longer valid",
SQLState=null, vendorCode=17073)
DRVR OPER OracleConnection.close()
so the prepareCall() is issued against an already closed connection and the
call fails.
I assume we have been using a JDBC 2.0 compliant driver. We tried out
drivers that Oracle supplies for 8.1.6, 8.1.7 versions. To be true I
couldn't find any information about the JDBC specification they conform to. Does it
mean that these drivers may not be 100%-compatible with JDBC 2.0 Spec?
How can I find out if they are 2.0 compliant?
Also I have downloaded Oracle 9.2.0.1 JDBC drivers. This seemed to work
fine until we found another incompatibility which made us return back to
8.1.7 driver:
UnitOfWork(7818028)--Connection(4434104)--INSERT INTO STATUSHISTORY (CHANGEDATE, FK_SET_STATUS_ID) VALUES ({ts '2002-10-17 16:46:54.529'}, 2)
INTERNAL EXCEPTION STACK:
java.sql.SQLException: ORA-00904: invalid column name
at oracle.jdbc.dbaccess.DBError.throwSqlException(DBError.java:134)
at oracle.jdbc.ttc7.TTIoer.processError(TTIoer.java:289)
at oracle.jdbc.ttc7.Oall7.receive(Oall7.java:573)
at oracle.jdbc.ttc7.TTC7Protocol.doOall7(TTC7Protocol.java:1891)
at oracle.jdbc.ttc7.TTC7Protocol.parseExecuteFetch(TTC7Protocol.java:1093
at oracle.jdbc.driver.OracleStatement.executeNonQuery(OracleStatement.jav
a:2047)
at oracle.jdbc.driver.OracleStatement.doExecuteOther(OracleStatement.java
:1940)
at oracle.jdbc.driver.OracleStatement.doExecuteWithTimeout(OracleStatemen
t.java:2709)
at oracle.jdbc.driver.OraclePreparedStatement.executeUpdate(OraclePrepare
dStatement.java:589)
at TOPLink.Private.DatabaseAccess.DatabaseAccessor.executeDirectNoSelect(
DatabaseAccessor.java:906)
at TOPLink.Private.DatabaseAccess.DatabaseAccessor.executeNoSelect(Databa
seAccessor.java:960)
at TOPLink.Private.DatabaseAccess.DatabaseAccessor.executeCall(DatabaseAc
cessor.java:819)
at TOPLink.Public.PublicInterface.UnitOfWork.executeCall(UnitOfWork.java:

Answer 1

Hello Yury,
I believe the problem is that TopLink's ServerSession by default executes read queries concurrently on the same connection. It does this to reduce the number of required connections for the read connection pool. Normally this is a good concurrency optimization, however some JDBC drivers have issues when this is done. I had thought that with Oracle JDBC 8.1.7 this issue no longer occurred, but perhaps it is only after version 9. I believe that the errors were only with the thin JDBC driver, not the OCI, so using the OCI driver should also resolve the problem. Using RAW instead of BLOB would also work.
You can configure TopLink to resolve this problem through using exclusive read connection pooling.
Example:
serverSession.useExclusiveReadConnectionPool(int minNumerOfConnections, int maxNumerOfConnections);
This will ensure that TopLink does not to try to concurrently execute read queries on the same connection.
I'm not exactly sure what your second problem with the 9.x JDBC drivers is. From the SQL and stack trace it would seem that the driver does not like the JDBC timestamp syntax. You can have TopLink print timestamp in Oracle's native SQL format to resolve this problem.
Example:
serverSessoin.getLogin().useNativeSQL();
Make sure you configure your server session before you login, or if using the TopLink Session Manager perform the customizations through a SessionEventListener-preLogin event.