a simple example of XA (not working)

Hi all,
I'm trying to build a simple "manual" example of the use of the XA extension,
the following is the code:

PGXADataSource dataSource = new PGXADataSource();
dataSource.setDatabaseName("hrpm");
dataSource.setUser("luca");
dataSource.setServerName("localhost");

XAConnection connection = dataSource.getXAConnection();
System.out.println("Connesso!");
XAResource resource = connection.getXAResource();
MyXid identifier = new MyXid();
identifier.setBranchQualifier(new byte[] {0x01, 0x02, 0x03, 0x04, 0x05});
identifier.setGlobalTransactionId(new byte[] {0x05, 0x04, 0x03, 0x02, 0x01});
identifier.setFormatId(100);

try{
resource.start(identifier, XAResource.TMNOFLAGS);


Connection jdbcConnection = connection.getConnection();
Statement statement = jdbcConnection.createStatement();
String sql = "INSERT INTO ...";
int inserted = statement.executeUpdate(sql);

resource.end(identifier, XAResource.TMSUCCESS);

int commit = resource.prepare(identifier);
if( commit == XAResource.XA_OK )
resource.commit(identifier, false);
else
resource.rollback(identifier);

now the exception that is raised when the program commits the transaction is
the following:

javax.transaction.xa.XAException: org.postgresql.util.PSQLException: ERROR:
prepared transaction with identifier "100_BQQDAgE=_AQIDBAU=" does not exist
at org.postgresql.xa.PGXAConnection.commitPrepared(PGXAConnection.java:412)
at org.postgresql.xa.PGXAConnection.commit(PGXAConnection.java:339)

and the log of the database says:

2007-06-16 16:05:48 CEST LOG: execute <unnamed>: INSERT INTO ....
2007-06-16 16:05:48 CEST LOG: execute <unnamed>: PREPARE
TRANSACTION '100_BQQDAgE=_AQIDBAU='
2007-06-16 16:05:48 CEST WARNING: non c'è nessuna transazione in corso
2007-06-16 16:05:48 CEST LOG: execute <unnamed>: COMMIT
PREPARED '100_BQQDAgE=_AQIDBAU='
2007-06-16 16:05:48 CEST ERROR: prepared transaction with
identifier "100_BQQDAgE=_AQIDBAU=" does not exist
2007-06-16 16:05:48 CEST STATEMENT: COMMIT PREPARED '100_BQQDAgE=_AQIDBAU='
2007-06-16 16:05:49 CEST LOG: unexpected EOF on client connection

that is an error since it seems no one transaction has been started. However,
debugging the application, I've seen that the driver disables the autocommit
mode, thus where is the problem here?
I'm sorry, I'm not a XA expert, just learning.

Thanks,
Luca