Cast java.sql.Connection to PGConnection

I want to upload a file in PostgreSQL using Tomcat:

@Resource(name = "jdbc/DefaultDB")
private DataSource ds;
Connection conn = ds.getConnection();

I tried to cast the conn object this way:

PGConnection pgCon = ((org.apache.commons.dbcp.DelegatingConnection)
conn).getInnermostDelegate();

I get

Caused by: java.lang.ClassCastException:
org.apache.tomcat.dbcp.dbcp2.PoolingDataSource$PoolGuardConnectionWrapper
cannot be cast to org.postgresql.PGConnection

I also tried:

Connection unwrap = conn.unwrap(Connection.class);
connSec = (org.postgresql.PGConnection) unwrap;

I get

java.sql.SQLException: Cannot unwrap to org.postgresql.PGConnection

LargeObjectManager pgCon = pgCon.getLargeObjectAPI();

What is the proper way to implement the code?

P.S

I tried this:

PGConnection pgConnection = null;

if (conn.isWrapperFor(PGConnection.class))
{
pgConnection = conn.unwrap(PGConnection.class);
}

LargeObjectManager lobj = pgConnection.getLargeObjectAPI();

But I get NPE at this line LargeObjectManager lobj =
pgConnection.getLargeObjectAPI();

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