conn.setAutoCommit(false);

// Get the Large Object Manager to perform operations with
LargeObjectManager lobj = ((org.postgresql.PGConnection)conn).getLargeObjectAPI();

PreparedStatement ps = con.prepareStatement("SELECT imgoid FROM imageslo WHERE imgname = ?");
ps.setString(1, "myimage.gif");
ResultSet rs = ps.executeQuery();
if (rs != null) {
    while (rs.next()) {
        // Open the large object for reading
        int oid = rs.getInt(1);
        LargeObject obj = lobj.open(oid, LargeObjectManager.READ);

        // Read the data
        byte buf[] = new byte[obj.size()];
        obj.read(buf, 0, obj.size());
        // Do something with the data read here

        // Close the object
        obj.close();
    }
    rs.close();
}
ps.close();

conn.commit();

http://www.postgresql.org/docs/7.4/static/jdbc-binary-data.html

Postgres: How to read oid value using JDBC API

Java