用java访问oracle大字段问题，有代码，请帮着看一下错误原因？

程序在执行到
      if (ors.next())
      {
        oracle.sql.BLOB blob=ors.getBLOB("filebody");
        //java.io.OutputStream outstream=blob.getBinaryOutputStream();
        //outstream.write(a);
        int j=blob.putBytes(1,a);
        System.out.println("j:"+j);
        conn.commit();
        ors.close();
      }中的int j=blob.putBytes(1,a);语句时，
发生如下错误：file length::30720read itotal::30720java.io.IOException: ORA-22920: 未锁定含有 LOB 值的行
ORA-06512: 在"SYS.DBMS_LOB", line 700
ORA-06512: 在line 1
at oracle.jdbc.dbaccess.DBError.SQLToIOException(DBError.java:531) at oracle.jdbc.driver.OracleBlobOutputStream.flushBuffer(OracleBlobOutputStream.java:179) at oracle.jdbc.driver.OracleBlobOutputStream.write(OracleBlobOutputStream.java:125)errror :java.io.IOException: ORA-22920: 未锁定含有 LOB 值的行
ORA-06512: 在"SYS.DBMS_LOB", line 700
ORA-06512: 在line 1
at java.io.OutputStream.write(OutputStream.java:61) at beelink.com.orablob.AccessBlob.main(AccessBlob.java:74)