Cannot update Clob to Oracle by using JDBC

[jammy chen]

I used the following codes to update clob to oracle by using jdbc

			conn = getConnection();
                        pStmt = conn.prepareStatement("update testtable set PLM_REFERENCE = 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa' where id = ?");
			pStmt.setLong(1, 1);
			pStmt.executeUpdate();
	
			//update
                        String data = "bbbbbbbbbbbbbbbbbb";

			pStmt = conn.prepareStatement("select PLM_REFERENCE from testtable where id = ? for update of PLM_REFERENCE");
			pStmt.setLong(1, 1);
			rset = pStmt.executeQuery();
			while (rset.next()) {
				clob = rset.getClob(1);
				Writer writer = clob .getCharacterOutputStream();
			        writer.write(data.toCharArray());
			        writer.flush();
			        writer.close();
			}

			pStmt = conn.prepareStatement("update testtable set PLM_REFERENCE = ? where id = ?");
			pStmt.setClob(1, clob);
			pStmt.setLong(2, 1);
			pStmt.executeUpdate();

after ran above codes, I checked "bbbbbbbbbbbbbbbbbbaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" was stored in the db, why? it should be "bbbbbbbbbbbbbbbbbb", right? As we can see the later string "bbbbbbbbbbbbbbbbbb" just appended. but I want it to be replaced.
I

[Rob Spoor]

Add this between flushing and closing the writer:

clob.truncate(data.length());