How to update empty string to oracle Clob

I know it works by using SQL

update activity set REFERENCE = EMPTY_CLOB() where id = ?

But I cannot do like this, I cannot hard coded 'EMPTY_CLOB()' in SQL. I used the way like the following:

String empty_string = "";
conn = getConnection();

pStmt = conn.prepareStatement("SELECT REFERENCE FROM activity WHERE ID = ? FOR UPDATE");
pStmt.setLong(1, 1);
rset = pStmt.executeQuery();
Clob clob = null;
while (rset.next()) {
    clob = rset.getClob(1);
    Writer writer = adapter.getCharacterOutputStream(clob);
    writer.write(empty_string);         
    writer.flush();
    writer.close();
}

pStmt = conn.prepareStatement("update activity set REFERENCE = ? WHERE ID = ?");
pStmt.setClob(1, clob);
pStmt.setLong(2, 1);
pStmt.executeUpdate();

But It didn't work. the clob didn't be updated to empty string, it still stored as previous value.

Any body can help me on this?

As I have already mentionued in your other question: in my experience getClob() and setClob() don't work properly.

Use setCharacterStream() instead:

StringReader clob = new StringReader("");
pStmt = conn.prepareStatement("update activity set REFERENCE = ? WHERE ID = ?");
pStmt.setCharacterStream(1, clob, 0);
pStmt.setLong(2, 1);
pStmt.executeUpdate();

That way you can also remove the unnecessary SELECT before updating, which will improve performance as well.

Another option would be to simply set that column to NULL

Edit:

With newer drivers (11.x) you might also want to try to use setString() and getString() on the CLOB column.

The locking of the row should only be necessary when you use a LOB locator that you intend to keep during a transaction that spans more than one statement (at least that's my understanding of the linked reference to the manual).