MERGE 

subquery 

NOT MATCHED statement

        MERGE INTO CAR_STOCK st
        USING CAR_PRODUCTO pro
        ON (pro.id = st.producto_id AND pro.ean = ?)
        WHEN MATCHED THEN
        UPDATE SET st.stockActual = ?
        WHEN NOT MATCHED THEN
        INSERT (stockActual, local_id, producto_id, activo)
        VALUES (?, ?, (SELECT id FROM car_producto WHERE ean = ?), 'S');

JDBC 

PreparedStatement

You can check this with a simple test:

- Create a function in that outputs some text
- Call that in your subquery
- Run a merge that doesn't insert anything
- See if the text from the function is in your output

Let's see what happens when we do this.

Setup

create or replace function f
  return int as 
begin
  dbms_output.put_line('executed');
  return 1;
end;
/
create table t (
  x int,
 y int
);

Test

SQL> merge into t
  2  using  (select 1 n from dual) d
  3  on   (t.x = d.n)
  4  when matched then update set t.y = 2
  5  when not matched then insert values (n, (select f from dual));
executed

1 row merged.

SQL>
SQL> merge into t
  2  using  (select 1 n from dual) d
  3  on   (t.x = d.n)
  4  when matched then update set t.y = 2
  5  when not matched then insert values (n, (select f from dual));

1 row merged.

So the answer is no.