Question

So I’m trying to refactor some code which creates JDBC objects in a loop and didn’t close them out cleanly. My first thought is to create a LinkedList to store the prepared statements, result sets, etc., and then close them in a loop inside a finally block. So, the approach is like:

Connection conn = null;
LinkedList<PreparedStatement> statements = new LinkedList<PreparedStatement>();
LinkedList<ResultSet> results = new LinkedList<ResultSet>();

    try {
        conn = database.getConnection(); 

        for (String i : arr1) {
            for (String j : arr2) {

                Statement stmt = conn.createStatement();
                statements.add(stmt);

                ResultSet rs = stmt.executeQuery(...);
                results.add(rs);

                // ...work...

            }
        }

    }
    catch(SQLException ex) {ex.printStackTrace();}
    finally {
        // close all result sets..
        for (ResultSet rs : (ResultSet[])results.toArray()) {
            if (rs != null)   try { rs.close();   } catch (SQLException ex) {ex.printStackTrace();}
        }

        for (Statement stmt : (Statement[])statements.toArray()) {
            if (stmt != null) try { stmt.close(); } catch (SQLException ex) {ex.printStackTrace();}
        }

        if (conn != null) try { conn.close(); } catch (SQLException ex) {ex.printStackTrace();}
   }

Is this a reasonable approach? Will this end up causing some kind of leak or problem? Thanks in advance, and please let me know if this belongs rather on codereview.se or somewhere else.

Answer 1

This is IMHO a bad idea for at least three reasons:

1. Resources aren’t cleaned up immediately when they are no longer used. ResultSet is an expensive resource and I am not even sure whether you can have several opened result sets on one connection (update: you can, see comments).

2. In this approach you are opening multiple resources at once, which might lead to excessive and unnecessary usage of database resources and peaks. Especially dangerous if the number of iterations is high.

3. A special case of previous point is memory – if either Statement or ResultSet holds a lot of memory, holding an unnecessary reference to several such objects wil cause excessive memory usage.

That being said consider using already built and safe utility classes like JdbcTemplate. I know it comes from Spring framework, but you can use it outside of the container (just pass an instance of DataSource) and never worry about closing JDBC resources again.