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From: Cecil Westerhof <Cecil@decebal.nl>
Newsgroups: comp.lang.java.programmer
Subject: Re: Is this the way to work with try with resources
Date: Wed, 09 Aug 2017 19:37:57 +0200
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Andreas Leitgeb <avl@logic.at> writes:

> Cecil Westerhof <Cecil@decebal.nl> wrote:
>> Andreas Leitgeb <avl@logic.at> writes:
>>> Cecil Westerhof <Cecil@decebal.nl> wrote:
>>>>     private static void statistics() {
>>>>         try (ResultSet rsTotal  = executeQuery(COUNT_TOTAL_PROV);
>>>>              ResultSet rsUnused = executeQuery(COUNT_UNUSED_PROV);
>>>>              ResultSet rsUsed   = executeQuery(COUNT_USED_PROV)) {
>>> and
>>>>     private static ResultSet executeQuery(String query) throws SQLException {
>>>>         return conn.createStatement().executeQuery(query);
>>>>     }
>>> So, if one of the queries fails, what will happen to the statements 
>>> created so far?
>>> Or, rather, what will *not* happen to them?
>> If I interpreted try with resources correctly they are destroyed and
>> nothing is done with them.
>
> This claim is self-contradictory. They cannot be both, "destroyed" and
> "nothing done with them" at the same time.

Sadly communication is not my strong suit. I mend: they are not used
to retrieve values and are destroyed. The latter is not true I
understand from your explanation.


> Anyway, I think, we're conflating two aspects here:
>   1.) your app is a simple client tool, which wouldn't really need any
>      error-handling with explicit cleanup at all. It could just System.exit()
>      for any exception, and all cleanup will be cared for.
>   2.) The discussion on how to do try-with-ressources *correctly*.

At the moment 1 is the case, but I also want to do 2: as the twig is
bent the tree is inclined.


> I'd prefer to stick with "2.)" and ignore the fact that your specific
> task doesn't really demand correctness. Ok? (Otherwise there's no point
> in participating in the discussion any longer.)

OK. :-D


> Now, in that snippet above, it would (3 times) create a statement, then execute 
> a query to get a resultset.  If one of the queries went wrong, then the
> try would auto-close any previously obtained resultsets, but it would not
> do anything on the statements - in particular: not clean them up.
>
> So, if instead it were a long running server app, it would leak up to
> three statements each time your proc is run, until it eventually 
> closes and reopens the connection.

Luckily I created a class SQLite3 and changed all my:
    ResultSet rsTotal  = executeQuery(COUNT_TOTAL_PROV)
to:
    ResultSet rsTotal  = sqlite3Conn.executeQuery(COUNT_TOTAL_PROV)

Where sqlite3Conn is an instance of SQLite3.

I had there the following function:
    public ResultSet executeQuery(String query) throws SQLException {
        return conn.createStatement().executeQuery(query);
    }
but I now changed that to:
    public ResultSet executeQuery(String query) throws SQLException {
        try (Statement stmt = conn.createStatement()) {
            return stmt.executeQuery(query);
        }
    }

This should take care of the leakage.


By the way is there a way to ‘find’ all open Statements? That could be
used to verify if a long running application is leaking resources.


>>>>             format = String.format("There are %%,%dd %%-6s proverbs.%%n",
>>>>                                    String.format("%,d", count).length());
>>>>             System.out.printf(format, count, "total");
>>> What's the point of determining the length of e.g. "1,234,567" just to
>>> set it as the length for a "%,d", which would obviously produce exactly
>>> that length, anyway?
>> Because I use it for the second and third.
>
> Ah, first row is always the widest value? Ok, that's a
> justification.

Yep, total = used + notUsed. So both cannot be longer as total.


> But even then I'd probably have used:
>  int maxLen = String.format("%,d", count).length(); // first count is largest
>  format = "There are %," + maxLen + "d %-6s proverbs.%n";
> for slightly better readability.

You are right: I implemented it.


Thanks for the clarification and the readability.

-- 
Cecil Westerhof
Senior Software Engineer
LinkedIn: http://www.linkedin.com/in/cecilwesterhof