Execute SQL Queries in multiple threads (HSQLDB)

Question

Let's say we've got an SQL database (hsqldb) and want to run a number of queries on it which do not modify the content. This takes a long time for some queries and I would like to run the queries in multiple threads. So my question is: what is the best way to implement this?

I did not find any good samples to do this so I came up with the following (which I would love to get some comments on).

First, very briefly in words: I use thread-safe collections to access the queries and to put the results in. The queries are executed in a number of worker threads. The results are processed in the main thread which checks for new results until all threads are finished.

Now the code:

Create thread-safe collections of queries and results:

ConcurrentLinkedQueue<String> queries = new ConcurrentLinkedQueue<String>()
ConcurrentLinkedQueue<ResultSet> sqlResults = new ConcurrentLinkedQueue<ResultSet>();

Create a number of threads and start them (edited):

ExecutorService executorService = Executors.newFixedThreadPool(4);
for(int i=0; i<4; i++){
    executorService.execute(new QueryThread(sqlResults, queries));
}

Within the thread class QueryThread a connection is opened and queries are executed as long as there are any left:

private class QueryThread implements Runnable {
    private ConcurrentLinkedQueue<ResultSet> sqlResults;
    private ConcurrentLinkedQueue<String> queries;

    public QueryThread(ConcurrentLinkedQueue<ResultSet> sqlResults, ConcurrentLinkedQueue<String> queries){
        this.sqlResults = sqlResults;
        this.queries = queries;
    }

    @Override
    public void run(){
        Connection connThr = null;
        try{
            try {
                connThr = DriverManager.getConnection(dbModeSave, "sa", "");
                connThr.setAutoCommit(false);
            } catch (SQLException e1) {
                // TODO Auto-generated catch block
                e1.printStackTrace();
            }

            String currentQuery;
            do {
                currentQuery = queries.poll(); // get and remove element from remaining queries
                if (currentQuery != null) { // only continue if element was found
                    try {
                        Statement stmnt = connThr.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_UPDATABLE);
                        try {
                            ResultSet resultSet = stmnt.executeQuery(currentQuery);
                            sqlResults.add(resultSet);
                        } catch (SQLException e) {
                            // (Do something specific)
                        } finally {
                            stmnt.close();
                        }
                    } catch (SQLException e) {
                        // (Do something specific)
                    }
                }
            } while (currentQuery != null);

        } finally {
            if (connThr != null) {
                try {
                    connThr.close();
                } catch (SQLException e) {
                    // Nothing we can do?
                }
            }
        }
    }
}

From the original thread I check, if the threads are all finished and therefore all queries were processed (edited).

while (!executorService.isTerminated()) {
    try {
        Thread.sleep(100);
    } catch (InterruptedException e) {
        Thread.currentThread().interrupt();
    }
    while (!sqlResults.isEmpty()) {
        ResultSet result = sqlResults.poll();
        //process result and close it in the end
    }
}

Answer 1

Java standard sulution for parallel processing is ThreadPoolExecutor. Try it.