Question

I have a panel with a JTabbedpane and in every tab you can set parameters to execute a query. When one query is busy retrieving his data from the database, you can already open a new tab to set the new parameters. To avoid overload on the database only one query may be executed at once. But when you click execute the program must remember which queries to execute in the right order. During the execution a loader icon is shown and the GUI may not be frozen, because there is a stop button you can click to stop the execution.

I used a swingworker to avoid the GUI from blocking while executing the query and that works fine. But now I want to prevent the next query to start before the previous has finished. In a model, common for the whole panel, I initialized a semaphore: private final Semaphore semaphore = new Semaphore(1, true);

This is the code which starts the swingworker (I’ve added println commands to see which is started, stopped or finished)

private void doStoredQuery() {
        try {
            semaphore.acquire();
            System.out.println(queryName + "started");
            worker.execute();
        } catch (InterruptedException e) {
           throw new RuntimeException(e);
        }
    }

And this is my swingworker (initializeWorker() is called from the constructor of the main class):

private SwingWorker<StoredQueryDataModel, Integer> initializeWorker() {
    worker = new SwingWorker<StoredQueryDataModel, Integer>() {
        @Override
        protected StoredQueryDataModel doInBackground() throws Exception {
            try {
                StoredQueryDataModel dataModel = null;
                publish(0);
                try {
                    dataModel = new StoredQueryDataModel(queryRunner, ldbName, queryName, params);
                } catch (S9SQLException e) {
                    // 
                } catch (Throwable e) {
                    showErrorMessage(e);
                }
                return dataModel;
            }
            finally {
                semaphore.release();
                System.out.println(queryName + "finished");
            }
        }

        @Override
        protected void process(List<Integer> chunks) {
            //ignore chunks, just reload loader icon
            panel.repaint();
        }

        @Override
        protected void done() {
            String error;
            try {
                result = get();
                error = null;
            } catch (Exception e) {
                error = e.getMessage();
            }

            if(result == null) {
                semaphore.release();
                System.out.println(queryName + " stopped");
            }

            if(error == null) {
                // process result
            }
            else {
                showErrorMessage(new Throwable(error));
            }
        }

    };
    return worker;
}

I’ve tried putting the acquire and release on other positions in the code, but nothing seems to work. I am bot in Swingworker and sempahores quite new… Can someone help?

Answer 1

I have found the problem: the semaphore had to be a static variable. In my code there were as many semaphores as there are tabs, which caused them to run at the same time instead of sequentially.