Run Service in background Thread

问题

Can I run Service in background thread?

I want to load data from database and during loading data i want progress bar to be indicating progress. First I have created task, run it in background thread and then update progress bar according to this task. However, I found out that this Task is not reusable, so next time i hit the button, it didn't work. Later on, I fount out that Service can run Tasks multiple times, So i have encapsulated those tasks inside of Service.

Now it works great - every time i hit button, table is reloaded - except progress bar is not moving.

How can I reach desired result?

回答1:

Okay... Here is completed code: https://mega.nz/#!yUsjgJyZ!DHfuBqsujAHurS-pQ_W5y8BAflOtvxsm48goRPkDsxA

First, i want to tell you what is my goal:

When I run program, i want the progress bar to be in indeterminate state. After pressing the button, i want progress bar to reflect progress of Service. After finishing the Service, i want the progress bar to be completed (100%).

Here is what i have found. It looks like service automatically runs it's task on background thread. I have built a "sand box" program where i was playing with services and progress bar. The program comprised of progress bar, two text fields with two buttons above them. First button can start service_countTo100 and second button can run service_getActualCount.

Now in fxml file i set progress bar to indeterminate default state. After pressing the button1, a counting has started (displayed in text_field1) and progress bar has changed according to actual progress. After pressing button2, actual value of count is displayed in text_field2. And here comes some issues. To demonstrate the issues, i will show you source code of Service:

// Create the service
public static Service<Integer> serviceTo100 = new Service<Integer>() {
    @Override
    protected Task<Integer> createTask() {
        Task<Integer> taskTo100 = new Task<Integer>() {
            @Override protected Integer call() throws Exception {

                int iterations;
                updateProgress(-1, 100);
                for (iterations = 0; iterations < 100; iterations++) {
                    if (isCancelled()) {
                        updateMessage("Cancelled");
                        break;
                    }

                    updateMessage("Iteration " + iterations);
                    updateProgress(iterations, 100);

                    // Now block the thread for a short time, but be sure
                    // to check the interrupted exception for cancellation!
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException interrupted) {
                        if (isCancelled()) {
                            updateMessage("Cancelled");
                            break;
                        }
                    }
                }

            //udpateProgress(100, 100);    
            return iterations;
            }
        };
    return taskTo100;
    }
};

As you can see, there is a simple loop which counts from 0 to 100 and there is also a updateProgress(iterations, 100); statement which updates progress. This service is located in Services class. In FXMLDocumentController.java is method called runService(Service service) defined as this:

public void runService(Service service){

        service.start();
        progressBar.progressProperty().bind(service.progressProperty());

        service.setOnSucceeded((event) -> {
            service.reset();
            progressBar.progressProperty().unbind();
            progressBar.setProgress(1);                                
        });
}

The initialize looks like this:

@Override
public void initialize(URL url, ResourceBundle rb) {
    btn_start1.setOnAction((event) -> {

        Service service = Services.serviceTo100;
        runService(service);
        txt_field1.textProperty().bind(service.messageProperty());                     
        System.out.printf("something\n");  

    });
    .
    .
    .
    }//end of initialize

Now the most interesting thing (at least for me, novice) comes into play.

In service definition, you can notice commented line //updateProgress(100,100). That was my try to set progress bar to completed state when counting has stopped. But java ignored that line. I tried to put System.out.print("Something"); and it worked, but updateProgress(100, 100) didn't. When counting has finished, progress bar was set to it's default state defined in fxml file, which was indeterminate.

Conclusion: Service class creates task defined inside of it and after task is completed, the task doesn't exists anymore, thus, there is no progress to be bound with progress bar.

Even though task does not exists anymore, you cannot set progress of the bounded progress bar with setProgress(double double) method. You first need to unbound it.

Now I got what i needed:

You run the program, progress bar is in indeterminate state, waiting for hit. You hit the button, service starts counting up to 100 and progress bar is progressing according to progressProperty of service. When counting is done, Progress bar is set to indeterminate state again, waiting for another start.

来源：https://stackoverflow.com/questions/50374913/run-service-in-background-thread

标签

multithreading

javafx

service

task

background-thread