NULL指针异常在Javafx中的服务中数据绑定值时

Question

我正在尝试获得一个简单的数据结合示例，以便使用Javafx服务类更好地理解，并可能将其纳入我正在从事的项目中，但是我无法将shoudl从服务(整数(返回到呼叫服务的整数中的值。

我已经检查了调试器，并确认任务实际上是返回值，但是我一直在获取一个null指针例外，指出我对具有绑定的代码行"无法绑定到null"，但是，其他属性，其他属性我弯曲似乎很好

主类

public class Main extends Application {
    private Service<ObservableValue<Integer>> service;
           @Override
           public void start(Stage primaryStage){

            /**********************************
             * Start of Multi-Threading Logic
             **********************************/
            service = new EmployeeService();
            ObjectProperty<Integer> size = new SimpleObjectProperty<>(0);
            size.bind(service.getValue()); // Null Pointer Exception Cause
            progressBar.progressProperty().bind(service.progressProperty());
            progressBar.visibleProperty().bind(service.runningProperty());
            progressLabel.textProperty().bind(service.messageProperty());
            progressLabel.setVisible(false);

            /*************************************************************
             * Start Task Button Event Handler
             * 
             * If the service state is succeeded or cancelled
             * then the service must first be reset before starting
             * 
             * if the service state is ready then start the service
             * 
             * if the service state is something other than the above 
             * like running or failed, then do nothing
             *************************************************************/
            startTaskButton.setOnAction(new EventHandler<ActionEvent>() {
                @Override
                public void handle(ActionEvent event) {
                    if(service.getState() == Service.State.SUCCEEDED
                            || service.getState() == Service.State.CANCELLED) {
                        service.reset();
                        service.start();
                    }else if(service.getState() == Service.State.READY) {
                        service.start();
                    }else {
                        System.out.println("Not doing anything");
                    }
                }   
            });

            /*************************************************************
             * Cancel Task Button Event Handler
             * 
             * The service should only be canceled if it is currently
             * running, else do nothing. 
             *************************************************************/
            cancelButton.setOnAction(new EventHandler<ActionEvent>() {
                @Override
                public void handle(ActionEvent event) {
                    if(service.getState() == Service.State.RUNNING) {
                        service.cancel();
                    }
                }
            });

服务Impl Class

/*************************************************************
 * Custom Service Impl class that creates the DisplayDataTask
 *************************************************************/
class EmployeeService extends Service<ObservableValue<Integer>>{
    @Override
    protected Task<ObservableValue<Integer>> createTask() {
        return new DisplayDataTask();
    }
}

任务Impl class

/*************************************************************
 * Custom Task Impl class that creates a basic list
 * and returns the length of the list in the call method
 *************************************************************/
class DisplayDataTask extends Task<ObservableValue<Integer>> {
    @Override
    protected ObservableValue<Integer> call() throws Exception {
        int randomNum = 71890;
        Thread.sleep(5000); // Sleep for 5 seconds to simulate a task 
        ObjectProperty<Integer> size = new SimpleIntegerProperty(randomNum).asObject();
        return size;
    }
    @Override
    protected void cancelled() {
        super.cancelled();
        updateMessage("Operation Cancelled");
        System.out.println("The " + this.getClass().getSimpleName() + " has been canceled.");
    }   
}   

Answer 1

Service.getValue()返回服务的当前值，在服务完成之前，它将返回null。

您几乎肯定不希望该服务在此处返回ObservableValue<Integer>，您希望它返回实际的Integer。因此，将服务重写为：

class EmployeeService extends Service<Integer>{
    @Override
    protected Task<Integer> createTask() {
        return new DisplayDataTask();
    }
}

class DisplayDataTask extends Task<Integer> {
    @Override
    protected Integer call() throws Exception {
        int randomNum = 71890;
        Thread.sleep(5000); // Sleep for 5 seconds to simulate a task 
        Integer size = randomNum ;
        return size;
    }
    @Override
    protected void cancelled() {
        super.cancelled();
        updateMessage("Operation Cancelled");
        System.out.println("The " + this.getClass().getSimpleName() + " has been canceled.");
    }   
} 

然后您只能做

size.bind(service.valueProperty());