即使单击按钮多次，Javafx 也仅启动一个线程

Question

我有一个按钮，它从字段中获取我的用户名和密码，并向后端发送请求以进行身份​​验证。我正在使用线程，因此我的按钮动画和场景不会像这样卡住:

Service<Void> service = new Service<Void>() {
            @Override
            protected Task<Void> createTask() {
                return new Task<Void>() {
                    @Override
                    protected Void call() throws Exception {
                        //do authentication
                        if(responseStatus != 200){
                            authenticated = false 
                        }else{
                            authenticate = true
                        }
                        Platform.runLater(() -> {
                        try{
                            if(authenticated) {
                                changeScene();
                            }
                        }finally{
                            latch.countDown();
                        }
                    });
                    latch.await();
                    return null;
                }
            };
        }
    };
    service.start();

private void changeScene(){
    try {
        Stage window = (Stage)loginPane.getScene().getWindow();
        LoggedFirstStyle.displayLoggedScene();
        window.close();

    } catch (IOException e) {
        e.printStackTrace();
    }
}

但问题是，如果我多次单击按钮，平台运行稍后会执行多次，changeScene 也是如此，并且会打开多个场景。我这样做的方式好吗？如果可以的话，我怎样才能防止在同一个方法中打开多个线程？

Answer 1

一个Service提供“重用”Task 的能力。我将“重用”放在引号中，因为真正发生的是 Service 每次启动时都会创建一个新的 Task。有关 Service 和 Task 之间的差异和用法的更多信息，您可以:

• 阅读 Service 的文档和 Task -也许Worker也是如此。
• 阅读 Concurrency in JavaFX .
• 请参阅此 Stack Overflow 问题:Task vs Service for database operations .

由于服务旨在重复使用，因此您应该只创建一个实例。它还维护状态，因此只能“一次”执行一次。换句话说，同一个 Service 不能并行执行多次。当Service完成时，它将处于SUCCEEDED、CANCELLED或FAILED状态；要再次启动Service，您必须调用restart()(将取消正在运行的Service)或调用reset() 再次调用 start() 之前。

当 Service 运行时，您需要禁用某些 UI 组件，以便用户无法尝试多次启动它。您可以通过监听器和/或绑定(bind)来完成此操作。如果需要，您还可以进行检查，以便您的代码不会尝试启动 Service(如果它已在运行)。是否需要这些检查取决于哪些代码可以启动Service以及如何执行它。

这是一个小例子。它使用 FXML 创建界面，但重要的部分是 LoginController 和 LoginService 类。根据您的应用程序，您可能还想添加一种取消登录的方法。

Main.java

package com.example;

import java.io.IOException;
import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;

public class Main extends Application {

    @Override
    public void start(Stage primaryStage) throws IOException {
        // Login.fxml is in the same package as this class
        Parent root = FXMLLoader.load(getClass().getResource("Login.fxml"));
        primaryStage.setScene(new Scene(root));
        primaryStage.setTitle("Service Example");
        primaryStage.show();
    }

}

LoginService.java

package com.example;

import javafx.beans.property.SimpleStringProperty;
import javafx.beans.property.StringProperty;
import javafx.concurrent.Service;
import javafx.concurrent.Task;

public class LoginService extends Service<Boolean> {

    private final StringProperty username = new SimpleStringProperty(this, "username");
    public final void setUsername(String username) { this.username.set(username); }
    public final String getUsername() { return username.get(); }
    public final StringProperty usernameProperty() { return username; }

    private final StringProperty password = new SimpleStringProperty(this, "password");
    public final void setPassword(String password) { this.password.set(password); }
    public final String getPassword() { return password.get(); }
    public final StringProperty passwordProperty() { return password; }

    @Override
    protected Task<Boolean> createTask() {
        return new LoginTask(getUsername(), getPassword());
    }

    private static class LoginTask extends Task<Boolean> {

        private final String username;
        private final String password;

        public LoginTask(String username, String password) {
            this.username = username;
            this.password = password;
        }

        @Override
        protected Boolean call() throws Exception {
            Thread.sleep(3_000L); // simulate long running work...
            return !isCancelled() && "root".equals(username) && "root".equals(password);
        }

    }

}

LoginController.java

package com.example;

import javafx.beans.binding.Bindings;
import javafx.beans.binding.BooleanBinding;
import javafx.concurrent.Worker;
import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.scene.Cursor;
import javafx.scene.control.Alert;
import javafx.scene.control.Button;
import javafx.scene.control.PasswordField;
import javafx.scene.control.TextField;
import javafx.scene.layout.GridPane;

public class LoginController {

    @FXML private GridPane root;
    @FXML private TextField userField;
    @FXML private PasswordField passField;
    @FXML private Button loginBtn;

    private LoginService service;

    @FXML
    private void initialize() {
        service = new LoginService();
        service.usernameProperty().bind(userField.textProperty());
        service.passwordProperty().bind(passField.textProperty());

        // Don't let user interact with UI while trying to login
        BooleanBinding notReadyBinding = service.stateProperty().isNotEqualTo(Worker.State.READY);
        userField.disableProperty().bind(notReadyBinding);
        passField.disableProperty().bind(notReadyBinding);
        loginBtn.disableProperty().bind(notReadyBinding);

        root.cursorProperty().bind(
                Bindings.when(service.runningProperty())
                        .then(Cursor.WAIT)
                        .otherwise(Cursor.DEFAULT)
        );

        service.setOnSucceeded(event -> serviceSucceeded());
        service.setOnFailed(event -> serviceFailed());
    }

    private void serviceSucceeded() {
        if (service.getValue()) {
            /*
             * Normally you'd change the UI here to show whatever the user needed to
             * sign in to see. However, to allow experimentation with this example
             * project we simply show an Alert and call reset() on the LoginService.
             */
            showAlert(Alert.AlertType.INFORMATION, "Login Successful", "You've successfully logged in.");
            service.reset();
        } else {
            showAlert(Alert.AlertType.ERROR, "Login Failed", "Your username or password is incorrect.");
            service.reset();
        }
    }

    private void serviceFailed() {
        showAlert(Alert.AlertType.ERROR, "Login Failed", "Something when wrong while trying to log in.");
        service.getException().printStackTrace();
        service.reset();
    }

    private void showAlert(Alert.AlertType type, String header, String content) {
        Alert alert = new Alert(type);
        alert.initOwner(root.getScene().getWindow());
        alert.setHeaderText(header);
        alert.setContentText(content);
        alert.showAndWait();
    }

    @FXML
    private void handleLogin(ActionEvent event) {
        event.consume();

        // isBlank() is a String method added in Java 11
        boolean blankUsername = userField.textProperty().getValueSafe().isBlank();
        boolean blankPassword = passField.textProperty().getValueSafe().isBlank();

        if (blankUsername || blankPassword) {
            showAlert(Alert.AlertType.ERROR, null, "Both username and password must be specified.");
        } else {
            service.start();
        }
    }

}

登录.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.geometry.Insets?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.control.PasswordField?>
<?import javafx.scene.control.TextField?>
<?import javafx.scene.layout.ColumnConstraints?>
<?import javafx.scene.layout.GridPane?>
<?import javafx.scene.layout.RowConstraints?>
<?import javafx.scene.text.Font?>

<GridPane fx:id="root" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" vgap="20.0"
          xmlns:fx="http://javafx.com/fxml/1" xmlns="http://javafx.com/javafx/10.0.1"
          fx:controller="com.example.LoginController">

    <columnConstraints>
        <ColumnConstraints hgrow="SOMETIMES" minWidth="-Infinity" percentWidth="50.0"/>
        <ColumnConstraints halignment="RIGHT" hgrow="SOMETIMES" minWidth="-Infinity" percentWidth="50.0"/>
    </columnConstraints>

    <rowConstraints>
        <RowConstraints minHeight="-Infinity" percentHeight="25.0" vgrow="SOMETIMES"/>
        <RowConstraints minHeight="-Infinity" percentHeight="25.0" vgrow="SOMETIMES"/>
        <RowConstraints minHeight="-Infinity" percentHeight="25.0" vgrow="SOMETIMES"/>
        <RowConstraints minHeight="-Infinity" percentHeight="25.0" vgrow="SOMETIMES"/>
    </rowConstraints>

    <children>
        <Button fx:id="loginBtn" defaultButton="true" mnemonicParsing="false" onAction="#handleLogin" text="Login"
                GridPane.columnIndex="1" GridPane.rowIndex="3"/>
        <Label minWidth="-Infinity" text="Welcome">
            <font>
                <Font name="Segoe UI" size="32.0"/>
            </font>
        </Label>
        <TextField fx:id="userField" prefColumnCount="20" promptText="Username" GridPane.columnSpan="2"
                   GridPane.rowIndex="1"/>
        <PasswordField fx:id="passField" prefColumnCount="20" promptText="Password" GridPane.columnSpan="2"
                       GridPane.rowIndex="2"/>
    </children>

    <padding>
        <Insets bottom="50.0" left="50.0" right="50.0" top="50.0"/>
    </padding>

</GridPane>