将Java项目转为Maven后JavaFX应用无法启动，求解决

我将Java项目转为Maven项目后，JavaFX应用无法启动，转Maven前应用可正常运行，推测问题出在MainView类中。以下是相关代码、配置及报错信息：

MainView代码 #

package view;
import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;
import model.*;
import java.util.ArrayList;
import java.util.Objects;

public class MainView extends Application implements Interface{

    @Override
    public void start(Stage stage) {
        try {
            Parent root = FXMLLoader.load(Objects.requireNonNull(getClass().getResource("MainView.fxml")));
            Scene scene = new Scene(root, 1000, 600);
            stage.setScene(scene);
            stage.show();

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

    public static void main(String[] args) {
        launch(args);
    }
}

pom.xml配置 #

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>groupId</groupId>
    <artifactId>program</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.release>14</maven.compiler.release>
        <javafx.version>16</javafx.version>
        <javafx.maven.plugin.version>0.0.6</javafx.maven.plugin.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.openjfx</groupId>
            <artifactId>javafx-controls</artifactId>
            <version>${javafx.version}</version>
        </dependency>
        <dependency>
            <groupId>org.openjfx</groupId>
            <artifactId>javafx-fxml</artifactId>
            <version>${javafx.version}</version>
        </dependency>
    </dependencies>
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.8.1</version>
                <configuration>
                    <release>${maven.compiler.release}</release>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.openjfx</groupId>
                <artifactId>javafx-maven-plugin</artifactId>
                <version>${javafx.maven.plugin.version}</version>
                <configuration>
                    <mainClass>view.MainView</mainClass>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

报错堆栈 #

java.lang.NullPointerException
    at java.base/java.util.Objects.requireNonNull(Objects.java:208)
    at view.MainView.start(MainView.java:18)
    at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$9(LauncherImpl.java:847)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runAndWait$12(PlatformImpl.java:484)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:457)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:399)
    at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:456)
    at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:184)
    at java.base/java.lang.Thread.run(Thread.java:833)

解决方法 #

报错核心是getClass().getResource("MainView.fxml")返回null，触发了Objects.requireNonNull的空指针异常。原因是Maven默认只编译src/main/java下的.java文件，不会将该目录下的.fxml等非Java文件复制到编译后的target/classes目录，导致运行时找不到fxml文件。

方式一：调整文件结构 #

将MainView.fxml移动到src/main/resources/view目录下（和MainView类的包路径view对应），Maven会自动将该目录下的资源文件打包到classpath中。

方式二：修改pom.xml配置 #

在<build>节点中添加资源配置，让Maven同时复制src/main/java下的.fxml文件到classes目录：

<build>
    <resources>
        <resource>
            <directory>src/main/java</directory>
            <includes>
                <include>**/*.fxml</include>
            </includes>
        </resource>
        <resource>
            <directory>src/main/resources</directory>
        </resource>
    </resources>
    <!-- 保留原有的plugins配置 -->
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.8.1</version>
            <configuration>
                <release>${maven.compiler.release}</release>
            </configuration>
        </plugin>
        <plugin>
            <groupId>org.openjfx</groupId>
            <artifactId>javafx-maven-plugin</artifactId>
            <version>${javafx.maven.plugin.version}</version>
            <configuration>
                <mainClass>view.MainView</mainClass>
            </configuration>
        </plugin>
    </plugins>
</build>

额外优化 #

可以将getResource调用改为从类加载器根目录查找，稳定性更强：

Parent root = FXMLLoader.load(Objects.requireNonNull(MainView.class.getClassLoader().getResource("view/MainView.fxml")));

内容的提问来源于stack exchange，提问作者larppiiis