How to get a resources path after jlink-ing?

Question

I am trying to make my JavaFx-Application executable using Maven and Visual Studio Code.

After some time spent on this topic, I found some posts mentioning jlink.

I am a newcomer when it comes to packaging Java/JavaFX applications, so I gave it a try.

Currently, I can at least execute the launcher for the package.

But immediately after starting the application, a NullPointerException is thrown: Cannot invoke "Object.toString()" because the return value of "java.lang.Class.getResourceAsStream(String)" is null.

For styling the components of my view I created some.css-files and put them inside a /style directory. This directory I placed this, according to the sample JavaFx application, inside a /resources directory created by Maven. In a similar manner, I proceeded with my sound and image files.

Here you can see an excerpt of my directory structure.

|
|--src/main
|  |
|  |-- java
|  |   | ...
|  |
|  |-- resources
|      |
|      |-- img
|      |   | ...    
|      |
|      |-- style
|      |   | ...
|      |
|      |-- sound
|          | ...
|
|-- target
    |
    |-- classes
    |   | ...
    |   |
    |   |-- img
    |   |   | ...
    |   |
    |   |-- style
    |   |   | ...
    |   |
    |   |-- sound
    |   |   | ...
    |
    |-- ...
    |
    |-- app
        |
        |-- bin
        |-- ...

Now I am trying to access my resources from within my application.

This was my first approach. It works just fine when running from VSCode.

    public static final String PATH_TO_STYLESHEET = App.class.getResource("/style").toString();
    public static final String PATH_TO_IMG = App.class.getResource("/img").toString();
    public static final String PATH_TO_SOUNDS = App.class.getResource("/sounds").toString();

But after running jlink, my application crashes, showing the NullPointerException mentioned earlier.

Here is my pom.xml:

<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/maven-v4_0_0.xsd">
    
    <modelVersion>4.0.0</modelVersion>
    <groupId>org.openjfx</groupId>
    <artifactId>App</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    
    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.release>19</maven.compiler.release>
        <javafx.version>19</javafx.version>
        <javafx.maven.plugin.version>0.0.8</javafx.maven.plugin.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.openjfx</groupId>
            <artifactId>javafx-controls</artifactId>
            <version>${javafx.version}</version>
        </dependency>
        <dependency>
            <groupId>org.openjfx</groupId>
            <artifactId>javafx-media</artifactId>
            <version>${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>
                    <release>${maven.compiler.release}</release>
                    <jlinkImageName>App</jlinkImageName>
                    <launcher>launcher</launcher>
                    <mainClass>com.test.App</mainClass>
                </configuration>
            </plugin>
        </plugins>
        <resources>
            <resource>
                <directory>src/main/resources</directory>
            </resource>
        </resources>
    </build>
    
</project>

And this is the command I have been using for creating the package.

mvn javafx:jlink -f pom.xml

Does anyone have an idea how I can get the path to my stylesheets, images, and sounds, after running jlink? The path is absolutely sufficient. I do not need a file itself.

Is there an option to copy the resources to a specific location?

Answer 1

From my testing, with my own application but your POM file, the problem seems to be with what "resource" you're trying to find. For example, you have:

 public static final String PATH_TO_IMG = App.class.getResource("/img").toString();

But /img is not a resource, it's a "package". Whether or not calling getResource with a path to a package is supposed to work, I'm not sure. It may be undefined behavior. Regardless, once packaged into a run-time image, the code will apparently start to return null in such cases. Yet your resources are being packaged correctly.

I suspect what you're trying to do is use these static final fields to do things like:

Image image = new Image(PATH_TO_IMG + "foo.png");

Possibly part of the goal is to avoid a bunch of SomeClass.class.getResource(...) calls everywhere. If I'm right, then you can still do something similar. Have your constants be the root of the resource name :

public static final String IMG_ROOT = "/img";

And then have a utility method:

public static String getImagePath(String imageName) {
    String resource = IMG_ROOT + "/" + imageName;
    URL url = App.class.getResource(resource);
    if (url == null) {
        throw new RuntimeException("could not find resource: " + resource);
    }
    return url.toString();
}

Which you can then use like so:

Image image = new Image(getImagePath("foo.png"));