Deep Dive into JAR Packages: Building Executable JARs with Gradle and Accessing Folders and Files Inside JARs

Preface

Java's cross-platform capability sounds tempting and delicious, billed as “Write Once, Run Everywhere,” but in reality it's “Run Once, Debug Everywhere”... In actual development you still run into all kinds of pitfalls. Having just solved a series of issues, I specifically wrote this article to summarize them.

Building JAR Packages with Gradle

Thanks to the almighty Gradle, which greatly improves Java development productivity~ In Gradle you can generate a JAR using the official plugin: application to simply generate a JAR, and there are also many different configurations you can customize. For details, please refer to the official Gradle documentation.

Here I'm using a Gradle plugin called shadow, which has the JAR-building configuration all laid out clearly—very convenient! Official docs: https://imperceptiblethoughts.com/shadow/configuration/#configuring-output-name

Below is a build.gradle configuration reference:

plugins {
    id 'com.github.johnrengelman.shadow' version '4.0.3'

    // Apply the java plugin to add support for Java
    id 'java'

    // Apply the application plugin to add support for building an application
    id 'application'
}

dependencies {
    implementation 'com.github.jengelman.gradle.plugins:shadow:4.0.3'
}

// Output to build/libs/name.jar
shadowJar {
    baseName = 'name'
    classifier = null
    version = null
}

apply plugin: 'com.github.johnrengelman.shadow'
apply plugin: 'java'

The specific configuration should be set according to the project's actual needs~ After setting up the shadow plugin, run gradle build and you can generate your executable JAR under the build/libs/ folder—super convenient. For more features you can check the shadow official documentation; it's written very clearly.

Accessing Resources Inside a JAR

Although a JAR has various directory structures, a JAR is still essentially a file, so you cannot access it with traditional methods. Things like the File class and the Class object's getResouce method won't work. You should use the ClassLoader's getResourceStream method to directly obtain the input stream of the resource file. For example:

InputStream is=this.getClass().getResourceAsStream("/resource/res.txt");
InputStream is=this.getClass().getClassLoader().getResourceStream("/resource/res.txt");

Note: The getResourceStream methods of the Class object and the ClassLoader object are also different. For the specific differences, you can check this note: Correctly Obtaining Java Project Resources

Accessing Folders Inside a JAR

When there are many resource files in a JAR, you can't possibly enter names one by one to fetch them—that's way too hacky. You definitely want an automated approach to improve productivity. In fact, accessing folders inside a JAR is quite troublesome, but I still found a clever workaround, and after trying it, it works pretty well. (Though it's best to add some caching)

The code is as follows:

String path = getClass().getProtectionDomain().getCodeSource().getLocation().getPath();
JarFile localJarFile = new JarFile(new File(path));

Enumeration<JarEntry> entries = localJarFile.entries();
while (entries.hasMoreElements()) {
      JarEntry jarEntry = entries.nextElement();
      String innerPath = jarEntry.getName();
      System.out.println(innerPath);
}

Use getClass().getProtectionDomain().getCodeSource().getLocation().getPath(); to get the path of the current JAR. If the code is not running inside a JAR, what you get is the path of the current class file. So before using it, it's best to add a check to see whether the program is running inside a JAR.

About JavaFX Media Resource Issues

JavaFX can play music, but unlike other Image and Font resources, the Media object's constructor can only accept a String parameter (i.e., a file URL), so there's no way to use the getResourceStream method to pass the file input stream into the object.

I looked it up on the official site and found a solution: just change the file URL to a JarURL. Docs: https://docs.oracle.com/javase/6/docs/api/java/net/JarURLConnection.html.

Simple example:

String path = String.format("jar:file:%s!/%s", jarPath, relativePath);
Media media = new Media(path);

Note: The form of relativePath is like media/hello.wav.

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