Reading a resource file from within jar

java

resource-management

best-practices

java-8

byAnton Shumikhin·Mar 4, 2025

In reading a resource file located within a JAR, use getClass().getResourceAsStream("/file.txt") to get it as an InputStream:

InputStream inputStream = getClass().getResourceAsStream("/file.txt");
Scanner scanner = new Scanner(inputStream, "UTF-8");
try {
    while (scanner.hasNextLine()) {
        String line = scanner.nextLine();
        // The scroll has been unrolled, let's decipher it line by line...
    }
} finally {
    scanner.close(); // Don't forget, a good archeologist always seals the tomb when they're done!
}

Avoid '/' if called directly from ClassLoader, but include '/' for paths within your package structure when called from getClass().

Pitfall patrol: Common errors

Here's a run-down of common gotchas when working with resources inside a JAR:

Avoid treating JAR resource as a File. Files can't navigate through the file labyrinth within the JAR using File API.
Within a JAR, a resource URI (jar:file:/example.jar!/file.txt) is not hierarchical - it can't be translated into a common file path.
Ensure resource auto-closure using try-with-resources for InputStream and BufferedReader.

Best practices: Resource handling

Master these patterns to become a Jedi of resource handling:

Stone-carve your resource files into the resources/ directory of your Java project to ensure they get packaged into the JAR without black magic.
Summon root-level and third-party library resource files with the ancient mantra: Thread.currentThread().getContextClassLoader().getResourceAsStream("file.txt").
In a Spring Boot application, conjure resources using PathMatchingResourcePatternResolver and ResourcePatternResolver.
If circumstances demand, transcribe resource contents into a temporary parchment (file), taking care to manage its lifecycle diligently.

Trails in development vs production

In the wild lands of IDE (like Eclipse) versus the production environment, resource paths can vary. Tips for a smooth migration:

In the oasis of your IDE, use direct paths for a simpler journey.
To ensure resource loading succeeds in both IDE and when packaged as a JAR, always travel on the classpath-relative paths.

The utility tool: Efficient resource loading

Consider the wisdom of encapsulating resource loading logic in a static utility method to avoid code repetition and errors:

public static InputStream loadResourceAsStream(String path) {
    return MyClass.class.getResourceAsStream(path);
    // Ta-da! You have the secret scroll now!
}

Visualization

Think about your JAR file as a vault, inside which resource files are stored:

🏺 - Your JAR file
📜 - The resource file inside

To access the stored resource:

getClass().getResourceAsStream("/path/to/scrollInsideJar.txt");

The secret revealed:

Before: 🏺🔒 (Vault sealed)
After:  🏺📜✨ (Resource file retrieved)

Using getClass().getResourceAsStream, the seal is unbroken and the contents are safe and sound!

Further practices: Safety and efficiency

Here are some more best practices to ensure safety and efficiency in managing resources:

Set up a temporary directory using JUnit's TemporaryFolder for any resources you need to extract for tests.
Always armor up against IOExceptions and other file-system traps.
After a stream has revealed its secrets, always close it to prevent leaks.
If you're adventuring into the world of plugins or modular apps, be aware of the plugin loader's context.