How to read text file from classpath in Java?

java

file-io

classpath

resource-management

byAlex Kataev·Oct 9, 2024

Get hold of the classpath resource stream with YourClass.class.getResourceAsStream("/file.txt") and then breathe life into this with a BufferedReader:

try (BufferedReader reader = new BufferedReader(new InputStreamReader(
     YourClass.class.getResourceAsStream("/file.txt"), StandardCharsets.UTF_8))) {
    reader.lines().forEach(System.out::println);  // Dancing on the beat of every line
}

Very important: The path /file.txt is relative to the root of all the classpath "forest". If the file lives in a specific package, dress it up nicely including the full path, like /com/yourcompany/interviews/file.txt.

Clearing the path: Pre-read steps

Before your knight embarks on this reading quest, be certain the file is there, waiting loyally on the classpath. Fear not! Maven and Gradle, those diligent tools obediently take care of the classpath, and place src/main/resources on the field.

Ponder this!

Directory Structure: Building a castle in our land? Match the layout of your package structure within src/main/resources. Helps the postman deliver mails faster.
Leading Slash (/): As wise men say, getResourceAsStream()'s / dictates to JVM to start from base of the kingdom (i.e., classpath's root). no-slash? Sure, the hunt begins from your house (the package carrying the class) itself.
Character Encoding: Ah, the universal language of understanding. Specify StandardCharsets.UTF_8 to avoid petty regional disputes (encoding issues) across kingdoms (platforms).

Flock of codes: Advanced techniques

The try-with-resources spell is potent for efficient and guaranteed resource closure:

try (InputStream is = YourClass.class.getResourceAsStream("/file.txt")) {
    // Your loyal InputStream, closing on duty once work is done
}

Faster horses, NIO (New I/O), lets your horses (code) run faster. Files.readAllLines is swift and crisp:

Path path = Paths.get(YourClass.class.getResource("/file.txt").toURI());
Files.readAllLines(path, StandardCharsets.UTF_8).forEach(System.out::println); // Reading magic words, one rune (line) at a time.

Little birdies whisper: toURI() helps to manage spaces and symbols in file paths.

When the goblins attack: Handling exceptions

File Not Found: Oh no! No carrier pigeon? Check classpath settings. Ensure the fortress (project) is built right, the message (resource) is sent correctly.
Spring Boot: The magical boot indeed! Use ClassPathResource for a smoother classpath resource access.
Leaking Resources: More tea, anyone? Always close the streams once the party is over!

External arcana: Libraries

Spring's Resource: Tame the wild resources with Spring's Resource. The omnipresence analog with a tang of flexibility at reach.
IOUtils: Apache's Commons IO library presents IOUtils.toString. Like the wand itself reading aloud the InputStream into a String.

Case of the alternate spell book

Spring Framework: Already in a Spring context? Use new ClassPathResource("file.txt").getInputStream() to make it rain data.
Explicit Class Loader: Threading in a labyrinth of modular applications? Use getClass().getClassLoader().
System Class Loader Caveat: Close the shades, keep ClassLoader.getSystemClassLoader() out. It may not see everything on the classpath.

Wisdom speaks: Tips & Tricks

Commons IO: Pair up getResourceAsStream() and Commons IO IOUtils for a perfect waltz of stream to string conversion.
File Path Consistency: For paths, always trust the forward slashes (/). Cross-platform harmony, they say.

Pro tips and guiding lights

Include the Knights: The brave knights (dependencies) like Spring or Commons IO must stand in your build configuration.
Dynamic swapping: Changing the underlying resources with a Resource object is as simple as a swift flick of your wand. No coding changes required.
Cleanliness Leads to Code-Godliness!: Clean, pious code helps while accessing files and seeking out bugs.
Choice of weapon: Weapons for a Knight, Resources for a coder. Choose sensibly as per context.