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What it looks like

java.io.FileNotFoundException: config/app.json (No such file or directory)

Extends IOException in java.io.

What it means

The text in parentheses is the operating system's reason, and it matters more than the exception name: No such file or directory, Permission denied, Is a directory or The system cannot find the path specified. Relative paths are resolved against the process working directory, which is not the project folder when the app runs from an IDE, a service or a container, and that is the single most common cause.

Common causes

• A relative path resolved against an unexpected working directory.
• The file is inside the jar: files under src/main/resources are classpath resources, not files on disk.
• File permissions or a read-only container file system.
• Windows versus Linux differences: case sensitivity, back slashes, drive letters.

How to fix it

1. Print new File(path).getAbsolutePath() to see what is really being opened.
2. Load packaged files with getResourceAsStream("/name") or Spring's ClassPathResource, not FileInputStream.
3. Use java.nio.file.Path and Files.exists and give the path through configuration.
4. In containers, mount the file or directory and check the user's permissions.

Broken and fixed

new FileInputStream("src/main/resources/data.csv"); // fails in the packaged jar

try (InputStream in = getClass().getResourceAsStream("/data.csv")) {
    // read from the classpath
}

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