Why Does My Classpath Resource Cause a FileNotFoundException When Running My JAR?

Classpath Resource Not Found When Running as JAR:

When running Java applications packaged as JARs (Java Archive files), attempts to load classpath resources using annotations like @Value may result in FileNotFoundException errors. While these resources may be present in the IDE (Integrated Development Environment) while developing, they might not be accessible from within the JAR.

Understanding the Issue:

Classpath resources are located in the project's src/main/resources directory by default. When running the application from an IDE, these resources are directly available to the application on the filesystem. However, when building a JAR, these resources are bundled within the JAR file and no longer exist as individual files on the filesystem.

Accessing Resources from JAR Files:

The method resource.getFile() used in the given code expects the resource to be available as a file on the filesystem. However, since classpath resources within JAR files are not accessible in this manner, an error is thrown. To resolve this issue, use resource.getInputStream() instead. This method retrieves the resource's content as an input stream, regardless of its location.

Example Code:

<code class="java">@Value("${message.file}")
private Resource messageResource;

public void run(String... arg0) throws Exception {
    // Read resource as input stream rather than file
    InputStream inputStream = messageResource.getInputStream();
    BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
    String line;
    while ((line = reader.readLine()) != null) {
        logger.debug("Line: " + line);
    }
}</code>

Using getInputStream() ensures that classpath resources can be accessed regardless of whether they are present on the filesystem or bundled within a JAR.

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