Why Do Classpath Resources Disappear When My Spring Boot App Is Packaged as a JAR?

Classpath Resources Not Found When Running as JAR: Unraveling the Mystery

When building a Java application with Spring Boot, utilizing classpath resources can enhance the functionality of your program. However, you may encounter the dreaded "FileNotFound" exception when attempting to access these resources after packaging your application into a JAR file. This discrepancy often arises due to the fundamental difference between how resources are handled during development and in a JAR environment.

The Problem

When running an application from an Integrated Development Environment (IDE), such as Spring Tool Suite (STS), resources are treated as being on the file system. This means that accessing a resource using resource.getFile() is straightforward and works seamlessly. However, when the application is packaged into a JAR, resources are no longer part of the file system hierarchy but are embedded within the JAR archive.

The Resolution: Unleashing the Power of getInputStream()

To gracefully handle resources in a JAR context, it's crucial to embrace a different approach. Instead of relying on resource.getFile(), which attempts to retrieve the file directly from the file system, switch to resource.getInputStream() instead. This method returns an input stream that allows you to read the contents of the resource, regardless of its location.

By employing resource.getInputStream(), you bypass the dependency on the file system and ensure flawless access to resources encased within the JAR archive. Thus, your application will perform as expected, whether in development or after being packaged and executed as a standalone JAR file.

Embracing the Elegance of getInputStream()

To illustrate the simplicity and efficacy of resource.getInputStream(), let's modify the example code presented earlier:

<code class="java">private void testResource(Resource resource) {
    try (InputStream is = resource.getInputStream()) {
        // Perform operations using the input stream...
        // ...
    } catch (IOException ex) {
        logger.error(ex.toString());
    }
}</code>

With this adjustment, your application will gracefully navigate classpath resources in both development and JAR environments, providing a seamless experience and eliminating the frustrations of "FileNotFound" exceptions.

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