Why Does Classpath Resource Missing When Running As JAR?

Classpath Resource Missing When Running As JAR

When loading classpath resources using the @Value annotation, one may encounter file not found exceptions when running the application as a JAR file. Despite the resource being present in the src/main/resources directory and confirmed within the JAR file, the issue arises due to a discrepancy in how classpath resources are handled in different contexts.

Understanding the Issue

The problem lies in the use of resource.getFile(). This method expects the resource to be accessible on the file system and cannot handle resources nested within JAR files. When running the application within STS, this method works because the resource is directly accessible on the file system. However, when running from the JAR file, the resource is located within the JAR file and is not directly accessible on the file system.

Alternative Solution

To resolve this issue, it is recommended to use resource.getInputStream() instead of resource.getFile(). This method allows you to access the resource's content regardless of its location. Here is an example:

<code class="java">try (BufferedReader reader = new BufferedReader(new InputStreamReader(messageResource.getInputStream()))) {
  // Read and process the resource's content here
}</code>

This approach will ensure that the resource is accessed correctly when running the application as a JAR file, eliminating the file not found exception.

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