Why Does getClass().getResource() Fail When Running from the Command Line?

File Loading by getClass().getResource()

The getClass().getResource() method can be employed to load resource files. When deployed from a development environment like Eclipse, this method typically functions seamlessly. However, running the application from the command line can sometimes result in a null pointer exception.

This issue arises because getClass().getResource() utilizes the class loader to retrieve the resource. To be accessible, the resource must be included in the classpath. When using Eclipse, the source folder and all its components, including resource files, are placed in the classpath by default. Hence, resources can be loaded from the same package or directory as the class.

In contrast, when executing from the command line, the classpath is not automatically configured. The file or directory containing the resource must be explicitly added to the classpath. One method to accomplish this is by setting the CLASSPATH environment variable to the file or directory's path.

It's crucial to note that directly using FileInputStream as shown in the code is not recommended for loading resources. Instead, use getResourceAsStream() to obtain an InputStream. This ensures compatibility when deploying as a JAR file or loading classes over a network.

Furthermore, the getResource() method allows you to specify a resource's location relative to the classpackage. For instance:

Foo.class.getResourceAsStream("Test.properties")

Loads Test.properties from the same package as Foo.

Foo.class.getResourceAsStream("/com/foo/bar/Test.properties")

Loads Test.properties from the com.foo.bar package.

By adhering to these guidelines, you can successfully load resource files using getClass().getResource() when running your application from the command line.

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