How Do I Access Resources Within a Java JAR File?

Accessing Resource Paths in Java JAR Files

When working with resources stored within a Java JAR file, obtaining a path to the resource can be challenging. Unlike regular files stored on the file system, resources within a JAR do not necessarily have a direct corresponding file.

The provided code snippet illustrates this issue:

ClassLoader classLoader = getClass().getClassLoader();
File file = new File(classLoader.getResource("config/netclient.p").getFile());

This code results in a FileNotFoundException because the resource is not accessible as a physical file on the file system. The JAR file may not have been expanded into individual files, and the classloader itself may not provide a file handle to the resource.

Alternative Solution

To obtain the contents of a resource within a JAR file, an alternative approach is to use classLoader.getResourceAsStream():

ClassLoader classLoader = getClass().getClassLoader();
PrintInputStream(classLoader.getResourceAsStream("config/netclient.p"));

This code reads and prints the contents of the resource directly, without requiring access to a file path. If it becomes absolutely necessary to access the resource as a file, you can copy the stream out into a temporary file:

File tempFile = File.createTempFile("temp", null);
try (
    InputStream inputStream = classLoader.getResourceAsStream("config/netclient.p");
    OutputStream outputStream = new FileOutputStream(tempFile);
) {
    IOUtils.copy(inputStream, outputStream);
}

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