private static boolean isExtractingRequired(URLConnection conn, File targetFile) {
        if (!targetFile.exists()) {
            // Extract anyway if the file doesn't exist
            return true;
        }

        // OK, if the file exists then compare last modified date
        // of this file to file in jar
        long targetLastModified = targetFile.lastModified();
        long sourceLastModified = conn.getLastModified();

        // Allow ~1 second range for OSes that only support low precision
        return Math.abs(sourceLastModified - targetLastModified) >= 1000;

        // Note extraction should also work fine if user who was using
        // a newer version of library, downgraded to an older version
        // which will make above check invalid and extract it again.
    }
Checks if library extraction is required by comparing source and target
last modified date. Returns true if target file does not exist.

@param conn the source file
@param targetFile the target file
@return false if target file exist and the difference in last modified date is
         less than 1 second, true otherwise
jme3-desktop/src/main/java/com/jme3/system/NativeLibraryLoader.java