IO-542: FileUtils#readFileToByteArray: optimize reading of files with known size (closes #38)

Author: ilmarmors

src/main/java/org/apache/commons/io/FileUtils.java

@@ -1849,7 +1849,9 @@ public static String readFileToString(final File file) throws IOException {
      */
     public static byte[] readFileToByteArray(final File file) throws IOException {
         try (InputStream in = openInputStream(file)) {
-            return IOUtils.toByteArray(in); // Do NOT use file.length() - see IO-453
+            long fileLength = file.length();
+            // file.length() may return 0 for system-dependent entities, treat 0 as unknown length - see IO-453
+            return fileLength > 0 ? IOUtils.toByteArray(in, fileLength) : IOUtils.toByteArray(in);
         }
     }