ScannerUseDelimiter
Scanner.useDelimiter is not an efficient way to read an entire InputStream

The problem

Scanner.useDelimiter("\\A") is not an efficient way to read an entire InputStream.

Scanner scanner = new Scanner(inputStream, UTF_8).useDelimiter("\\A");
String s = scanner.hasNext() ? scanner.next() : "";

Scanner separates its input into “tokens” based on a delimiter that is a regular expression. The regular expression \A matches the beginning of the input, only, so there is no later delimiter and the single token consists of every character read from the InputStream.

This works, but has multiple drawbacks:

• You need a special case for an empty InputStream. In that case there is no token after \A. That’s why the extract above checks hasNext(). If you forget to do that, you get NoSuchElementException in the empty case.

• It swallows IOException. Quoting the Scanner specification:

  A scanner can read text from any object which implements the Readable interface. If an invocation of the underlying readable’s read() method throws an IOException then the scanner assumes that the end of the input has been reached. The most recent IOException thrown by the underlying readable can be retrieved via the ioException() method.

• It is much slower than calling inputStream.readAllBytes().

Instead, prefer one of the following alternatives:

Since Java 9, it has been possible to write this:

String s = new String(inputStream.readAllBytes(), UTF_8);

On Android, that does require API level 33, though. Guava’s ByteStreams.toByteArray(inputStream) is equivalent to inputStream.readAllBytes().

Suppression

Suppress false positives by adding the suppression annotation @SuppressWarnings("ScannerUseDelimiter") to the enclosing element.