JEP400 vs new Scanner(System.in)

[Ron Pressler]

Hi.

The appropriate list is core-libs-dev, where this discussion should continue.

System.in is the standard input, which may or may not be the keyboard. For keyboard input, take a look at the java.io.Console class [1], in particular its charset and reader methods.

[1]: https://docs.oracle.com/en/java/javase/19/docs/api/java.base/java/io/Console.html

— Ron

On 13 Oct 2022, at 16:20, Reinier Zwitserloot <reinier at zwitserloot.com<mailto:reinier at zwitserloot.com>> wrote:

PREAMBLE: I’m not entirely certain amber-dev is the appropriate venue. If not, where should this be discussed? It’s not quite a bug but nearly so, and not quite a simple feature request either.

JDK18 brought JEP400 which changes the default charset encoding to UTF-8. This, probably out of necessity, goes quite far, in that Charset.defaultCharset() is now more or less a constant - always returns UTF_8. It’s now quite difficult to retrieve the OS-configured encoding (the ’native’ encoding).

However, that does mean one of the most common lines in all of java’s history, is now necessarily buggy: new Scanner(System.in) is now broken. Always, unless your docs specifically state that you must feed the app UTF_8 data. Linting tools ought to flag it down as incorrect. It’s incorrect In a nasty way too: Initially it seems to work fine, but if you’re on an OS whose native encoding isn’t UTF-8, this is subtly broken; enter non-ASCII characters on the command line and the app doesn’t handle them appropriately. A bug that is literally utterly undiscoverable on macs and most linux computers, even. How can you figure out your code is broken if all the machines you test it on use UTF-8 as an OS default?

This affects beginning java programmers particularly (who tend to be writing some command line-interactive apps at first). In light of Brian Goetz’s post “Paving the Onramp” (https://openjdk.org/projects/amber/design-notes/on-ramp) - the experience for new users is evidently of some importance to the OpenJDK team. In light of that, the current state of writing command line interactive java apps is inconsistent with that goal.

The right way to read system input in a way that works in both pre- and post-JEP400 JVM editions appears to be, as far as I can tell:


Charset nativeCharset = Charset.forName(System.getProperty("native.encoding", Charset.defaultEncoding().name());
Scanner sc = new Scanner(System.in, nativeCharset);

I’ll risk the hyperbole: That’s.. atrocious. Hopefully I’m missing something!

Breaking _thousands_ of blogs, tutorials, stack overflow answers, and books in the process, everything that contains new Scanner(System.in). Even sysin interaction that doesn’t use scanner is likely broken; the general strategy then becomes:


new InputStreamReader(System.in);

which suffers from the same problem.

I see a few directions for trying to address this; I’m not quite sure which way would be most appropriate:


  *   Completely re-work keyboard input, in light of Paving the on-ramp. Scanner has always been a problematic API if used for keyboard input, in that the default delimiter isn’t convenient. I think the single most common beginner java stackoverflow question is the bizarre interaction between scanner’s nextLine() and scanner’s next(), and to make matters considerably worse, the proper fix (which is to call .useDelimiter(“\\R”) on the scanner first) is said in less than 1% of answers; the vast majority of tutorials and answers tell you to call .nextLine() after every .nextX() call. A suboptimal suggestion (it now means using space to delimit your input is broken). Scanner is now also quite inconsistent: The constructor goes for ‘internet standard’, using UTF-8 as a default even if the OS does not, but the locale does go by platform default, which affects double parsing amongst other things: scanner.nextDouble() will require you to use commas as fractions separator if your OS is configured to use the Dutch locale, for example. It’s weird that scanner neither fully follows common platform-independent expectations (english locale, UTF-8), nor local-platform expectation (OS-configured locale and OS-configured charset). One way out is to make a new API for ‘command line apps’ and take into account Paving the on-ramp’s plans when designing it.
  *   Rewrite specifically the new Scanner(InputStream) constructor as defaulting to native encoding even when everything else in java defaults to UTF-8 now, because that constructor is 99% used for System.in. Scanner has its own File-based constructor, so new Scanner(Files.newInputStream(..)) is quite rare.
  *   Define that constructor to act as follows: the charset used is the platform default (i.e., from JDK18 and up, UTF-8), unless arg == System.in is true, in which case the scanner uses native encoding. This is a bit bizarre to write in the spec but does the right thing in the most circumstances and unbreaks thousands of tutorials, blogs, and answer sites, and is most convenient to code against. That’s usually the case with voodoo magic (because this surely risks being ’too magical’): It’s convenient and does the right thing almost always, at the risk of being hard to fathom and producing convoluted spec documentation.
  *   Attach the problem that what’s really broken isn’t so much scanner, it’s System.in itself: byte based, of course, but now that all java methods default to UTF-8, almost all interactions with it (given that most System.in interaction is char-based, not byte-based) are now also broken. Create a second field or method in System that gives you a Reader instead of an InputStream, with the OS-native encoding applied to make it. This still leaves those thousands of tutorials broken, but at least the proper code is now simply new Scanner(System.charIn()) or whatnot, instead of the atrocious snippet above.
  *   Even less impactful, make a new method in Charset to get the native encoding without having to delve into System.getProperty(). Charset.nativeEncoding() seems like a method that should exist. Unfortunately this would be of no help to create code that works pre- and post-JEP400, but in time, having code that only works post-JEP400 is fine, I assume.
  *   Create a new concept ‘represents a stream that would use platform native encoding if characters are read/written to it’, have System.in return true for this, and have filterstreams like BufferedInputStream just pass the call through, then redefine relevant APIs such as Scanner and PrintStream (e.g. anything that internalises conversion from bytes to characters) to pick charset encoding (native vs UTF8) based on that property. This is a more robust take on ‘new Scanner(System.in) should do the right thing'. Possibly the in/out/err streams that Process gives you should also have this flag set.


If it was up to me, I think a multitude of steps are warranted, each relatively simple.


  *   Create Charset.nativeEncoding(). Which simply returns Charset.forName(System.getProperty(“native.encoding”). But with the advantage that its shorter, doesn’t require knowing a magic string, and will fail at compile time if compiled against versions that predate the existence of the native.encoding property, instead of NPEs at runtime.
  *   Create System.charIn(). Which just returns an InputStreamReader wrapped around System.in, but with native encoding applied.
  *   Put the job of how java apps do basic command line stuff on the agenda as a thing that should probably be addressed in the next 5 years or so, maybe after the steps laid out in Paving the on-ramp are more fleshed out.
  *   In order to avoid problems, before the next LTS goes out, re-spec new Scanner(System.in) to default to native encoding, specifically when the passed inputstream is identical to System.in. Don’t bother with trying to introduce an abstracted ‘prefers native encoding’ flag system.

 --Reinier Zwitserloot

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