PREAMBLE: Due to not being sure where to post it, this was posted to amber-dev before. I have updated it to take into account Ron Pressler’s notes on System.console, and Brian Goetz’s notes on steering clear of shoving deadlines in debate posts like this one. —
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 <https://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 <https://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. - (based on feedback from Ron Pressler in amber-dev) Try to move the community away from treating System.in and System.out as the streams to be used for ‘command line apps’, and towards using System.console() instead, which is already char based, and is better positioned to take care of picking the right charset for you. However, this is quite a big job, given that virtually all tutorials, books, q&a sites like Stack Overflow talk about System.in/out and not about Console. Even if somehow the message gets out and these start using Console instead, the experience for java developers would be deplorable, given that *no IDE supports Console!* - possibly because it is maybe difficult for them to set it up properly? At any rate, just like the JDBC group works together with DB vendors to ensure JDBC actually is fit for purpose, there would have to be something set up to ensure tool developers like the eclipse team or IntelliJ update their templates and support Console for their run/debug-inside-IDE features. An open question then comes up: How does the OpenJDK team move the community in the direction that the OpenJDK wants them to move? “Build it and they will come”? I highly doubt that would work here; System.in works well enough for the base case at first glance. At the very least a statement by the OpenJDK that new Scanner(System.in) is a bad idea would help to start the decades-long work of trying to break down established Stack Overflow answers, mark tutorials as obsolete, etc. I have no idea if the OpenJDK even wants to meddle with community interaction like this, but if it does not, then “it’s fine, Console exists, it’s not our problem the community doesn’t use it” seems a bit hollow. 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, 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. - Contact IntelliJ, eclipse, and possibly maven/gradle (insofar that Console doesn’t work when using mvn run and the like) and ask them what they need to add console support, keeping in mind encoding is important, and possibly, to rewire their syso (eclipse) and sysout (intellij) template shortcuts away from System.out.println and towards System.console().printf instead. --Reinier Zwitserloot
