神秘

Question

神秘

在准确探索Java标识符中允许哪些字符时，我偶然发现了一些非常好奇的东西，似乎几乎肯定是一个bug。

我希望发现Java标识符符合以下要求：它们以具有Unicode属性ID_Start的字符开头，后面跟着具有属性ID_Continue的字符，并且允许例外领先的下划线和美元符号。事实证明并非如此，而且我发现与我听说过的普通标识符或其他任何其他想法极为不同。

简短演示

请考虑以下演示，证明Java标识符中允许使用ASCII ESC字符（八进制033）：

$ perl -le 'print qq(public class escape { public static void main(String argv[]) { String var_\033 = "i am escape: \033"; System.out.println(var_\033); }})' > escape.java
$ javac escape.java
$ java escape | cat -v
i am escape: ^[

但是，情况甚至更糟。实际上，几乎无限恶化。甚至允许NULL！还有数千个甚至不是标识符字符的其他代码点。我在Solaris，Linux和运行Darwin的Mac上测试了这一点，并且都给出了相同的结果。

长演示

这是一个测试程序，它将显示Java非常不允许作为合法标识符名称一部分的所有这些意外代码点。

#!/usr/bin/env perl
# 
# test-java-idchars - find which bogus code points Java allows in its identifiers
# 
#   usage: test-java-idchars [low high]
#   e.g.:  test-java-idchars 0 255
#
# Without arguments, tests Unicode code points
# from 0 .. 0x1000.  You may go further with a
# higher explicit argument.
#
# Produces a report at the end.
#
# You can ^C it prematurely to end the program then
# and get a report of its progress up to that point.
#
# Tom Christiansen
# tchrist@perl.com
# Sat Jan 29 10:41:09 MST 2011

use strict;
use warnings;

use encoding "Latin1";
use open IO => ":utf8";

use charnames ();

$| = 1;

my @legal;

my ($start, $stop) = (0, 0x1000);

if (@ARGV != 0) {
    if (@ARGV == 1) {
        for (($stop) = @ARGV) { 
            $_ = oct if /^0/;   # support 0OCTAL, 0xHEX, 0bBINARY
        }
    }
    elsif (@ARGV == 2) {
        for (($start, $stop) = @ARGV) { 
            $_ = oct if /^0/;
        }
    } 
    else {
        die "usage: $0 [ [start] stop ]\n";
    } 
} 

for my $cp ( $start .. $stop ) {
    my $char = chr($cp);

    next if $char =~ /[\s\w]/;

    my $type = "?";
    for ($char) {
        $type = "Letter"      if /\pL/;
        $type = "Mark"        if /\pM/;
        $type = "Number"      if /\pN/;
        $type = "Punctuation" if /\pP/;
        $type = "Symbol"      if /\pS/;
        $type = "Separator"   if /\pZ/;
        $type = "Control"     if /\pC/;
    } 
    my $name = $cp ? (charnames::viacode($cp) || "<missing>") : "NULL";
    next if $name eq "<missing>" && $cp > 0xFF;
    my $msg = sprintf("U+%04X %s", $cp, $name);
    print "testing \\p{$type} $msg...";
    open(TESTPROGRAM, ">:utf8", "testchar.java") || die $!;

print TESTPROGRAM <<"End_of_Java_Program";

public class testchar { 
    public static void main(String argv[]) { 
        String var_$char = "variable name ends in $msg";
        System.out.println(var_$char); 
    }
}

End_of_Java_Program

    close(TESTPROGRAM) || die $!;

    system q{
        ( javac -encoding UTF-8 testchar.java \
            && \
          java -Dfile.encoding=UTF-8 testchar | grep variable \
        ) >/dev/null 2>&1
    };

    push @legal, sprintf("U+%04X", $cp) if $? == 0;

    if ($? && $? < 128) {
        print "<interrupted>\n";
        exit;  # from a ^C
    } 

    printf "is %s in Java identifiers.\n",  
        ($? == 0) ? uc "legal" : "forbidden";

} 

END { 
    print "Legal but evil code points: @legal\n";
}

以下是仅在前三个代码点上运行该程序的示例，该代码点既不是空格也不是标识符字符：

$ perl test-java-idchars 0 0x20
testing \p{Control} U+0000 NULL...is LEGAL in Java identifiers.
testing \p{Control} U+0001 START OF HEADING...is LEGAL in Java identifiers.
testing \p{Control} U+0002 START OF TEXT...is LEGAL in Java identifiers.
testing \p{Control} U+0003 END OF TEXT...is LEGAL in Java identifiers.
testing \p{Control} U+0004 END OF TRANSMISSION...is LEGAL in Java identifiers.
testing \p{Control} U+0005 ENQUIRY...is LEGAL in Java identifiers.
testing \p{Control} U+0006 ACKNOWLEDGE...is LEGAL in Java identifiers.
testing \p{Control} U+0007 BELL...is LEGAL in Java identifiers.
testing \p{Control} U+0008 BACKSPACE...is LEGAL in Java identifiers.
testing \p{Control} U+000B LINE TABULATION...is forbidden in Java identifiers.
testing \p{Control} U+000E SHIFT OUT...is LEGAL in Java identifiers.
testing \p{Control} U+000F SHIFT IN...is LEGAL in Java identifiers.
testing \p{Control} U+0010 DATA LINK ESCAPE...is LEGAL in Java identifiers.
testing \p{Control} U+0011 DEVICE CONTROL ONE...is LEGAL in Java identifiers.
testing \p{Control} U+0012 DEVICE CONTROL TWO...is LEGAL in Java identifiers.
testing \p{Control} U+0013 DEVICE CONTROL THREE...is LEGAL in Java identifiers.
testing \p{Control} U+0014 DEVICE CONTROL FOUR...is LEGAL in Java identifiers.
testing \p{Control} U+0015 NEGATIVE ACKNOWLEDGE...is LEGAL in Java identifiers.
testing \p{Control} U+0016 SYNCHRONOUS IDLE...is LEGAL in Java identifiers.
testing \p{Control} U+0017 END OF TRANSMISSION BLOCK...is LEGAL in Java identifiers.
testing \p{Control} U+0018 CANCEL...is LEGAL in Java identifiers.
testing \p{Control} U+0019 END OF MEDIUM...is LEGAL in Java identifiers.
testing \p{Control} U+001A SUBSTITUTE...is LEGAL in Java identifiers.
testing \p{Control} U+001B ESCAPE...is LEGAL in Java identifiers.
testing \p{Control} U+001C INFORMATION SEPARATOR FOUR...is forbidden in Java identifiers.
testing \p{Control} U+001D INFORMATION SEPARATOR THREE...is forbidden in Java identifiers.
testing \p{Control} U+001E INFORMATION SEPARATOR TWO...is forbidden in Java identifiers.
testing \p{Control} U+001F INFORMATION SEPARATOR ONE...is forbidden in Java identifiers.
Legal but evil code points: U+0000 U+0001 U+0002 U+0003 U+0004 U+0005 U+0006 U+0007 U+0008 U+000E U+000F U+0010 U+0011 U+0012 U+0013 U+0014 U+0015 U+0016 U+0017 U+0018 U+0019 U+001A U+001B

这是另一个演示：

$ perl test-java-idchars 0x600 0x700 | grep -i legal
testing \p{Control} U+0600 ARABIC NUMBER SIGN...is LEGAL in Java identifiers.
testing \p{Control} U+0601 ARABIC SIGN SANAH...is LEGAL in Java identifiers.
testing \p{Control} U+0602 ARABIC FOOTNOTE MARKER...is LEGAL in Java identifiers.
testing \p{Control} U+0603 ARABIC SIGN SAFHA...is LEGAL in Java identifiers.
testing \p{Control} U+06DD ARABIC END OF AYAH...is LEGAL in Java identifiers.
Legal but evil code points: U+0600 U+0601 U+0602 U+0603 U+06DD

问题

任何人都可以解释这个看似疯狂的行为吗？整个地方有很多很多其他令人费解的许可代码点，从U + 0000开始，这可能是最奇怪的。如果在第一个0x1000代码点上运行它，则会看到某些模式出现，例如允许使用属性Current_Symbol的任何和所有代码点。但是太多其他事情是完全无法解释的，至少是我。

Answer 1

Java Language Specification section 3.8推迟到Character.isJavaIdentifierStart()和Character.isJavaIdentifierPart()。后者在其他条件中具有Character.isIdentifierIgnorable()，它允许非空白控制字符（包括整个C1范围，请参阅列表的链接）。

Answer 2

另一个问题可能是：为什么Java不允许在其标识符中使用控制字符？

设计语言或其他系统时的一个好原则是，在没有正当理由的情况下不要禁止任何事情，因为你永远不知道如何使用它，实施者和用户必须应对的规则越少越好。

确实，您当然不应该利用这一点，通过将转义实际嵌入到您的变量名中，并且您将看不到任何公开的类，其中包含带有空字符的类。

当然，这可能会被滥用，但是语言设计师的工作不是以这种方式保护程序员，而是通过强制适当的缩进或精心选择的变量名称。

Answer 3

我不知道什么是重要的。无论如何它对你有何影响？

如果开发人员想要模糊他的代码，他可以使用ASCII。

如果开发人员想让他的代码可以理解，他将使用该行业的通用语言：英语。不仅标识符只是ASCII，还有普通的英文单词。否则，没有人会使用或阅读他的代码，他可以使用他喜欢的任何疯狂角色。

为什么Java允许控制字符在其标识符中？

神秘

简短演示

长演示

问题

3 个答案: