We'd like to parse JSON5 documents in the JVM, but the JSON5-specific libraries available on GitHub all appear to have very low support. As parsers are often magnets for security vulnerabilities, we'd prefer not to use a parser that isn't already being widely used and scrutinised by the community.
This got us to wondering: Can the optional features of the Jackson JSON parser be used to parse JSON5?
By enabling the following optional Jackson parser features:
ALLOW_UNQUOTED_FIELD_NAMESALLOW_TRAILING_COMMAALLOW_SINGLE_QUOTESALLOW_BACKSLASH_ESCAPING_ANY_CHARACTERALLOW_NON_NUMERIC_NUMBERSALLOW_JAVA_COMMENTSALLOW_LEADING_DECIMAL_POINT_FOR_NUMBERSit seems possible to support all of JSON5's headline features except for:
Jackson has a number of optional features which can be enabled on a parser to make it more lenient in the way it parses.
Comparing these parser options to the headline features of JSON5, we find that the following are supported:
Object keys may be an ECMAScript 5.1 IdentifierName.
✅ Supported with the ALLOW_UNQUOTED_FIELD_NAMES feature
Objects may have a single trailing comma.
Arrays may have a single trailing comma.
✅ Both supported with the ALLOW_TRAILING_COMMA feature
Strings may be single quoted.
✅ Supported with the ALLOW_SINGLE_QUOTES feature
Strings may span multiple lines by escaping new line characters.
Strings may include character escapes.
✅ Both of these appear to be supported by Jackson's ALLOW_BACKSLASH_ESCAPING_ANY_CHARACTER feature, though the meaning or purpose of escaping here may differ subtly. DYOR.
Numbers may be IEEE 754 positive infinity, negative infinity, and NaN.
✅ Supported with the ALLOW_NON_NUMERIC_NUMBERS feature
Single and multi-line comments are allowed.
✅ Supported with the ALLOW_JAVA_COMMENTS feature.
The following JSON5 feature is partially supported by Jackson:
Numbers may have a leading or trailing decimal point.
✅ Leading decimal points are supported with the ALLOW_LEADING_DECIMAL_POINT_FOR_NUMBERS feature.
⛔️ Jackson doesn't appear to have support for trailing decimal points.
There doesn't currently (mid-2021) appear to be any way to configure Jackson to permit the following JSON5 features:
Numbers may be hexadecimal.
Numbers may have a trailing decimal point.
Numbers may begin with an explicit plus sign.
Additional white space characters are allowed.