Is this a known issue? Apex able to deserialize an ill-formed json


It looks like it is a "feature".

Anonymous Apex (note the extra closing brace right after the "v3" value of "k3" key:

System.debug(JSON.deserializeUntyped('{"k1": "efcc1129","k2": "v2", "k3": "v3"},"beingIgnored": "ignored"}'));


18:23:58.39 (39602600)|CODE_UNIT_STARTED|[EXTERNAL]|execute_anonymous_apex 18:23:58.39 (40572794)|USER_DEBUG|[1]|DEBUG|{k1=efcc1129, k2=v2, k3=v3}

Node js

JSON.parse('{"k1": "efcc1129","k2": "v2", "k3": "v3"},"beingIgnored": "ignored"}') Uncaught SyntaxError: Unexpected non-whitespace character after JSON at position 42

By visual, it is obvious that the json is malformed. It seems like Apex is ignoring the rest and stopped at the error position gracefully. Note that the last "element" was dropped possibly due to the pre-mature closing brace.

{"k1": "efcc1129","k2": "v2", "k3": "v3"},"beingIgnored": "ignored"}

Best Answer

Apex uses a custom JSON parser. This parser stops as soon as it parses a valid JSON object, despite more string being left. Here's some more fun examples.

System.debug(JSON.deserializeUntyped('[] See my nice, empty array?'));
System.debug(JSON.deserializeUntyped('{} This object is no bother.'));
System.debug(JSON.deserializeUntyped('"Hello World" now available in JSON!'));
System.debug(JSON.deserializeUntyped('42 The answer to life, the universe, and everything!'));
System.debug(JSON.deserializeUntyped('true ... this also works!'));
System.debug(JSON.deserializeUntyped('null : if you gaze long into an abyss, the abyss will also gaze into thee'));

This is certainly a bug in the world of strict JSON deserialization. However, this behavior has always been around in Apex, if my memory serves me correctly. I think I discovered this bug shortly after Apex was introduced. In a sense, this is technically an optimization, as it is returning as soon as a valid object is parsed.

I don't think this will be fixed, but we can at least report it to and see what they say.

If you're concerned about having strict JSON, you could theoretically write a regular expression to check for validity before trying to parse, but that's overkill for most use cases--we almost always have valid JSON to begin with, so this extra check isn't strictly necessary most of the time.

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