RFR: 8273370: Preferences.exportSubtree() generates invalid XML if value contains control char

[Joe Wang]

On Thu, 24 Mar 2022 18:21:05 GMT, Joe Wang <joehw at openjdk.org> wrote:

> The issue was caused by the difference on handling control characters between the parser and serializer. The parser rejected control characters while the serializer converted them to NCRs. The fix is for the later to be aligned with the parser, reporting error upon encountering control characters. 
> 
> Mach5 tier2 passed.

For XML 1.0, it's known that they were invalid. For prefs, it would be nice to add InvalidPreferencesFormatException to exportSubtree to match with that of importPreferences. But it's such an edge case, for this patch, I opted for no signature change.

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PR: https://git.openjdk.java.net/jdk/pull/7945