WW-3364 Use registered type converters in JSON serialization (opt-in)

[lukaszlenart]

Fixes WW-3364

Summary

The JSON plugin never consulted the type converters an application registers, so a value type rendered through a converter in the HTML view (e.g. Money → "12.50 EUR") came out as a nested bean in the JSON response.

This adds an opt-in constant, struts.json.writer.useTypeConverters (default false). When enabled, StrutsJSONWriter serializes a value that is not a built-in JSON type with the converter registered for its class — or a superclass/interface, via XWorkConverter.lookup() — and writes the result as a JSON string.

<constant name="struts.json.writer.useTypeConverters" value="true"/>

Behaviour

• Off by default — existing output is unchanged.
• Built-in types are never converted. The check sits after the Boolean/Number/String/Map/Iterable/Date/Temporal/Locale/Enum branches, so @JSON(format) and struts.json.dateformat keep working. It runs before the record branch, so a record value type can have a converter.
• Only per-type (application-wide) converters (struts-conversion.properties / xwork-conversion.properties, @TypeConversion(type = ConversionType.APPLICATION)) are used; per-property Foo-conversion.properties mappings need context the writer does not have.
• java.io.File is skipped, so the framework's own UploadedFileConverter is never applied.
• @JSONFieldBridge keeps precedence — it is applied at the property level before the value is written.
• Failure falls back — if a converter throws or returns a non-String, the value is serialized as a bean/record as before (logged at debug).
• A converted value is written as a single string: nested includeProperties/excludeProperties patterns and @JSON annotations on its members no longer apply to it. This will be stated in the JSON plugin docs.

Out of scope: using converters on the read side (JSONPopulator).

Test plan

• New StrutsJSONWriterTypeConverterTest (container-backed, StrutsTestCase): default off, converted value, nested property, interface-registered converter, record, no converter, field-bridge precedence, failing converter fallback, built-in types untouched, File skipped
• Each guard test was mutation-checked: removing the flag check, the exception fallback, the File exclusion, the built-in ordering, or the field bridge makes the corresponding test fail
• ./mvnw test -DskipAssembly -pl plugins/json — 177 tests, 0 failures

Docs: a paragraph for the JSON plugin page follows in apache/struts-site.

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
92.3% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud