fix: warn when Kotlin class loses field values due to missing kotlin-reflect

[e5l]

What / Why

When a Kotlin class with constructor parameters (e.g. any data class) is
deserialized and kotlin-reflect is not on the classpath, FastJSON2 cannot
resolve the constructor parameter names. The result is a silent failure: an
object is constructed, but its fields are left at their default/null values with
no error and no diagnostic. Users only discover the data loss downstream, and it
looks like a FastJSON2 bug rather than a missing dependency.

This is the worst kind of failure mode — wrong data, no signal.

Root cause

KotlinUtils.getConstructor(...) only resolves parameter names when
STATE == 2, i.e. when kotlin.reflect.jvm.ReflectJvmMapping (shipped in
kotlin-reflect) is present:

if (creatorParams != 0 && STATE == 2) {
    // ... resolve names via Reflection.getOrCreateKotlinClass(...)
}

When kotlin-reflect is absent, STATE == 1 (only kotlin-stdlib is present),
the block is skipped, beanInfo.createParameterNames stays null, and binding
falls back to defaults with no indication of why.

Change

Adds a diagnostic warning when — and only when — this exact condition is hit:
the class is Kotlin (the call site in ObjectReaderBaseModule is already guarded
by beanInfo.kotlin), it has constructor parameters, kotlin-reflect is
unavailable, and parameter names were not resolved by any other means
(annotations / @JSONCreator):

} else if (creatorParams != 0 && beanInfo.createParameterNames == null) {
    System.err.println("fastjson2 warning: class " + clazz.getName()
        + " is a Kotlin class with constructor parameters, "
        + "but kotlin-reflect is not on the classpath. ...");
}

The message names the class and tells the user to add
org.jetbrains.kotlin:kotlin-reflect.

No behavior change beyond the warning; the fallback path is unchanged. Java
classes are unaffected (guarded by beanInfo.kotlin), and users who already
supply parameter names are not warned (guarded by createParameterNames == null).

[CLAassistant]

All committers have signed the CLA.

[wenshao]

Reviewed — no blockers. Suggestion-level recommendations are in the Suggestion summary comment below.

[wenshao]

Suggestions — commit ee938bbaca4449d2f4512c75ccbb0c95a025044a

File	Issue	Suggested fix
KotlinUtils.java:105-111	No deduplication guard — warning fires once per distinct Kotlin class. An app with N Kotlin data classes emits N separate System.err.println lines at startup.	Add a static volatile Set<Class<?>> or volatile boolean guard to warn at most once per class (or once globally).
KotlinUtils.java:107	Misleading message for serialization path — getConstructor is called from both ObjectReaderBaseModule (deserialization) and ObjectWriterBaseModule (serialization), but the message says "will cause deserialization to silently lose field values".	Soften to "may cause deserialization issues" or only emit the warning from the reader path.
KotlinUtils.java:83-103	kotlin-reflect runtime failure goes unwarned — when STATE==2 but kotlin-reflect fails at runtime (e.g., JDK 17+ module restrictions), catch (Throwable) silently swallows the error. The else if is never reached since the if was already true.	After the catch block, check if createParameterNames is still null and emit a warning indicating kotlin-reflect was found but failed.
KotlinUtils.java:105	No test coverage — the new else if branch is never exercised by existing tests (all Kotlin tests run with kotlin-reflect present, STATE=2).	Add a test that simulates STATE < 2 or mocks a BeanInfo with null createParameterNames.
KotlinUtils.java:105	System.err.println inconsistent with codebase — this is the only System.err.println in all of core/src/main/java/. It bypasses logging frameworks and cannot be suppressed.	Use java.util.logging.Logger at WARNING level (JDK-standard, zero extra dependency).

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[wenshao]

No issues found. LGTM! ✅

Downgraded from Approve to Comment: CI still running.

— qwen3.7-max via Qwen Code /review