Fix Kotlin bson decoding of optionals

[rozza]

Previously, DataClassCodec pre-populated all constructor parameters with null before reading the document, which prevented callBy from using Kotlin default parameter values.

Now optional parameters missing from the document are left absent from the args map so callBy invokes their defaults, and a clear CodecConfigurationException is thrown when a required non-nullable field is missing.

Ported the bson-kotlin test cases to bson-kotlinx to ensure coverage and prevent future regressions.

JAVA-6162

[Copilot]

Pull request overview

Fixes Kotlin BSON decoding of data classes so that missing optional constructor parameters can correctly fall back to Kotlin default parameter values, while missing required non-nullable fields fail with a clear exception. Adds/updates test coverage (ported from bson-kotlin to bson-kotlinx) around defaults + null-handling to prevent regressions.

Changes:

• Update DataClassCodec.decode to stop pre-populating constructor args with null, and to validate missing required fields after reading the document.
• Add a new DataClassWithDefaultsAndNulls sample in both bson-kotlin and bson-kotlinx.
• Expand tests in both modules to explicitly assert decoding behavior for missing fields vs explicit nulls vs defaults.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
bson-kotlinx/src/test/kotlin/org/bson/codecs/kotlinx/samples/DataClasses.kt	Adds DataClassWithDefaultsAndNulls sample for kotlinx-serialization codec tests.
bson-kotlinx/src/test/kotlin/org/bson/codecs/kotlinx/KotlinSerializerCodecTest.kt	Adds decode assertions for defaults/nulls and a missing-required-field failure case.
bson-kotlin/src/test/kotlin/org/bson/codecs/kotlin/samples/DataClasses.kt	Adds DataClassWithDefaultsAndNulls sample for reflection-based DataClassCodec tests.
bson-kotlin/src/test/kotlin/org/bson/codecs/kotlin/DataClassCodecTest.kt	Expands tests to assert decoding behavior for defaults/nulls and missing required field exception.
bson-kotlin/src/main/kotlin/org/bson/codecs/kotlin/DataClassCodec.kt	Core logic change: don’t prefill args with null; validate missing required fields and preserve defaults via callBy.

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[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

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[strogiyotec]

LGTM , checked these changes locally, thanks Ross

[strogiyotec]

@rozza the only thing I noticed is a small behavior change

    @Test
    fun testMissingRequiredNonNullableFieldThrows() {
        // Before this PR: args map was pre-seeded with nulls, so a missing non-nullable field
        // would silently end up as `null` in the constructed object (reflection bypasses
        // Kotlin's null-safety), or surface as an opaque callBy failure.
        //
        // After this PR: the codec detects the missing non-nullable field and throws a clear
        // CodecConfigurationException naming the field.
        val incomplete = BsonDocument.parse(
            """{"char": "c", "byte": 0, "short": 1, "int": 22,
             | "long": {"$numberLong": "42"}, "float": 4.0,
             | "double": 4.2, "boolean": true}""".trimMargin()
            // note: "string" field is missing
        )

        val codec = DataClassCodec.create(DataClassWithSimpleValues::class, registry())

        val ex = assertThrows<CodecConfigurationException> {
            codec?.decode(BsonDocumentReader(incomplete), DecoderContext.builder().build())
        }
        assertEquals(
            "Required field 'string' is missing from the document for DataClassWithSimpleValues data class",
            ex.message
        )
    }

data class DataClassWithSimpleValues(
    val char: Char,
    val byte: Byte,
    val short: Short,
    val int: Int,
    val long: Long,
    val float: Float,
    val double: Double,
    val boolean: Boolean,
    val string: String
)

With this PR , the test case will throw CodecConfigurationException with error message

Required field 'string' is missing from the document for DataClassWithSimpleValues data class

Previously this test case created an instance of DataClassWithSimpleValues with null string

I guess it's worth mentioning in the release notes

[rozza]

@strogiyotec

Previously this test case created an instance of DataClassWithSimpleValues with null string

In this case the call to return primaryConstructor.callBy(args) fails and we throw:

org.bson.codecs.configuration.CodecConfigurationException: Unable to invoke primary constructor of DataClassWithSimpleValues data class

So the change here is the error message is more specific and helpful for the user.