Codecs

Overview

In this guide, you can learn about Codecs and supporting classes that handle the encoding and decoding of Kotlin objects to and from BSON data. The Codec abstraction allows you to map any Kotlin type to a corresponding BSON type. You can use this feature to map your domain objects directly to and from BSON instead of using classes such as Document or BsonDocument.

You can learn how to specify custom encoding and decoding logic using the Codec abstraction and view example implementations in the following sections:

• Codec

• CodecRegistry

• CodecProvider

• Custom Codec Example

Codec

The Codec interface contains abstract methods for serializing and deserializing Kotlin objects to BSON data. You can define custom conversion logic in your implementation of this interface.

To implement the Codec interface, override the encode(), decode(), and getEncoderClass() abstract methods.

enum class PowerStatus {
    ON,
    OFF
}

class PowerStatusCodec : Codec<PowerStatus> {
    override fun encode(
        writer: BsonWriter, value: PowerStatus, encoderContext: EncoderContext
    ) = writer.writeBoolean(value == PowerStatus.ON)
    override fun decode(
        reader: BsonReader, decoderContext: DecoderContext)
    : PowerStatus {
        return when (reader.readBoolean()) {
            true -> PowerStatus.ON
            false -> PowerStatus.OFF
        }
    }
    override fun getEncoderClass(): Class<PowerStatus> = PowerStatus::class.java
}

CodecRegistry

A CodecRegistry is an immutable collection of Codec instances that encode and decode Kotlin classes. You can use any of the following CodecRegistries class static factory methods to construct a CodecRegistry from the Codec instances contained in the associated types:

• fromCodecs(): Creates a registry from Codec instances

• fromProviders(): Creates a registry from CodecProvider instances

• fromRegistries(): Creates a registry from CodecRegistry instances

The following code shows how to construct a CodecRegistry by using the fromCodecs() method:

val codecRegistry = CodecRegistries
    .fromCodecs(IntegerCodec(), PowerStatusCodec())

The preceding example assigns the CodecRegistry the following Codec implementations:

• IntegerCodec: Codec that converts Integers. It is part of the BSON package.

• PowerStatusCodec: Sample Codec from the preceding section that converts Kotlin enum values to BSON booleans.

You can retrieve the Codec instances from the CodecRegistry instance by using the following code:

val powerStatusCodec = codecRegistry.get(PowerStatus::class.java)
val integerCodec = codecRegistry.get(Integer::class.java)

If you attempt to retrieve a Codec instance for a class that is not registered, the codecRegistry.get() method raises a CodecConfigurationException exception.

For more information about the classes and interfaces in this section, see the following API documentation:

• CodecRegistries

• IntegerCodec

CodecProvider

A CodecProvider is an interface that contains abstract methods to create Codec instances and assign them to CodecRegistry instances. Similar to the CodecRegistry interface, the BSON library uses the Codec instances retrieved by the CodecProvider.get() method to convert between Kotlin and BSON data types.

However, in cases that you add a class that contains fields requiring corresponding Codec objects, ensure that you instantiate the Codec objects for the class' fields before you instantiate the Codec for the entire class. You can use the CodecRegistry parameter in the CodecProvider.get() method to pass any of the Codec instances that the Codec relies on.

To see a runnable example that demonstrates read and write operations using Codec classes, see the Custom Codec Example section of this guide.

val newRegistry = CodecRegistries.fromRegistries(
    CodecRegistries.fromCodecs(MyEnumCodec()),
    MongoClientSettings.getCodecRegistry()
)

val bsonTypeClassMap = BsonTypeClassMap()
val clazz = bsonTypeClassMap[BsonType.ARRAY]
println("Kotlin class name: " + clazz.name)

Kotlin class name: java.util.List

val replacements = mutableMapOf<BsonType, Class<*>>(BsonType.ARRAY to MutableSet::class.java)
val bsonTypeClassMap = BsonTypeClassMap(replacements)
val clazz = bsonTypeClassMap[BsonType.ARRAY]
println("Class name: " + clazz.name)

Kotlin class name: java.util.Set

Custom Codec Example

This section demonstrates how you can implement Codec and CodecProvider interfaces to define the encoding and decoding logic for a custom Kotlin class. It shows how you can specify and use your custom implementations to perform read and write operations.

The following code defines the sample data class Monolight:

data class Monolight(
    var powerStatus: PowerStatus = PowerStatus.OFF,
    var colorTemperature: Int? = null
) { 
    override fun toString():
        String = "Monolight { powerStatus: $powerStatus, colorTemperature: $colorTemperature }"
}

This class contains the following fields that each require a corresponding Codec to handle encoding and decoding:

• powerStatus: Describes whether the device light is "ON" or "OFF". For this field, use the PowerStatusCodec which converts the PowerStatus enum values to BSON booleans.

• colorTemperature: Describes the color of the device light in kelvins as an Int value. For this field, use the IntegerCodec provided in the BSON library.

The following code shows how to implement a Codec for the Monolight class. The constructor expects an instance of CodecRegistry from which it retrieves the Codec instances needed to encode and decode the class fields:

class MonolightCodec(registry: CodecRegistry) : Codec<Monolight> {
    private val powerStatusCodec: Codec<PowerStatus>
    private val integerCodec: Codec<Int>
    init {
        powerStatusCodec = registry[PowerStatus::class.java]
        integerCodec = IntegerCodec()
    }
    override fun encode(writer: BsonWriter, value: Monolight, encoderContext: EncoderContext) {
        writer.writeStartDocument()
        writer.writeName("powerStatus")
        powerStatusCodec.encode(writer, value.powerStatus, encoderContext)
        writer.writeName("colorTemperature")
        integerCodec.encode(writer, value.colorTemperature, encoderContext)
        writer.writeEndDocument()
    }
    override fun decode(reader: BsonReader, decoderContext: DecoderContext): Monolight {
        val monolight = Monolight()
        reader.readStartDocument()
        while (reader.readBsonType() != BsonType.END_OF_DOCUMENT) {
            when (reader.readName()) {
                "powerStatus" -> monolight.powerStatus =
                    powerStatusCodec.decode(reader, decoderContext)
                "colorTemperature" -> monolight.colorTemperature =
                    integerCodec.decode(reader, decoderContext)
                "_id" -> reader.readObjectId()
            }
        }
        reader.readEndDocument()
        return monolight
    }
    override fun getEncoderClass(): Class<Monolight> = Monolight::class.java
}

To ensure that the Codec instances for the fields are available for the Monolight class, implement a custom CodecProvider as shown in the following code example:

class MonolightCodecProvider : CodecProvider {
    @Suppress("UNCHECKED_CAST")
    override fun <T> get(clazz: Class<T>, registry: CodecRegistry): Codec<T>? {
        return if (clazz == Monolight::class.java) {
            MonolightCodec(registry) as Codec<T>
        } else null // Return null when not a provider for the requested class
    }
}

After defining the conversion logic, you can perform the following actions:

• Store instances of Monolight in MongoDB

• Retrieve documents from MongoDB as instances of Monolight

The following code assigns the MonolightCodecProvider to the MongoCollection instance by passing it to the withCodecRegistry() method. The example class also inserts and retrieves data by using the Monolight class:

    val mongoClient = MongoClient.create(uri)
    val codecRegistry = CodecRegistries.fromRegistries(
        CodecRegistries.fromCodecs(IntegerCodec(), PowerStatusCodec()),
        CodecRegistries.fromProviders(MonolightCodecProvider()),
        MongoClientSettings.getDefaultCodecRegistry()
    )
    val database = mongoClient.getDatabase("codec_test")
    val collection = database.getCollection<Monolight>("monolights")
        .withCodecRegistry(codecRegistry)
    // Insert instances of Monolight
    val monolights = listOf(
        Monolight(PowerStatus.ON, 5200),
        Monolight(PowerStatus.OFF, 3000)
    )
    collection.insertMany(monolights)
    // Retrieve instances of Monolight
    val results =  collection.find()
    results.forEach { l ->
        println(l)
    }

Monolight { powerStatus: ON, colorTemperature: 5200 }
Monolight { powerStatus: OFF, colorTemperature: 3000 }

For more information about the methods and classes mentioned in this section, see the following API documentation:

• withCodecRegistry()

• fromRegistries()