Serialization issues with Kotlin classes in Spring Boot: Maven vs Gradle

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Serialization of Kotlin classes in Spring Boot can lead to issues with Maven and Gradle. This article examines the differences between the two build tools and how they handle Kotlin classes during compilation. Specifically, the article explores why Gradle-generated Kotlin classes can cause a “no Creators, like the default constructor, exist” error during deserialization, whereas Maven-generated Kotlin classes do not. The article also provides solutions to resolve the issue, such as annotating the primary constructor with @JsonCreator or explicitly defining a no-arg constructor in the Kotlin class.

Overview of Serialization in Spring Boot

Serialization is the process of converting an object’s state into a byte stream. This byte stream can then be transferred over the network or stored in a file. In Spring Boot, serialization is used when transferring data between a client and a server.

Maven’s approach to generating bytecode for Kotlin classes

Maven uses the Kotlin compiler to generate bytecode for Kotlin classes. The bytecode generated by Maven is compatible with Jackson, the default JSON library used in Spring Boot. Therefore, Kotlin classes generated by Maven do not face any serialization issues when used in Spring Boot.

Gradle’s approach to generating bytecode for Kotlin classes

Gradle uses the Kotlin Gradle plugin to generate bytecode for Kotlin classes. The bytecode generated by the Kotlin Gradle plugin is not compatible with Jackson. As a result, Kotlin classes generated by Gradle can cause serialization issues when used in Spring Boot.

Understanding the “no Creators, like default constructor, exist” error

The “no Creators, like default constructor, exist” error occurs when a Kotlin class lacks a no-arg constructor or a constructor annotated with @JsonCreator. This error can occur during deserialization, where Jackson needs to create an instance of the Kotlin class from a JSON string.

Solutions to resolve the error

There are two ways to resolve the “no Creators, like default constructor, exist” error:

Annotating the primary constructor with @JsonCreator

One way to resolve the error is to annotate the primary constructor of the Kotlin class with @JsonCreator. This annotation tells Jackson to use the primary constructor to create an instance of the Kotlin class.

Here’s an example:

data class Person @JsonCreator constructor(
    @JsonProperty("name") val name: String,
    @JsonProperty("age") val age: Int
)

In the example, the primary constructor of the Person class is annotated with @JsonCreator. The @JsonProperty annotation is used to map the JSON properties to the class properties.

Explicitly defining a no-arg constructor in the Kotlin class

Another way to resolve the error is to explicitly define a no-arg constructor in the Kotlin class. This no-arg constructor is used by Jackson to create an instance of the Kotlin class during deserialization.

Here’s an example:

data class Person(val name: String, val age: Int) {
    constructor() : this("", 0)
}

In the example, a no-arg constructor is explicitly defined in the Person class. The constructor initializes the properties of the class with default values.

Conclusion

As the saying says, “With great power comes great responsibility.” In the world of software development, Kotlin has proven to be a powerful programming language. But as with any tool, challenges arise. One such challenge is serialization issues when using Kotlin with Spring Boot and build tools like Gradle. Fear not! While these challenges can seem daunting, they can be overcome with the proper understanding and configuration. By staying current with the latest best practices and tools, developers can continue to create exceptional applications using Kotlin and Spring Boot. So don’t be afraid to tackle these challenges head-on, and unlock the full potential of this dynamic duo!