MyBatis Kotlin Examples

This project demonstrates use of MyBatis with the Kotlin language. This is a work in progress.

Initially, we are concentrated on how to use MyBatis with Kotlin without substantial changes to any existing library.

Subsequently, we will investigate modifications to MyBatis, or potential Kotlin extensions, that may lead to a more idiomatic Kotlin integration.

Note that in most cases we are foregoing XML and working solely with the Java API for MyBatis. Hopefully it will be clear how to translate the concepts to XML if so desired.

All the code in this project is pure Kotlin. As is typical, source code is in the /src/main/kotlin directory and test code is in /src/test/kotlin. The examples described below have associated packages in the source trees. So code for example01 is in /src/main/kotlin/example01, etc.

Using MyBatis As Is

One of MyBatis' core strengths is mapping SQL result sets to an object graph. MyBatis is good at these mappings for many different types of class hierarchies. MyBatis can create a deeply nested object graph and its group by function provides support for creating an object graph that avoids the need for N+1 queries. However, the implementation of this result set mapping is predicated on the idea that objects can be instantiated and later modified - and this is not strictly compatible with idiomatic Kotlin. With idiomatic Kotlin, objects are created through their constructors and are immutable after creation.

example01 - Auto Mapping

This example shows that MyBatis is able to create simple objects (basic data types, no nested classes) with idiomatic Kotlin. The "model" classes are Kotlin data classes with a mix of nullable and non-nullable types. No special mapping is need in MyBatis. MyBatis can auto discover class constructors in this case. The important distinction is:

1. Only simple data types are used
2. There are no nested classes or group by functions

example02 - Type Handlers and Advanced Auto Mapping

This example shows the use of TypeHandlers. With this example we have moved beyond simple types. We are still using immutable types, but in this case we have a non standard type that requires a TypeHandler. In addition to creating the TypeHandler itself, we need to annotate the class constructor with the @AutomapConstructor annotation and also register the type handler. Important code changes:

1. Look in /src/main/kotlin/example02/Example02Model.kt to see how to annotate the class
2. Look in /src/main/kotlin/util/YesNoTypeHandler.kt to see how to write the type handler
3. Look in /src/test/kotlin/example02/Example02Test.kt to see how to register the type handler

example03 - Nested Objects

This example shows how to write Kotlin code for a class hierarchy where a class (Person in this case) has an attribute that is another class (Address in this case). With this example we can no longer use Kotlin data classes because of the way that MyBatis constructs an object graph in cases like this.

example04 - Join Queries with Nested Collections

This example shows how to use MyBatis support for N+1 query mitigation. This involves creating a result map with a nested collection. Currently, this is only supported in MyBatis XML.

example05 - MyBatis Dynamic SQL

This example shows how to use Kotlin to interact with the "MyBatis Dynamic SQL" library. This example also shows how a future version of MyBatis Generator might generate code for Kotlin.