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MyBatis Kotlin Examples

This project demonstrates use of MyBatis with the Kotlin language. This is a work in progress, and we will update as our learnings progress and/or new capabilities in MyBatis become available.

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 with Kotlin

One of MyBatis' core strengths is mapping SQL result sets to an object graph. MyBatis is good at these mappings for many 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. The examples (especially example03 and example04) will show patterns of dealing with this.

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 needed in MyBatis. MyBatis can auto discover class constructors in this case. The important distinction is 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. Important code changes:

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

example02.oldmybatis - Type Handlers and Advanced Auto Mapping in Older MyBatis Versions

This example shows the use of classes that need TypeHandlers in older versions of MyBatis. In MyBatis versions prior to 3.5.0, MyBatis sometimes had difficulty finding constructors on classes that included types with TypeHandlers. In that case, 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/oldmybatis/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/oldmybatis/Example02Test.kt to see how to register the type handler

example03 - Nested Objects

This example shows how to write Kotlin 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 immutable 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 using XML to define the result map.

example04.lazy - Lazy Loaded Nested Collections

This example shows how to use MyBatis support for lazy loaded associations. This forces an N+1 query (or worse), so be careful with lazy loading. There are also issues with lazy loading and Kotlin due to the way that Javassist works (MyBatis uses Javassist to create dynamic proxies when you use lazy loading). Any class that will be lazy loaded, and any field in that class that will be lazy loaded, must be declared "open" in Kotlin. We can code queries like this without XML which is good, but the performance may be worse - tradeoffs.

example05 - MyBatis Dynamic SQL

This example shows how to use Kotlin to interact with the "MyBatis Dynamic SQL" library. This example also shows how MyBatis Generator creates code for Kotlin.

example06 - MyBatis Dynamic SQL (Generated Values)

This example shows how to use Kotlin to interact with the "MyBatis Dynamic SQL" library when a table contains generated values. This example also shows how MyBatis Generator creates code for Kotlin.