KPOI A Kotlin DSL for working with Apache POI

Sometimes in my line of work I have the pleasure of trying to programmatically generate spreadsheets or get to work on someone else’s code which generates spreadsheets. In Java land the go to library for doing this is Apache POI. The API for generating spreadsheets is very imperative and side effect based. I could not help be curious about what it might look like if the API was re-imagined with a Domain Specific Language written in Kotlin. So like most normal humans I decided would to spend a couple weekends writing code that helps generate spreadsheets.

What is a DSL?

For those that not familiar with the concept, a Domain Specific Language is a “computer language specialized to a particular application domain”. For example SQL (when you exclude the many extensions that have been made to it over the years) is a DSL specifically for querying databases. Contrast this to general purpose programming languages like Java, Python, Kotlin, and Haskell which are designed to cover a wide variety of problems.

Some languages provide the ability to extend the syntax of the language to create DSLs within the language itself. This is fairly common in the ML (thats “Meta Language” not “Machine Learning”) family of languages (Standard ML, OCaml, F Sharp) and languages that take inspiration from ML to varying degrees (Haskell, Scala, Rust) as well as modern languages not directly related to ML (Kotlin, Swift, Groovy). An example of a DSL you might have within a language is a declarative way of generating HTML.

This post is designed to:

  1. Highlight the language features in Kotlin which make wrapping/calling existing Java code a pleasant experience.
  2. Giving a medium scale example creating a Kotlin DSL.

Some great additional guides on creating DSLs are also available:

At the end of this article we can take Apache POI code in Java which looks like this:

And change it to the Kotlin DSL which looks like:

Designing the Apache POI DSL:

The design goals for the DSL were born from real world usage of POI:

  • Focus the DSL on generating workbooks rather than parsing existing workbooks
  • POI provides multiple spreadsheet implementations with different version compatibility and performance characteristics. I wanted to build a single DSL for all for all the implementations that allows the underlying sheet type to be changed as needed.
  • It should provide defaults for common operations but allow the defaults to be easily overridden. For example you will not need to supply a row number when writing contiguous rows but you can set a specific row number if you wish.

In order to build off Apache POI we need to understand the existing API. It is built off a hierarchy of concepts:

  • You start off by creating a Workbook
  • You can create Sheets from a Workbook (these are the tabs you would see at the bottom in Excel). After creating you can set Sheet level configuration.
  • You can create Rows from a Sheet. After creating you can set Row level configuration.
  • You can create Cells from a Row. After creating you set the value of a Cell and can set various styling information

You can see the POI API is very imperative. You create each component from a previous component and then after creation you modify it is as needed. I want the DSL to instead allow you to define the workbook declaratively by creating a nested structure that mirrors the relation between the classes.

Implementing the DSL:

So how do we go about implementing a DSL to wrap an existing Java library? Well Kotlin provides some nice language features that we can build off of.

Higher Order Functions, Lambdas as Blocks, Lambda as last parameter

One set of fundamental language features are lambdas and higher order functions. These concepts will be familiar for those who have worked in Java 8. The basic idea is that functions are “first class” can be passed as parameters to other functions.

In our case lets say we want a method called “workbook” which will create a new Workbook for us. The method is passed a lambda which modifies the workbook and then returns the new workbook. In Java this would look like:

Where as in Kotlin this would look like:

So what on earth just happen in that last example method call? Basically if you have a method that only takes a single parameter which is a lambda then you can drop a lot of the syntax you would normally have to type out in Java. This is key to making a DSL! If you look closely at the final example you can see that it looks just like we are using a expression in the language such as the “if” expression. This is why an internal DSL is often described as extending the language. We are able to create new syntax which feels like its a part of the language itself!

Receiver Types

A second piece of syntax we can clean up in the DSL is the explicit reference to the Workbook. In the previous example we skipped naming the parameter we passed into the lambda and instead opted to use the default parameter name of “it”. We can further simplify things by making use of receiver types. A receiver type lets you make the explicit workbook implicit so you can call the methods on it directly without having to reference the workbook by name. This means we can update the previous example as follows:

Now we can call setHidden and setSelectedTab directly without referring to “it”.

Property Access Syntax

Why do we use getters and setters in Java? One reason is that accessors enable encapsulation: if the underlying implementation of a class changes and we are directly referring to fields on a class then we must update all the places were we access fields that have changed. If we use accessor methods then we can change the underling implementation, update the methods and the external code can stay the same. Additionally getters and setters allow us to do things like check if the underlying value is initialized or do some form of type conversion.

However these examples typically are the exception rather than the norm. Unless you are writing a library that you are exposing to others then you have the ability to refactor both the class and the code that calls a class. In fact the change can be easy and comprehensive when done with a modern IDE. It would be preferable then to instead allow access to a field directly but to provide a way to override the accessor’s behavior if you encounter a circumstance when you need to. This is what language like C# and Kotlin allow you to do.

In Kotlin you directly refer to a field on a class. Then if you need to you can define a getter or setter. However instead of explicitly referring to the accessor method, the method will instead be implicitly used when accessing the field:

Additionally since Kotlin aims for a pleasant Java interop experience you can use Kotlin’s property access syntax on Java classes that have a getter. Kotlin will handle the conversion for you. So if you have a Plain Old Java Object with getters and setters you can use it in Kotlin as if its a Kotlin class in most cases. This means we can update our previous example to:

Default/Optional Parameters

In Kotlin you can set default values for method parameters. This makes the parameters optional. In the previous examples we have been hard-coding the type of Workbook used as HSSFWorkbook (this an Excel ‘97 compatible workbook, .xls). What if a user wants to use a XSSFWorkbook (Excel 2007, .xlsx) instead? It would be nice if a user could provide the workbook to use if they wanted to. With a default parameter value we can enable this behavior:

Extension Methods

So after all that work we now have a nice way to create and modify Workbooks. What about Sheets, Rows and Cells though? For Workbooks we were able to create a default Workbook or use a provided Workbook. Will that work for the other parts? Remember that with Apache POI you need a parent object to create a child object:

Lets think back to the example from the beginning we would like to create:

You will notice that inside the lambda block passed to the “workbook” expression we want to have a similar looking “sheet” expression (starting on line 2). If we think back to the explanation of how the “workbook” expression works we realize that the “sheet” expression is actually a call to a method named “sheet” on the implicit Workbook object. However Workbook is a class from Apache POI that we do not have control over and it does not have any method like that! So what we need is a clean mechanism to enhance the existing Java classes from the API.

If we were working in Java some options to achieve this are:

  • Use inheritance to extend the classes in the library to add new functionality. In this case it would generally be frowned upon. Item 18 in Effective Java (Favor composition over inheritance) makes the important distinction between using inheritance within your own projects vs across project boundaries:

    It is safe to use inheritance within a package, where the subclass and the superclass implementations are under the control of the same programmers. It is also safe to use inheritance when extending classes specifically designed and documented for extension (Item 19). Inheriting from ordinary concrete classes across package boundaries, however, is dangerous … Unlike method invocation, inheritance violates encapsulation [Snyder86]. In other words, a subclass depends on the implementation details of its superclass for its proper function. The superclass’s implementation may change from release to release, and if it does, the subclass may break, even though its code has not been touched. As a consequence, a subclass must evolve in tandem with its superclass, unless the superclass’s authors have designed and documented it specifically for the purpose of being extended.

  • Use composition to create a new object that wraps the original object and adds new functionality. Its perfectly acceptable to create the wrapper object but it can add extra syntax to the DSL since you will need to reach into the wrapper to get the inner class (littering the code with “wrapper.innerObject” references). Alternatively you could write the wrapper so that it has methods that cover every use case for the inner object but that would tightly couple the wrapper to the inner object and break encapsulation.
  • Use a static utility class to contain the new functionality. An example of this idea would be Apache Commons which provides many utility classes. For example the StringUtils class contains static methods like containsIgnoreCase which evaluates whether a given string contains a specific string while ignoring case. Static utility classes provides a clean separation of the original class (String) and the new functionality (the static methods) but adds noise to the syntax as you must now write something like StringUtils.containsIgnoreCase(bookText, searchPhrase) (or containsIgnoreCase(bookText, searchPhrase) if you use a static import). This unfortunately is not the clean call to just “sheet” that we want.

It would be nice if there was a way to enhance the original class with new functionality similar to a static utility class (to avoid breaking encapsulation) but be able to retain the calling conventions we would get if we extended the class. Born from this need Kotlin provides extension methods. Under the hood extensions methods basically generate a static utility class for you while allowing you too call the methods as if they were a true method belonging to the class.

For example, in Apache POI you can merge cells in a sheet by defining the region that bounds the cells to merge and then saying you want to merge that region:

In Kotline we can create an extension method called “merge” which directly takes the bounded region and handles creating the CellRangeAddress for you:

Here you will noticed we defined the method as Sheet.merge. This is the special syntax for an extension method. It says we are defining the “merge” method on the existing “Sheet” type from Apache POI.

Extension methods are a powerful basic language feature that we need to use to build a DSL for an existing library. Going back to the previous question of how to implement the “sheet” method we can do something like this:

This leverages extensions methods as well as all the techniques from the previous sections. I even threw in some other Kotlin features on top of that (sorry not sorry). Breaking it down:


  • This is an extension method adding “sheet” to the Workbook class

name: String? = null

  • You can optionally pass a first parameter “name”
  • Why is the parameters type “String?” and not “String”? This is a very important Kotlin feature that I’m glossing over for now. Basically the type “String” can never be null. “String?” is a “String” that can also be null. This means null safety can be checked and enforced at compile time forcing authors to think about how to handle nulls.

block: Sheet.() -> Unit = {}

  • You can optionally pass a parameter “block”. If you don’t supply it then a lambda that does nothing is used. The block also uses a receiver type of Sheet.

return if (name != null) { createSheet(name) } else { createSheet() }

  • In Kotlin if statements with else branches are actually expressions. This means the statement will be equal to the final value of the branch that is evaluated (similar to the ternary operator “?:” in Java). In this example if name is not null it will create a sheet with that name. Otherwise it will create a sheet using the default name. Kotlin actually has a lot of very cool operators to handle null safety but I opted not to use them here for clarity.


  • In Java we have Object that all objects inherit from. It gives us sad methods likes the default equals and default toString. In Kotlin we have Any which classes inherit from and gives us a bunch of useful methods like apply. Apply will call a supplied method on an object and then return the object. This is great for modifying a value and then passing it on. By using apply hear we avoid needing to make the block method return its input so you don’t have to worry about returning anything when you write the block to pass into this method!

Putting that all together, the sheet method essentially adds a new method to Workbook which creates a new Sheet from the workbook, applies the given lambda to it and then returns the Sheet.

Whew… wait we still need row and cell. They leverage the same design more or less:

There are some more Kotlin features here that could use more complete explanations but I’m glossing over since this is about DSLs and not a full Kotlin language tutorial:

  • What are the “rowNumber ?: physicalNumberOfRows” and “column ?: physicalNumberOfCells” doing? Well remember those null operators I mentioned? “?:” is the “elvis operator”. If the first value is null then the operator evaluates to second value instead.
  • What is that “when” part? That is a “When Expression”. Think of it like a better and safer switch statement. There are actually a couple different ways to use it. In this scenario the cell method is optionally taking a “value” parameter which can be any of the different types that Apache POI supports for cell values. The Cell class has overloaded the setCellValue method with a different implementation for each of the types it supports. This unfortunately breaks the automatic property access syntax so we use a When Expression to handle it. The branches in the When Expression matches the different types for “value” using the “is” keyword (similar to instanceOf in Java). Then we can call “cell.setCellValue(value)”. Note that we do not need to explicitly cast the type of value to the type we matched. This is because Kotlin has smart casts. Since the branch already checks the type the compiler knows its correct and does not require us to specify it. Features like this are how you know that Kotlin was written by people who have spent many days writing in Java and wishing for something basically the same but less soul crushing.

With that we now can use the example that was given in the beginning! Along with the extension methods above I have also implemented some additional extension methods for common scenarios. They can all be viewed in the repository

Testing Realistic Use Cases for kpoi

While the documentation for Apache POI can sometimes be sparse there does exist the great Busy Developers’ Guide to HSSF and XSSF Features. It contains a series of code examples for common use cases and problems. To build out the tests for the DSL I took the example Java code from the guide and use it to generate workbooks. Then I create the equivalent workbooks using the Kotlin DSL and compare the two. This helps provide some test cases that are modeled after real world usage.

An unfortunate aspect of Apache POI is that none of the classes implement Equals() due to the authors deciding there was not a clear concept for “equivalent workbooks”. Because of this I had to resort to using reflection based comparison for the workbooks. My personal preference would be to use AssertJ but there is currently ongoing work around the recursive comparison API so instead I opted to use Unitil’s assertReflectionEquals which recursively uses reflection to compare objects field by field.

Potential future work

Note that this DSL is specifically designed to cover declaratively creating workbooks. It has nothing to do with reading in notebooks and manipulating which is another use case for Apache POI. There still is interesting work that could be done around the read side to make it more pleasant.