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Language Implementation

This handbook provides an overview of the process that should be followed when looking to implement support for a new programming language in jsii. It attempts to provide a step-by-step procedure, while drawing the reader's attention on points that have been found to cause problems in the past.


Implementing a new language in jsii is not just a matter of implementing code generation. Mapping the jsii type system to a new programming language means finding how to represent an API originally designed in TypeScript to a form that is as idiomatic as possible in the new language. This is a craft that often requires trial and error, and the best (if not only) way to validate a proposal is to put it in front of users and seek feedback. As a consequence, this endeavor should be expected to span months, not weeks.

Scoping & Planning

The first step of most successful projects is to start by scoping work out and establishing a baseline plan to execute on. For contributors not yet familiar with jsii, the specification document is a great place to start. In particular, the New Language Intake document provides a high-level view of the recommended process for implementing new language support.

The work of implementing support for a new language involves many different components:

  • The [jsii] compiler emits warnings when a language's reserved words are used to name types, methods or properties; as this will later require slugification or escaping in the generated code - usually resulting in a degraded developer experience.
  • The [jsii-pacmak] tool includes code generators for all supported languages, and a new implementation must be provided for the new language.
  • Code generation usually requires specific configuration to be provided in order to be able to generate valid packages (for example, the Java code generator requires a base java package to generate into, as well as a Maven group and artifact ID for the package). The [jsii-config] tool needs to be updated with support generating a configuration block with the required entries for the new code generator.
  • [jsii-rosetta] tool translates TypeScript example code found in the original documentation into the new target language. A new translation implementation needs to be added for the new language.
  • Building and publishing infrastructure elements are provided by [aws-delivlib] to make it easier for jsii users to publish their libraries to all supported package registries.

Language Proposition RFC

The recommended way to formalize the initial plan is to write it into an RFC hosted in the CDK RFC repository. Enough time has to be spent considering the requirements in order to get the work scoped and planned well, ensuring smooth execution.

An additional benefit of following the RFC process is that it makes it easier to track learnings accumulated through the implementation process, as those will be tracked as comments or iterations on the RFC document.

It is possible (and sometimes desirable) to start prototyping code-generation for the new language, as this can highlight implementation challenges that need to be discussed in the RFC document. In any case, examples of the API signatures that are expected to be rendered allow early feedback to be provided by possible future users, and still helps identify challenges.

The following questions should be answered as early as possible in the process, to avoid surprises later on that result in significant re-engineering effort:

  • What do the generated APIs look like, for the typical API idioms?
  • Classes (constructors, properties, methods, inheritance strategy, abstract members, ...)
    • The AWS CDK (one of the main consumers of jsii) uses specific patterns to offer a better experience in many programming languages. For example, constructor signatures where the last argument is a jsii struct allows for keyword argument lifting in Python, and convenient Builder APIs in Java.
  • Enums
  • Interfaces and Structs (properties, methods, inheritance strategy, implementation, ...). In particular, how are new optional properties handled (those are not considered breaking change within the jsii type system).
  • Structs (properties, inheritance strategy, implementation, ...)
  • What information is needed in order for the code-generator to produce artifacts? What should the configuration block look like?
  • What is the standard way to publish packages for the new language?
  • Are there any requirements (code signature, special metadata, ...) that need to be implemented in order to publish valid packages?
  • How are dependencies modeled? If semantic versioning is not the norm, what is the strategy to correctly represent semantic version ranges?
  • What are the toolchain and platform requirements?
  • For example, Java requires an OpenJDK 8 distribution and maven, Python requires python 3.8 or above, etc...

Code Generation

First, implement a first version of the code generation for the new language before getting too far into the host library implementation. This top-down approach ensures the requirements for the lower level parts of the implementation are well-defined before they are implemented (reducing the chances that significant re-work has to be done), and enables using the Standard Compliance Suite to ensure the overall implementation is correct according to the specification (since the code necessary to implement the test cases will be available right from the start).

This work happens within the [jsii-pacmak] package.

Focus initially on the API signatures before getting into their implementation. The first version may even throw a not implemented exception when called.

The [jsii-calc] package, can be used as a sample consuming library which uses jsii to generate code in all target languages. Start by making sure a decent API is generated from this package and its dependencies, and use those to implement the tests from the Standard Compliance Suite. You'll also get a feeling for whether the generated code achieves a good developer experience or not.

Host Library

Now that we are generating "empty shell" APIs that represent the necessary entities to back the Standard Compliance Suite tests, start implementing the host library and update the code generator until all the tests pass. It is possible to publish artifacts even when tests in the suite are failing. As soon as basic features are working, work on Building and Packaging can start, so early feedback can be gathered.


A standard architecture for the host library has not been documented yet. Upcoming language implementations should contribute to this process by documenting a general architecture that should be implementable in any programming languages (and thus, abstracting away language specificities).

Building & Packaging

The necessary toolchains should be added to he [] Docker image, so that the [jsii-pacmak] generation can be changed to support building ready to publish artifacts instead of just code.

Before publishing any artifacts, ensure all packages (the host library as well as generated artifacts) are designated as experimental (e.g: Python packages were annotated with the Development Status :: 4 - Beta trove classifier on PyPI, and NuGet packages were published with a pre-release version such as 1.2.3-pre).

Additionally, [aws-delivlib] needs to be augmented to support publishing artifacts to the language's package repository.


The package publishing is being extracted from [aws-delivlib] into a standalone library, currently hosted at eladb/jsii-release.


Before releasing the new language support to Developer Preview, basic documentation needs to be produced to explain how to configure a jsii project to support the new language, and any peculiarities in working with libraries generated by [jsii-pacmak] for this language.

Support for example code translation should also be built into [jsii-rosetta].

Developer Preview

Once the full Standard Compliance Suite passes (possibly with the exception of certain fringy features), and the documentation covering all aspects of using the language bindings have been produced, the new language can be released to Developer Preview.

It is recommended that new languages stay in Developer Preview for a minimum of 4 weeks, ideally until they have received sufficient usage to have built confidence that there are no major usability concerns: once out of Developer Preview, it will no longer be possible to introduce breaking changes to the generated code in order to address usability issues or bugs.

In order to improve the chances of catching usability issues, focused user experience studies will be conducted with an audience composed of developers with varied degrees of experience with the new language.


A user experience template will be provided to ensure coverage of critical aspects of the experience. Any critical user experience issue (for example, issues that required breaking changes to the generated code) discovered but not covered in the template should be added to the template so that subsequent language implementations do not fall to the same problem.

General Availability

Once the new language has been in Developer Preview without any significant usability issues or bugs for a sufficient amount of time and is used in real-world use-cases such as for AWS CDK applications, it becomes a candidate to be declared Generally Available. At this point, breaking changes are no longer possible on the generated code.