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API for converting JVM objects to representations by MIME type, for the Jupyter ecosystem.


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JVM Repr

Build Status JVM repr on jitpack

Configuration | Usage for Library Authors | Usage for Kernel Authors

Standardizing object representation and inspection across JVM kernels (Scala, Clojure, Groovy, ...).


JVM languages use various conventions to convert REPL outputs to forms that can be displayed. This project is an attempt to standardize by providing an API that libraries can use to register display methods for any jupyter frontend, whether console, notebook, or dashboard.

This API has two main uses:

  • For library authors to provide a way to convert from a library's JVM objects to useful representations by MIME type
  • For kernel authors to convert any JVM object to useful representations by MIME type

As it is with IPython, this is a contract between the libraries within the ecosystem, the kernel that inspects the objects, and the frontends that display them.


See instructions on JitPack for gradle, maven, sbt, or leiningen.


Usage - Library authors

Library authors can register conversion code either

  • by implementing a Displayer and registering it with Displayers, or
  • by having classes extend AsDisplayData, whose display method returns how an instance should be displayed.

For example, the following will register a displayer for Vegas graphs:

import java.util.Map
import jupyter.Displayer
import jupyter.Displayers
import scala.collection.JavaConverters._
import vegas.DSL.ExtendedUnitSpecBuilder


    new Displayer[ExtendedUnitSpecBuilder] {
      override def display(plot: ExtendedUnitSpecBuilder): Map[String, String] = {
        val plotAsJson = plot.toJson
          "text/plain" -> plotAsJson,
          "application/json" -> plotAsJson,
          "text/html" -> new StaticHTMLRenderer(plotAsJson).frameHTML()

The following has Thing be represented as simple text in Jupyter front-ends, like Thing(2) for new Thing(2):

import java.util.HashMap;
import java.util.Map;

class Thing implements AsDisplayData {
  private int n;
  public Thing(int n) {
    this.n = n;
  public Map<String, String> display() {
    Map<String, String> result = new HashMap<>();
    result.put(MIMETypes.TEXT, "Thing(" + n + ")");
    return result;

Any kernel implementation can use the method to display Vegas graphs for the DSL objects.

Library authors can optionally implement setMimeTypes(String...) to receive hints for the MIME types that the kernel or front-end supports. It is recommended that library authors use these hints to avoid expensive conversions.

Usage - Kernel authors

Kernel authors can use this API to display registered objects:

import java.util.Map

// ...

  Object result = interpreter.eval(code);
  Map<String, String> resultByMIME = Displayers.display(result);

Kernel authors can optionally call Displayers.setMimeTypes(String...) to send hints to display implementations with the set of MIME types that can be used by the kernel or front-end.


API for converting JVM objects to representations by MIME type, for the Jupyter ecosystem.



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