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This project is no longer actively maintained.

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A port of Github's refactoring tool Scientist in Java




This Java port supports most of the functionality of the original Scientist library in Ruby, however its interface is a bit different.

The core component of this library is the Experiment<T> class. It's recommended to use this class as a Singleton. The main usage is as follows:

Basic Usage

You can either run a synchronous experiment or an asynchronous experiment.

For a synchronous experiment, the order in which control and candidate functions are run is randomized.

To run a synchronous experiment:

Experiment<Integer> e = new Experiment("foo");, this::candidateFunction);

For an asynchronous experiment, the two functions are run asynchronously.

To run an asynchronous experiment:

Experiment<Integer> e = new Experiment("foo");
e.runAsync(this::controlFunction, this::candidateFunction);

There could be a situation where you want to continue to perform the work of existing functions on the same thread and run an experiment on a different thread.For example, if you deploy a web application with Tomcat Tomcat has its own thread pool, and you don't want the experiment to affect existing functionality.

To run control on the same thread but different for candidate:

Experiment<Integer> e = new Experiment("foo");
e.runAsyncCandidateOnly(this::controlFunction, this::candidateFunction);

Behind the scenes the following occurs in both cases:

  • It decides whether or not to run the candidate function
  • Measures the durations of all behaviors
  • Compares the result of the two
  • Swallows (but records) any exceptions raised by the candidate
  • Publishes all this information.


Scientist4J ships with support for two common metrics libraries—Dropwizard metrics and Micrometer. As each of these is optional, you’ll need to add your choice as an explicit dependency to your project:




The following metrics are reported, with the form scientist.[experiment name].*:

  • duration of default (control) behavior in ns
  • duration of candidate behavior in ns
  • counter of total number of users going through the codepath
  • counter of number of mismatches
  • counter of candidate exceptions

You may also implement your own MetricsProvider, to meet your specific needs.

Optional Configuration

Users can optionally override the following functions:

  • publish (to publish results of an experiment, if you want to supplement the MetricsProvider’s publishing mechanism)
  • compareResults (by default this library just uses equals between objects for equality, but in case you want to special case equality between objects)
  • enabled (to limit what % of users get exposed to the new code path - by default it's 100%)
  • runIf (to enforce conditional behavior on who should be exposed to the new code path)
  • isAsync (force using the async for legacy code or move to runAsync method)

License: MIT


A port of Github's Refactoring tool Scientist in Java







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