Meet bloop, a fast tool to compile and test your project

Thursday 30 November 2017

Jorge Vicente Cantero and Martin Duhem

Martin Duhem and I are happy to announce Bloop, a tool that Scala developers can use to compile and test their projects faster without being tied to their stock build tool.

We started this project after the feedback we got from this tweet, in which some of you encouraged us to work on compiler/build performance:

What is bloop?

Bloop is a command-line tool for fast edit/compile/test workflows. Its primary goal is to give you the fastest possible experience specifically in the edit/compile/test loop. Bloop is not meant to replace your current build system. The name stands for “build loop”.

Why bloop?

Build tools are complex applications that need to cover a wide range of use cases, from building and testing your project to deploying it, integrating with external tools, resolving dependencies and any build-related tasks.

Such a vast array of requirements make build tools like sbt, Maven and Gradle create their own configuration models and DSLs to be customizable, whether that is programmatically or declaratively. It is easy that in the process of creating a build tool, performance is sacrificed by ease of customization, readability or maintenance.

On the other hand, there is also the problem that build tools move slowly and have a hard time upgrading to new changes upstream (for example, in compiler APIs). As a result, Gradle, Maven, Leiningen, and CBT have not yet been migrated to Zinc 1.0, which offers faster incremental compilations than its predecessor. As a result, users of these tools are not benefiting from the latest state of the art.

bloop aims to fix both problems:

  1. It’s specialized on only providing as fast as possible edit/compile/test workflows, rather than covering all of the needs of general build tool like sbt.
  2. It allows other build tools to integrate with it.

You can think of bloop as a powered CLI for Zinc, rather than as a brand new build tool – bloop does not aim to replace your stock build tool, rather complement it.

In practice, this means that when users are only going to compile and test their project, they can use bloop instead of a full-blown resource-hungry build tool; and that generic build tools can delegate to bloop to compile and test users’ projects instead of creating their own Scala and Java integrations, which are hard to maintain.

Numbers on compilation performance

We created bloop because we observed that Scala compile times slow down when within sbt compared to isolated benchmarks. This observation was at the beginning just an intuition, but when we sat down to measure a prototype we did see a significant difference.

Next, we introduce you some numbers that compare compilation times between sbt and bloop in different medium-to-large open-source projects. In all of them, bloop is significantly faster than sbt at compiling and testing your project.

Project name sbt (version) bloop speedup
sbt/sbt 21s (1.0.4) 15s 1.17
guardian/frontend 37s (0.13.16) 28s 1.32
apache/spark 159s (0.13.16) 107s 1.4
scala/scala 65s (0.13.16) 50s 1.14

These numbers have been obtained on an isolated machine, measuring at the fifth hot clean-compile iteration (of all the projects and subprojects of the build without counting the tests) and with 2 gigabytes of heap for both sbt and bloop (the shell, not the nailgun integration). When benchmarking sbt, we have made sure that dependency resolution and formatting are not measured.

To put those numbers into context, the speedup observed in apache/spark is equal to the compilation speedup that Jason Zaugg introduced in the 2.12.x series. That means that when Spark developers migrate to 2.12 and use bloop, they will experience a compilation speedup close to 60% (40% from upgrading to Scala 2.12.x, 40% from using bloop).

There are two important observations to make from these numbers:

  • In practice, sbt is slower than the previous numbers because dependency resolution is often triggered by compilation, slowing down the start of the compile.
  • Some of the benchmarked builds use sbt 0.13.16 instead of 1.0.4. bloop would be even faster if these builds were migrated to 1.0.4. The refined precision that Zinc 1.0 offers in its incremental mode has a price of ~3% compared to Zinc 0.13.16.

The current status

As of this announcement, bloop is capable of compiling and testing your project, and it integrates with sbt. It supports all the test frameworks sbt supports (JUnit, Scalatest, Scalacheck, utest and more).

bloop is a young project with only three weeks of life. That means that there’s a lot of work left before we support all different platforms and needs. In our next release, we want to focus on:

  • Stabilizing the internals of the project.
  • Supporting console, a faster testQuick, and built-in file watching.
  • Making more performance improvements to our integration, to be even faster.
  • Exploring integrations with other build tools like Maven and Gradle.
  • Supporting Dotty and out-of-the-box cross-compilation to Scala.js and Scala Native.

First beta release 0.1

The first beta version of bloop can be used following the instructions in scalacenter/bloop.

Preserving hot compilers

An important feature of bloop is its nailgun integration. Nailgun is a client, protocol, and server for running Java programs from the command line without incurring the JVM startup overhead. With it, bloop ensures that you always have a hot compiler in the background to compile your project. This brings significant improvements indeed: a hot compiler can compile up to 18 times faster than a cold compiler.

Keeping a hot compiler in the background is crucial if one takes compilation speed and productivity seriously. This is often difficult and takes space of mind. For example, sbt throws away a hot compiler every time users quit (Ctrl-C) and restart, but also when they do reload after several executions of compile. By this rule, the more users modify their sbt build or have bad habits, the less productive they are.

In an effort to address the problem at its root, we have preferred to add a built-in nailgun integration to bloop that always keeps hot compilers in the background.

How does it work?

When bloop starts up, it loads a build from a configuration directory passed in by the user, which defaults to .bloop-config. This directory contains a configuration file per every project, and every configuration file contains basic information about the project: name, base directory, classpath, source directories, classes directory, Scala and Java options and a few more.

bloop comes with an sbt plugin that allows you to generate these configuration files from your existing, functional sbt build. When the generation is done, you will have a .bloop-config in the base directory of your sbt project. With this configuration, you can close your sbt instance and run your build with bloop.

You will need to reach out to sbt again in the following scenarios:

  • You have changed your build (a library dependency, a project name, a scalac option). In which case, you need to rerun the generation of configuration files.
  • You need to execute build tasks unrelated to compilation and test execution.

This configuration file is currently a Java properties file. In the next weeks, we’d like to migrate from it to a more established configuration format. As this information is often required by other developer tools (for example, language servers, presentation compilers, linters), we would like the authors of these tools to agree on a configuration file that all these can understand.

In the future, we want to explore making bloop a server you can ask compilation and test-related information from different language servers, presentation compilers and linters. We’re also interested in rethinking our configuration files and agree with the tooling community on a configuration schema that all the tools in the ecosystem (language servers, build tools, presentation compilers and linters) can use. We believe these initiatives can make tooling much easier to install and upgrade in the future.


We’re happy to announce bloop, a tool with which sbt users can compile their projects faster and execute tests without being tied to an sbt instance. Users of other build tools will get support for bloop in the upcoming weeks.

Bloop’s end goal is to deliver a snappy developer experience to those developers writing Scala code. With this effort, the Scala Center continues its commitment to improve the tooling in the Scala ecosystem and help open source developers and companies alike be more productive.

If you have a question or want to get involved, come to our forums and chat rooms to talk!