7. Bootstrap


The Fantom compiler is written in Fantom itself - which presents a chicken and the egg problem. How do you compile the compiler without having a compiler? The problem is further compounded by the fact that the build scripts themselves are written in Fantom. To solve this problem, the bootstrap process requires two Fantom installations:

  1. rel: known good Fantom installation (typically the last build)
  2. dev: development environment to build (probably pulled from github)

By convention we structure our development directory tree like this:


The "rel" directory always contains the last released build. The "fan" directory contains our main development code branch.

Bootstrap Script

The easiest way to perform a bootstrap build from git is to use the "bootstrap.fan" script:

  1. Download latest released build from https://fantom.org/
  2. Unzip that build to {relHome}
  3. Ensure git is installed in your path (see Git)
  4. Verify java_home env var points to your JDK installation
  5. Execute {relHome}/bin/fan {relHome}/adm/bootstrap.fan
  6. Go grab a cup of coffee

NOTE: the default setup requries JDK 1.8 to recompile Fantom.

By default devHome is assumed to be a peer to relHome called "fan". If you'd like to use another directory then specify the "-devHome" option.

This script will perform a number of steps:

  1. Verifies your environment
  2. Clones git repo or if repo exists performs a pull
  3. Configures your etc files
  4. Rebuilds your devHome environment from scratch

The rest of this chapter covers all the internal details required to fully understand bootstrap building.

Dev Home

By default the build assumes devHome to be the home directory of Fantom installation. For example if you rebuild the Java runtime, then the output goes into "devHome/lib/java/sys.jar". In the case of the rel installation we don't want this default because we will overwrite ourselves (which leads to some nasty problems). So you need to set the devHome prop in "etc/build/config.props" of your rel installation to reference the dev directory using a URI (not OS path):

// Must be configured boot build in substitute/release installation

If you forget to do this, then you will get a build error which looks something like this:

ERR: Must update 'devHome' for bootstrap build


On a clean machine with only source code, we don't have any pods compiled such as sys, build, or compiler. In order to run the build scripts to compile these pods, we need to use our rel installation.

All bootstrap build scripts should have the following unix shebang line as the first line in the script. This should be done even if you are only building on Windows.

#! /usr/bin/env fansubstitute

On Unix, the fansubstitute script explicitly sets FAN_HOME to the value of the FAN_SUBSTITUTE variable before launching. On Windows, the launcher script looks for this line in the build script you are about to run. If found, it sets FAN_HOME to the value of the FAN_SUBSTITUTE before launching. So, in either case, you will need to export FAN_SUBSTITUTE to reference your rel installation. And of course you have to run your scripts as executables so that the shebang takes effect.


You need JDK 1.8 to compile from source.

In order to compile from source you will need to setup some config properties to reference your JDK home directory:


This paths should be formatted as file system Uris (not OS paths). This property should be configured in "etc/build/config.props" of both your rel and dev environments.


The "buildall.fan" script is the top level build script for compiling the Fantom distribution. We commonly run this command to rebuild everything and run tests on every pod:

buildall full test

The "buildall" script is executed by the rel substitute runtime and in turn launches two sub-scripts. The "buildboot.fan" script manages rebuilding the core runtime modules:


Once the bootstrap modules are compiled, the development environment is self hosting and can be used to compile the remainder of itself. This is done via the "buildpods.fan" script.


The bootstrap issue can cause some confusing dependency issues which are summarized here:

  • The rel compiler will be generating the sys, compiler, and build pod files. This means that the rel compiler must be able to generate fcode that the dev runtime can read. It also means that the rel compiler must be able to read any new syntax used by dev versions of sys, compiler, and build.
  • The rel compiler will actually use the dev versions of the pods to resolve dependencies. For example dev compiler can reference new sys APIs defined in dev but not rel. Under the covers this works because the compiler and build pod's build scripts specify a non-default dependsDir.

Because of these restrictions, adding new language features and fcode changes require some careful planning.


You can use the "dumpEnv" build target to dump key aspects of your build script environment. You can run target on "buildall.fan", although a more concise report is to dumpEnv on "buildboot.fan" and one of the non bootstrap pods like "testSys/build.fan".

Key things to note about your environment:

  1. all scripts should be using dev for devHomeDir
  2. bootbuild scripts should be using rel for fanHome
  3. non-bootbuild scripts like testSys should be using dev for fanHome
  4. verify javaHome for sys/javafan/build.fan


In summary, you want to make sure of a couple key things:

  1. setup your rel installation and never touch it (consider it readonly)
  2. ensure jdkHome is configured in both rel and dev etc/build/config.props
  3. ensure rel etc/build/config.props devHome points to your dev installation
  4. make sure the FAN_SUBSTITUTE environment variable points to the rel installation
  5. only put dev bin your path and always run your scripts from the dev installation
  6. never use a working repo for bootstrap (use only the boot repo)