Bootstrapping

[jgneff]

2022-09-22 - Moved this section out of the README file.

Building the JDK requires that you already have a JDK for your target operating system and architecture. The JDK used to build the JDK is called the Boot JDK. Furthermore, the minimum version required for the Boot JDK is either the previous version or, for an early-access build, the most recently released version. This requirement presents a challenge when you're just getting started and want to build OpenJDK using only trusted sources.

The most trusted source of software for Debian-based distributions is the set of system package repositories. Snap packages have access to the repositories of either Ubuntu 18.04 LTS, using the core18 base, or Ubuntu 20.04 LTS, using the core20 base. When the initial Snap package was built, the current release was JDK 15 but the latest release of the Ubuntu OpenJDK package was JDK 11 for core18 and JDK 14 for core20.

That gave me two options to create a chain of trusted builds for OpenJDK 15. The first option was to use the core20 base with Ubuntu OpenJDK 14 as the initial Boot JDK:

Option 1

Ubuntu 20.04 LTS with GLIBC 2.31 (core20 base)
Ubuntu OpenJDK 14 (openjdk-14-jdk-headless)
    ↳ Snap OpenJDK 15 (openjdk/latest/candidate)
        ↳ Snap OpenJDK 16 (openjdk/latest/beta)
        ↳ Snap OpenJDK 17 (openjdk/latest/edge)

The second option was to use the core18 base with Ubuntu OpenJDK 11 as the initial Boot JDK:

Option 2

Ubuntu 18.04 LTS with GLIBC 2.27 (core18 base)
Ubuntu OpenJDK 11 (openjdk-11-jdk-headless)
    ↳ Snap OpenJDK 12 (bootjdk/latest/candidate)
        ↳ Snap OpenJDK 13 (bootjdk/latest/beta)
            ↳ Snap OpenJDK 14 (bootjdk/latest/edge)
                ↳ Snap OpenJDK 15 (openjdk/latest/candidate)
                    ↳ Snap OpenJDK 16 (openjdk/latest/beta)
                    ↳ Snap OpenJDK 17 (openjdk/latest/edge)

I started this project using the first option, but now all of the Snap packages are built on the older Ubuntu 18.04 LTS release and were bootstrapped using the second option.

The problem with the first option is that the programs end up requiring very recent versions of the Linux kernel and GNU C library. Building on the core20 base creates Snap packages that require GLIBC version 2.29 or later. Building on the core18 base, on the other hand, lowers the GLIBC requirement to version 2.27.

The lower requirement lets the package fully support older systems such as Ubuntu 18.04 LTS and Fedora 28 when running in an unconfined environment. For even older systems, you can run the JDK programs using the confined and self-contained package commands or aliases.