Bootstrapping OpenJDK 8

sudo emerge -a1v =dev-java/icedtea-3.21.0
sudo emerge -av dev-java/openjdk:8
sudo emerge -a --depclean # Remove old software that was used for bootstrapping

Notes:

1. you need to do it in two steps as currently ant-core:bootstrap is not co-installable with ant-core:0 from the Gentoo tree.
2. Last tested on GCC 11. New version of GCC are known to occasionally break builds and might need additional patches.

Bootstrapping OpenJDK 11

sudo emerge -a1v =dev-java/openjdk-11.0.9_p11

At this point you can remove this overlay and install OpenJDK from main tree:

sudo emerge -a dev-java/openjdk:11

Bootstrapping story

We first compile fastjar and jikes java compiler both of which are written in C++. This is enough to compile an old version of GNU Classpath 0.93. Then we compile an old version of JamVM 1.5.1. This serves as our initial Java Runtime Environment.

We use this to compile ant 1.8.1 which then allows us to build ecj-3.2. This allows us to compile GNU Classpath 0.99 which comes with more java tools (e.g. javah). Then we compile JamVM 2.0.0. This serves as our second Java environment.

Finally, we compile git snapshot of GNU Classpath which has a much better support for Java 1.6. Then we compile a slightly patched ecj-4.2. Together with JamVM 2.0.0 this will serve as Java environment that can bootstrap Icedtea 2 (Java 7). Then we use it to build Icedtea 3 and subsequent OpenJDK versions.