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Bootstrapping OpenJDK 8
# If you are on arm64, you first need to unmask app-arch/fastjar
sudo emerge -a1v =dev-java/icedtea-3.7.0
sudo emerge -av dev-java/openjdk:8
sudo emerge -a --depclean # Remove old software that was used for bootstrapping
Note: 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.
Bootstrapping OpenJDK 11
sudo emerge -a1v =dev-java/openjdk-11.0.5_p10
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). Unfortunately, I was only able to build Java 7 with JamVM as VM (which is slower), Hotspot build failed. JamVM with OpenJDK 7 classes can build Icedtea 3 with Hotspot VM.