Java 8 and Java 11 in the same Bazel build via Transitions

POC to show how Bazel's transitions feature allows you to build Java code using multiple versions of Java in the same repo

Proof that it works

% bazel build -k //src:bin8_deploy.jar //src:bin11_deploy.jar
DEBUG: /Users/keithl/java-transition-test/src/defs.bzl:26:10: VALUE: 
DEBUG: /Users/keithl/java-transition-test/src/defs.bzl:27:10: {"eight": [<target //src:lib>, <target //src:lib2>], "eleven": [<target //src:lib>, <target //src:lib2>]}
DEBUG: /Users/keithl/java-transition-test/src/defs.bzl:29:10: [<target //src:lib>, <target //src:lib2>]
DEBUG: /Users/keithl/java-transition-test/src/defs.bzl:26:10: VALUE: 
DEBUG: /Users/keithl/java-transition-test/src/defs.bzl:27:10: {"eight": [<target //src:lib>, <target //src:lib2>], "eleven": [<target //src:lib>, <target //src:lib2>]}
DEBUG: /Users/keithl/java-transition-test/src/defs.bzl:29:10: [<target //src:lib>, <target //src:lib2>]
INFO: Analyzed 2 targets (0 packages loaded, 0 targets configured).
INFO: Found 2 targets...
INFO: Elapsed time: 0.807s, Critical Path: 0.64s
INFO: 7 processes: 1 internal, 4 darwin-sandbox, 2 worker.
INFO: Build completed successfully, 7 total actions

% javap -verbose -cp bazel-bin/src/bin8_deploy.jar com.stripe.keithtest.Dep3 | grep version 
  minor version: 0  
  major version: 52

% javap -verbose -cp bazel-bin/src/bin11_deploy.jar com.stripe.keithtest.Dep3 | grep version
  minor version: 0
  major version: 55
  
% for i in 8 11; do echo -n "$i: "; bazel-bin/src/bin$i --print_javabin; echo; done
8: /Users/keithl/java-transition-test/bazel-bin/src/bin8.runfiles/zulu_jdk8/bin/java
11: /Users/keithl/java-transition-test/bazel-bin/src/bin11.runfiles/local_jdk/bin/java

% /Users/keithl/java-transition-test/bazel-bin/src/bin8.runfiles/zulu_jdk8/bin/java -version
openjdk version "1.8.0_181"
OpenJDK Runtime Environment (Zulu 8.31.0.1-macosx) (build 1.8.0_181-b02)
OpenJDK 64-Bit Server VM (Zulu 8.31.0.1-macosx) (build 25.181-b02, mixed mode)

% /Users/keithl/java-transition-test/bazel-bin/src/bin11.runfiles/local_jdk/bin/java -version
openjdk version "11.0.11" 2021-04-20 LTS
OpenJDK Runtime Environment Corretto-11.0.11.9.1 (build 11.0.11+9-LTS)
OpenJDK 64-Bit Server VM Corretto-11.0.11.9.1 (build 11.0.11+9-LTS, mixed mode)

Notes about bazel-bin

Bazel only creates one bazel-bin symlink, which usually points to the JDK11 output dir. To access outputs of JDK8 targets (without bazel run) you'll need to find the bazel-out directory corresponding to JDK8. At Stripe we solve this by creating a bazel-bin-jdk8 symlink. We use a Bazel aspect to write a JSON file, and a small Bazel wrapper script (in tools/bazel) to parse those and create/update the bazel-bin-jdk8 symlink. Please reach out if you'd like some sample code for this.