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Add toolchain type for Java bootstrap runtime
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The `bootclasspath` rule in `rules_java` will soon use this toolchain type instead of the regular Java runtime toolchain type, which naturally carries a constraint on the target platform.

Also adds more detailed explanations of the now three Java toolchain types.

Work towards bazelbuild#17085
Work towards bazelbuild#18265
Work towards bazelbuild/rules_java#64
Split off from bazelbuild#18262

Closes bazelbuild#18841.

PiperOrigin-RevId: 545756139
Change-Id: Ib9dd7a1c20c32315375722d6a023a89859daea9c
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fmeum committed Aug 10, 2023
1 parent 7978458 commit dad8759
Showing 1 changed file with 32 additions and 2 deletions.
34 changes: 32 additions & 2 deletions tools/jdk/BUILD.tools
Original file line number Diff line number Diff line change
Expand Up @@ -18,12 +18,42 @@ load("//tools/python:private/defs.bzl", "py_binary", "py_test")

package(default_visibility = ["//visibility:public"])

# Used to distinguish toolchains used for Java development, ie the JavaToolchainProvider.
# A single binary distribution of a JDK (e.g., OpenJDK 17 for Windows arm64) provides three
# different types of toolchains from the perspective of Bazel:

# The compilation toolchain, which provides the Java runtime used to execute the Java compiler, as
# well as various helper tools and settings.
#
# Toolchains of this type typically have constraints on the execution platform so that their Java
# runtime can run the compiler, but not on the target platform as Java compilation outputs are
# platform independent.
#
# Obtain the associated JavaToolchainInfo via:
# ctx.toolchains["@bazel_tools//tools/jdk:toolchain_type"].java
toolchain_type(name = "toolchain_type")

# Used to distinguish toolchains used for Java execution, ie the JavaRuntimeInfo.
# The Java runtime that executable Java compilation outputs (e.g., java_binary with
# create_executable = True) will run on.
#
# Toolchains of this type typically have constraints on the target platform so that the runtime's
# native 'java' binary can be run there, but not on the execution platform as building an executable
# Java target only requires copying or symlinking the runtime, which can be done on any platform.
#
# Obtain the associated JavaRuntimeInfo via:
# ctx.toolchains["@bazel_tools//tools/jdk:runtime_toolchain_type"].java_runtime
toolchain_type(name = "runtime_toolchain_type")

# The Java runtime to extract the bootclasspath from that is then used to compile Java sources.
#
# As the bootclasspath is platform independent, toolchains of this type may have no constraints.
# Purely as an optimization to prevent unnecessary fetches of remote runtimes for other
# architectures, toolchains of this type may have constraints on the execution platform that match
# those on the corresponding compilation toolchain.
#
# Toolchains of this type are only consumed internally by the bootclasspath rule and should not be
# accessed from Starlark.
toolchain_type(name = "bootstrap_runtime_toolchain_type")

# Points to toolchain[":runtime_toolchain_type"] (was :legacy_current_java_runtime)
java_runtime_alias(name = "current_java_runtime")

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