Fix path mapping logic in `bootclasspath` for generated files

Copybara Import from https://github.com/bazelbuild/rules_java/pull/195

BEGIN_PUBLIC
Fix path mapping logic in `bootclasspath` for generated files (#195)

If the files comprising the Java runtime are not source files, the path to the Java home needs to be path mapped.

Work towards bazelbuild/bazel#22366

Closes #195
END_PUBLIC

COPYBARA_INTEGRATE_REVIEW=https://github.com/bazelbuild/rules_java/pull/195 from fmeum:java-home-path-map 43323b73d0f1bfc5ee24de64c2a9037746780879
PiperOrigin-RevId: 648662342
Change-Id: Idba93798982878d3f542b54d69c9536efd721db2

toolchains/default_java_toolchain.bzl

diff --git a/toolchains/default_java_toolchain.bzl b/toolchains/default_java_toolchain.bzl
index 020b101..b3d4d28 100644
--- a/toolchains/default_java_toolchain.bzl
+++ b/toolchains/default_java_toolchain.bzl
@@ -214,6 +214,9 @@
 # https://github.com/bazelbuild/bazel/commit/a239ea84832f18ee8706682145e9595e71b39680
 _SUPPORTS_PATH_MAPPING = {"supports-path-mapping": "1"}
 
+def _java_home(java_executable):
+    return java_executable.dirname[:-len("/bin")]
+
 def _bootclasspath_impl(ctx):
     exec_javabase = ctx.attr.java_runtime_alias[java_common.JavaRuntimeInfo]
 
@@ -252,10 +255,18 @@
     args.add(bootclasspath)
 
     any_javabase = ctx.toolchains[_JAVA_BOOTSTRAP_RUNTIME_TOOLCHAIN_TYPE].java_runtime
-    args.add(any_javabase.java_home)
+    any_javabase_files = any_javabase.files.to_list()
+
+    # If possible, add the Java executable to the command line as a File so that it can be path
+    # mapped.
+    java_executable = [f for f in any_javabase_files if f.path == any_javabase.java_executable_exec_path]
+    if len(java_executable) == 1:
+        args.add_all(java_executable, map_each = _java_home)
+    else:
+        args.add(any_javabase.java_home)
 
     system_files = ("release", "modules", "jrt-fs.jar")
-    system = [f for f in any_javabase.files.to_list() if f.basename in system_files]
+    system = [f for f in any_javabase_files if f.basename in system_files]
     if len(system) != len(system_files):
         system = None