Bazel: Support building with Java 9

Bazel@HEAD supports Java 9 and the upcoming Bazel release 0.15.0 will
include Java 9 JDK.

Test Plan:

  $ bazel test \
      --host_java_toolchain=@bazel_tools//tools/jdk:toolchain_java9 \
      --java_toolchain=@bazel_tools//tools/jdk:toolchain_java9 //...

Change-Id: I9d6b716391bbf96b0894a8f3be900bcf8ef48903

BUILD

@@ -3,6 +3,13 @@ package(default_visibility = ["//visibility:public"])
 load("//tools/bzl:genrule2.bzl", "genrule2")
 load("//tools/bzl:pkg_war.bzl", "pkg_war")
 
+config_setting(
+    name = "java9",
+    values = {
+        "java_toolchain": "@bazel_tools//tools/jdk:toolchain_java9",
+    },
+)
+
 genrule(
     name = "gen_version",
     outs = ["version.txt"],

Documentation/dev-bazel.txt

@@ -14,6 +14,28 @@ To build Gerrit from source, you need:
 * zip, unzip
 * gcc
 
+[[Java 9 support]]
+Java 9 is supported through alternative java toolchain
+link:https://docs.bazel.build/versions/master/toolchains.html[Bazel option].
+The Java 9 support is backwards compatible. Java 8 is still the default.
+To build Gerrit with Java 9, specify JDK 9 java toolchain:
+
+```
+  $ bazel build \
+      --host_java_toolchain=@bazel_tools//tools/jdk:toolchain_java9 \
+      --java_toolchain=@bazel_tools//tools/jdk:toolchain_java9 \
+      :release
+```
+
+Note that the following option must be added to `container.javaOptions`
+in `$gerrit_site/etc/gerrit.config` to run Gerrit with Java 9:
+
+```
+[container]
+  javaOptions = --add-modules java.activation \
+      --add-opens=jdk.management/com.sun.management.internal=ALL-UNNAMED
+```
+
 [[build]]
 == Building on the Command Line
 

tools/bzl/junit.bzl

@@ -67,7 +67,18 @@ def junit_tests(name, srcs, **kwargs):
     _GenSuite(name = s_name,
               srcs = srcs,
               outname = s_name)
+    jvm_flags = kwargs.get("jvm_flags", [])
+    jvm_flags = jvm_flags + select({
+        "//:java9": [
+            # Enforce JDK 8 compatibility on Java 9, see
+            # https://docs.oracle.com/javase/9/intl/internationalization-enhancements-jdk-9.htm#JSINT-GUID-AF5AECA7-07C1-4E7D-BC10-BC7E73DC6C7F
+            "-Djava.locale.providers=COMPAT,CLDR,SPI",
+            "--add-modules java.activation",
+            "--add-opens=jdk.management/com.sun.management.internal=ALL-UNNAMED",
+        ],
+        "//conditions:default": [],
+    })
     native.java_test(name = name,
                      test_class = s_name,
                      srcs = srcs + [":"+s_name],
-                     **kwargs)
+                     **dict(kwargs, jvm_flags=jvm_flags))