Refactor JDK 7 build to use the vendored version of it

Along the path, fix the build for JDK 7 and get rid of
most ugliness in the JDK 7 build. Now simply setting
JAVA_VERSION to 1.7 will build a JDK 7 compatible version.

Fixes #1159.

--
Change-Id: I9599283844a57d9e053f12d37445907f22a9232e
Reviewed-on: https://bazel-review.googlesource.com/#/c/3452
MOS_MIGRATED_REVID=120332747

3 files changed, 60 insertions, 2 deletions

diff --git a/third_party/java/jdk/README.md b/third_party/java/jdk/README.md
new file mode 100644
index 0000000000..02c794ae20
--- /dev/null
+++ b/third_party/java/jdk/README.md
@@ -0,0 +1,23 @@
+# Java compilers in Bazel
+
+Bazel compiles Java code using a custom builder. This builder is called
+JavaBuilder and its code lies in //src/java_tools/buildjar. To build Java
+code, JavaBuilder use the Java compiler from the JDK. To support
+[ErrorProne](http://errorprone.info) checks, we vendor a custom build
+of the Java compiler code. This is the raw version of the Java compiler
+from [OpenJDK](https://openjdk.java.net) but compiled for a lower
+version of the JRE. Those builds are vendored in
+//third_party/java/jdk/langtools.
+
+Current Bazel supports running on a JRE 8 only because the default Java
+compiler used (//third_party/java/jdk/langtools/javac.jar) is the
+Java compiler of OpenJDK 9 compiled to run on a JRE 8. This cannot
+be built to run on a JRE 7 because of code incompatiblity. Bazel's
+JavaBuilder at HEAD cannot be linked with earlier version of the
+Java compiler (it depends on some internals of the Java compiler).
+
+To build a version of Bazel that can run on a JRE 7, we need to rely
+on the version of JavaBuilder provided with Bazel 0.1.0
+(//third_party/java/jdk/javabuilder/JavaBuilder_0.1.0_deploy.jar) which works
+with a Java compiler of OpenJDK 8 compiled to run on a JRE 7
+(//third_party/java/jdk/langtools/javac7.jar).
diff --git a/third_party/java/jdk/javabuilder/BUILD b/third_party/java/jdk/javabuilder/BUILD
new file mode 100644
index 0000000000..f670c45bcd
--- /dev/null
+++ b/third_party/java/jdk/javabuilder/BUILD
@@ -0,0 +1,13 @@
+package(default_visibility = ["//visibility:public"])
+
+licenses(["notice"])  # Apache 2.0 license
+
+filegroup(
+    name = "srcs",
+    srcs = glob(["**"]),
+)
+
+filegroup(
+    name = "JavaBuilder_0.1.0",
+    srcs = ["JavaBuilder_0.1.0_deploy.jar"],
+)
diff --git a/third_party/java/jdk/langtools/BUILD b/third_party/java/jdk/langtools/BUILD
index a542ae28cb..e87fa27c1d 100644
--- a/third_party/java/jdk/langtools/BUILD
+++ b/third_party/java/jdk/langtools/BUILD
@@ -4,7 +4,16 @@ licenses(["restricted"])  # GNU GPL v2 with Classpath exception
 
 filegroup(
     name = "srcs",
-    srcs = glob(["**"]),
+    srcs = select({
+        "//tools/jdk:jdk7": [
+            "BUILD",
+            "javac7.jar",
+        ],
+        "//conditions:default": [
+            "BUILD",
+            "javac.jar",
+        ],
+    }),
 )
 
 java_import(
@@ -14,5 +23,18 @@ java_import(
 
 filegroup(
     name = "javac_jar",
-    srcs = ["javac.jar"],
+    srcs = select({
+        "//tools/jdk:jdk7": ["javac7.jar"],
+        "//conditions:default": ["javac.jar"],
+    }),
+)
+
+java_import(
+    name = "javac7",
+    jars = ["javac7.jar"],
+)
+
+filegroup(
+    name = "javac7_jar",
+    srcs = ["javac7.jar"],
 )