[build] option to filter by JI_MAX_JDK on make prepare

[jonathanpeppers]

Context:
https://stackoverflow.com/questions/47627499/does-android-studio-3-support-java-9-for-android-development

At the current time, we will not be able to use JDK 9 for Android. Some
of our build agents (VSTS), now have JDK 9 installed, so we need to
make a few changes to make sure JDK 8 is picked up instead.

Changes:

• Create a JI_MAX_JDK option, as a way for xamarin-android to
  exclude JDK 9
• Create new JI_JAVAC_PATH and JI_JAR_PATH make variables, which
  will be the full path to javac and jar
• Use awk to filter on <= JI_MAX_JDK
• Use sed to find the JDK version number, see options of folder names
  below
• sort -n should be used to sort numerically
• Set JI_JAVAC_PATH and JI_JAR_PATH to their full paths
• Support both Darwin and Linux, Windows support is handled in
  xamarin-android currently

Known JDK folder names

macOS:

1.6.0.jdk
jdk1.7.0_79.jdk
jdk1.8.0_101.jdk
jdk1.8.0_152.jdk
jdk-9.0.1.jdk

Ubuntu:

java-8-openjdk-amd64
java-9-openjdk-amd64

[atsushieno]

Are you ignoring Windows and Linux?

[jonpryor]

I like the sentiment. I don't like the idea.

The underlying idea behind Java.Interop is that it should be Java version agnostic, to the degree that this is supportable and makes sense.

It certainly makes sense that we be able to run Java.Interop against JDK 9.

The problem with JDK 9 isn't Java.Interop; it's xamarin-android, and the Android SDK. Hardcoding this restriction into Java.Interop sounds like a violation of separation of concerns.

What I'd prefer -- and offhand I have no idea how this would actually work in practice -- is that the make prepare target "somehow" take a "JDK filter", which xamarin-android could provide. This would allow Java.Interop to pick the latest-and-greatest JDK running around, while allowing xamarin-android to restrict things to just JDK 8.

[atsushieno]

Is this filtering correct? On Linux I have I have /usr/lib/jvm/java-8-openjdk-amd64 which would be filtered out by this if this were also applied to Linux.

[jonpryor]

This is within an ifeq ($(OS),Darwin) block, so this change won't impact Linux.

[atsushieno]

I keep writing "if it were... on Linux".

[jonathanpeppers]

I think @atsushieno is asking if 1.8 will work on Linux since his folder says 8.

Looked on ubuntu and I have symbolic links:

$ ls -la /usr/lib/jvm/
...
lrwxrwxrwx   1 root root   20 Oct 23 17:43 java-1.8.0-openjdk-amd64 -> java-8-openjdk-amd64
lrwxrwxrwx   1 root root   20 Aug 24 12:15 java-1.9.0-openjdk-amd64 -> java-9-openjdk-amd64
drwxr-xr-x   5 root root 4096 Dec 18 22:58 java-8-openjdk-amd64
drwxr-xr-x   8 root root 4096 Dec 18 23:00 java-9-openjdk-amd64

I just did apt install open-jdk-8-headless and open-jdk-9-headless.

But still, I need to:

• Make the Java 9 restriction an option that can be passed in by xamarin-android
• Fix this for Linux, too
• On Windows, JdkInfo.props is generated by an MSBuild task in xamarin-android, so I won't have to deal with it until then

[jonathanpeppers]

@jonpryor this has the sed/regex changes we discussed today, should be good to go