Allow/use JAVA_HOME

[jonpryor]

The default xamarin-android build uses whatever java is in $PATH, that the manner to override this choice is ill-defined. Additionally, the default xamarin-android build artifacts (xabuild) will likewise use whatever java is in $PATH.

Unfortunately, on macOS it is rather "weird" to control the the default JDK which is used when more than one is installed, making the "use whatever java is in $PATH" behavior sub-optimal.

There is already a way to specify the JDK to use for app builds: $HOME/.config/xbuild/monodroid-config.xml contains a /monodroid/java-sdk/@path attribute which is the path to the JDK to use.

However, defaults matter, and the default is $PATH.

We should do something smarter here, so that when we encounter a machine with both JDK 1.8 and JDK 9 installed, we prefer JDK 8.

[directhex]

Dumb question: why not try a few built-in paths after JAVA_HOME, before $PATH?

Like

Why not call the fully-qualified path from a common location?

/Library/Java/JavaVirtualMachines/jdk1.8.0*/Contents/Home/bin/
/usr/lib/jvm/java-8-openjdk*/bin/
/usr/lib/jvm/java-1.8.0*/jdk/bin/
C:\Program Files\Java\jdk1.8.0\bin\
C:\Program Files (x86)\Java\jdk1.8.0\bin\

That way you don't need to care about the user's default.

[jonpryor]

This should be fixed as of dotnet/java-interop@4bd9297 and dotnet/java-interop@bed2958 and 9bc51ae.

[awattar]

@jonpryor Are there any plans to support other standard Java/Android variables for SKD and NDK?