Commits on Jan 23, 2018

1. [build] Cursory JDK 9 support …

   We have a VSTS+macOS build machine which has JDK 9 as the default JDK,
   thus causing *all* xamarin-android builds to fail, as `gradlew` and
   JDK 9 don't *directly* mix:
   
   	Executing: ./gradlew assembleDebug --stacktrace
   	...
   	org.gradle.api.ProjectConfigurationException: A problem occurred configuring project ':library'.
   	  ...
   	Caused by: org.gradle.internal.event.ListenerNotificationException:
   	Failed to notify project evaluation listener.
   	  ...
   	Caused by: java.lang.NoClassDefFoundError: javax/xml/bind/annotation/XmlSchema
   	  ...
   	Caused by: java.lang.ClassNotFoundException: javax.xml.bind.annotation.XmlSchema
   
   When using JDK 9, `gradlew` fails because *something* within it
   attempts to use the deprecated type
   `javax.xml.bind.annotation.XmlSchema`, and JDK 9 doesn't provide
   deprecated modules in the default `$CLASSPATH`.
   
   Knowing that the VSTS machine *also* has JDK 8 installed, we've tried
   to "filter out" JDK 9 so that it wouldn't be used; see e.g. a3c4358.
   Unfortunately that doesn't work, because on macOS the default JDK used
   is [always the JDK with the highest version number][macOS-jdk], and
   this can't be easily changed system-wide because changing this
   behavior requires "removing" a `Info.plist` file to prevent the JDK
   from being "seen" by `/usr/bin/java`:
   
   [macOS-jdk]: https://stackoverflow.com/a/44169445
   
   	# Don't use JDK 9 by default on macOS:
   	$ cd /Library/Java/JavaVirtualMachines/jdk-9.0.4.jdk/Contents
   	$ mv Info.plist Info.plist.disabled
   
   (Renaming `Info.plist.disabled` to `Info.plist` will restore JDK 9 as
   the default JDK on the machine.)
   
   A "proper" fix would presumably involve using and exporting
   `$JAVA_HOME` *everywhere*, which isn't currently the case.
   [This is something that *should* be explored.][xa-1213]
   
   [xa-1213]: dotnet#1213
   
   In the meantime, a question: *Can* we build with JDK 9?
   *Is it even possible in the first place*?
   
   Turns out, *yes*, it *is* possible. The above `gradlew` error can be
   fixed by providing the (new-in-JDK9) `--add-modules` option, so that
   the `java.xml.bind` module is accessible:
   
   	$ ANDROID_HOME=... JAVA_OPTS="--add-modules java.xml.bind" ./gradlew assembleDebug --stacktrace --no-daemon
   	# ...works!
   
   Unfortunately, *other* parts xamarin-android get in the way, e.g. the
   `<ResolveSdks/>` task doesn't correctly parse the version number out
   of `java -version`, and JDK 9 `javac` requires `-source` and `-target`
   when `-bootclasspath` is used.
   
   Update the build system and tests so that things *can* be built under
   JDK 9.
   
   Additionally, cleanup the internal build system a bit: instead of
   having `javac` and `jar` calls strewn throughout *7* different
   projects -- all of which would need to be updated to provide usable
   `javac -source` and `javac -target` values (JDK 9 doesn't support
   `-target 1.6` anymore!), introduce `build-tools\scripts\Jar.targets`
   and the new `BuildTestJarFile` target. This takes all
   `@(TestJarEntry)` files, compiles them with `javac`, and `jar`s the
   compiled files into `%(TestJarEntry.OutputFile)`.
   
   **Note**: This does ***NOT*** mean that building and/or using JDK 9
   will be commercially supported in *any* way. (It might not even work!)

   

   jonpryor committed Jan 23, 2018
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