implement jshell support

[rogerkeays]

$ jshell -C-Xplugin:fluent
An exception has occurred in the compiler (17.0.2). Please file a bug against the Java compiler via the Java bug reporting page (http://bugreport.java.com) after checking the Bug Database (http://bugs.java.com) for duplicates. Include your program, the following diagnostic, and the parameters passed to the Java compiler in your report. Thank you.
java.lang.NoClassDefFoundError: sun/misc/Unsafe
	at com.sun.tools.javac.comp.Fluent.init(Fluent.java:26)
	at jdk.compiler/com.sun.tools.javac.api.BasicJavacTask.initPlugin(BasicJavacTask.java:255)
	at jdk.compiler/com.sun.tools.javac.api.BasicJavacTask.initPlugins(BasicJavacTask.java:229)
        ...

We need to replace Unsafe to be future proof anyway.

Unsafe source: https://github.com/openjdk/jdk/blob/master/src/jdk.unsupported/share/classes/sun/misc/Unsafe.java

[rogerkeays]

We can replace com.sun.misc with jdk.internals.misc, and use -J--add-opens=java.base/jdk.internal.misc=ALL-UNNAMED to start the plugin, but still fails with jshell:

$ jshell -C-Xplugin:fluent -J--add-opens=java.base/jdk.internal.misc=ALL-UNNAMED
Exception in thread "main" java.lang.InternalError: Exception during analyze - java.lang.NullPointerException: Cannot invoke "java.io.InputStream.available()" because "<local3>" is null
	...
Caused by: java.lang.NullPointerException: Cannot invoke "java.io.InputStream.available()" because "<local3>" is null
	at com.sun.tools.javac.comp.Fluent.reload(Fluent.java:57)
	at com.sun.tools.javac.comp.Fluent.init(Fluent.java:40)
	... 27 more

so, a classloading problem.

[rogerkeays]

• jshell inits the plugin for every invocation: easily fixed with a null check

• jshell uses it's own custom resolver: not so easy to fix:

  Exception in thread "main" java.lang.InternalError: Exception during analyze - java.lang.ClassCastException: class com.sun.tools.javac.comp.Fluent$FluentResolve cannot be cast to class jdk.jshell.TaskFactory$DisableAccessibilityResolve (com.sun.tools.javac.comp.Fluent$FluentResolve is in module jdk.compiler of loader 'app'; jdk.jshell.TaskFactory$DisableAccessibilityResolve is in module jdk.jshell of loader 'app')