JVM (JNI) bindings for Go
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README

what??
======
	Sometimes a tiny (or substantial) bit of data exists in a 'Java' world;  This might include 
3rd-party Java libraries, or an implementation that go's not ready to handle yet ('extreme XML'
comes to mind).

	Using GoJVM, a dynamically linked JVM can be loaded, and Java objects and classes can be manipulated,
allowing Go to 'call into' instantiated classes, and manage portions of logic w/r/t Java.

why-in-gods-name
================
	To be honest, I just got frustrated proxying java calls in arbitrary ways to
a go method, in order to handle a vendor library that only shipped in 
C++ or Java.  Since go's C++ support is pretty terrible but Java's C support is
surprisingly good, this is the route I've chosen for gojvm's design.

running
=======
	If (and you will), get errors about the binaries not able to find their libjni...so, you will need
to export a dynamic-loader path.

	Most systems do not ship with their native Java .so's on their dynamic load path.  On _my_ (debianish)
system, commands that will load the JVM will need to be prefixed, simliar to:

LD_LIBRARY_PATH=/usr/lib/jvm/default-java/jre/lib/amd64/server/		gotest

	(alternatively, wrapped in a tiny shell script [not included] that does same)

disclaimer
==========
	- The local use case is very primative Java, so uses beyond are entirely unsupported (though patches welcome!)
		- this can be mitigated with 'primitive shims' that re-state complex java ideas into something simpler for GoJVM to understand.
	- I can all but guarantee that there are bugs, crashes, and JVM nonos somewhere in the code.
  - I am 100% certain that certain actions leak memory or references on the JVM side, so if this is a concern to you,
be sure you understand JNI references better than I did when I started writing it :)

reflection
==========
	Fairly weak right now, but support for basic types & very basic slices are supported

	Go provides no way (to my knowledge) to dynamically create a struct, so magic go is
impossible w/o a magic intermediate compilation phase.

	Java reflection is fairly strong, but also provides no way of dynamially defining a class
(without access to a .class compiler);  (TODO: Java includes one IIRC, why not make use of it <g>).

	The implementation is generally naieve about results -- it holds a reference but may not provide
a trivial way to access its semantics.  Part of this derives from certain JVM limitations, some from
Go, but basically, generalized reflection is very verbose code (and not idiomatic in either Java or Go),
so that a caller (or wrapper) will typically have to know more details about the expected underlying 
structure of an Object than would be gleanable by trivial refletion at run-time.

sizes
=====
	We make certain assumptions in reflection: that a go int can be casted to a
java int (same for long/int64, etc).  On my system they seem to prove nominally
fine (so far), but tests have not been written that really excercise boundary
conditions on types.

strings
=======
	Java uses UTF-16, Go uses UTF-8;  When using the CallString and other
functions, all references and conversions are handled, otherwise the 
user of the Object is responsible for dereferencing and UTF conversions.

	As odd as it seems - strings are one of the worst types for GoJVM, since
both Java and Go have native UTF string types, but differ in terms of
default semantics.  A guaranteed low-bit ascii string is trivial, but
to ensure portability and safety, all explicit string methods are converted by
either the Go or Java VM (currently the Java VM handles all conversions).

	If the Go side needs only the reference and not the content of the string, 
it is recommended to use the XXXObj functions rather than the XXXString,
as no automatic conversion will be made at that time.

tests
=====
	Will fail unless you've built (or distributed) the java class-files.
		'make java_classes' should accomplish this on a 'standard' system..
	[ a patch/how to make 'gotest': depend on this would be appreciated]

	Generally, an x86 style-platform is assumed, and mainline build & testing is done against the Sun JRE (6),
however the design is from JNI spec and interestingly, the lack of a strong Java heritage makes our 'naive' 
implementation more robust against non-Java approaches on the JVM.
	Basic testing of reference handling is done, however as a private/new project, it likely has areas that
leak like scieves or perform like sludge. As the projects understanding of the JNI (and real-world use) happens,
it is expected some performance improvements will be gleaned, but it is NOT a goal of gojvm to match native
go or Java speeds to cross go/Java speeds.

benchmarks
==========
	In many of the current benchmarks, we've included a 'reference' go 
implementation.  To use them as a comparison against the 'JVM interactive' 
tests is an expressly unfair comparison.  While Java is  'write once, run 
anywhere that happens to match the semantics and limitations of my test 
environment', JNI is not.  Java has to do a great deal of work to handle 
requests of the JNI interface, whereas a strictly Java or Go implementation
has no such 'border' concerns.

	The results serve as a basis point for general ratios between native and cross
calls, but do NOT indicate that Go native would be superior to Java native without
more conclusive measures.

performance suggestions
=======================
	As gojvm improves, certain caching properties will improve, making the 
performance /closer to C calling into JNI/, but it is NOT expected to ever 
perform better than code that stays in the JVM or Go entirely, since the
performance of GoJVM depends on coordination from both.

	That's not to say you can't benefit from gojvm, but that the border wall
is not 'invisible', no matter how much syntactic sugar is on top of it.
The 'JNI book' says keep your interfaces small, but that is not
entirely correct -- performing a loop that requires un/de/re/marshalling
across languages will suffer far more than if one created a stub in the
source language to do the loop, and pass the results to the other.

	If your goal is to interact with a Java process under your control, 
in a non performance-critical way - the trivial interfaces will probably
suffice with some wrapping.

	If your goal is to maximize performance of critical sections across
a varied language set, be _very_ careful to check your performance metrics.

	For 'serious' performance between go and the JVM, it is recommended
that you establish a communications channel that is NOT directly through
the JNI interface (e.g., passing an file-descriptor between Go and Java),
and using that for communications.  At that point Java and Go are operating
in independant threads, and /shouldn't/ step on each other.

	Ultimately, you will want proxies on each side that make sense in each
context, and GoJVM only aims to provide a means and examples of that,
not 'the final way you will access the JVM from go'.  As (if) the project
grows, I imagine more general & derivable solutions will be made, but without
changes to the go or jvm (or use of the compilers from at least one side),
I do not anticipate ever having a trivial 'obj.DoSomething(withThis)' being
valid go (without a wrapper)..

jvm notes & observations
========================

	Trivial types (int, long, etc), are generally converted safely, but
there are (probably) cases that if the JVM sizes don't match the expected go 
size that precision could be arbitrarily lost.  I don't know of such a system,
but if this paragraph scares you, then let me know the 'right' way to handle
yours.

	jBooleans are stored as bytes, zero being false and all others being true,
thus a direct (and portable) comparison to C.JNI_TRUE is not correct;  consider
the case of a JNI method interacting with a (different) JNI method.  If one
writes true as 0x01, and the other as 0xff, it is likely that using 'TRUE' 
comparisons  would lead to unexpected failure.

portability
===========

	If compiled on the host system, the only constraints are mentioned under
'sizing' (e.g., long=int64, int=int(32), short=int16, etc). 

	If you are shipping product, note that the host must provide an appropriate 
set of JNI .so/.dll's to link to, and the user will need to provide an 
LD_LIBRARY_PATH (probably).	Both of which may be host/architecture specific,
amd should not be hard coded (as they are in the examples...) GoJVM does not 
(by itself) produce a cross-platform binary, but a compiled GoJVM binary can 
rely on any arch-appropriate or arch-independant java .class files.

	Callbacks (from Java to Go) use and abuse the C variadic macros.  This
seems to work fine on Amd64 (and I believe should on i386), but I am
not convinced this approach is portable.  Do let me know if you test it
on other architectures.

threading
=========
	Access to the environment must be on the threads designated *Environment 
pointer.  If you are uncertain, it is safe to call the contexts 
'AllocCurrentThread', but an uncertain caller should _NOT_ Dealloc the 
current thread.

	GoJVM does not specifically track deallocations/dereferences, so an
unsafe call betwixt will blow up go or the JVM in interesting, but generally
unpleasant and non-stack-tracey ways.  You are encouraged to use defer
judiciously. (Dev-tip: If you've wonked the JVM and  ^C and ^\ don't 
work, ^Z sometimes does;  you MUST reap the process via an  appropaite 
'kill %' or kill -KILL %' in order to reap them if the JVM is in
a state that 'requires' this behaviour.

exceptions
==========
	Published calls through *Environment (on the appropriate thread) shall
catch their own exceptions and they shall be returned on err;  The state of
the JVM on return with exception is /cleared/ of exceptions, however the 
jthrowable is in the opaque portion of the *Exception object.

	If you use *Environment JVM calls directly, you MUST provide a deferral
to unref any mess you leave out of the JVM on success/error/exception, 
or you will leak.

	For as good as the 'JNI book' is, there is some vagueness about the safety
of certain Exception calls in non-exceptional states.  Still, all exceptional
functions are considered safe to call during an exception, so I expect the worst
case is that you'd possibly generate an exception through misuse, rather than
wedge the JVM.

Code, Copyright & License
=========================
	All reasonably holdable copyrights are held (c) 2011, James D. Nurmi, Abneptis LLC.

	Current code is licensed to you under the terms of the  GPLv3+A.