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ParparVM – Open Source JVM for iOS

ParparVM

The safe simple & easy way to build native iOS apps in Java

ParparVM – Open Source JVM for iOS ParparVM is the VM developed by Codename One to replace the defunct XMLVM with which it was originally built. We took our extensive experience both in JIT’s and native OS development and built something that is both simple, concervative and performant.

Usage

The ByteCodeTranslator and JavaAPI projects are designed as a NetBeans project although it should be possible to work with any Java IDE or ant directly. It requires asm 5.0.3 which you can find in thecn1-binaries project.

You can run the translation process using:

java -jar ByteCodeTranslator.jar ios path_to_stub_class:path_to/vm/JavaAPI/build/classes;path_to_your_classes  dest_build_dir MainClassName com.package.name "Title For Project" "1.0" ios none

Once the translation process succeeds you should have a valid xcode project that you can run and use as usual. You will need a Mac for this to work.

The main class name is expected to have a public static void main(String[]) method and it is assumed to reside in the com.package.name directory (figuratively, you need to replace com.package.name with your actual package passed to the translator).

Why Another VM for iOS?

There are many VM’s on the market and a few open source ones but none of the ones that translate to C are actively maintained.

J2ObjC is an excellent tool for porting libraries to Objective-C but it isn’t designed to be a full scale VM (no GC etc.).

The other VM’s (Avian & RoboVM) are pretty impressive technically since they compile directly to ARM/LLVM bitcode. That is a very problematic approach:

  • While this is an impressive feat Apple doesn’t officially support this route. That means that with every transition Apple makes (64bit, bitcode etc.) hurdles occur. This isn’t some theoretical issue but something that has created serious stumbling blocks to such projects.

    ParparVM had a seamless migration to iOS 9 (no code changes!) and had a relatively easy 64 bit transition! By comparison RoboVM’s CEO wrote this :

"Our work to add full support for iOS 9 in time for its public release was one of the most daunting challenges we’ve faced in our existence"

  • These VM’s use the entire Android class libraries resulting in relatively large apps. They also take longer to compile.

  • Xcode’s tools can’t be used to their full extent with such tools, with ParparVM you can just use Xcodes amazing profiler and related tools out of the box since all the code is C code

  • Hacking these tools requires some deep VM/ASM/LLVM knowledge. ParparVM is trivial by comparison and requires a bit of Java bytecode knowledege and some C.

Besides these advantages ParparVM also embeds the a concurrent GC logic directly into the code, this is very similar to ARC in some aspects. The ParparVM GC is concurrent and doesn’t need to "stop the world" for typical code, its relatively trivial and written entirely in C.

Performance

Since the Xcode C compiler is VERY fast the performance of ParparVM should be pretty good. However, it performs badly and sometimes horribly in microbenchmarks. However, its performance in "real life" situations is generally very good.

GC Overhead

Normally VM’s separate the GC to a completely separate thread and disconnect from it. ParparVM doesn’t do that and so you end up with an overhead that is paid in user threads in order to reduce GC stalls.

This is a price we gladly pay, it removes the UI stalls normally associated with GC code in favor of theoretical method performance.

Method Overhead & Stack

ParparVM uses code that mimics the JVM stack bytecodes, this makes the translation process very fast and simple. It also makes the GC easy since the stacks are maintained "as is" but maintaining a software stack has a slight overhead that prevents the compiler from truly optimizing away some cases.

We have handcoded optimization methods that optimize away common use cases, e.g. getters/setters are effectively free under ParparVM. But this work should probably be extended further.

Java Level Support

We aimed the VM at Java 5 support and overlay the Java 8 support with retrolambda, it should work for some Java 8 syntax out of the box simply because of ASM’s ability to parse newer class files.

The API is relatively limited in scope to keep the size low, we would like to add additional API’s ideally with very concise implementations to avoid bringing over the full JDK.

Relation To Codename One

ParparVM is used by Codename One internally, its open source and we have no intention to change that.

Parpar has no dependency on Codename One itself that we know of but some might exist inadvertantly since Codename One is the only target we have.

Support

We try to answer all questions tagged codenameone on stackoverflow but since these questions might be esoteric its possible that we won’t be able to answer some of them. The Codename One discussion group is generally aimed at Codename One development and not at using the source code/native. Since these are advanced topics they might dillute the discussion there.

Areas Of Interest

Performance

We would like to improve the performance of the VM further while keeping source/binary size down and reducing compilation time (tall order). This can be accomplished by:

  • Additional optimization options – stack elimination, method init elimination.
  • Exception processing – currently we don’t rely on CPU code for exception detection. We can rely on some processor specific behavior to implement null pointer, array index out of bounds etc.
  • Better dead code elimination and better inlining logic
  • Better handling of interfaces which have a big overhead

Crash handling

Currently VM crashes arn’t graceful, it should be pretty easy to extract VM state and log it to a file that can be used on next launch. Since most of the code related to the stack tracking is in C it should be accessible and easy to log this on a crash.

Ports

When we started this work we envisioned a C# compilation target as well. However, with the changes to Windows Phone 8 C might be a better target for Windows development. Porting the VM to other OS’s could be very interesting, we tried to keep the code portable but quite a few things are specific to Objective-C it should be possible to adapt most of those things.

The Name

Parpar is a butterfly in Hebrew.

Initially when we worked at Sun and produced LWUIT we wanted to call it Morpho (butterfly) but marketing shot it down. Unfortunately MorphoVM is already taken so we decided to go with ParparVM instead.

原文  https://github.com/codenameone/CodenameOne/tree/master/vm

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