A Standalone Android Runtime: Running HelloWorld The Hard Way

Whichever way HelloWorld gets launched there is a point at which the runtime will start to execute the code in the HelloWorld package HelloWorld.apk. Which is a problem. HelloWorld.apk is a standard Android package built using the Android development tools so what is in it, as you might expect, is a classes.dex file, which is nice if you are the Dalvik VM but not so great if you are another kind of JVM that does not talk Dalvik bytecode, and ours does not.

So why not put the class files in the package when its being built and use them ? Where’s the fun in that ? The contents of classes.dex started out as some class files, so lets turn it back into the class files on demand !

To do this we need a specialized ClassLoader which is pretty simple, in this case at least, a Dex loader, likewise, and a DVM to JVM bytecode compiler, which is a bit trickier. Fortunately the HelloWorld application is doing nothing much at all so there are not a large number or variety of instructions to compile.

Loading the Dex file and compiling the code back into JVM bytecode on-the-fly is not necessarily how you would solve this problem for real but it will work in this case.

Compiling is not the most visual of activities so instead here is the trace from the compiler which is invoked by the ClassLoader when the first class in the HelloWorld application is accessed by the runtime. To make life simpler at this point we load the entire Dex file and compile everything in sight, which is not a lot as you can see. The compiler is printing each DVM instruction as it compiles it.

Process[25001]:
Process[25001]: Class standalone/helloworld/HelloWorldActivity
Process[25001]:
Process[25001]: compiling <init>
Process[25001]:
Process[25001]: invoke-direct Landroid/app/Activity; <init> ()V
Process[25001]: return-void
Process[25001]:
Process[25001]: compiling onCreate
Process[25001]:
Process[25001]: invoke-super Landroid/app/Activity; onCreate (Landroid/os/Bundle;)V
Process[25001]: const #+2130903040
Process[25001]: invoke-virtual Lstandalone/helloworld/HelloWorldActivity; setContentView (I)V
Process[25001]: return-void
Process[25001]:
Process[25001]:
Process[25001]:
Process[25001]: Class standalone/helloworld/R$attr
Process[25001]:
Process[25001]: compiling <init>
Process[25001]:
Process[25001]: invoke-direct Ljava/lang/Object; <init> ()V
Process[25001]: return-void
Process[25001]:
Process[25001]:
Process[25001]:
Process[25001]: Class standalone/helloworld/R$drawable
Process[25001]:
Process[25001]: compiling <init>
Process[25001]:
Process[25001]: invoke-direct Ljava/lang/Object; <init> ()V
Process[25001]: return-void
Process[25001]:
Process[25001]:
Process[25001]:
Process[25001]: Class standalone/helloworld/R$layout
Process[25001]:
Process[25001]: compiling <init>
Process[25001]:
Process[25001]: invoke-direct Ljava/lang/Object; <init> ()V
Process[25001]: return-void
Process[25001]:
Process[25001]:
Process[25001]:
Process[25001]: Class standalone/helloworld/R$string
Process[25001]:
Process[25001]: compiling <init>
Process[25001]:
Process[25001]: invoke-direct Ljava/lang/Object; <init> ()V
Process[25001]: return-void
Process[25001]:
Process[25001]:
Process[25001]:
Process[25001]: Class standalone/helloworld/R
Process[25001]:
Process[25001]: compiling <init>
Process[25001]:
Process[25001]: invoke-direct Ljava/lang/Object; <init> ()V
Process[25001]: return-void

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Copyright (c) 2009 By Simon Lewis. All Rights Reserved.

A Standalone Android Runtime, Or Look Ma No Dalvik !

Having experimented with getting the Dalvik VM to run independently of the Android OS and application runtime, I thought I would try and turn the problem on its head and see whether it was possible to get the Android application runtime to start-up and run a simple HelloWorld app independently of the Android OS and the Dalvik VM, using a standard Java environment, to begin with a desktop JSE, but possibly CDC/whatever later on. To make it more interesting I wanted to do it without running any native (non-Java) code, at least initially. After all the Android runtime is written in Java so hard could it be ?

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Copyright (c) 2009 By Simon Lewis, All Rights Reserved.

Workaround

It looks as though the problem with mmap() is that the filesystem doesn’t support making mmap()ed files writable which prevents Dalvik as currently implemented from optimizing its Dex file, which prevents it from starting. A workaround for this is to provide Dalvik with an optimized Dex file we prepared earlier. If you do then you get this

Dalvik Running Standalone (but not quite working) on an N800

A screenshot of Dalvik running in an xterm on an N800

The error (which has scrolled off the top) seems to be a failed call to mmap()

Dalvik Running Standalone On MacOSX

When the source for Google’s Dalvik Java VM became available as part of the Android open source release I thought I would see whether it was possible to build and run it standalone, i.e not as part of Android.

The answer is yes, and here’s a shot of it running in the debugger under Xcode on MacOSX to prove it

Unfortunately the canonical HelloWorld application on its own doesn’t make for particularly interesting viewing

With verbose logging on the output is certainly a lot more voluminous.

Here’s the debugger window scrolled to different positions to show all of it