SquirrelJME As A Runtime


There are two portions of SquirrelJME, the SquirrelJME Virtual Machine and the SquirrelJME Runtime. The virtual machine is the one which executes the program and provides an environment that runs on the operating system or other bare hardware. The runtime is the class libraries which provide a standard set of classes and interfaces which are used by Java programs. This document describes the requirements which are needed to have a virtual machine which can use the SquirrelJME runtime.

Note that J2ME was renamed to Java ME, so any mentions of Java ME retroactively mentions J2ME.

There are two cases where the runtime is used, those two cases are pure and hosted. There are two major differences between the two:

SquirrelJME operates in one of two fashions: Single Program Mode where only a single program is ran at once which eventually terminates, and Launcher Mode where a launcher is available and multiple programs can be launched and ran at the same time.

Requirements of Java ME

Java ME is different from Java SE and operates in a slightly different fashion. However, every conforming Java SE JVM can run Java ME programs but the same is not possible in most cases because Java ME is a subset of Java SE.

JAR Resource Lookup

When using Class.getResourceAsStream() in Java ME, there is a strict method in how resource lookup is performed. A single JAR is considered to be a single unit where resources and classes are located. A class within one unit is not able to access the resources in another unit. Class files should not be visible to this method and not accessible as resources, the reason for this is that output executables may be ROMized which would destroy the class files that executable code is derived from.

As an example, here is a set of two JAR files:

This would be the result of multiple Class.getResourceAsStream() calls from each class:

This reason for this is that in each JAR, there is a resource called META-INF/MANIFEST.MF. This resource is used and looked up my programs which are MIDlets in order to obtain their application properties. It also is used by the run-time to determine what a JAR is and what it supports.

Class Loading And Lookup

Unlike Java SE, there are no ClassLoaders. Java ME operates entirely on a single two tier approach. The first tier are classes which are built-in and available to every program. The second tier are classes which are not built-in and which have been loaded dynamically from the launcher. When a class is looked up, the order is always built-in classes first. If a program is currently being executed then it may only look up classes which exist in its execution context. If two programs are loaded they are both in two different execution contexts and they cannot lookup each others classes. Thus if two JARs have the same class, it will only use the class that is in their same JAR.

Required Methods

For simplicity, sanity, and portability; SquirrelJME has a very simple means to provide support for many systems and is designed in a way to reduce the amount of work that is required to support various systems. This means that porting SquirrelJME will be very simple and straightforward once the base semantics of the virtual machine are implemented.

The package net.multiphasicapps.squirreljme.unsafe is a very special package which contains the methods which are used as a bridge to native code or other bridging code. The package is designed in a public and private fashion. Since the methods will be needed across many methods in other packages, the first major interface are the public interfaces. The public interfaces allow for checks to be performed so that sane values and such are used on input for the private implementation. The private implementation are the ones that porters will implement to provide functionality, all of these classes are in the form of __Ext_foo__ where foo is the functionality group. These classes are only visible to the net.multiphasicapps.squirreljme.unsafe package which means that they do not have to worry as much of being exposed for exploitive purposes. This also allows for virtual functionality to be provided and a means for backwards compatibility and migration.

The public classes call into the private classes, but due to the hidden nature it is not required for the classes to actually exist, just that they have the functionality provided. For example, the JVM can replace any instance of a call in the unsafe package to the private class with just a function pointer while disregarding the actual class. This would mean that these private classes could just be declared native and be linked by the JVM as required.

Since SquirrelJME's JVM is not being used, there are some requirements of how some of these classes are implemented.

SystemEnvironment (Ext_systemenvironment`)

SquirrelJME uses a basic service system similar to ServiceLoader except it provides services on a system level. Unlike ServiceLoader which provides multiple services, only single services are available at a time and once a service object is initialized it will remain available to the program until it exits always returning the same instance (that is, it is a singleton).

As an example, the JVM may provide a framebuffer which needs to be accessed by SquirrelJME's LCDUI API. In this case the net.multiphasicapps.squirreljme.lcdui.DisplayManager class needs to be implemented. In this example environment, that class is implemented as foo.FooDisplayManager. In order for the runtime to find that class, the full name of the class must be returned in the __Ext_systemenvironment__.mapService() method. When that service is requested, that class will be loaded and stored and returned so that the LCDUI code may use it to provide a display interface.

SystemVM (__Ext_systemvm__)

__Ext_systemvm__.isSquirrelJMEJVM() should always return false because using just the run-time is not using the JVM.