Squawk: A Java VM for Wireless Sensor Networks
TS-1598

Squawk is the Java Virtual Machine for the Sun Small Programmable Object Technology (Sun SPOT) wireless sensor/actuator device designed by Sun Labs. Industry and academia have received this device with much excitement, because it brings Java technology to the world of wireless sensor/actuators, allowing developers to use standard development tools to work directly on device. The current state of the art requires developers to program in low-level languages with little or no debugging support while running on device.

Squawk, a research project at Sun Labs to investigate the challenges of supporting Java programming for embedded devices, implements a full Java Platform, Micro Edition (Java ME) --formerly J2ME platform--VM. Squawk's tiny size--80 KB RAM and 270 KB flash, including CLDC and hardware libraries--makes it ideal for deployment on the Sun SPOT device. Squawk is one of the smallest publicly known Java ME platform VM implementations that enables Java technology on wireless sensor devices.

Squawk's reference implementation is written in the Java programming language. The interpreter and garbage collector are translated to C for speed on the Sun SPOT device. Squawk was designed for small, memory-constrained devices and deploys Java technology-based applications on the Sun SPOT device that are one third the size of their classfile counterparts.

By supporting interrupts and device drivers in Java technology, Squawk is able to run on the bare metal without an underlying operating system, thus further reducing the overall footprint.

Squawk also provides a research implementation of JSR 121 (isolates), and multiple isolates can be run by the VM, enabling multiple applications to be run on the Sun SPOT device at the same time, without the extra memory overhead of standard JVM software implementations.

Further, applications (isolates) are allowed to migrate from one Sun SPOT device to another, providing for ease of reprovisioning in the field, remote debugging, and more. This presentation discusses the internals of the Squawk VM: Squawk's split VM architecture, design decisions to make application code smaller, how isolates are implemented and migrated within Sun SPOT devices, how interrupts and device drivers are supported by the VM, and extensions to the general connection framework (GCF) to support the radio (i.e., radio://).

The session includes several demonstrations and example code to illustrate particular features of the VM in a deployed wireless sensor application context.

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