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Understanding the ActionScript Virtual Machine for Java Developers

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The ActionScript Virtual Machine 2 (AVM2) executes ActionScript 3.0 (AS3) bytecode in the Flash Player 9 runtime. ActionScript 3 is an Object Oriented programming language, used by developers to build Flash based applications in Adobe Flex and AIR. AS3 is fully compliant with the Third Edition of the ECMAScript standard.

For the Java developer, the concept of a virtual machine is nothing new, but there are a number of AVM2 features worth reviewing. Per Olesen published a blog last week discussing a number of the AVM2 notable features, including: Strong Typing, Method Closures, Just-in-Time (JIT) Compiler, and Garbage Collection.

Type Information
AS3 supports strong typing, discussed by Olesen:
Prior to AS3, all type information was stripped out of the code when compiled. At runtime, everything was just dynamically typed atoms. Starting with AS3, we can get the type information all the way down to the runtime.

… Using explicitly typed variables can improve performance and reduce memory consumption.
Method Closures
AS3 supports Method Closures, described by Olesen:
In AS3, we got method closures, which also mean we can create variables that are functions, and pass them around, while the function still retains the environment, it was created within ("this" is still what "this" was, when defined).
In Gary Grossman's and Emmy Huang's article, ‘Action Script 3.0 Overview,’ they describe a key usage of Method Closures:
Event handling is simplified in ActionScript 3.0 thanks to method closures, which provide built-in event delegation.

myButton.addEventListener("click", someMethod);
Just-in-Time (JIT) Compiler
JIT compilation is a technique where bytecode is converted into native machine code to improve performance. Adobe’s article ActionScript Virtual Machine 2 (AVM2) Overview details the impact on development:
In practice, the AVM2 may transform the code at run-time by means of a JIT, but this does not affect the semantics of execution, only its performance.
Olesen does note one impact developers should consider:
There is one thing to take notice of with JIT in AVM2. Constructors are not JIT'ed, so if you have performance intensive code in a class, take it out of the constructor.
Garbage Collection
Olesen provides a high level view of the Garbage Collection in AVM2:
The memory management and garbage collector is to be found in the separate MMgc project. This is a Deferred Reference Counting (DRC) mechanism combined with an incremental, conservative mark/sweep collector. Of course, the garbage collector implementation has been tuned for best client performance, with small (30ms) time slices.
For those looking for additional resources, , Farata Systems has a detailed blog entry comparing the language syntax between Java 5 and ActionScript 3. Also, more information can be found on AVM2 by reviewing the Mozilla Tamarin Project. Tamarin is the AVM2 implementation used in the Flash Player 9 and AIR platform. In addition, Adobe has a community Flex site dedicated to Java Developers at,

For Java developers, there are clearly a number of similarities in the ActionScript syntax and virtual machine architecture that should ease the learning process.

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