Question

I need to run Java applications on top of Linux on my Beaglebone. I know that ARM cores do have support for Jazelle technology to execute Java bytecode in hardware. Anyway it is not clear to me what I need to exploit this technology. Do I need to “activate” Jazelle in some way? Do I need a Jazelle aware Java Virtual Machine? What is the flow for deploying and running Java software which exploits Jazelle technology? Is a license required for using Jazelle?

Thanks.

Answer 1

Jazelle is an execution mode in ARM architecture which “provides architectural support for hardware acceleration of bytecode execution by a Java Virtual Machine (JVM)” – quoted from ARM Architecture Manual.

It also looks like there are two flavours of Jazelle; DBX and RCT. Following text is from Cortex-A Series Programmer’s Guide (v3).

2.2.2
Jazelle
Jazelle-DBX (Direct Bytecode eXecution) was introduced in ARMv5TEJ to accelerate Java
performance while conserving power. A combination of increased memory availability and
improvements in just-in-time (JIT) compilers have since reduced its value in application
processors. As a result, many ARMv7-A processors do not implement this hardware
acceleration.

Jazelle-DBX is best suited to providing high performance Java in systems with very limited
memory (for example, feature phone or low-cost embedded use). In today’s systems, it is mainly used for backwards compatibility.

2.2.3 Thumb Execution Environment (ThumbEE)
Introduced and required in ARMv7-A, ThumbEE is sometimes referred to as Jazelle-RCT
(Runtime Compilation Target). It involves small changes to the Thumb instruction set that make
it a better target for code generated at runtime in controlled environments (for example, by
managed languages like Java, Dalvik, C#, Python or Perl).
ThumbEE is designed to be used by Just-In-Time (JIT) or Ahead-Of-Time (AOT) compilers,
where it can reduce the code size of recompiled code. Compilation of managed code is outside
the scope of this document.

To my knowledge most of Jazelle-DBX’s use is in Java Card implementations where you really want to have lots of performance supplied by a tiny chip. This really restricted domain can take advantage of some extra boost by executing bytecode on cpu directly.

However most of the modern embedded systems like your Beaglebone already have more memory and cpu power than when Java made its debut on typical desktops of those times which renders Jazelle-DBX not necessary. That’s mostly because you get JIT compilation to native host. (JIT implementations would be too big for Java Cards.)

ThumbEE / Jazelle-RCT’s usefulness is also questionable. Quoting from an android-porting discussion:

Going through all that to get free null-pointer checks doesn’t make
much sense.

To conclude, I am not aware of any Java VM implementation utilizing any Jazelle flavour including Dalvik of Android.
If I had to run Java on an embedded device, I would choose between; Dalvik, JamVM or Java SE Embedded.