May 2004, Issue 166

Embedded Java Controllers

PROGRAMMING TOOLS

AJile provides a few applications to program and debug your JStamp/JStik. These come with the development versions of the hardware.

Jembuilder takes Java class files and converts them to a form that can be loaded into the controller. This is a type of linker. Charade loads this file and acts as an interactive terminal/debugger.

JSwat is a debugger. I haven’t used it, so I can’t comment on it. Although I admit to occasionally writing buggy code, I’ve learned other methods of debugging Java.

In addition, you need a Java compiler. Sun’s Java works fine. A text editor to create the source files is useful, although some people prefer an IDE. All of the applications run in Windows.

EMBEDDED JAVA

If you’re used to running Java on a larger machine, the connected limited device configuration (CLDC) version of Java is limited. Some things are left out because they aren’t needed (e.g., the entire java.awt.* hierarchy is gone). However, most of the core libraries remain, although in a somewhat smaller form. Many of the methods and classes not needed in an embedded environment have been removed. For instance, the main hierarchy is reduced to java.io, java.lang, java.util, and javax.microedition.io. Each of these packages is pruned to have only the classes needed for embedded systems. You can download the latest version of the CLDC standard from Sun.

The main thing you need to know is that the JStamp and JStik do not support normal preemptive threading. For all normal threads, you must use Thread.yield(), Thread.sleep(), or some sort of blocking I/O in order to switch threads. This makes for a much more defined real-time behavior when threading is used.

The other main difference is that the I/O packages are much more limited. There are no java.net.* packages. Replacing this is the javax.microedition.io package, which deals with I/O at a somewhat lower level than standard Java. However, the javax.comm package works fine when dealing with the serial ports.

Note that the CLDC is one of the configurations of Java 2 Micro Edition. AJile has pledged to eventually provide the other configuration, but it isn’t available at this time.