We've just published Sanjay Dasgupta's new article Simplify Native Code Access with JNA. In this article, Sanjay describes the path through which Java developers can apply Java Native Access to directly interact with native operating system APIs, presenting several working examples that access Microsoft Windows native APIs, and also delving into the complexities of memory mapping and alignment issues.
Sanjay has packaged up his examples into a single zip file that you can download and unpack into a directory. His package includes the jna.jar file, a build.bat file that you execute from within a Windows command window, and individual *.bat files that you run from within a Windows command window to launch the individual applications.
The applications include a "Hello World" app that signals "Hello World" in Morse code using the Windows kernel's Beep() function; a Java implementation of the Windows LockWorkStation() function; a program that brings into Java the Windows list of available logical drives; and an app that applies TWAIN to connect to a scanning or image device.
I may have greater interest in "low-level" issues than many developers. After all, I started out counting bits and bytes passed over RS-232 serial wires and GPIB cables in the late 1970s and early 1980s. I also spent plenty of time and effort working on the type of memory issues that come into play when you try to integrate someone's legacy library written in a language other than the one that your controlling system is written in, where you have to control and manage their software from a different language.
So, the latter portion of Sanjay's article, where he describes and analyzes memory alignment issues that can arise when you attempt to link Java to native Windows API functions via JNA -- I guess I find that discussion both interesting and comforting. Because in that section, Sanjay makes clear the type of not-obvious problems you may have to deal with when you interface what are basically "alien" languages (Java and a native operating system API). Operating systems and their APIs assume things regarding bits and bytes. And different systems and languages, choosing among the possible options, implement things differently.
Memory is fundamental to operating systems and languages. If you have a memory block that is subdivided into various elements, what are the rules for those subdivisions? Should each element start at a "word" boundary? But, then, what's a "word"? The definition can differ for different OSs. Do you pad with unused bytes if one element ends at a memory location that is not a natural boundary? Or do you start the next element in the middle of that memory "word"? But wouldn't it be weird to do that? Well, to some language and OS designers, that seemed weird, to others it didn't (why waste memory?).
The fact that someone had to make these decisions for each OS and language, and the fact that a toss of the coin was sometimes as good a means of making a choice as anything else (since what's "best" really isn't clear), means that communications between "aliens" (like Java and a native OS API) can be fraught with misinterpretation due to differing assumptions about memory boundaries, memory application, and memory alignment.
I really like Sanjay's treatment of these issues! It's one of the clearest presentations I've seen.
If you need to interface Java with APIs from a native OS, for example, Windows -- or if you're just curious about how you'd go about accomplishing this -- take a look at Sanjay Dasgupta's Simplify Native Code Access with JNA. It's an excellent presentation of the relevant issues!
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