TweetFollow Us on Twitter

Writing Java Cross-Platform

Volume Number: 14 (1998)
Issue Number: 5
Column Tag: Javatech

Writing Java on the Mac for All Platforms

by Ken Ryall

Here are several of the pitfalls of writing multi-platform Java code

It's easier than ever to write your Java code on the Mac for multi-platform use. You can write once on the Mac, then run anywhere, but you'll learn a lot about the way Java is implemented on each platform along the way.

Ironically, the Mac is a good Pure Java starting point. If it works on the Mac it should work on other Java platforms. However, there are differences, often subtle ones. These differences between Java on the Mac and Java on other platforms are often reflections of each underlying operating system. In addition to differences in how each virtual machine is implemented, other potential problem areas include threads, networking, file systems, and user interface elements.

Java Threads

Unlike most other Java platforms, Macintosh Java uses cooperative threads rather than preemptive ones. Java's preemptive threads are simulated on the Mac by a cooperative VM. It will yield time to other Java threads and to the Mac OS when a Java thread's state becomes "not runnable". This happens when a thread is waiting for I/O to complete, is put to sleep, is suspended and in a few other cases.

This difference in underlying thread implementation means you should avoid making assumptions about thread scheduling, priority or timing beyond what is specified in the Java API. Threads of equal priority will get very different treatment depending on the VM.

Also, be aware of what kind of code will let the VM run other threads. For example, when opening a socket and listening for connection requests, the VM will run other threads while waiting for Socket.accept() to return a connected socket.

// bind a socket on port 3000
ServerSocket mySocket = new ServerSocket(3000);
// listen for connection requests
Socket clientSocket = mySocket.accept();

However, the loop below contains no calls to I/O and may block other threads on the Mac. On platforms such as Windows NT other threads would still be allowed to run.

int count = 5000000; // busy pointless loop
while (count > 0)
   count -; // While in loop VM may run other threads, maybe not.

If you have CPU intensive loops like this you'll need to call Thread.yield() to allow the VM to service other threads.

If your application uses threads in more than a casual way you'll need to experiment to find best cross-platform performance. For example, here is a thread with an idle loop that checks things now and again and shares time with other threads by calling Thread.yield():

public void run()
{
   boolean keepGoing = true;
   while(keepGoing)
   {
      // do some stuff...
      // yield time to other threads
      yield();
   }
}

Calling Thread.yield() while looping seems to work fine on the Mac. However, on Windows NT constantly calling Thread.yield() will leave the VM hogging as much of the CPU time as it can get. Putting the thread to sleep for short periods is a better cross platform solution.

public void run()
{
   boolean keepGoing = true;
   while(keepGoing)
   {
      // do some stuff...
      // sleep for ten seconds, this will let other threads run
      sleep(1000 * 10);
   }
}

Developing on the Mac will also make you keep your use of threads simple: more than a few running Java threads hurts performance more than on other platforms.

Networking

The network classes in the java.net package use whatever native TCP/IP services the host machine offers. This means your Java networking code will pick up any eccentricities in the native libraries. A common complaint of cross platform Java developers is that their network sockets work with one platform/VM combination but not another. Results can even vary from machine to machine and network to network.

Networking problems can include socket/stream combinations that don't return all the data until asked multiple times. Other sockets don't seem to want to send their data until closed, even after being flushed.

In one case, my server was returning some data by writing to a stream and closing the socket. This worked fine on Mac OS and Windows NT but sent empty packets from some Windows 95 machines on some networks. The fix? Insert a tiny delay between flushing the stream and closing the socket. How did I find it? Desperate trial and error.

Most of these problems are related to bugs in the various VMs, whose intensive revision has produced evolving levels of functionality and stability. Reviewing some of the many Java networking examples can help identify problems and workarounds. In particular, the web server Jigsaw http://www.w3.org/jigsaw/ is a good example of a complex multi-threaded application. Jigsaw works cross-platform and has undergone more extensive testing than most examples available.

Unix Subtleties

Java includes cross-platform classes for handling files and tries to present a platform neutral API, but it helps to remember that the whole thing came from the Unix world. Nowhere is this more apparent than in the way the File class works. In fact, the Java File class probably creates more cross-platform problems than any other because of its assumptions about the native file system.

To begin with, file names can be longer than the 31 characters that the current Mac file system allows. ( HFS+ supports longer file names internally, but there isn't an API to access them yet.) Lots of Java sample code that has never seen a Mac has files like: "MyReallyCoolJavaThreadedNetworkingClass.java". VMs on the Mac have no problem with longer names in JAR files, you just can't have the original source files. This can be a problem if you're working with developers on other platforms who like very long file names. Until the long filenames in HFS+ are accessible, you'll either have to get them to be less verbose, or build their work into libraries so you won't have to work with their source files directly.

Various VMs also make different assumptions about case-sensitivity of file names and allowable characters in file names. Remember that most virtual machines began life as Unix source code and may be incompletely ported to their new native file systems.

In Java, files are specified by path name, but trying to build a path name in a platform neutral fashion is difficult because VMs return inconsistent and conflicting values for descriptions of parent directories and separator characters. Making a path name that will work with one VM isn't too difficult, making one that is cross platform friendly is often much more difficult.

Cross platform Java developers are frustrated by this and usually try to simplify file specification so they don't have to build path names. If you do, be sure to use File.pathSeparator() instead of hard coding a path separator. Also, examine your file naming schemes and remove characters that aren't allowed on any of your target platforms.

User Interface

Differences in platform user interface mean that a some of your interface may not come out looking like you intended. Fortunately you can often tweek placements, sizes etc. in a platform neutral way, you'll just need to try out the results on each of your target platforms as you go. Be sure to use layout managers, not absolute locations, to place items and avoid assumptions about things like font metrics and screen size.

Also, every VM on every platform has its own AWT bugs and display glitches, and these change as the VMs are revised. Most of these you'll be able to see and work around.

Don't forget that while you can write a lot of pure java cross platform code, you can still determine what platform is running your Java code and act accordingly.

      String whichOS = System.getProperty("os.name");
      if (whichOS.contains("Mac"))
      {
         // do mac specific stuff here
      }

Testing

Testing cross-platform Java applications is simple if tedious; you just have to try it with every VM/platform combination you hope to use it with. In particular, exercise the features that use networking, threads, and deal with files. Check the UI carefully for missing elements and overlapping items.

Some of the Java tools are less mature than their C++ equivalents so there aren't as many debuggers and other testing utilities to work with. Of course, there is the old fashioned way: System.out.println(). Finding out how things work with virtual machine X on platform Y is mostly a matter of trial and error.

It's also helpful to find other developers who are sharing their experiences with the VMs you are targeting. For Mac VMs a good place to start is the MRJ mailing list at http://www.lists.apple.com/mrj.html and there is also the Metrowerks newsgroup at news://news.metrowerks.com/comp.sys.mac.programmer.codewarrior.

Installing

Java's platform independence quickly vanishes when its time to install and launch your application. Each platform has its own VM installation and method for launching a Java application. If you want to look just like any other professional application you'll need to build an installer for each platform that includes a VM and puts all your pieces where you want them: maybe the "Startup Items" folder on the Mac or as a service on NT.

This can be complex and tough to maintain, but is no worse than installing a complex cross platform C++ application. A more Java-oriented solution is InstallAnywhere, available from Zero G Software http://www.zerog.com. InstallAnywhere is written in Java and lets you build an single universal installer that will work on any platform, including setting the runtime environment, and installing a VM.

Write Once...

Developing cross platform Java code on the Mac really works if you'll keep a few things in mind. You can write once and run anywhere as long as you test and make a few adjustments. Finding solutions to cross-platform problems will often reveal problem code that slipped by the original complier or VM and needs to be reworked anyway. If you'll willing to deal with a few platform-specific problems and a wider variety of VM behavior, you'll have the satisfaction of developing your code on the Mac, and the ability to drop the result on an NT machine and find that it all just works.


Ken Ryall, kryall@outer.net, has been developing on the Mac since 1985 and is currently working on the debugger at Metrowerks in Austin, TX. When not at work he's usually out flying or at home trying to convince his border terrier not to chew up the ISDN line again.

 
AAPL
$105.22
Apple Inc.
+0.39
MSFT
$46.13
Microsoft Corpora
+1.11
GOOG
$539.78
Google Inc.
-4.20

MacTech Search:
Community Search:

Software Updates via MacUpdate

OS X Server 4.0 - For OS X 10.10 Yosemit...
Designed for OS X and iOS devices, OS X Server makes it easy to share files, schedule meetings, synchronize contacts, develop software, host your own website, publish wikis, configure Mac, iPhone,... Read more
TotalFinder 1.6.12 - Adds tabs, hotkeys,...
TotalFinder is a universally acclaimed navigational companion for your Mac. Enhance your Mac's Finder with features so smart and convenient, you won't believe you ever lived without them. Tab-based... Read more
BusyCal 2.6.3 - Powerful calendar app wi...
BusyCal is an award-winning desktop calendar that combines personal productivity features for individuals with powerful calendar sharing capabilities for families and workgroups. BusyCal's unique... Read more
calibre 2.7 - Complete e-library managem...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital... Read more
Skitch 2.7.3 - Take screenshots, annotat...
With Skitch, taking, annotating, and sharing screenshots or images is as fun as it is simple.Communicate and collaborate with images using Skitch and its intuitive, engaging drawing and annotating... Read more
Delicious Library 3.3.2 - Import, browse...
Delicious Library allows you to import, browse, and share all your books, movies, music, and video games with Delicious Library. Run your very own library from your home or office using our... Read more
Art Text 2.4.8 - Create high quality hea...
Art Text is an OS X application for creating high quality textual graphics, headings, logos, icons, Web site elements, and buttons. Thanks to multi-layer support, creating complex graphics is no... Read more
Live Interior 3D Pro 2.9.6 - Powerful an...
Live Interior 3D Pro is a powerful yet very intuitive interior designing application. View Video Tutorials It has every feature of Live Interior 3D Standard, plus some exclusive ones: Create multi... Read more
The Hit List 1.1.7 - Advanced reminder a...
The Hit List manages the daily chaos of your modern life. It's easy to learn - it's as easy as making lists. And it's powerful enough to let you plan, then forget, then act when the time is right.... Read more
jAlbum Pro 12.2.4 - Organize your digita...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code!... Read more

Latest Forum Discussions

See All

Rami Ismail Opens Up distribute​() for D...
Rami Ismail Opens Up distribute​() for Developers Posted by Jessica Fisher on October 24th, 2014 [ permalink ] Rami Ismail, Chief Executive of Business and Development at indie game studio | Read more »
Great Hitman GO Goes on Sale and Gets Ne...
Great Hitman GO Goes on Sale and Gets New Update – Say That Three Times Fast Posted by Jessica Fisher on October 24th, 2014 [ permalink ] | Read more »
Rival Stars Basketball Review
Rival Stars Basketball Review By Jennifer Allen on October 24th, 2014 Our Rating: :: RESTRICTIVE BUT FUNUniversal App - Designed for iPhone and iPad Rival Stars Basketball is a fun mixture of basketball and card collecting but its... | Read more »
Rubicon Development Makes Over a Dozen o...
Rubicon Development Makes Over a Dozen of Their Games Free For This Weekend Only Posted by Jessica Fisher on October 24th, 2014 [ permalink ] | Read more »
I Am Dolphin Review
I Am Dolphin Review By Jennifer Allen on October 24th, 2014 Our Rating: :: NEARLY FIN-TASTICUniversal App - Designed for iPhone and iPad Swim around and eat nearly everything that moves in I Am Dolphin, a fun Ecco-ish kind of game... | Read more »
nPlayer looks to be the ultimate choice...
Developed by Newin Inc, nPlayer may seem like your standard video player – but is aiming to be the best in its field by providing high quality video play performance and support for a huge number of video formats and codecs. User reviews include... | Read more »
Fighting Fantasy: Caverns of the Snow Wi...
Fighting Fantasy: Caverns of the Snow Witch Review By Jennifer Allen on October 24th, 2014 Our Rating: :: CLASSY STORYTELLINGUniversal App - Designed for iPhone and iPad Fighting Fantasy: Caverns of the Snow Witch is a sterling... | Read more »
A Few Days Left (Games)
A Few Days Left 1.01 Device: iOS Universal Category: Games Price: $3.99, Version: 1.01 (iTunes) Description: Screenshots are in compliance to App Store's 4+ age rating! Please see App Preview for real game play! **Important: Make... | Read more »
Toca Boo (Education)
Toca Boo 1.0.2 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0.2 (iTunes) Description: BOO! Did I scare you!? My name is Bonnie and my family loves to spook! Do you want to scare them back? Follow me and I'll... | Read more »
Intuon (Games)
Intuon 1.1 Device: iOS Universal Category: Games Price: $.99, Version: 1.1 (iTunes) Description: Join the battle with your intuition in a new hardcore game Intuon! How well do you trust your intuition? Can you find a needle in a... | Read more »

Price Scanner via MacPrices.net

Weekend sale: 13-inch 128GB MacBook Air for $...
Best Buy has the 2014 13-inch 1.4GHz 128GB MacBook Air on sale for $849.99, or $150 off MSRP, on their online store. Choose free home shipping or free local store pickup (if available). Price valid... Read more
Nimbus Note Cross=Platform Notes Utility
Nimbus Note will make sure you never forget or lose your valuable data again. Create and edit notes, save web pages, screenshots and any other type of data – and share it all with your friends and... Read more
NewerTech’s Snuglet Makes MagSafe 2 Power Con...
NewerTech has introduced the Snuglet, a precision-manufactured ring designed to sit inside your MagSafe 2 connector port, providing a more snug fit to prevent your power cable from unintentional... Read more
Apple Planning To Sacrifice Gross Margins To...
Digitimes Research’s Jim Hsiao says its analysts believe Apple is planning to sacrifice its gross margins to save its tablet business, which has recently fallen into decline. They project that Apple’... Read more
Who’s On Now? – First Instant-Connect Search...
It’s nighttime and your car has broken down on the side of the highway. You need a tow truck right away, so you open an app on your iPhone, search for the closest tow truck and send an instant... Read more
13-inch 2.5GHz MacBook Pro on sale for $949,...
Best Buy has the 13″ 2.5GHz MacBook Pro available for $949.99 on their online store. Choose free shipping or free instant local store pickup (if available). Their price is $150 off MSRP. Price is... Read more
Save up to $125 on Retina MacBook Pros
B&H Photo has the new 2014 13″ and 15″ Retina MacBook Pros on sale for up to $125 off MSRP. Shipping is free, and B&H charges NY sales tax only. They’ll also include free copies of Parallels... Read more
Apple refurbished Time Capsules available sta...
The Apple Store has certified refurbished Time Capsules available for up to $60 off MSRP. Apple’s one-year warranty is included with each Time Capsule, and shipping is free: - 2TB Time Capsule: $255... Read more
Textilus New Word, Notes and PDF Processor fo...
Textilus is new word-crunching, notes, and PDF processor designed exclusively for the iPad. I haven’t had time to thoroughly check it out yet, but it looks great and early reviews are positive.... Read more
WD My Passport Pro Bus-Powered Thunderbolt RA...
WD’s My Passport Pro RAID solution is powered by an integrated Thunderbolt cable for true portability and speeds as high as 233 MB/s. HighlightsOverviewSpecifications Transfer, Back Up And Edit In... Read more

Jobs Board

*Apple* Solutions Consultant - Apple Inc. (U...
…important role that the ASC serves is that of providing an excellent Apple Customer Experience. Responsibilities include: * Promoting Apple products and solutions Read more
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Project Manager / Business Analyst, WW *Appl...
…a senior project manager / business analyst to work within our Worldwide Apple Fulfillment Operations and the Business Process Re-engineering team. This role will work Read more
*Apple* Retail - Multiple Positions (US) - A...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.