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
$102.50
Apple Inc.
+0.25
MSFT
$45.43
Microsoft Corpora
+0.55
GOOG
$571.60
Google Inc.
+2.40

MacTech Search:
Community Search:

Software Updates via MacUpdate

VueScan 9.4.41 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
Cloud 3.0.0 - File sharing from your men...
Cloud is simple file sharing for the Mac. Drag a file from your Mac to the CloudApp icon in the menubar and we take care of the rest. A link to the file will automatically be copied to your clipboard... Read more
LibreOffice 4.3.1.2 - Free Open Source o...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
SlingPlayer Plugin 3.3.20.505 - Browser...
SlingPlayer is the screen interface software that works hand-in-hand with the hardware inside the Slingbox to make your TV viewing experience just like that at home. It features an array of... Read more
Get Lyrical 3.8 - Auto-magically adds ly...
Get Lyrical auto-magically add lyrics to songs in iTunes. You can choose either a selection of tracks, or the current track. Or turn on "Active Tagging" to get lyrics for songs as you play them.... Read more
Viber 4.2.2 - Send messages and make cal...
Viber lets you send free messages and make free calls to other Viber users, on any device and network, in any country! Viber syncs your contacts, messages and call history with your mobile device,... Read more
Cocktail 7.6 - General maintenance and o...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
LaunchBar 6.1 - Powerful file/URL/email...
LaunchBar is an award-winning productivity utility that offers an amazingly intuitive and efficient way to search and access any kind of information stored on your computer or on the Web. It provides... Read more
Maya 2015 - Professional 3D modeling and...
Maya is an award-winning software and powerful, integrated 3D modeling, animation, visual effects, and rendering solution. Because Maya is based on an open architecture, all your work can be scripted... Read more
BBEdit 10.5.12 - Powerful text and HTML...
BBEdit is the leading professional HTML and text editor for the Mac. Specifically crafted in response to the needs of Web authors and software developers, this award-winning product provides a... Read more

Latest Forum Discussions

See All

Rhonna Designs Magic (Photography)
Rhonna Designs Magic 1.0 Device: iOS Universal Category: Photography Price: $1.99, Version: 1.0 (iTunes) Description: Want to sprinkle *magic* on your photos? With RD Magic, you can add colors, filters, light leaks, bokeh, edges,... | Read more »
This Week at 148Apps: August 25-29, 2014
Shiny Happy App Reviews   | Read more »
Qube Kingdom – Tips, Tricks, Strategies,...
Qube Kingdom is a tower defense game from DeNA. You rally your troops – magicians, archers, knights, barbarians, and others – and fight against an evil menace looking to dominate your kingdom of tiny squares. Planning a war isn’t easy, so here are a... | Read more »
Qube Kingdom Review
Qube Kingdom Review By Nadia Oxford on August 29th, 2014 Our Rating: :: KIND OF A SQUARE KINGDOMUniversal App - Designed for iPhone and iPad Qube Kingdom has cute visuals, but it’s a pretty basic tower defense game at heart.   | Read more »
Fire in the Hole Review
Fire in the Hole Review By Rob Thomas on August 29th, 2014 Our Rating: :: WALK THE PLANKUniversal App - Designed for iPhone and iPad Seafoam’s Fire in the Hole looks like a bright, 8-bit throwback, but there’s not enough booty to... | Read more »
Alien Creeps TD is Now Available Worldwi...
Alien Creeps TD is Now Available Worldwide Posted by Ellis Spice on August 29th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Dodo Master Review
Dodo Master Review By Jordan Minor on August 29th, 2014 Our Rating: :: NEST EGGiPad Only App - Designed for the iPad Dodo Master is tough but fair, and that’s what makes it a joy to play.   | Read more »
Motorsport Manager Review
Motorsport Manager Review By Lee Hamlet on August 29th, 2014 Our Rating: :: MARVELOUS MANAGEMENTUniversal App - Designed for iPhone and iPad Despite its depth and sense of tactical freedom, Motorsport Manager is one of the most... | Read more »
Motorsport Manager – Beginner Tips, Tric...
The world of Motorsport management can be an unforgiving and merciless one, so to help with some of the stress that comes with running a successful race team, here are a few hints and tips to leave your opponents in the dust. | Read more »
CalPal Update Brings the App to 2.0, Add...
CalPal Update Brings the App to 2.0, Adds Lots of New Stuff Posted by Ellis Spice on August 29th, 2014 [ permalink ] | Read more »

Price Scanner via MacPrices.net

The Rise of Phablets
Carlisle & Gallagher Consulting Group, a businesses and technology consulting firm focused solely on the financial services industry, has released an infographic depicting the convergence of... Read more
Eddy – Cloud Music Player for iPhone/iPad Fre...
Ukraine based CapableBits announces the release of Eddy, its tiny, but smart and powerful cloud music player for iPhone and iPad that allows users to stream or download music directly from cloud... Read more
A&D Medical Launches Its WellnessConnecte...
For consumers and the healthcare providers and loved ones who care for them, A&D Medical, a leader in connected health and biometric measurement devices and services, has launched its... Read more
Anand Lal Shimpi Retires From AnandTech
Anand Lal Shimpi, whose AnandTech Website is famous for its meticulously detailed and thoroughgoing reviews and analysis, is packing it in. Lal Shimpi, who founded the tech site at age 14 in 1997,... Read more
2.5GHz Mac mini, Apple refurbished, in stock...
The Apple Store has Apple Certified Refurbished 2.5GHz Mac minis available for $509, $90 off MSRP. Apple’s one-year warranty is included, and shipping is free. Read more
13-inch 2.5GHz MacBook Pro on sale for $999,...
B&H Photo has the 13″ 2.5GHz MacBook Pro on sale for $999.99 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Read more
Labor Day Weekend MacBook Pro sale; 15-inch m...
B&H Photo has the new 2014 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 Desktop... Read more
Labor Day Weekend iPad mini sale; $50 to $100...
Best Buy has the iPad mini with Retina Display (WiFi models) on sale for $50 off MSRP on their online store for Labor Day Weekend. Choose free shipping or free local store pick up. Price is for... Read more
13-inch 1.4GHz MacBook Air on sale for $899,...
Adorama has the new 2014 13″ 1.4GHz/128GB MacBook Air on sale for $899.99 including free shipping plus NY & NJ tax only. Their price is $100 off MSRP. Read more
It’s Official: Apple Issues Invitations To Se...
Apple has issued one of its characteristically cryptic press invitations for a special event to be held at the Flint Center for the Performing Arts in hometown Cupertino on Sept. 9, 2014 at 10:00 am... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the 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
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.