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Thursday, March 15, 2007

Ruby script for creating new Tapestry 5 projects

One thing with building lots of demos for upcoming presentations, labs, & etc. is that I'm having to use mvn archetype:create a lot and that command line is just hideous.

So I took a deep breath, stepped back, and wrote a Ruby script, newproj to simplify the process:


require 'getoptlong'

opts = GetoptLong.new(
  [ "--group", "-g", GetoptLong::REQUIRED_ARGUMENT ],
  [ "--artifact", "-a", GetoptLong::REQUIRED_ARGUMENT ],
  [ "--package", "-p", GetoptLong::OPTIONAL_ARGUMENT ],
  [ "--version", "-v", GetoptLong::OPTIONAL_ARGUMENT ]

group = nil
artifact = nil
package = nil
version = "1.0.0-SNAPSHOT"
error = false

  opts.each do | opt, arg |
    case opt
      when "--group" 
        group = arg
      when "--artifact" 
        artifact = arg
      when "--package" 
        package = arg
      when "--version" 
        version = arg
rescue GetoptLong::Error
  error = true

if error || ARGV.length != 0 || group == nil || artifact == nil
  puts "newproj: --group groupId --artifact arifactId [--package package] [--version version]"
  exit 0

package = package || "#{group}.#{artifact}"

command = "mvn archetype:create -DarchetypeGroupId=org.apache.tapestry -DarchetypeArtifactId=quickstart -DarchetypeVersion=5.0.3"
command << " -DgroupId=#{group} -DartifactId=#{artifact} -DpackageName=#{package} -Dversion=#{version}"

puts command


I'll eventually add to this; it needs the option to control the mvn -o (offline) flag, and further in the future, the ability to choose the correct archetype (once we add more than just quickstart). But this sure beats copying and pasting out of the documentation, like I've been doing.

Monday, March 05, 2007

Q: What's your toughest coding challenge? A:...

I've seen this question before on resumes and other people's blogs. I've never had a specific example that worked well. Previously, I've had vague stories of struggling with, say, Javassist ... but the best story here would involve my own code.

A few weeks ago, I noticed that some of my tests, the integration tests, didn't run very well on the Tapestry Bamboo server (our continuous integration server). Occasionally, even when developing on my Mac, I'd see these anomalous errors.

When things work in unit tests and fail in integration tests, one of the first places to look is for concurrency issues. Tapestry has a lot of code related to concurrency: synchronizing, caching, clearing of caches, on-demand instantiation, the works.

I started seeing things that made me question Java reality. For instance, this method is only invoked from a dynamic proxy, from a synchronized method, and the proxy only invokes the method once, yet I was seeing multiple calls:

public class OneShotServiceCreator implements ObjectCreator
    private final ServiceDef _serviceDef;

    private final ObjectCreator _delegate;

    private boolean _locked;

    public OneShotServiceCreator(ServiceDef serviceDef, ObjectCreator delegate)
        _serviceDef = serviceDef;
        _delegate = delegate;

     * We could make this method synchronized, but in the context of creating a service for a proxy,
     * it will already be synchronized (inside the proxy).
    public Object createObject()
        if (_locked)
            throw new IllegalStateException(IOCMessages.recursiveServiceBuild(_serviceDef));

        _locked = true;

        return _delegate.createObject();


The code that calls this looks something like:

private synchronized SomeService _delegate()
  if (_delegate == null) {
    _delegate = (SomeService) _creator.createObject();
    _creator = null;

  return _delegate;

You can see how this would tend to drive you a bit crazy. Eventually, I realized what was going on ... sometimes a runtime exception (an OutOfMemoryError) would be thrown, so the proxy would never complete _delegate(), and would (later, possibly in a different thread), re-invoke this the createObject() method ... and fail, because the lock was set. Solution:

    public Object createObject()
        if (_locked)
            throw new IllegalStateException(IOCMessages.recursiveServiceBuild(_serviceDef));

        // Set the lock, to ensure that recursive service construction fails.

        _locked = true;

            return _delegate.createObject();
        catch (RuntimeException ex)
            _log.error(IOCMessages.serviceConstructionFailed(_serviceDef, ex), ex);

            // Release the lock on failure; the service is now in an unknown state, but we may
            // be able to continue from here.

            _locked = false;

            throw ex;


I also started seeing bizarre error messages, like: Unable to resolve page 'MyPage' to a component class name. Available page names: Start, MyPage, YourPage.. What the hell was going on there ... was something modifying the underlying CaseInsensitiveMap? But the access methods are synchronized.

Once you start seeing bizarre concurrent behavior, and after listening to a few Brian Goetz talks about concurrency (not to mention his great book) ... well, you can start getting paranoid. Maybe its a bug in the JVM (trust me, it's never going to be a bug in the JVM). Maybe I'm not understanding access to effectively immutable objects outside of synchronized blocks (that's a bit more reasonable).

Then I saw something even more bizarre:

    public  T getService(String serviceId, Class serviceInterface, Module module)
        notBlank(serviceId, "serviceId");
        notNull(serviceInterface, "serviceInterface");
        // module may be null.

        ServiceDef def = _moduleDef.getServiceDef(serviceId);

        if (def == null) throw new IllegalArgumentException(IOCMessages.missingService(serviceId));

        if (notVisible(def, module))
            throw new RuntimeException(IOCMessages.serviceIsPrivate(serviceId));

        Object service = findOrCreate(def);

            return serviceInterface.cast(service);
        catch (ClassCastException ex)
            // This may be overkill: I don't know how this could happen
            // given that the return type of the method determines
            // the service interface.

            throw new RuntimeException(IOCMessages.serviceWrongInterface(serviceId, def
                    .getServiceInterface(), serviceInterface));

See the comment about how that code is not reachable? It was reached! It took a while, but I found out that sometimes I was getting back the wrong ServiceDef object out of the ModuleDef. Rarely, but sometimes.

Then it struck me ... all of this strange behavior was traced to my CaseInsensitiveMap implementation. Sure it has a 100% code coverage test suite ... but when I double checked the code I found some sloppiness: it uses a couple of instance variables as a scratch pad while searching. This means that, under the right timing, even reads of the effectively immutable Map by different threads would interfere with each other, with Thread A getting the result from Thread B's query. Here's a diff of the solution, which basically moved those two scratch variables into their own inner class.

Things are working perfectly (for now), which is a great relief. This bug hunt was a distraction from other things, but better to tackle it now than later. This is also a good example of the need for a continuous integration server ... the fact that the server is on a different OS and JDK ensured that the tests would fail pretty consistently on the CI server even though they mostly worked on my Mac. Brian states this as well is his book ... test on all sorts of hardware with all sorts of memory configurations, because you don't know which one aligns with your bugs and brings them out into the open.

Saturday, March 03, 2007

T5 Spring Integration -- Hibernate next?

I've put together a first pass at Tapestry 5/Spring integration as another new module: tapestry-spring-integration.

It's small and to the point, leveraging the normal Spring configuration for web applications, just making beans available for injection into Tapestry components and services. Also, it makes accessing the Spring beans (from the Tapestry side) case insensitive.

Next up will be some form of Tapestry 5 / Hibernate integration ... however, due to the conflicting licenses, I may take a pass at tapestry-ejb3.

That licensing is driving me crazy; I've checked repeatedly with the Lords of Apache Licensing, and they maintain that the ASL is not compatible with the LGPL, that by linking to LGPL code (importing LGPL classes, in Java terms) the LGPL "infects" the ASL code, adding an unwanted restriction not present in ASL.

From the sidelines, it's funny and disturbing: The ASL folks talk about "fauxpen source licenses" as if openness was purely black and white, and the least restriction was a total betrayal. Meanwhile, the FSF camp keeps saying the licenses are compatible. Go figure.

I've had discussions with people who really got heated over ASL vs. LGPL. Andrew Oliver, for one, really tried to sell me on the idea that the ASL was a boon for corporations over individuals. From my position, the theoretical taking of "Tapestry" over by, say, IBM and rebranding it as "IBM Web Presentation Objects" (or something) would be laughable ... and even if it did happen, I think it would still be good for Tapestry, which is good for me, and good for the Tapestry community. True open source forks are really rare and look more like a straw man argument than a real consideration.