Archive for September, 2009

Object Graphs, the GC, and System.disposeXML()

AIR 1.5.2 contains a new, performance-related API that heavy users of XML may find handy, System.disposeXML(), that is an instructive example regarding use (and abuse) of the AIR garbage collector.

When an XML document is loaded, it’s stored in memory as a graph of individual objects representing elements, text, attributes, and so on. Each pair of related objects–for example, a parent/child element parent–are linked, in the underlying representation, with pointers going in both directions.

This is a convenient representation for traversal, but can be problematic for the garbage collector. As I mentioned in my earlier post about referencing counting and ActionScript objects, the garbage collector will eventually find and discard the entire set of objects after the last external reference is removed. However, it takes more work for the GC than less-connected data structures.

In practice, the amount of time it takes the garbage collector to do its job is related both to the number of allocated objects and the number of pointers between them. So while it is nice that the GC is capable of finding even these highly-connected graphs of objects, using them is also asking it do a lot of work. You can reduce this workload by explicitly disconnecting the pointers between objects.

For graphs of ActionScript objects, you can ease the GC workload by simply nulling out reference between the objects. Note that you don’t have to find every last reference for this to be helpful. The GC will still do it’s job in the end, and every bit of clearing references helps. For ActionScript objects this can be particularly helpful because, if their reference count then goes to zero, they can be cleaned up even before the GC sweep completes.

XML objects are a trickier case because the API doesn’t allow developers to traverse the actual object graph. That’s where System.disposeXML() comes in: It traverses the internal object graph backing the XML object, setting all of the internal points to null. Afterwords, the XML object is empty and, presumably, useless. But then, since you were done with it anyway, that shouldn’t be a problem.

Find Out About New AIR Deployment Options at MAX 2009

MAX wouldn’t be half as exciting without new stuff to announce, and this year there will be plenty of news. And while developers tend to focus mostly on new APIs when we think about updates to AIR, there are some new deployment options coming, too–options that may enable key new scenarios for some applications. I’ll be talking about these new options as well as providing an overview of the existing optios at my MAX 2009 session, “Explore Deployment and Distribution Options for Adobe AIR Applications”.

Why AIR fetches a Thawte CRL no matter who signed your app

I was recently asked why AIR fetches a Thawte CRL (from http://tss-geotrust-crl.thawte.com/ThawteTimestampingCA.crl) every time an application is installed–even if the application was signed with a certificate from another CA. It turns out there is a good reason for this, and the clue is in the URL itself.

By default, all AIR application signatures are timestamped. Timestamps are created by a timestamp server, and are themselves signatures. When AIR validates the timestamp signature, it downloads the CRL associated with the timestamp signing certificate–just like it would when validating any other signature. And the Thawte timestamp server uses a certificate that–no surprise here–has a CRL hosted by Thawte.

This default is easy to override using the “-tsa” option to the AIR file packaging tool, adt. A value of “-tsa none” turns off timestamps entirely. To specify an alternate timestamp server, specify the URL of that server after the -tsa flag.

For more on AIR code signing, including why timestamping is used, take a look at Code Signing in Adobe AIR.

Downloading Historical Adobe AIR Releases

Interested in checking behavior of an older release of AIR? Or perhaps checking out how seamlessly we effect the upgrade to a newer version? You can download installers for the older releases if you know where to look:

  • Mac OS: http://airdownload.adobe.com/air/mac/download/<version>/AdobeAIR.dmg
  • Windows: http://airdownload.adobe.com/air/win/download/<version>/AdobeAIRInstaller.exe
  • Linux: http://airdownload.adobe.com/air/lin/download/<version>/AdobeAIRInstaller.bin

Substitute an actual version string in place of “<version>”, i.e, “1.5” or “1.5.2”.