| Andrew Cooke | Contents | Latest | RSS | Twitter | Previous | Next

C[omp]ute

Welcome to my blog, which was once a mailing list of the same name and is still generated by mail. Please reply via the "comment" links.

Always interested in offers/projects/new ideas. Eclectic experience in fields like: numerical computing; Python web; Java enterprise; functional languages; GPGPU; SQL databases; etc. Based in Santiago, Chile; telecommute worldwide. CV; email.

Personal Projects

Lepl parser for Python.

Colorless Green.

Photography around Santiago.

SVG experiment.

Professional Portfolio

Calibration of seismometers.

Data access via web services.

Cache rewrite.

Extending OpenSSH.

Last 100 entries

Ssh, tty, stdout and stderr; Feathers falling in a vacuum; Santiago 30m Bike Route; Mapa de Ciclovias en Santiago; How Unreliable is UDP?; SE Santiago 20m Bike Route; Cameron's Rap; Configuring libxml with Eclipse; Reducing Combinatorial Complexity With Occam - AI; Sentidos Comunes (Chilean Online Magazine); Hilary Mantel: The Assassination of Margaret Thatcher - August 6th 1983; NSA Interceptng Gmail During Delivery; General IIR Filters; What's happening with Scala?; Interesting (But Largely Illegible) Typeface; Retiring Essentialism; Poorest in UK, Poorest in N Europe; I Want To Be A Redneck!; Reverse Racism; The Lost Art Of Nomography; IBM Data Center (Photo); Interesting Account Of Gamma Hack; The Most Interesting Audiophile In The World; How did the first world war actually end?; Ky - Restaurant Santiago; The Black Dork Lives!; The UN Requires Unaninmous Decisions; LPIR - Steganography in Practice; How I Am 6; Clear Explanation of Verizon / Level 3 / Netflix; Teenage Girls; Formalising NSA Attacks; Switching Brakes (Tektro Hydraulic); Naim NAP 100 (Power Amp); AKG 550 First Impressions; Facebook manipulates emotions (no really); Map Reduce "No Longer Used" At Google; Removing RAID metadata; New Bike (Good Bike Shop, Santiago Chile); Removing APE Tags in Linux; Compiling Python 3.0 With GCC 4.8; Maven is Amazing; Generating Docs from a GitHub Wiki; Modular Shelves; Bash Best Practices; Good Emergency Gasfiter (Santiago, Chile); Readings in Recent Architecture; Roger Casement; Integrated Information Theory (Or Not); Possibly undefined macro AC_ENABLE_SHARED; Update on Charges; Sunburst Visualisation; Spectral Embeddings (Distances -> Coordinates); Introduction to Causality; Filtering To Help Colour-Blindness; ASUS 1015E-DS02 Too; Ready Player One; Writing Clear, Fast Julia Code; List of LatAm Novels; Running (for women); Building a Jenkins Plugin and a Jar (for Command Line use); Headphone Test Recordings; Causal Consistency; The Quest for Randomness; Chat Wars; Real-life Financial Co Without ACID Database...; Flexible Muscle-Based Locomotion for Bipedal Creatures; SQL Performance Explained; The Little Manual of API Design; Multiple Word Sizes; CRC - Next Steps; FizzBuzz; Update on CRCs; Decent Links / Discussion Community; Automated Reasoning About LLVM Optimizations and Undefined Behavior; A Painless Guide To CRC Error Detection Algorithms; Tests in Julia; Dave Eggers: what's so funny about peace, love and Starship?; Cello - High Level C Programming; autoreconf needs tar; Will Self Goes To Heathrow; Top 5 BioInformatics Papers; Vasovagal Response; Good Food in Vina; Chilean Drug Criminals Use Subsitution Cipher; Adrenaline; Stiglitz on the Impact of Technology; Why Not; How I Am 5; Lenovo X240 OpenSuse 13.1; NSA and GCHQ - Psychological Trolls; Finite Fields in Julia (Defining Your Own Number Type); Julian Assange; Starting Qemu on OpenSuse; Noisy GAs/TMs; Venezuela; Reinstalling GRUB with EFI; Instructions For Disabling KDE Indexing; Evolving Speakers; Changing Salt Size in Simple Crypt 3.0.0; Logarithmic Map (Moved)

© 2006-2013 Andrew Cooke (site) / post authors (content).

Nested Handler Mappings in Spring

From: "andrew cooke" <andrew@...>

Date: Tue, 14 Mar 2006 19:20:02 -0400 (CLT)

The MVC support in Spring includes the idea of handler mappings - an
incoming HTTP request is passed to an (ordered) set of handlers, and each
in turn attempts to provide a controller.  This is nice, because
controllers are typically associated with URLs, so you can put the URL to
controller login in the handler (note to self - maybe I can do more of
that).

I have generic handlers that do things like pull parameters from the end
of URLs (a simple kind of URL rewriting), for example.  Others that make
decisions based on the state of the database (for URLs associated with
objects that have several significantly different states -
design/preparation and use, for example).

However, there are times when you really want to chain Handlers.  This
isn't possible with standard Spring because (1) the thing returned by a
handler should be a controller, not another handler and (2) every bean
that is an instance of the handler interface is pulled from the context
and called directly - you don't specify the handler list explicitly.

The second point means that there's a risk that your nested handlers are
called directly.

However, one useful wrinkle - handlers return Object.  So there's nothing
in the method signature to stop one handler returning another.

So, what to do?  I tried writing a different interface, but that didn't
work so well - sometimes it's not clear whether a handler should be a top
level handler or a nested level handler (if it's generic, it may be both).
 And a lot of functionality was provided by Spring superclasses, so
starting a new hierarchy lost much of that.

I couldn't find a perfect solution, but in the end I've gone with the
following, which works pretty well:

- an IterationHandler that drives the iterative process.  This is a top
level handler has a subhandler which it calls and, if that returns a
handler itself, it calls that handler in turn.

- the IterationHandler also propogates interceptors along the chain.  This
saves you from having to define the same bunch of interceptors for all
nested handlers - you just put them in the top IteratorHandler.

- a NestableHandler abstract class extends AbstractHandlerMapping and
checks the Session attributes to see if IterationHandler has set a flag. 
This indicates whether or not the subclass is being called at the top
level, and is compared against a configuration parameter.  If a handler is
intended only for nested use then it returns null when called directly -
this is the normal way for handlers to "pass".

- the NestedHandler implements AbstractHandlerMapping's getHandlerInternal
interface as final, passing checked calls to a second, abstract method. 
In this way any subclass automatically (and unavoidably) is checked.

So it works pretty much as required - the only nuisance is that extra
calls may be made from the top level to handlers that will not return
controllers.  I think this can be avoided by making the bean anonymous
(nesting its definition in the IterationHandler bean within Spring). 
However, that is not always desirable.

Andrew

Comment on this post