Ask Ben: Instantiating Nested Java Classes In ColdFusion
Posted October 1, 2008 at 9:58 AM
I'm trying to create an instance of: java.awt.geom.Point2D.Double in coldfusion:
http://java.sun.com/j2se/1.5.0/docs/api/java/awt/geom/Point2D.html
Point2D.Double is a nested class inside of the abstract class Point2D. I have tried to instantiate the class, but get, "This fails because Coldfusion cannot find the class." And which does not work because Point2D is an abstract class and there is not a public constructor on which you can call PointClass.init(x,y). Right now, I've resorted to making my own Point class that wraps the Point2D.Double class so that I can instantiate it in Coldfusion. I don't think this is ideal and am looking for ideas about how to directly create a Point2D.Double class in Coldfusion. I'm also using Coldfusion 8.
Working with nested Java classes in ColdFusion is easy, as long as you know the secret hand shake (which is kept very hush hush). If you try to instantiate the nested classes using dot-notation in ColdFusion, as you have tried:
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- <!--- Create the point object. --->
- <cfset objPoint = CreateObject(
- "java",
- "java.awt.geom.Point2D.Double"
- ).Init(
- JavaCast( "double", 5 ),
- JavaCast( "double", 10 )
- )
- />
... you will get the following error:
Object Instantiation Exception. Class not found: java.awt.geom.Point2D.Double
The trick here is that when referring nested classes in the class path, you have to use the dollar sign ($) rather than the dot (.):
java.awt.geom.Point2D$Double
Knowing that, when we run this code:
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- <!--- Create the point object. --->
- <cfset objPoint = CreateObject(
- "java",
- "java.awt.geom.Point2D$Double"
- ).Init(
- JavaCast( "double", 5 ),
- JavaCast( "double", 10 )
- )
- />
-
- <!--- Output the point string. --->
- <cfdump var="#objPoint.ToString()#" />
... we get the following output:
Point2D.Double[5.0, 10.0]
... which indicates that the Point class was proparly instantiated and utilized. Hope that helps.
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Reader Comments
Thanks for the tip, Ben. Definitely useful.
I didn't learn this trick until Mark Mandel showed it to me while I was at CFUnited this year (chatting him up on IM since he didn't make the trip). It is a huge life saver when you finally figure it out, and I had been meaning to blog it for a while now. Thanks for putting it out there for those that may not know it!
@Todd,
Glad to help out with spreading the word :) I think I only found out when I first starting working with POI and realized that the available font Colors were nested classes. Before that, I had never seen this syntax.
I heard that CF9 was going to support some sort of import when working with components the same as you can do in java when working with packages. I wonder how things like this will be effected.
Thank you, thank you, thank you.
I've been looking for this for ages.
This isn't so much a secret, it's rather just how Java inner and anonymous classes are implemented. To be compatible with older versions of Java, when they added the nested classes feature, they decided to just name mangle the classes and generate accessor functions to simulate accessing variables you didn't really have access to.
Essentially, when you nest classes, the compiler renames your nested class as the root class, with the nested class path separated by dollar signs.
org.my.company.AwesomeList$Node$Element$0
That's a class AwesomeList with a nested class Node, that has a nested class Element, that has an anonymous class inside it.
If your nested class accessed a private variable the compiler might generate a function public varname$0() or some such and implicitly call it whenever you accessed that variable in your code.
Technically you could name your own classes and methods this way with dollar signs, but that'd be bad since the dollar is reserved for code generation, and now used for inner classes.
This talks more about this:
<http://www.retrologic.com/innerclasses.doc7.html>
Java's full of these quirky implementation choices to maintain backwards compatibility. For instance, generics are also implemented using a compiler trick (type erasure).
Elliot,
That may well be. But I'm not interested in Java except what it gives me withing CF. Same with any other component or tool. Give me the API that I can use within CF. If it doesn't have it, or if I have to spend weeks hunting for it, or spend weeks learning an entirely new language to do something that shouldn't take more than 20 minutes to do, it's not useful.
It may not be a secret to Java programmers, but I'm not one, nor am I interested in being one. C++ has gotten me anywhere I need to go.
How did I not know about this until today?!? Man, you saved my bacon today Ben. Of course I had to do the whole banging-my-head-into-a-wall thing for about 6 hours before finding it!
@Dave,
Ha ha ha ha, I hope I saved you some brain cells :)
