Creating Multiple Instances From The Same Java Class In ColdFusion

Well, yeah I might have asked "how did you expect it to work," =) except that I see the issue where I think confusion can arise. I would have thought (if I had not ever done it) that create object created the object and returned it, and thus you would be calling the constructor of an already constructed object, since thats how we do it in CF.

Its too bad they didn't just make constructors a default part of the language. I see why it might be difficult to do now (although, I think someone who thought about it long enough might find a way to do it that won't break anything). Then, we wouldn't have a disconnect between the way it works for Java and the way it works for CF.

I had never thought to look until you just asked. So, I looked and I'm getting funny things happening.

If I do this (notice no initialization of sb):

<cfset sb = CreateObject("java","java.lang.StringBuffer")/>
<cfset sb2 = sb.init()>
<cfset sb.append("hello")>
<cfset sb2.append("wassup?")>

<cfoutput>
#sb.ToString()#<br/>
#sb2.ToString()#
</cfoutput>

Both sb and sb2 are the same object.

But if I do this (addition of sb3):
<cfset sb = CreateObject("java","java.lang.StringBuffer")/>
<cfset sb2 = sb.init()>
<cfset sb3 = sb.init()>
<cfset sb3.append("hello")>
<cfset sb2.append("wassup?")>

<cfoutput>
#sb3.ToString()#<br/>
#sb2.ToString()#
</cfoutput>

They are different objects.

And finally, if I do this:

<cfset sb = CreateObject("java","java.lang.StringBuffer")/>
<cfset sb2 = sb.init()>
<cfset sb3 = sb.init()>
<cfset sb3.append("hello")>
<cfset sb2.append("wassup?")>

<cfoutput>
#sb.ToString()#<br/>
#sb2.ToString()#
</cfoutput>

(notice we are outputting sb, NOT sb3) they are different objects with sb == sb3 != sb2.

So in short, I'm clueless as to what's actually going on. But I would have guessed at something like createobject was returning an object of type Class which represented StringBuffer (not sure if I'm saying that correctly or not, but I think you get the idea)

Oh, and I should have said (in the last bit) that clearly, that doesn't appear to be the case (createObject returning an object of type Class that represented StringBuffer).

Hi Ben,
CreateObject for a java class, simply creates a proxy for the class. Thus at this point, only the class gets loaded. Its only when you call a constructor explicitly using 'init' or when you call a non-static method, the class gets intantiated.
Probably the reasons it is like this are

1. If all you need to call is a static method, why to instantiate the object unnecessarily. Object instantiation is costly.
2. We wanted to keep the syntax clean. For createObject to return the actual object, it would require all the constructor arguments here itself, which will complicate the syntax.

Rupesh

Rupesh, that makes sense. Is that documented somewhere?

"I guess the CreateObject() returns a class definition and then the Init() method is reflectively calling the "new" constructor on that class definition or something? It's all voodoo to me."

You guessed correctly.

This behavior is what allows you to manipulate the static methods and data in a Java class within ColdFusion.

There's a gotcha to be aware of tho'... once you have your class (definition), if you attempt to reference any non-static method or data before calling init(), ColdFusion will automatically call init() with no arguments for you. That's how you can access some Java objects without explicitly calling init() with no arguments.

I don't know whether it will still allow you to call init() explicitly to create a new instance but it's something to be careful about...

Hey Ben,

What resources (or books if any) would you be making use of to study?

Hi,

Once an external class file has been instantiated through cfobject it remains cached until the CF App service is restarted. Are you aware of any way to flush out the cached class (under web-inf/classes) without restarting the CF app service?

Thanks,

Ben

Sorry about the confusion. What I was asking was THAT pertinent to the topic, but I've been researching this all over and have found very little. Anyway, here is some background and what I'm trying to figure out (I'm including some sample code that you can test this with as well).

Scenario:
Create an external Class file. Copy and save the compiled file to the cf_root/wwwroot/web-inf/classes folder. Now instantiate it with cfobject. Great! You can read the file and see it via your CFM page. Now go ahead and make a change to the java file, compile it and save over the new "modified" class file to the cf_root/wwwroot/web-inf/classes folder. Run your CFML code again and it will read the initial cached file. Heck, delete your class file in the cf_root/wwwroot/web-inf/classes folder and CF will still read the cached file. The only way to force CF to read the new file is to restart the CF App service.

Here is some sample code:
Java:
===============
package ClassReload;

public class ReloadTest{
public ReloadTest(){

}
public String GetReload(){
return "This be version UNO!!";
}
}
===============

save the resulting class file to the web-inf/classes folder and run the following CF code:

<cfobject action="create" type="java" class="ClassReload.ReloadTest" name="TestClassReloading">
<cfdump var="#TestClassReloading.GetReload()#">

Great! You'll see "This be version UNO!!"

Now go ahead and change your return value to be something else "This be version TWO!!". Recompile and save to the web-inf/classes folder. Run your CF code again and you'll still see "This be version UNO!!". The only way I can see the new class file is by restarting the CF App service.

I have followed the suggestions at the following to no avail:
http://kb.adobe.com/selfservice/viewContent.do?externalId=tn_18228&sliceId=2
or
http://livedocs.adobe.com/coldfusion/8/htmldocs/help.html?content=Java_2.html
In both cases scroll to where you see the "class loading" or "dynamic class loading" part.

Sorry for the long post, but I hope it makes sense.

Sorry, I meant it was NOT that pertinent to the topic :)