RE: Xalan Extensions: using a single instance rather than new i nstantiation

["Voytenko, Dimitry" <DV...@SECTORBASE.COM>]

Hi Anthony,

I mostly use extensions as x-path functions (like URLDecoder.decode,
URLEncoder.encode, etc)

-----Original Message-----
From: Richardella, Anthony [mailto:Anthony_Richardella@sra.com]
Sent: Friday, December 15, 2000 09:57
To: 'xalan-dev@xml.apache.org'
Subject: RE: Xalan Extensions: using a single instance rather than new i
nstantiation


I've been reading this discussion and I've been intrigued. I don't know
anything about Xalan extension, but do I have this correct, you can call
Java code inside a stylesheet? If so, what would it be used for?

Anthony.

-----Original Message-----
From: Bassi Suk [mailto:bassi.s@mellon.com]
Sent: Friday, December 15, 2000 11:22 AM
To: 'xalan-dev@xml.apache.org'
Cc: 'Gary L Peskin'
Subject: RE: Xalan Extensions: using a single instance rather than new i
nstantiation


Gary,

Thanks a lot for your assistance.

We have got it to work with your suggestion on making 'myMethod' static.

However, the second suggestion (still using the singleton approach) did not
seem to work - we got an exception when we passed the instance in as the
first parameter of any subsequent calls.

I think l was a bit too sketchy on the attributes of the java extension.
Here is more detail as to the java code:

public class MyDBFactory{
	private static MyDBFactory instance = null; 
	private static DataSourceImpl ds = null;

	private MyDBFactory(){
	}

	public static synchronized MyDBFactory getInstance() {
		if (instance == null) {
			instance = new myDBFactory();
			instance.init();
		}
		return instance;
	}

	private void init() {
		....
		// set data source from naming service lookup 
		ds = ....etc
		
	}

	public synchronized Connection getConnection() {
		...
		// returns a database connection from a pool of connections
managed by an external resource
	}


	public String getCustomer(String s1, String s2) {
		....
		uses getConnection to get DB connection, issues a sql select
statement
		and returns result
	}
}


When we put the following in the xslt:
...
<xsl:variable name="my-singleton" select="xyzExt:getInstance()"/>
<xsl:variable name="value"
select="xyzExt:getCustomer($my-singleton,$variable1,$variable2)"/>
...
....

we get the following error message:

....
...
Call to extension function failed: method call/new failed:
java.lang.reflect.InvocationTargetException target exception:
java.lang.Exception: MyDBFactory::Get connection, process, or close
statement exception: null: 
....
....

I think this may be because the inital getInstance call is a static method
but the subsequent call getCustomer is not statically declared.

To summarise, we have a solution that works based on your first suggestion,
but I just wondered if you had any ideas on how we could get the other
option to work as this method of calling fits the pattern we have used a lot
in our system.

regards,
Suk.




> -----Original Message-----
> From:	Gary L Peskin [SMTP:garyp@firstech.com]
> Sent:	Thursday, December 14, 2000 5:54 PM
> To:	Bassi Suk
> Cc:	'xalan-dev@xml.apache.org'
> Subject:	Re: Xalan Extensions: using a single instance rather than
> new   instantiation
> 
> Bassi Suk wrote:
> > Therefore, in the XSLT stylesheet we want to call the
> > mySingleton.getInstance() and get an instance of mySingleton and then be
> > able to use the mySingleton.myMethod().
> > 
> > The QUESTION is, what should the XSLT syntax be for the above to occur.
> 
> Suk --
> 
> This depends on whether you're using XalanJ1 or XalanJ2, as I'll explain
> below.  Let's start with XalanJ1 since that's the most stable version,
> for now.
> 
> First of all, I recommend not using the lxslt:component/script
> construct.  It's not needed for java language extensions.  Second, you
> don't need the extension-element-prefixes attribute.  That's only if you
> have extension -elements- and your example is limited to an extension
> -function-.
> 
> In XalanJ1, you should be able to prefix your namespace URI with
> "class:" to prevent instantiation of the class by Xalan.  This is not
> necessary in XalanJ2.
> 
> Finally, you might want to consider just making myMethod a static
> method.  That way you wouldn't need to instantiate any instances of
> mySingleton at all.
> 
> Assuming myMethod is static, your java would look like:
> 
> public class mySingleton {
>   private mySingleton() {}
>   public static String myMethod() {
>     return "We are humbled";
>   }
> }
> 
> You could invoke this method with a stylesheet that looks like this:
> 
> <?xml version="1.0" ?> 
> <xsl:stylesheet 
>         xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
>         xmlns:xyzExt="class:mySingleton"
>         version="1.0">
> 
> <xsl:variable name="value" select="xyzExt:myMethod()"/>
>  ...
> </xsl:stylesheet>
> 
> If you wanted to stick with instantiating a singleton object, you java
> code would look like that in your email and the stylesheet could look
> like this:
> 
> 
> <?xml version="1.0" ?> 
> <xsl:stylesheet 
>         xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
>         xmlns:xyzExt="class:mySingleton"
>         version="1.0">
> 
> <xsl:variable name="my-singleton" select="xyzExt:getInstance()"/>
> <xsl:variable name="value" select="xyzExt:myMethod($my-singleton)"/>
>  ...
> </xsl:stylesheet>
> 
> When calling an instance method, you need to supply the object as the
> first argument to the method call.  The extension mechanism uses the
> first argument to determine on what object the method is being invoked. 
> It then strips off that first argument and passes the remaining
> arguments to the actual java function.
> 
> HTH,
> Gary
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