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Date: Mon, 20 Jul 2009 14:38:11 +0200
Message-ID: <327858f40907200538x58e73727tf861a299adc73cf2@mail.gmail.com>
Subject: Re: New Tag - Looking for Comments (Maintain user code during
	regeneration of files)
From: Leon Rosenberg <rosenberg.leon@googlemail.com>
To: Commons Users List <user@commons.apache.org>
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Hello Jason,

thank you very much for the detailed answer.

On Mon, Jul 20, 2009 at 12:42 AM, Jason
Weinstein<jasonweinstein@hotmail.com> wrote:
>
> Hi Leon,
>
> Often generated files are only a starting point.

An interesting point. Actually the mess that happens if you try to
"regenerate" edited generated file is the main agrument against code
generation (see also rod johnsons j2ee development without ejb). So
why would someone use a system that actually relies on changing
generated code?
Giving 2 examples: How many real life projects REALLY use code
generated by rational rose et al?
But, millions of sites are using jsps on the daily basis, as an
example of code generator jsp -> jsp.java and i think that noone
actually edits and changes code generated by jasper or other jsp
generator / compiler.
So maybe instead of allowing user to alter generated code and even
persist those changes over generation cycles, it makes more sense to
parametrize either the generator or the generated code, so there is no
need to edit it? Staying with your it would be :
public class Factory {
    private String className =3D RuntimeConfiguration.get("className");

    public Object create() throws Exception {
        return Class.forName(className).newInstance();
    }
This way you at least can rely that the result of a generation is
predictable and repeatable for the defined version of the generator.

regards
Leon


>
> Example
>
> Template
>
> public class Factory {
>
> =A0 =A0private String className =3D "${className}";
>
>
>
> =A0 =A0public Object create() throws Exception {
>
> =A0 =A0 =A0 =A0return Class.forName(className).newInstance();
>
> =A0 =A0}
>
> }
>
>
> First time template generates:
>
> public class Factory {
> =A0 =A0private String className =3D "foo";
>
> =A0 =A0public Object create() throws Exception {
> =A0 =A0 =A0 =A0return Class.forName(className).newInstance();
> =A0 =A0}
> }
>
> User may change this generated file to be:
>
> public class Factory {
>
> =A0 =A0private String className =3D "foo";
>
>
>
> =A0 =A0public Object create() throws Exception {
> =A0 =A0 =A0 =A0Object o =3D Class.forName(className).newInstance();
>
> =A0 =A0 =A0 =A0System.out.println("creating object " =A0+ o);
> =A0 =A0 =A0 =A0Statistic.createdObject(className, +1);
> =A0 =A0 =A0 =A0return o;
>
> =A0 =A0}
>
> }
>
> Template may be updated to generate:
>
> public class Factory {
>
> =A0 =A0private String className =3D "foo";
>
>
>
> =A0 =A0public Object create() throws Exception {
>
>
> =A0 =A0 =A0 =A0return Class.forName(className).newInstance();
>
>
> =A0 =A0}
>
>
>
> =A0 =A0public Object create(ClassLoader cl) throws Exception {
>
>
> =A0 =A0 =A0 =A0return Class.forName(className, cl, true).newInstance();
>
>
> =A0 =A0}
>
>
> }
>
> When user code is maintained the generated file would be
>
> public class Factory {
>
> =A0 =A0private String className =3D "foo";
>
>
>
> =A0 =A0public Object create() throws Exception {
> =A0 =A0 =A0 =A0Object o =3D Class.forName(className).newInstance();
>
>
> =A0 =A0 =A0 =A0System.out.println("creating object " =A0+ o);
>
> =A0 =A0 =A0 =A0Statistic.createdObject(className, +1);
>
> =A0 =A0 =A0 =A0return o;
>
>
> =A0 =A0}
>
>
>
> =A0 =A0public Object create(ClassLoader cl) throws Exception {
>
>
>
> =A0 =A0 =A0 =A0return Class.forName(className, cl, true).newInstance();
>
>
>
> =A0 =A0}
>
>
>
> }
>
> And not (missing user code)
>
> public class Factory {
>
>
> =A0 =A0private String className =3D "foo";
>
>
>
>
>
> =A0 =A0public Object create() throws Exception {
>
>
>
> =A0 =A0 =A0 =A0return Class.forName(className).newInstance();
>
>
>
> =A0 =A0}
>
>
>
>
>
> =A0 =A0public Object create(ClassLoader cl) throws Exception {
>
>
>
> =A0 =A0 =A0 =A0return Class.forName(className, cl, true).newInstance();
>
>
>
> =A0 =A0}
>
>
>
> }
>
>
> Many times generated files need updating. Sometimes generated files are j=
ust starting points meant for the user to fill in and sometimes they are me=
ant to be final results but developers/users may wish to make mods to them =
anyways.
>
> The above is a simplistic example, but a user mod could be anything from =
adding an annotation to a log statement to employing an optimization such a=
s a cache to adding a new method, etc.
>
> What i presented, was a simple practical way to allow user mods to be mai=
ntained during file regeneration.
>
> In conclusion, very often not everything is accounted for during generati=
on. Sometimes the files need to be regenerated as templates change. What wa=
s presented was a way to maintain user changes in the generated files as fi=
les are regenerated. When combined with a compile step this also allows an =
easy way to flag incompatible user changes, or simply flag that the file wa=
s changed by the user.
>
>
> These types of comments often need work arounds in real systems as genera=
ted code does not always meet user requirements or may even have bugs or ot=
her limitations.
>
> // user generated code - do not modify - overwrites
>
>
>
>
>
>
>
> A couple side thoughts
>
> <use as template> - way to use different template during codegen, useful =
as workaround for end user in prod system
> <crc file> - way to test for user mods, and flag errors, may skip whitesp=
ace, etc
>
>> Date: Sun, 19 Jul 2009 14:04:53 +0200
>> Subject: Re: New Tag - Looking for Comments (Maintain user code during =
=A0 =A0 =A0 =A0regeneration of files)
>> From: rosenberg.leon@googlemail.com
>> To: user@commons.apache.org
>>
>> On Fri, Jul 17, 2009 at 11:11 AM, Jason
>> Weinstein<jasonweinstein@hotmail.com> wrote:
>> >
>> > A common problem with templating systems is that they overwrite change=
s made
>> > to the generated files, when things are regenerated.
>>
>> Hello,
>>
>> maybe I don't understand your point, but why should someone change
>> generated files?
>>
>> Leon
>>
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>
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