Global concept

Discussion:

Global concept

Malcolm Edgar

2002-12-30 10:59:29 UTC

After Howards email about Visit object names including Global, I started
thinking it would be really good to have a shared global object in Tapestry.

I want to kick off a discussion before putting in a Feature Request.

Regards Malcolm

Global
======

Aim
---
Provide a shared global object that can be used for globally accessed data
or utility methods. Currently it is recommended that the SimpleEngine be
subclassed to provide a point to hang shared data off, or access global data
via the ServletContext accessed from the IRequestCycle.

Conceptually it simpler to have a global object available throughout the
framework.

Design
------
A global bean object modelled on the access style of the Visit object.

The global object would have an accessor method added to IPage interface:

public Object getGlobal();

This object would be instantiated via a no args condtructor, with the class
name specified in the Application.specification as is done with the visit
object. This object should follow a singleton style design pattern, so their
is only ever one instance available.

This object should be stored in the ServletContext under the key
'net.sf.tapestry.global' so that it can potentially be shared with other
Servlet components.

Scope
-----
Global object: shared application data
Visit object: user specific data
Persistent page properties: user page specific data

Example Usage
-------------
Application properties
Application constants
Cached data
Connection DataSources

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Howard M. Lewis Ship

2002-12-30 11:37:36 UTC

Permalink

A very good idea, I think.

----- Original Message -----
From: "Malcolm Edgar" <malcolm_edgar-***@public.gmane.org>
To: <tapestry-contrib-5NWGOfrQmneRv+***@public.gmane.org>
Sent: Monday, December 30, 2002 5:59 AM
Subject: [Tapestry-contrib] Global concept

Post by Malcolm Edgar
After Howards email about Visit object names including Global, I started
thinking it would be really good to have a shared global object in Tapestry.
I want to kick off a discussion before putting in a Feature Request.
Regards Malcolm
Global
======
Aim
---
Provide a shared global object that can be used for globally accessed data
or utility methods. Currently it is recommended that the SimpleEngine be
subclassed to provide a point to hang shared data off, or access global data
via the ServletContext accessed from the IRequestCycle.
Conceptually it simpler to have a global object available throughout the
framework.
Design
------
A global bean object modelled on the access style of the Visit object.
public Object getGlobal();
This object would be instantiated via a no args condtructor, with the class
name specified in the Application.specification as is done with the visit
object. This object should follow a singleton style design pattern, so their
is only ever one instance available.
This object should be stored in the ServletContext under the key
'net.sf.tapestry.global' so that it can potentially be shared with other
Servlet components.
Scope
-----
Global object: shared application data
Visit object: user specific data
Persistent page properties: user page specific data
Example Usage
-------------
Application properties
Application constants
Cached data
Connection DataSources
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Post by Malcolm Edgar
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p***@public.gmane.org

2002-12-30 14:40:36 UTC

Permalink

hi,

perhaps i'm a little daft, but i dont get it... if - as you said below -
you're creating it as a singleton, why do you even need framework support
for it, instead of just writing MyFoo.getInstance()?

i'm aware, there are some classloading issues with a singleton (although
this has not come up as a problem for me in recent containers), so i keep
my stuff in the ServletContext as it's supposed to be - but since that
would be inconvenient to access, i usually wrap my stuff around with some
code that sets it up as a ThreadLocal (that is statically accessible)...
so it becomes very convenient to access (anywhere with no casting), since
i can still say MyFoo.getInstance(), and have the flexibility as to where
it's really coming from (eg. it doesnt strictly need to be "global")...
also, i have some concerns about having only one global... you would
probably need a Map of them... and you still need to the casting
everywhere you need it...

so, i wouldn't bother to add this to the Tapestry framework itself, it
already supports what you need, here's an example:
http://flock.sourceforge.net/xref/net/sf/flock/webapp/FlockTapestryEngine.html
http://flock.sourceforge.net/xref/net/sf/flock/webapp/FlockContext.html

... and you set it up in the application servlet init() for example, or
lazy-init, or whatever works for you.

best regards,
viktor

On Mon, 30 Dec 2002 06:37:36 -0500, "Howard M. Lewis Ship"

Post by Howard M. Lewis Ship
A very good idea, I think.
----- Original Message -----
Sent: Monday, December 30, 2002 5:59 AM
Subject: [Tapestry-contrib] Global concept

Post by Malcolm Edgar
After Howards email about Visit object names including Global, I started
thinking it would be really good to have a shared global object in

Tapestry.

Post by Malcolm Edgar
I want to kick off a discussion before putting in a Feature Request.
Regards Malcolm
Global
======
Aim
---
Provide a shared global object that can be used for globally accessed data
or utility methods. Currently it is recommended that the SimpleEngine be
subclassed to provide a point to hang shared data off, or access global

data

Post by Malcolm Edgar
via the ServletContext accessed from the IRequestCycle.
Conceptually it simpler to have a global object available throughout the
framework.
Design
------
A global bean object modelled on the access style of the Visit object.
public Object getGlobal();
This object would be instantiated via a no args condtructor, with the

class

Post by Malcolm Edgar
name specified in the Application.specification as is done with the visit
object. This object should follow a singleton style design pattern, so

their

Post by Malcolm Edgar
is only ever one instance available.
This object should be stored in the ServletContext under the key
'net.sf.tapestry.global' so that it can potentially be shared with other
Servlet components.
Scope
-----
Global object: shared application data
Visit object: user specific data
Persistent page properties: user page specific data
Example Usage
-------------
Application properties
Application constants
Cached data
Connection DataSources

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h***@public.gmane.org

2002-12-30 15:11:16 UTC

Permalink

Yes, it's a common pattern, a singleton-as-servlet-context-attribute.

Another way to do it is to create an application extension.

Where we've been headed with Tapestry is to ease adoption. I like this
suggestion, because it formalizes something you end up doing anyway. For
example, the Visit concept could be done (and re-done) in every application,
but moving it into IEngine/AbstractEngine means that its done once and done
right ... easier to explain how to leverage the existing tools that to describe
how to build your own tools.

As Tapestry evolves, I'm continuing to see improvements where we replace
inheritance with aggregation.

Having the framework formalize this means that the framework takes care of
instantiation and provides a consistent, formal reference.

One idea I've been playing with is that the Visit (and, perhaps, the Globals)
should have optional interfaces they can implement. For example, if the Visit
implements IEngineAware, then setEngine() will be invoked. This gives the
Visit a chance to hold a reference to the engine, or otherwise initialize using
the engine.

--
hlship-***@public.gmane.org

http://tapestry.sf.net

Post by p***@public.gmane.org
hi,
perhaps i'm a little daft, but i dont get it... if - as you said below -
you're creating it as a singleton, why do you even need framework support
for it, instead of just writing MyFoo.getInstance()?
i'm aware, there are some classloading issues with a singleton (although
this has not come up as a problem for me in recent containers), so i keep
my stuff in the ServletContext as it's supposed to be - but since that
would be inconvenient to access, i usually wrap my stuff around with some
code that sets it up as a ThreadLocal (that is statically accessible)...
so it becomes very convenient to access (anywhere with no casting), since
i can still say MyFoo.getInstance(), and have the flexibility as to where
it's really coming from (eg. it doesnt strictly need to be "global")...
also, i have some concerns about having only one global... you would
probably need a Map of them... and you still need to the casting
everywhere you need it...
so, i wouldn't bother to add this to the Tapestry framework itself, it
http://flock.sourceforge.net/xref/net/sf/flock/webapp/FlockTapestryEngine.html
http://flock.sourceforge.net/xref/net/sf/flock/webapp/FlockContext.html
... and you set it up in the application servlet init() for example, or
lazy-init, or whatever works for you.
best regards,
viktor
On Mon, 30 Dec 2002 06:37:36 -0500, "Howard M. Lewis Ship"

Post by Howard M. Lewis Ship
A very good idea, I think.
----- Original Message -----
Sent: Monday, December 30, 2002 5:59 AM
Subject: [Tapestry-contrib] Global concept

Post by Malcolm Edgar
After Howards email about Visit object names including Global, I started
thinking it would be really good to have a shared global object in

Tapestry.

data

class

Post by Malcolm Edgar
name specified in the Application.specification as is done with the visit
object. This object should follow a singleton style design pattern, so

their

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or over the web
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Malcolm Edgar

2002-12-30 21:06:07 UTC

Permalink

The advantages I see with the global object include:

* Convenient access. This object can be accessed in pages using property
paths as is done with the Visit object.

* More OO than the stashing various objects in ServletContext bucket under
various keys. Similiar in concept to Visit vs HttpSession attributes

* Slightly better type safety than ServletContext bucket method, as you are
only casting to one class. Similiar in concept to Visit vs HttpSession
attributes

* Avoids classloader issues of Singletons. I have been burnt doing this in
the past with dynamic classloaders. However, if it works for you that is
great.

* Global object doesn't have to worry about serialization/transient stuff,
as does instance variables in SimpleEngine sub classses.

I was doing similar stuff to your FlockTapestryEngine in my code. I was
initializing a PersistenceManager in the Servlet.init(), placing it in the
ServletContext, placing it into a SimpleEngine subclass instance variable
after grabbing it service method, having a BasePage subclass provide an
convenience method.

What I was hoping was to avoid having to write all the above code, and just
declaring the thing's classname in the Application.specification and then
accessing via IPage.

regards Malcolm

p.s. Marvin is cool

Subject: Re: [Tapestry-contrib] Global concept
Date: Mon, 30 Dec 2002 09:40:36 -0500
hi,
perhaps i'm a little daft, but i dont get it... if - as you said below -
you're creating it as a singleton, why do you even need framework support
for it, instead of just writing MyFoo.getInstance()?
i'm aware, there are some classloading issues with a singleton (although
this has not come up as a problem for me in recent containers), so i keep
my stuff in the ServletContext as it's supposed to be - but since that
would be inconvenient to access, i usually wrap my stuff around with some
code that sets it up as a ThreadLocal (that is statically accessible)...
so it becomes very convenient to access (anywhere with no casting), since
i can still say MyFoo.getInstance(), and have the flexibility as to where
it's really coming from (eg. it doesnt strictly need to be "global")...
also, i have some concerns about having only one global... you would
probably need a Map of them... and you still need to the casting
everywhere you need it...
so, i wouldn't bother to add this to the Tapestry framework itself, it
http://flock.sourceforge.net/xref/net/sf/flock/webapp/FlockTapestryEngine.html
http://flock.sourceforge.net/xref/net/sf/flock/webapp/FlockContext.html
... and you set it up in the application servlet init() for example, or
lazy-init, or whatever works for you.
best regards,
viktor
On Mon, 30 Dec 2002 06:37:36 -0500, "Howard M. Lewis Ship"

Post by Howard M. Lewis Ship
A very good idea, I think.
----- Original Message -----
Sent: Monday, December 30, 2002 5:59 AM
Subject: [Tapestry-contrib] Global concept

Post by Malcolm Edgar
After Howards email about Visit object names including Global, I

started

Post by Howard M. Lewis Ship

Post by Malcolm Edgar
thinking it would be really good to have a shared global object in

Tapestry.

data

Post by Howard M. Lewis Ship

Post by Malcolm Edgar
or utility methods. Currently it is recommended that the SimpleEngine

Post by Howard M. Lewis Ship

Post by Malcolm Edgar
subclassed to provide a point to hang shared data off, or access

global

Post by Howard M. Lewis Ship
data

Post by Malcolm Edgar
via the ServletContext accessed from the IRequestCycle.
Conceptually it simpler to have a global object available throughout

the

Post by Howard M. Lewis Ship

Post by Malcolm Edgar
framework.
Design
------
A global bean object modelled on the access style of the Visit object.
The global object would have an accessor method added to IPage
public Object getGlobal();
This object would be instantiated via a no args condtructor, with the

class

Post by Malcolm Edgar
name specified in the Application.specification as is done with the

visit

Post by Howard M. Lewis Ship

Post by Malcolm Edgar
object. This object should follow a singleton style design pattern, so

their

Post by Malcolm Edgar
is only ever one instance available.
This object should be stored in the ServletContext under the key
'net.sf.tapestry.global' so that it can potentially be shared with

other

Post by Howard M. Lewis Ship

Post by Malcolm Edgar
Servlet components.
Scope
-----
Global object: shared application data
Visit object: user specific data
Persistent page properties: user page specific data
Example Usage
-------------
Application properties
Application constants
Cached data
Connection DataSources

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