Re: Bean Choices (code question)

[Michael Stacey]

I have a lot of informational beans that hold static reference data (think
ISO codes, State abbreviations, etc...) With these types of beans, I have
the choice of instantiating a singleton object reference to the bean and
accessing the methods through the object, or making all the methods public
static so I can access them via BeanName.method(). For the former, the bean
does all of it's initialization and database access in it's constructor and
my JSP's have something like the following at the top:

Isn't this a prime candidate for application-level beans?  These are 
instantiated
when the servlet is started and reused, no?

-- Michael 

RE: Bean Choices (code question)

[Cox, Charlie]

Title: RE: Bean Choices (code question)





there is no gc on static objects. static objects(methods,fields, or classes) are created once on startup and there is only one instance of the method or field ever created that all requestors to your class will use. But with Tomcat I do believe that if you have the same class in 2 different contexts, that it will load two separate instances of your bean due to the different class loaders.

This also means that if you are writing to the static fields on the first(or any) call, make sure to synchronize the write, in case 2 calls occur at the same time.

Charlie


-Original Message-
From: Jeff Kilbride [mailto:[EMAIL PROTECTED]]
Sent: Thursday, May 17, 2001 8:56 PM
To: [EMAIL PROTECTED]
Subject: Re: Bean Choices (code question)



Yeah, I know, but it's tempting to write all the methods as public static.
Then I don't have to create the object at all, but I don't know the affects
on garbage collection.


--jeff


- Original Message -
From: Ross Dyson [EMAIL PROTECTED]
To: [EMAIL PROTECTED]
Sent: Thursday, May 17, 2001 5:24 PM
Subject: RE: Bean Choices (code question)



 If you make the bean class have a private constructor and a static
reference
 to an instance of its own class, then there will always be a strong
 reference from the class itself to the instance of the class. (The first
 call to bean.getInstance() instantiates the private static member variable
 which is your singleton instance of your bean, the contructor does the db
 work etc.)

 Therefore, it won't get gc'd.

 That's the way I do my classes that do that kind of stuff, anyway.

 -Original Message-
 From: Jeff Kilbride [mailto:[EMAIL PROTECTED]]
 Sent: Friday, 18 May 2001 9:35 AM
 To: [EMAIL PROTECTED]
 Subject: Bean Choices (code question)


 I'm sorry if this is a little off-topic, since it is more about java code,
 but I thought Tomcat users would be interested/knowledgeable about the way
 beans work.

 I have a lot of informational beans that hold static reference data (think
 ISO codes, State abbreviations, etc...) With these types of beans, I have
 the choice of instantiating a singleton object reference to the bean and
 accessing the methods through the object, or making all the methods
public
 static so I can access them via BeanName.method(). For the former, the
bean
 does all of it's initialization and database access in it's constructor
and
 my JSP's have something like the following at the top:

 %!
 private static BeanName bean = BeanName.getInstance();
 %

 For the latter, the bean does all of it's initialization and database
access
 in a static block:

 static {
 // bean initialization and DB access
 }

 and my JSPs reference the static methods:

 %= BeanName.method() %

 My question concerns the way beans with all public static methods are
 instantiated and garbage collected. Because these beans hold unchanging
 reference info, I only want the data loaded from the database once the
first
 time the bean is referenced. For beans with all public static methods,
 since there are no object references to them, when are they garbage
 collected? Is it possible that the bean will be unloaded and the code in
my
 static block will be re-run when the bean is next referenced? Or do these
 all static objects somehow stick around forever? For the singleton, I know
 the object will last as long as the JSP is loaded, because there is a
 definite object reference to the singleton. I'm a little unclear as to
what
 happens with a bean that has all public static methods, though.

 Can anyone shed some light on this?

 Thanks,
 --jeff

Re: Bean Choices (code question)

[Jeff Kilbride]

Hi Charlie,

Thanks for confirming this! I was under the impression that static methods
were loaded once and never unloaded, but I wasn't sure.

On the same subject, are class constructors called when static methods are
accessed? Right now, in my all static method classes, I'm doing my
initialization in static blocks. I'm wondering if I can move that
initialization into the class constructor, but I'm not sure if/when the
constructor is called when a static method is called.

Any insight on this point, also, is greatly appreciated.

Thanks,
--jeff

- Original Message -
From: Cox, Charlie [EMAIL PROTECTED]
To: [EMAIL PROTECTED]
Sent: Friday, May 18, 2001 4:28 AM
Subject: RE: Bean Choices (code question)


 there is no gc on static objects. static objects(methods,fields, or
classes)
 are created once on startup and there is only one instance of the method
or
 field ever created that all requestors to your class will use. But with
 Tomcat I do believe that if you have the same class in 2 different
contexts,
 that it will load two separate instances of your bean due to the different
 class loaders.

 This also means that if you are writing to the static fields on the
first(or
 any) call, make sure to synchronize the write, in case 2 calls occur at
the
 same time.

 Charlie

 -Original Message-
 From: Jeff Kilbride [mailto:[EMAIL PROTECTED]]
 Sent: Thursday, May 17, 2001 8:56 PM
 To: [EMAIL PROTECTED]
 Subject: Re: Bean Choices (code question)


 Yeah, I know, but it's tempting to write all the methods as public
static.
 Then I don't have to create the object at all, but I don't know the
affects
 on garbage collection.

 --jeff

 - Original Message -
 From: Ross Dyson [EMAIL PROTECTED]
 To: [EMAIL PROTECTED]
 Sent: Thursday, May 17, 2001 5:24 PM
 Subject: RE: Bean Choices (code question)


  If you make the bean class have a private constructor and a static
 reference
  to an instance of its own class, then there will always be a strong
  reference from the class itself to the instance of the class.  (The
first
  call to bean.getInstance() instantiates the private static member
variable
  which is your singleton instance of your bean, the contructor does the
db
  work etc.)
 
  Therefore, it won't get gc'd.
 
  That's the way I do my classes that do that kind of stuff, anyway.
 
  -Original Message-
  From: Jeff Kilbride [mailto:[EMAIL PROTECTED]]
  Sent: Friday, 18 May 2001 9:35 AM
  To: [EMAIL PROTECTED]
  Subject: Bean Choices (code question)
 
 
  I'm sorry if this is a little off-topic, since it is more about java
code,
  but I thought Tomcat users would be interested/knowledgeable about the
way
  beans work.
 
  I have a lot of informational beans that hold static reference data
(think
  ISO codes, State abbreviations, etc...) With these types of beans, I
have
  the choice of instantiating a singleton object reference to the bean and
  accessing the methods through the object, or making all the methods
 public
  static so I can access them via BeanName.method(). For the former, the
 bean
  does all of it's initialization and database access in it's constructor
 and
  my JSP's have something like the following at the top:
 
  %!
  private static BeanName bean = BeanName.getInstance();
  %
 
  For the latter, the bean does all of it's initialization and database
 access
  in a static block:
 
  static {
  // bean initialization and DB access
  }
 
  and my JSPs reference the static methods:
 
  %= BeanName.method() %
 
  My question concerns the way beans with all public static methods are
  instantiated and garbage collected. Because these beans hold unchanging
  reference info, I only want the data loaded from the database once the
 first
  time the bean is referenced. For beans with all public static methods,
  since there are no object references to them, when are they garbage
  collected? Is it possible that the bean will be unloaded and the code in
 my
  static block will be re-run when the bean is next referenced? Or do
these
  all static objects somehow stick around forever? For the singleton, I
know
  the object will last as long as the JSP is loaded, because there is a
  definite object reference to the singleton. I'm a little unclear as to
 what
  happens with a bean that has all public static methods, though.
 
  Can anyone shed some light on this?
 
  Thanks,
  --jeff
 
 

Re: Bean Choices (code question)

[Bo Xu]

Jeff Kilbride wrote:

 Hi Charlie,
 [...]
 On the same subject, are class constructors called when static methods are
 accessed? Right now, in my all static method classes, I'm doing my
 initialization in static blocks. I'm wondering if I can move that
 initialization into the class constructor, but I'm not sure if/when the
 constructor is called when a static method is called.

 Any insight on this point, also, is greatly appreciated.

 Thanks,
 --jeff
 [...]

Hi :-)  I did a test, I find:
- when I invoke a static method with className.methodName(...),
  constructor and non-static init-block will not be invoked

 -  it doesn't metter that if this class is a all static method style class
 or Not,i.e., this class can have all of the following:
% static init-block/non-static init-block
% static field/non-static(instance) filed
% static method/non-static method

- only when this class is being loaded into the classloader,
  static init-block will be invoked(i.e., normally, static init-block
  only will be invoked One time), now we only have a Class object
  of my-class, we don't have the instance object of my-class.

- I am not sure if the above is righ when Java Reflection is used to
  find/load this class(like a Servlet container does)



Bo
May.18, 2001

RE: Bean Choices (code question)

[Cox, Charlie]

Title: RE: Bean Choices (code question)







-Original Message-
From: Bo Xu [mailto:[EMAIL PROTECTED]]
Sent: Friday, May 18, 2001 1:47 PM
To: [EMAIL PROTECTED]
Subject: Re: Bean Choices (code question)


Jeff Kilbride wrote:

 Hi Charlie,
 [...]
 On the same subject, are class constructors called when static methods are
 accessed? Right now, in my all static method classes, I'm doing my
 initialization in static blocks. I'm wondering if I can move that
 initialization into the class constructor, but I'm not sure if/when the
 constructor is called when a static method is called.

 Any insight on this point, also, is greatly appreciated.

 Thanks,
 --jeff
 [...]

Hi :-) I did a test, I find:
- when I invoke a static method with className.methodName(...),
 constructor and non-static init-block will not be invoked



That's what I thought. The constructor is not invoked because there is no instance of the class. Basically, if it's not static, it won't/can't run until an instance is created. Therefore static methods can only call other static methods in the same class.


 - it doesn't metter that if this class is a all static method style class
 or Not, i.e., this class can have all of the following:
 % static init-block/non-static init-block
 % static field/non-static(instance) filed
 % static method/non-static method

- only when this class is being loaded into the classloader,
 static init-block will be invoked(i.e., normally, static init-block
 only will be invoked One time), now we only have a Class object
 of my-class, we don't have the instance object of my-class.

- I am not sure if the above is righ when Java Reflection is used to
 find/load this class(like a Servlet container does)

I think this is where the classloader(s) issue comes into account and that separate instances in different contexts would be considered different classes. There was a very good explaination of tomcat's classloading discussed in a previous thread : 

http://www.mail-archive.com/tomcat-user@jakarta.apache.org/msg13260.html




Bo
May.18, 2001



Charlie

Re: Bean Choices (code question)

[Jeff Kilbride]

That's what I thought, too. I guess I'll have to leave my initialization
code inside the static blocks.

Right now, I'm only using these classes in one context, so the classloader
issues don't apply. (yet...)

Thanks Charlie and Bo!

--jeff

- Original Message -
From: Cox, Charlie [EMAIL PROTECTED]
To: [EMAIL PROTECTED]
Sent: Friday, May 18, 2001 11:21 AM
Subject: RE: Bean Choices (code question)



 
 -Original Message-
 From: Bo Xu [mailto:[EMAIL PROTECTED]]
 Sent: Friday, May 18, 2001 1:47 PM
 To: [EMAIL PROTECTED]
 Subject: Re: Bean Choices (code question)
 
 
 Jeff Kilbride wrote:
 
  Hi Charlie,
  [...]
  On the same subject, are class constructors called when static methods
 are
  accessed? Right now, in my all static method classes, I'm doing my
  initialization in static blocks. I'm wondering if I can move that
  initialization into the class constructor, but I'm not sure if/when the
  constructor is called when a static method is called.
 
  Any insight on this point, also, is greatly appreciated.
 
  Thanks,
  --jeff
  [...]
 
 Hi :-)  I did a test, I find:
 - when I invoke a static method with className.methodName(...),
   constructor and non-static init-block will not be invoked
 

 That's what I thought. The constructor is not invoked because there is no
 instance of the class. Basically, if it's not static, it won't/can't run
 until an instance is created. Therefore static methods can only call other
 static methods in the same class.


  -  it doesn't metter that if this class is a all static method style
 class
  or Not,i.e., this class can have all of the following:
 % static init-block/non-static init-block
 % static field/non-static(instance) filed
 % static method/non-static method
 
 - only when this class is being loaded into the classloader,
   static init-block will be invoked(i.e., normally, static init-block
   only will be invoked One time), now we only have a Class object
   of my-class, we don't have the instance object of my-class.
 
 - I am not sure if the above is righ when Java Reflection is used to
   find/load this class(like a Servlet container does)
 
 I think this is where the classloader(s) issue comes into account and that
 separate instances in different contexts would be considered different
 classes. There was a very good explaination of tomcat's classloading
 discussed in a previous thread :
 http://www.mail-archive.com/tomcat-user@jakarta.apache.org/msg13260.html

 
 
 Bo
 May.18, 2001
 

 Charlie

Bean Choices (code question)

[Jeff Kilbride]

I'm sorry if this is a little off-topic, since it is more about java code,
but I thought Tomcat users would be interested/knowledgeable about the way
beans work.

I have a lot of informational beans that hold static reference data (think
ISO codes, State abbreviations, etc...) With these types of beans, I have
the choice of instantiating a singleton object reference to the bean and
accessing the methods through the object, or making all the methods public
static so I can access them via BeanName.method(). For the former, the bean
does all of it's initialization and database access in it's constructor and
my JSP's have something like the following at the top:

%!
private static BeanName bean = BeanName.getInstance();
%

For the latter, the bean does all of it's initialization and database access
in a static block:

static {
// bean initialization and DB access
}

and my JSPs reference the static methods:

%= BeanName.method() %

My question concerns the way beans with all public static methods are
instantiated and garbage collected. Because these beans hold unchanging
reference info, I only want the data loaded from the database once the first
time the bean is referenced. For beans with all public static methods,
since there are no object references to them, when are they garbage
collected? Is it possible that the bean will be unloaded and the code in my
static block will be re-run when the bean is next referenced? Or do these
all static objects somehow stick around forever? For the singleton, I know
the object will last as long as the JSP is loaded, because there is a
definite object reference to the singleton. I'm a little unclear as to what
happens with a bean that has all public static methods, though.

Can anyone shed some light on this?

Thanks,
--jeff

RE: Bean Choices (code question)

[Ross Dyson]

If you make the bean class have a private constructor and a static reference
to an instance of its own class, then there will always be a strong
reference from the class itself to the instance of the class.  (The first
call to bean.getInstance() instantiates the private static member variable
which is your singleton instance of your bean, the contructor does the db
work etc.)

Therefore, it won't get gc'd.

That's the way I do my classes that do that kind of stuff, anyway.

-Original Message-
From: Jeff Kilbride [mailto:[EMAIL PROTECTED]]
Sent: Friday, 18 May 2001 9:35 AM
To: [EMAIL PROTECTED]
Subject: Bean Choices (code question)


I'm sorry if this is a little off-topic, since it is more about java code,
but I thought Tomcat users would be interested/knowledgeable about the way
beans work.

I have a lot of informational beans that hold static reference data (think
ISO codes, State abbreviations, etc...) With these types of beans, I have
the choice of instantiating a singleton object reference to the bean and
accessing the methods through the object, or making all the methods public
static so I can access them via BeanName.method(). For the former, the bean
does all of it's initialization and database access in it's constructor and
my JSP's have something like the following at the top:

%!
private static BeanName bean = BeanName.getInstance();
%

For the latter, the bean does all of it's initialization and database access
in a static block:

static {
// bean initialization and DB access
}

and my JSPs reference the static methods:

%= BeanName.method() %

My question concerns the way beans with all public static methods are
instantiated and garbage collected. Because these beans hold unchanging
reference info, I only want the data loaded from the database once the first
time the bean is referenced. For beans with all public static methods,
since there are no object references to them, when are they garbage
collected? Is it possible that the bean will be unloaded and the code in my
static block will be re-run when the bean is next referenced? Or do these
all static objects somehow stick around forever? For the singleton, I know
the object will last as long as the JSP is loaded, because there is a
definite object reference to the singleton. I'm a little unclear as to what
happens with a bean that has all public static methods, though.

Can anyone shed some light on this?

Thanks,
--jeff

Re: Bean Choices (code question)

[Jeff Kilbride]

Yeah, I know, but it's tempting to write all the methods as public static.
Then I don't have to create the object at all, but I don't know the affects
on garbage collection.

--jeff

- Original Message -
From: Ross Dyson [EMAIL PROTECTED]
To: [EMAIL PROTECTED]
Sent: Thursday, May 17, 2001 5:24 PM
Subject: RE: Bean Choices (code question)


 If you make the bean class have a private constructor and a static
reference
 to an instance of its own class, then there will always be a strong
 reference from the class itself to the instance of the class.  (The first
 call to bean.getInstance() instantiates the private static member variable
 which is your singleton instance of your bean, the contructor does the db
 work etc.)

 Therefore, it won't get gc'd.

 That's the way I do my classes that do that kind of stuff, anyway.

 -Original Message-
 From: Jeff Kilbride [mailto:[EMAIL PROTECTED]]
 Sent: Friday, 18 May 2001 9:35 AM
 To: [EMAIL PROTECTED]
 Subject: Bean Choices (code question)


 I'm sorry if this is a little off-topic, since it is more about java code,
 but I thought Tomcat users would be interested/knowledgeable about the way
 beans work.

 I have a lot of informational beans that hold static reference data (think
 ISO codes, State abbreviations, etc...) With these types of beans, I have
 the choice of instantiating a singleton object reference to the bean and
 accessing the methods through the object, or making all the methods
public
 static so I can access them via BeanName.method(). For the former, the
bean
 does all of it's initialization and database access in it's constructor
and
 my JSP's have something like the following at the top:

 %!
 private static BeanName bean = BeanName.getInstance();
 %

 For the latter, the bean does all of it's initialization and database
access
 in a static block:

 static {
 // bean initialization and DB access
 }

 and my JSPs reference the static methods:

 %= BeanName.method() %

 My question concerns the way beans with all public static methods are
 instantiated and garbage collected. Because these beans hold unchanging
 reference info, I only want the data loaded from the database once the
first
 time the bean is referenced. For beans with all public static methods,
 since there are no object references to them, when are they garbage
 collected? Is it possible that the bean will be unloaded and the code in
my
 static block will be re-run when the bean is next referenced? Or do these
 all static objects somehow stick around forever? For the singleton, I know
 the object will last as long as the JSP is loaded, because there is a
 definite object reference to the singleton. I'm a little unclear as to
what
 happens with a bean that has all public static methods, though.

 Can anyone shed some light on this?

 Thanks,
 --jeff