What is this POJO

[Kamal Mettananda]

In a project I was using DAO and EJB (session), while it was called that the project was using POJO.
But I could not really see something different than using DAO pattern.

Would you be able to ellaberate why this POJO word is there.

[Anonymous]

POJO : Plain Old Java Objects
but depending on the context, one may have referred EJBs also as POJOs

[Paul Sturrock]

Originally posted by Yilmaz Mete:
POJO : Plain Old Java Objects
but depending on the context, one may have referred EJBs also as POJOs

If you did, you would be making a mistake. EJBs need the container to run, POJOs don't, so EJBs are not Plain java objects.

[Pradeep bhatt]

In the EJB 3.0 version all bean classes will be POJO.

[Paul Sturrock]

...unless of course you are talking about Session EJBs. Or Message Driven EJBs.
[ January 24, 2006: Message edited by: Paul Sturrock ]

[Kamal Mettananda]

No I'm not querying about EJB.

My question is why there is a concept like POJO? Anyway from the very begging of Java we were using Java objects but now those Java objects are given the name as POJO. But it's just the same Java objects.

So are we using a new word without saying just Java Objects?

[Manoj Kumkumath]

Kamal,
Googling gave me this
POJO

Looks like �What's is there in a name� has became deprecated.

[Kamal Mettananda]

Mmm.. Great Manoj, it has something like this.

"We wondered why people were so against using regular objects in their systems and concluded that it was because simple objects lacked a fancy name. So we gave them one, and it's caught on very nicely."

If that's just talking about Java Objects do we have to think anything more? It's just like learning J2SE. Is that correct?

[Fintan Conway]

Originally posted by Kamal Mettananda:
If that's just talking about Java Objects do we have to think anything more? It's just like learning J2SE. Is that correct?

Hi Kamal,

In the beginning there were only ordinary Java objects. Then along came EJBs and the like, which are more complicated Java objects. Everybody seemed to jump on the EJB bandwagon, and all objects created for an enterprise system were being created as EJBs. In order to differentiate between the ordinary objects and EJBs, the term POJO was created to refer to "ordinary" Java objects, i.e. non-EJB objects.

You are right that POJOs are written in J2SE, and do not include J2EE concepts like servlets, EJBs, etc., nor does a POJO include graphic concepts like AWT, Swing which are included in J2SE. A POJO is an object which simply contains data and methods, and nothing fancier.

Regards,

Fintan

[Kamal Mettananda]

Hi Fintan,

So there are two types, POJO and EJB.

A POJO is an object which simply contains data and methods, and nothing fancier.

Ok good. But there's still a word for those saying Java Beans which is used to express about the objects that contain data and getters and setters.

So the main difference between a Java Bean and a POJO is that it has some business methods other than the getters and setters.

That is Java Bean is a subset of POJO..

Hope this is correct.

[Manoj Kumkumath]

A POJO is an object which simply contains data and methods, and nothing fancier.

I think what Fintan mean is that POJO doesn't use any fancy stuff like container for getting thing done. I think we can not say that we use/don't use POJO at class level. It should be taked in a higher lever like how our application is persisting the data to db. Is it using plain java classes or Is it using fancy stuff like an CM persistence.

[Mike Firkser]

To put it in laymen's terms, POJO is just using basic Java - not getting fancy. Am I right?

[Macy]

I never heard POJO, I would like to find out more about it.

[Chris Richardson]

Hello,

See the post I made in http://www.coderanch.com/t/216003/ORM/java/Chris-Richardson-do-you-call that discusses the definition of a POJO.

The big idea behind POJOs is separation of concerns. So rather than write code that implements business logic and infrastructure concerns such as transactions, security etc, you separate it out.

Your POJOs implement the business logic. They are written using simple Java without implement infrastructure framework-specific interfaces or calling framework-specific APIs.

You then use non-invasive frameworks such as Spring, Hibernate and EJB3 to provide persistence, transactions et etc.

The benefit of this approach is that you can develop and test your business logic without worrying (so much) about application server, the database and the infrastructure frameworks. As a result, development is simpler and faster. You can focus on one thing at a time. You don't have to be an infrastructure framework expert to work on the code. Moreover, you can also switch/upgrade frameworks without having to rewrite your code.

POJOs and JavaBeans are two orthogonal concepts. A JavaBean doesn't have to a POJO and a POJO does not have to follow the JavaBean naming conventions. In fact, well-designed objects should not expose their fields via getters and setters unless it really is required. But that's a whole other thread.

Chris
[ January 24, 2006: Message edited by: Chris Richardson ]

[Pradeep bhatt]

You then use non-invasive frameworks such as Spring, Hibernate and EJB3 to provide persistence, transactions et etc.

Doesn't using annotations invasive ?

[Chris Richardson]

Pradip

Yes. Annotations are invasive. See my message about this topic in this thread: http://www.javaranch.com

Chris

[Kamal Mettananda]

Hi Chris,

According to ur explanation it is clear that java objects are called POJO when they use the basic J2SE level rather than using Framework specific classes or jars. And they basically target on the business logic.

So is it correct to say that POJO is used in the DAO Pattern where the objects are independent of the DBMS?

But one thing is not that clear for me.

Originally posted by Chris Richardson:
POJOs and JavaBeans are two orthogonal concepts.

I feel that both are a bit similar compared to EJB as non of those use any framework specific parts, except that Java Beans are used to store data while POJO are used to store data & business logic.
[ January 25, 2006: Message edited by: Kamal Mettananda ]

[Billy Tsai]

POJO=PO?
PO is Persistence Object

[Chris Richardson]

Originally posted by Billy Tsai:
POJO=PO?
PO is Persistence Object

Some POJOs (Plain Object Java Objects) are persistent where as others are not. e.g. an entity such as Account could be persistent where as a service such as MoneyTransferService is not.

Chris

[Chris Richardson]

Originally posted by Kamal Mettananda:
Hi Chris,

According to ur explanation it is clear that java objects are called POJO when they use the basic J2SE level rather than using Framework specific classes or jars. And they basically target on the business logic.

My book POJOs in Action is about the backend (business and database access tiers) but the idea of POJO also applies to the presentation tiers. e.g. POJOs are not coupled to the Servlet API where as non-POJOs are. Compare, for example, WebWork actions with Struts Actions.

Originally posted by Kamal Mettananda:
So is it correct to say that POJO is used in the DAO Pattern where the objects are independent of the DBMS?

DTOs are typically POJOs but they are not particularly interesting objects since they contain only data, no behavior.

Originally posted by Kamal Mettananda:
But one thing is not that clear for me.

I feel that both are a bit similar compared to EJB as non of those use any framework specific parts, except that Java Beans are used to store data while POJO are used to store data & business logic.

JavaBeans that are used as DTOs are typically POJOs as I mentioned above. However, strictly speaking:
* JavaBeans - a naming convention
* DTO - the role of an object
* POJO - a class that does not implement special interfaces

Chris
[ January 25, 2006: Message edited by: Chris Richardson ]

[Billy Tsai]

Assuming I have POJO for persistence purpose and one POJO inherits an class ( extends) and that POJO has one to many relationship with another POJO do u think the POJO on the many side should automatically extends the same class too?

[Chris Richardson]

Only if makes sense in your object model.

Chris

[Kamal Mettananda]

Hi Chris,

This is quite interesting.

Originally posted by Chris:
* POJO - a class that does not implement special interfaces

So I understand that Plain in POJO really means it's plain.

[sinasi susam]

"Plain old"

I think the chriteria to explain the stuation is they are the simplest objects to store and delegate the records that is related...

"Plain" and "old" like we begin to write java classes at first.Plain and old when we were beginner.I think those words have chosen because sun and everybody else are developing other complex things to persist datas.

Before EJB 3.0 there was too much arguments to persist the datas..
I think "plain old" is an answer such as "no need to complexity.No needed to implement too much complex interfaces and classes.)

[mayur dhawan]

POJO's are called Plain Old Java Objects. They are very similar to the regular java objects we use.Its just a name to differentiate it from EJB's