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Generic DAO disadvantages?

I want to create a DAO layer for my application. After some googling I found that many peoples uses Generic DAO approach (Don't repeat the DAO!).
But I did not like this approach.
What if I need slightly different interfaces between DAO for different DAO implementations? (i.e. methods in generic interface not exactly same which I want to create in my DAO implementations)
What if my entity's primary key consists of more than one attribute?

If you need a slightly different DAO for a particular entity, you can always extend a generic one(MySpecificDAO <....> extends GenericDAO<....>). Primary key can be composite itself, but it's impossible to have 2 primary keys .

Straight from the article you linked to:
Extending GenericDAO
The interface for each DAO is, of course, based on the GenericDao interface. I just need to adapt the interface to a specific domain class and extend it to include my finder methods. In Listing 6, you can see an example of the GenericDao interface extended for a specific purpose
Regarding your last question: by definition, an entity has one and only one primary key.

Disadvantage: you still have to implement the DAO. Stop following advice from 6 years ago, and use Spring Data repositories instead. Then you don't have to write any implementations at all.

What if I need slightly different interfaces between DAO for different
DAO implementations?
you can override the method in your GenericDaoImpl class. or create a new method.
What if my entity have 2 or more primary keys?
I guess you meant compound-key scenario. Note that usually the findOne/readOne/getOne method in GenericDao would expect a parameter, (T key) the T here is type, it could be composite primary key.
for example:
class PersonPK{
private String name;
private Date birthday;
.....
}

You can find here a Generic DAO a working and improved implementation of that very article. Just checkout the Example.java at the bottom of the page. In this example you can see how you can define "slightly different interfaces between DAO for different DAO
implementations".

Using AspectJ to extend an Entity

How can I use AspectJ to extend an Entity. For example say I have one entity Product with attributes like mfgDate,prodIdentifier. For some cases I would like to extend this entity to add new attributes like year,warranty.
Can I do this using AspectJ? The reason I want to do this is so that I can deliver the generic Product and then ask the other team to extend the generic product using Aspect.
Thanks

I suppose you're talking about Inter-type Declarations.

Hibernate - derived java classes for tables

I generated mapping files and POJOs in Netbeans instead of writing them myself. Is it possible to use a derived class in a place of an inherited class? An example would be something like this:
Person.hbm.xml - mapping file
Person.java - generated class (strategy class per table)
PersonExtended - class that extends Person.java
So when I create a new object:
PersonExtended personextended = new PersonExtended(<parameters>);
Would I be able to call methods like:
session.save(personextended) or session.delete(personextended)
?
Is this scenario sensible or should I add any code that I need to in a generated class? Thanks in advance for help or suggestions.
-------Edit--------
In my database I don't have the typical structure that would be possible to be mapped as an inheritance. I merely want to keep the additional methods separate from the main java class for an entity.
Best Regards,
sass.

you will have to tell hibernate how your extended classes should be mapped via a the hbm.xml file. Depending on the strategy Hibernate should use for polymorphism you might have to assign a descriminator value. there are 3 different strategies when using subclasses known as "table per class" "table per concrete class" and "table per subclass".
You can define subclasses in the hbm-xml file by using the <subclass> or <joined-subclass> elements
if you correctly defined your hbm.xml file you can then use session.save(new PersonExtended()) or sth.
you can read up on this here:
http://docs.jboss.org/hibernate/core/3.3/reference/en/html/inheritance.html
hope that helped..

Best way to share Java implementation between concrete JPA classes?

I have about 10 different entities in my J2EE application that right now share the exact same implementation. They all inherit from the same generic abstract class that has been annotated as a #MappedSuperclass, but this class contains none of the implementation I have repeated in all the concrete subclasses.
If I could, I'd put all the various fields and collections on this abstract superclass and therefore put the implementation methods there too -- all in one place instead of 10. However, due to JPA restrictions I cannot add JPA annotations to generic fields or accessors.
While I normally favor delegation to implementation inheritance anyway, due to another JPA restriction that says you can't have an embedded entity with a collection the idea of using a delegate also won't work.
When I had only 3-4 of these entities and 2-3 methods, it wasn't a big deal, but now I have about 10 -- and about 7-8 methods each...and some of the methods are getting really complex. And the "cut-copy-paste" inheritance I am using really sucks.
Any other brilliant ideas out there?

Double-check whether these "restrictions" actually hold true for your JPA provider. I've had embedded objects with collections and it has been fine (with Hibernate). And I've had a #MappedSuperclass with mapped fields.
You can try omitting the #MappedSuperclass, and make the superclass abstract and an #Entity with the proper inheritance hierarchy.

It turns out you can use either implementation inheritance or delegation as long as you are sure to use property access mode for your JPA annotations. I was using field access mode for JPA annotations and that was causing me no end of suffering because I could not annotate a generic field type.
However with property access mode I simply create my generic abstract implementation without annotating it as an entity, mappedsuperclass, embeddable, or anything. This way JPA will ignore it. Then, in the concrete subclasses, I create protected getter and setters methods as needed and put the JPA annotation on those.
Turned out to be deceptively simple.

Best way to add annotations to inherited methods

I have a number of abstract superclasses from which my concrete class inherit various methods. Some of these methods need to have JPA or JAXB annotations placed on them in the concrete class. Currently I do this via the following:
#MyLocalAnnotations
#Override
public method inheritedMethodHere (yadda yadda)
{
super.inheritedMethodHere(yadda yadda);
}
Is there a way to do this without overriding the method? It seems like such a waste to override a method only to supply local annotations.

Unfortunately, there isn't a better way than what you are doing now. To do what you are describing you will have to override the method, considering that your JPA annotation will need information specific to the concrete class.
With JPA annotations, you actually have two options -- you can annotate the methods, or you can annotate the properties. On our project we've standardized on annotating properties rather than methods, but this won't help you either, as the properties are presumably part of the concrete class. (If they are somehow shared in the super-class, then you REALLY will need to annotate the methods, and override them).
Its hard to make a recommendation without seeing your schema, but if your entity classes have so much in common that they share many properties in the super-class -- might they simply be stored in the same table, perhaps with a differentiating type column?
Or alternatively if they are not nearly identical, might you just reproduce the common properties in each concrete class? You might be causing yourself more work rather than saving yourself by trying to capture the common properties in the super class. Since you will have to annotate them individually in the concrete classes, just declare them in the concrete classes. If you need common methods that interact with these properties, a separate Utility class to capture those functions could be the answer.
On our project we DO sometimes use a common super class for entities, but it captures some meta-data about the entity -- nothing that would impact persistence logic. Therefore, no JPA annotations are needed (nor would they make sense) in the abstract class.