I'm using Jackson to map JSON to a Java object.
Is the created Java object a DAO since its an object representation of the JSON structure and so is providing access to the JSON data?
It is not a DAO design pattern.
DAO pattern stands for providing the access to the Data Layer.
I will rather say it Value Object.
No, i would say its a data transfer object.
DAOs are used to persist data.
Since you are mapping from one instance to another instance, I think you are using more of a Converter or Adapter (take a look at the adapter pattern here).
To my knowledge, the purpose of the DAO (Data Access Object) is to provide a layer, or an object, which provides some sort of access to your data storage.
It is a Memento, for "Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later" I wouldn't argue with calling it Value Object but it's definitly not a DAO which the defining characteristic is the abstraction of a persistent data story.
Related
I don't understand the difference between POJO and DTO in Java. I have read the article here: What is the Difference Between POJO (Plain Old Java Object) and DTO (Data Transfer Object)? . But I still don't understand the code implementation between them, what makes them different. Can you give the code example for each of them? Thank you so much before!
POJO means Plain Old Java Object. A POJO is an object that doesn't need to implement any specific interface or extend some specific class to be used by a framework. The term has been coined, if I remember correctly, to react against EJB1 and EJB2 which imposed a lot of constraints on the way you had to write beans to make them EJBs. POJO is more a marketing term than a technical term.
DTO means Data Transfer Object. A DTO is an object that is used to transfer data between layers of an application, typically over the network. It often consists in a POJO (i.e. a simple Java class) containing fields and getters.
A Plain Old Java Object or POJO is a term initially introduced to designate a simple lightweight Java object,
not implementing any javax.ejb interface in contrast to Entity beans
Now a days , POJO term is used for any simple object with no extra fancy annotation like third party annotations etc
DTO is an object whose responsibility is just to encapsulate data in a value object . It will just have instance members and corresponding getters and setters. Generally they used to represent the row in data store. While pojo include some more methods that doing some business processing on them
Actually, POJO is more general. Any class which has private serializable fields, getters and setter for each field and doesnt extend other class or implement an interface may be called POJO.
DTO is an object which uses some data object (for example, JPA entity object) to transfer data to another tier of your application (for example, if entity1 object has reference to other entity2, DTO can replace entity2 reference by its entity2.id.toString or other entity2 field).
While many stackoverflow answers regarding this question exists, rarely do they clearly distinguish the difference between the two. As a result, I am having confusion in understanding them.
Here I am working with this pattern
Referring to the figure 9.1, both business object and transfer object are used. While definition of both are given along the lines as:
generally considered to be a class that represents an Entity, e.g. a Book or a Store. Such a class has certain properties like price, colour, width, isbn number etc. In Java or .NET, it consists of so-called setters and getters
Now DTOs have same definition. It appears to me like a Bean representing an object. So in a standalone application what could possibly be business object and DTO. Please distinguish between the two in terms of the pattern link above.
A DTO is used to transport the data from one layer to other, (for example from the data access layer to the model).
A BO contains the Business Logic.
But the most important thing in this patterns is to separate the layer, in order to make the software more easily maintained.
For example, if you separate the data access layer, it doesn't matter if you are using a database to get the data, or a socket, or a plain text file separated with pipelines, you can change this and it will not affect the rest of the layers.
That article defines:
The BusinessObject represents the data client. It is the object that requires access to the data source to obtain and store data. A BusinessObject may be implemented as a session bean, entity bean, or some other Java object, in addition to a servlet or helper bean that accesses the data source.
and
This represents a Transfer Object used as a data carrier. The DataAccessObject may use a Transfer Object to return data to the client. The DataAccessObject may also receive the data from the client in a Transfer Object to update the data in the data source.
Surely you can see the difference between a class that wants to obtain and store data in a data source, and an object that simply carries data between layers?
Is a Data Transfer Object the same as a Value Object or are they different? If they are different then where should we use a DTO and where should we use a VO?
The programming language we are talking about is Java and the context is - there is a web application, which fetches data from a database and then processes it and ultimately the processed information is displayed on the front-end.
A value object is a simple object whose equality isn't based on identity.
A data transfer object is an object used to transfer data between software application subsystems, usually between business layers and UI. It is focused just on plain data, so it doesn't have any behaviour.
A Data Transfer Object is a kludge for moving a bunch of data from one layer or tier to another, the goal is to minimize the number of calls back and forth by packing a bunch of stuff into the same data structure and sending it together. Some people also use it, like Michael points out in his post here, so that the classes used by one layer are not exposed to the layer calling it. When I refer to DTO as a kludge, I mean there's not a precise abstract concept getting implemented, it's a practical workaround for helping with communication between application layers.
A Value Object is something where we're only interested in its value, like a monetary amount, a date range, or a code from a lookup table. It does not have an identity, meaning you would not be concerned, if you had several of them, of keeping track of which is which, because they are not things in themselves.
Contrast Value Objects to things that do have a unique identity in your system, which are called Entities. If you have a system where it tracks a customer making a payment, the customer and the payment are entities, because they represent specific things, but the monetary amount on the payment is just a value, it doesn't have an existence by itself, as far as your system is concerned. How something relates to your system determines if it is a Value Object or an Entity.
use a DTO at the boundary of your services if you don't want to send the actual domain object to the service's clients - this helps reduce dependencies between the client and service.
values objects are simply objects whose equality isn't based on identity e.g. java.lang.Integer
DTOs and value objects aren't really alternatives to each other.
They are different, but I've even used the two interchangeably in the past, which is wrong. I read that DTO (Data Transfer Object) was called a VO ( Value Object) in the first edition of the Core J2EE Patterns book, but wasn't able to find that reference.
A DTO, which I've sometimes called a Dumb Transfer Object to help me remember it's a container and shouldn't have any business logic is used to transport data between layers and tiers. It just should be an object with attributes that has getters/setters.
A VO however is similar to a JAVA Enum and represents a fixed set of data. A VO doesn't have object identity (the address of the object instance in memory), it is identified by its value and is immutable.
Martin Fowler, talking about Data Transfer Objects (DTOs):
Many people in the Sun community use the term "Value Object" for this pattern. I use it to mean something else.
So the term "Value Object" has been used to mean DTO, but as of him (and the other posters), its use as a DTO seems discouraged.
Good detailed answer in Matthias Noback article Is it a DTO or a Value Object?
In short a DTO:
Declares and enforces a schema for data: names and types
Offers no guarantees about correctness of values
A value object:
Wraps one or more values or value objects
Provides evidence of the correctness of these values
Maybe because of lack of experience, but I would put it this way: It's the matter of scope.
DTO has word transfer in it so it means some parts of the system will communicate using it.
Value object has smaller scope, you will pass set of data in value object instead in array from one service to the other.
As much as I understood niether of them is "object whose equality isn't based on identity".
We can use a class object for transfering data to another class. what is the speciality of data transfer objects? How to create them ? Is it just like class objects?
The primary difference is that DTOs, by design, don't have any business logic in them. They're just data structures.
For instance: You might have a database in which you store "users", and if using DTOs, you might use a UserBean to store and retrieve user objects. But your business logic may have a User object (possibly derived from the bean, more likely using the bean via aggregation) that not only has the data, but additional methods for things that User can do.
I believe this should be true:
assertTrue(POJO == DTO)
The only special thing about DTO is that they should not contain any behavior.
1, The class object maybe contains too many references to other objects, thus too big to be serialized to transfer. DTO only selects the interesting parts, this can be a performance gain.
2, In Hibernate, the entity object may contain lazy-initialized references, these objects need session context to do initialization. These entity objects looks like "smart objects", DTO here convert these "smart objects" to "plain objects", because transfer "smart objects" is meaningless when the session context is no more existed.
Personally, I don't like DTO, it introduce another layer of redundant, but sometime (especially when working with Hibernate ORM) I can't live without it.
A DTO class is an ordinary Java class with a just special meaning - just like a Observer, a Factory or a Model. The name is coming from a core J2EE design pattern (the Transfer Object pattern) and the pattern proposes a common way to transfer information between a database and java-classes-based model.
In brief, a DTO is a java class where the class name maps to a database table name and each database column maps to a class attribute. Then it contains getter and setter methods.
Here is one explanation of the (Data) Transfer Object pattern.
I cannot find difference between them. Does anyone know how to differentiate them?
POJO or "Plain Old Java Object" is a name used to describe "ordinary" Java objects, as opposed to EJBs (originally) or anything considered "heavy" with dependencies on other technologies.
DTO or "Data Transfer Object" is an object for... well... transferring data, usually between your "business" classes and persistence layer. It typically is a behavior-less class much like a C-style struct. They are an outdated concept.
A POJO is just a simple Java object, the acronym is used to emphasize that it really is nothing special.
A DTO is a Data Transfer Object which is used to encapsulate data that is transferred over a connection between layers or subsystems. See the wikipedia article, it's also a Core J2EE pattern (http://www.oracle.com/technetwork/java/transferobject-139757.html).
http://en.wikipedia.org/wiki/Data_transfer_object
All DTOs are POJOs, but not all POJOs are DTOs. An example of POJO that is not a DTO is a business class that contains state and behavior (business logic).
DTO (Data transfer object) : Is a Core J2EE design pattern used for transferring data within the system.DTO Pattern
POJO (Plain Old Java Object) : It is just an acronym people use for suggesting that is a simple java object (which nowadays is heavily annotated for doing some meaning full work).
DTO Pattern
J2EE Pattern Catalog
DTO is pojo, but pojo is not dto, because pojo can have more behavior but DTO just basically no behavior
Oracle document has clear description.
A POJO can have behavior. The book POJOs in Action details use of POJOS for application development.
DTOs are data containers that help transfer data from one layer to another. DTOs aren't supposed to contain any behavior.
I could understand the difference between POJO and DTO from this sentence of DTO's wiki:
DTOs are simple objects that should not contain any business logic but may contain serialization and deserialization mechanisms for transferring data over the wire.
Also, DTO is perfectly visualized and described in detail in Martin Fowler's Catalog of Patterns of Enterprise Application Architecture
POJO = Plain Old Java Object
DTO = Data Transfer Object
-- Edit
Well, this is assuming you don't know what the acronyms mean. A Pojo is just an object that is free from any sort of inheritance chain. A DTO exists in your data model, so probably follows a strict chain relating it to a given entity.