I have some persistence capable Java objects with all the #annotations in my web application. All these objects reside in a data layer. Is it a best practice to use these persistence objects as data transfer objects?
For example, if I want to pass back the data fetched from data store, should I directly return those persistence objects or should I manually copy data to a intermediary DTO and pass it back to other layers? Which approach would you suggest?
I would say that it is OK to do so (in fact the major advantage for these ORMs were to use these domain objects in different layers w/o having unnecessary DTOs) if you follow the following guidelines:
You don't extend the session boundaries i.e., any changes related to database should always be done using the Data Access Layer you have defined and not through these passed objects in other layers.
Any data that you need in other layer (layers above the Data Access Layer like Business Logic Layer and Presentation Layer) is pre-populated in these objects otherwise you will get exceptions according to the ORM behavior.
Don't extend the session boundaries for resolving the issue mentioned in No. 2
If you are absolutely certain no copies will end up in the Session storage or will be persisted/migrated to another instance/whatever, there is no need to do it.
If you need to keep these objects over multiple sessions/requests then it makes sense.
Another use-case is when you need to decouple loginc and persistence layer very thoroughly (i.e. to swap different persistence layers) then the coupling through the annotations could be troublesome.
I have never needed the extra level of abstraction provided by copying the persistence instance to a different DTO class.
Related
Why should I use DTOs/Domain objects when I can just put all business classes in a class library, use them in the business logic, then also pass those same business objects on to boundary classes?
UPDATE:
All are great points, thanks for the help. A follow up question:
Where do you typically place these DTOs? Alongside the Domain objects i.e. in the same namespace?
namespace MedSched.Medical
{
public class MedicalGroup
{
//...
}
public class MedicalGroupDTO
{
//...
}
}
The DTO's provide an abstraction layer for your domain model. You can therefore change the model and not break the contract you have for your service clients. This is akin to a common practice in database design where views and procedures become the abstraction layer for the underlying data model.
Serialization - you may over expose data and send bloated messages over the wire. This may be mitigated by using serialization attributes, but you may still have extra information.
Implicit vs. Explicit Contracts - by exposing domain objects you leave it up to the client to interpret their usage since they do not have the full model at their disposal. You will often make updates to domain objects implicitly based on the presence or removal of associations, or blindly accept all changes. DTOs explicitly express the usage of the service and the desired operation.
Disconnected Scenarios - having explicit messages or DTOs would make it easier for you to implement messaging and messaging patterns such as Message Broker, etc.
DDD - pure DDD demands that domain objects are externally immutable, and therefore you must offload this to another object, typically a DTO.
I can think of 2 basic scenarios for using DTOs:
You're creating your business object from data that's incomplete and will fail validation. For example, you're parsing a CSV file or an Excel file from where your business object is created. If you use data directly from these objects to create your business object, it is quite possible to fail several validation rules within the object, because data from such files are prone to errors. They also tend to have a different structure that you have in your final business object: having a placeholder for that incomplete data will be useful.
You're transporting your business object through a medium that is bandwidth intensive. If you are using a web service, you will need to use DTOs to simplify your object before transport; otherwise the CLR will have a hard time trying to serialize all your data.
DTOs are Data Transfer Objects, with Transfer being the key word. They are great when you want to pass your objects across the wire and potentially communicate with a different language because they are "light weight" (ie no business logic.)
If your application isn't web service based, DTOs aren't really going to buy you anything.
So deciding to use or not use DTOs should be based on the architecture of your application.
There are times when the data you want to pass around doesn't map exactly to the structure the business objects, in which case you use DTOs.
Ex: when you want to pass a subset of the data or a projection.
You know that for an API project there are many patterns to retrieve and serve data to client. Controller <-> Service <-> Dao pattern is one of the these patterns.
Is it good to return any type of object from service layer or service layer return objects must be related entity?
For example, we have User, UserService, UserDao.
Should UserService return types be User or any type of object is suitable?
Having logical operations in service layer force me to use other objects as return type.
What is the best practice for that case?
I've found that I need 3 types of objects to keep things manageable (I'm still in a battle trying to come up with a decent naming strategy). It's sometimes beneficial to have:
Objects returned from your data layer - These objects will be dictated by your data implementation. For example, they could change if you decide to store your data differently. Perhaps you decide to serialise an object and store it as JSON rather than storing it in a separate table, or you move from SQL to NOSQL.
Objects returned from your domain layer - These are application objects and should model your domain. They shouldn't contain anything that is specific to your data implementation.
Objects sent to and from you API - It's useful to have separate serialisation objects to shape your API. It means you can tweak your API without having to touch the business logic. On some projects, I have multiple versions of the API objects for v1, v2 etc.
The widely accepted practise involve the service to have all business logic in it and return the Data Transfer Object(DTO)/ Business Object like User in your case. The Service may call DAO, any other datasources to fetch the entities, and a mapper/converter util can be used to convert the entities to DTO objects.
The DTO objects returned from Service can be embedded in a ResponseEntity(In the case of Spring MVC) and returned from the controller.
This makes your application into 3 separated layers like web, service, dataaccess. This supports design principles like Separation of Concerns, Single Responsibility. This also makes your unit testing and code management easier.
It is common in Java applications to have :
an IHM layer
a service layer
a dao layer
It is common that each layer has it's own objects and relies on converters to transform objects from a layer to the other.
In my case, I use Sing MVC as the IHM layer, and MyBatis as the doa layer.
Since Spring MVC and MyBatis use only beans without any annotations on that beans :
Is it necessary to have differents objects on the 3 layers ?
Is it a good practice to share the same object between the 3 layers ?
Or why it is not recommended to do so ?
In most cases it is a good practice to keep a full seperation between layers.
Lets say you have an object called UserDAO that holds user data, and after writing quite a lot of code on all layers you decide to update sureName to be lastName in the object, now you have to go through all the layers and change every reference to that object field.
On the other hand, if you kept this object on the DB layer and mapped it to other objects on the upper layers, all you need to change is the mapping:
somObj.setUserLastName(dao.getSureName());
to:
somObj.setUserLastName(dao.getLastName());
of course this is a silly example, but it is just for demonstration.
On the other hand, seperating every object on every layer might cause a lot of redundency and duplication and can turn into a maintenance horror, so you need to think of the pros and cons and decide where it is most suitable to seperate and where not.
I have worked on both projects that have a model for each layer and projects that have a good domain model.
My preference goes to a domain model where the developers and architects took the time to develop it and think about the use of each field.
This kind of domain model is much easier to maintain and update.
I remember having to add a field to the DAO layer which implied I had to update 3 models and 4 converters. The risk of introducing bugs is major. Especially if each layer has, for example, another implementation of the Date object. (Dao --> java.sql.Timestamp, Service --> java.util.Date).
So I advise a good domain model shared across all layers.
I am using MVC pattern in my web application. In which I have three layers
Control Layer
Manager Layer
Dao Layer
And I am using DTOs from control layer to manger and then to Dao layer and same as opposite.
My question is that what is the main purpose of DTO?
Can I use DTOs to map our relational database table or should I go with 'Bean'?
If I use DTOs between layers then how can I represent a database table in an object because DTOs among layers can contain properties which are not related to the database table.
There is no problem in using DTOs and map them to your database tables. But you'll have to do the mapping by yourself (using JDBC, Spring JDBC, etc).
Another option is to use an ORM to do the mapping of your DTOs to database. You can even create properties that are not mapped to your tables. Take a look at JPA.
The choice between those two options is something personal. The first will be more laborous at first, while the second option have a bigger learning curve. If you are well versed into SQL, I would go with JDBC.
My question is that what is the main
purpose of DTO?
The main purpose of a DTO is to transmit data between two layers. It has no real functionality other than to act as a basket for shipping data.
That's why they call it a Data Transfer Object.
Can i use DTO's to map our relational
database table or should i go with
'Bean'?
Whether you decide to use JavaBean formatted accessors or your own accessors really doesn't matter with a DTO. Both sides of the transfer must be in agreement; but, if you have a setName(...) setter or a name(...) setter it will not affect functionality.
Although it may not matter in a functionality sense, it is best to stick to established naming conventions for ease of revisiting the code and lack of confusion when training new maintainers. Also, a few libraries might assume you are using bean conventions (or require them). If you are uncertain, best to stick with standard JavaBean conventions, as your new conventions are probably not as tested (or as formal).
If i use DTO's between layers then how
can i represent a database table in
object because DTO's among layers can
contain properties which are not
related to database table.
DTOs have nothing to do with database tables. Don't make your DTO look like your database table unless it's the most natural thing to do.
The main purpose of DTO's is to reduce the overhead when transferring data across layers.
If you didnt have a DTO, what you would have is a class containing data as well as logic which would be getting passed across layers. Using DTO's ensures that you pass only what is needed i.e the data across layers.
Definitely, you do have the option of mapping your DTO's to your database tables and having the bean design which more closely represents the domain objects.
That is one way of doing it.
Conversely, depending on your database design, your DTO's could be more in line with your actual business entities - just without the logic
My question is that what is the main
purpose of DTO?
Like the expansion (Data Transfer Object) implies, DTOs are meant to transfer structured data across various tiers. DTOs enable you to decouple the protocol specific implementations that represent data, so that data from different sources can be abstracted before communication across tiers.
For example, DTOs will allow you to decouple the data present in a HttpServletRequest object from its internal storage, so that you can send the data to a service in the business logic layer. The same applies for DTOs used to abstract the results obtained from a SQL query and residing in a ResultSet object. In short, DTOs allow you to transmit data without holding onto the source - you can forget about the HTTP response and the JDBC connections, while you work on the data.
Can i use DTO's to map our relational
database table or should i go with
'Bean'?
You can adopt the second approach of using Beans. In fact, with JPA you do not require your DTOs at all. The JPA managed beans themselves represent data in various tables, and can be de-linked from the database state, so that you can use them for data transmission.
If i use DTO's between layers then how
can i represent a database table in
object because DTO's among layers can
contain properties which are not
related to database table.
That depends on how you want to couple the DTO with the database table. It is preferable to have a one-to-one mapping between the DTO and the database table, and choose another DTO for the purpose of transmitting properties not related to the table. After all, DTOs like every other object should have a single responsibility. If the responsibility is to reflect a database table, then it should contain other "irrelevant" properties.
To extend the recommendation of using JPA in this context, it is poor design to have unrelated attributes in a JPA entity, especially when that unrelated attribute should be marked as transient and adds no value to the behavior of the entity.
I've recently overheard people saying that data transfer objects (DTOs) are an anti-pattern.
Why? What are the alternatives?
Some projects have all data twice. Once as domain objects, and once as data transfer objects.
This duplication has a huge cost, so the architecture needs to get a huge benefit from this separation to be worth it.
DTOs are not an anti-pattern. When you're sending some data across the wire (say, to an web page in an Ajax call), you want to be sure that you conserve bandwidth by only sending data that the destination will use. Also, often it is convenient for the presentation layer to have the data in a slightly different format than a native business object.
I know this is a Java-oriented question, but in .NET languages anonymous types, serialization, and LINQ allow DTOs to be constructed on-the-fly, which reduces the setup and overhead of using them.
"DTO an AntiPattern in EJB 3.0" (original link currently offline) says:
The heavy weight nature of Entity
Beans in EJB specifications prior to
EJB 3.0, resulted in the usage of
design patterns like Data Transfer
Objects (DTO). DTOs became the
lightweight objects (which should have
been the entity beans themselves in
the first place), used for sending the
data across the tiers... now EJB 3.0
spec makes the Entity bean model same
as Plain old Java object (POJO). With
this new POJO model, you will no
longer need to create a DTO for each
entity or for a set of entities... If
you want to send the EJB 3.0 entities
across the tier make them just
implement java.io.Serialiazable
OO purists would say that DTO is anti-pattern because objects become data table representations instead of real domain objects.
I don't think DTOs are an anti-pattern per se, but there are antipatterns associated with the use of DTOs. Bill Dudney refers to DTO explosion as an example:
http://www.softwaresummit.com/2003/speakers/DudneyJ2EEAntiPatterns.pdf
There are also a number of abuses of DTOs mentioned here:
http://anirudhvyas.com/root/2008/04/19/abuses-of-dto-pattern-in-java-world/
They originated because of three tier systems (typically using EJB as technology) as a means to pass data between tiers. Most modern day Java systems based on frameworks such as Spring take a alternative simplified view using POJOs as domain objects (often annotated with JPA etc...) in a single tier... The use of DTOs here is unnecessary.
Some consider DTOs an anti-pattern due to their possible abuses. They're often used when they shouldn't be/don't need to be.
This article vaguely describes some abuses.
The question should not be "why", but "when".
Definitively it's anti-pattern when only result of using it is higher cost - run-time or maintenance. I worked on projects having hundreds of DTOs identical to database entity classes. Each time you wanted to add a single field you ad to add id like four times - to DTO, to entity, to conversion from DTO to domain classes or entities, the inverse conversion, ... You forgot some of the places and data got inconsistent.
It's not anti-pattern when you really need different representation of domain classes - flatter, richer, ...
Personally I start with a domain class and pass it around, with proper checks at the right places. I can annotate and/or add some "helper" classes to make mappings to database, to serialization formats like JSON or XML ... I can always split a class to two if I feel the need.
It's about your point of view - I prefer to look at a domain object as a single object playing various roles, instead of multiple objects created from each other. If the only role an object is to transport data, then it's DTO.
If you're building a distributed system, then DTOs are certainly not an anti pattern. Not everyone will develop in that sense, but if you have a (for example) Open Social app all running off JavaScript.
It will post a load of data to your API. This is then deserialized into some form of object, typically a DTO/Request object. This can then be validated to ensure the data entered is correct before being converted into a model object.
In my opinion, it's seen as an anti-pattern because it's mis-used. If you're not build a distributed system, chances are you don't need them.
DTO becomes a necessity and not an ANTI-PATTERN when you have all your domain objects load associated objects EAGERly.
If you don't make DTOs, you will have unnecessary transferred objects from your business layer to your client/web layer.
To limit overhead for this case, rather transfer DTOs.
I think the people mean it could be an anti-pattern if you implement all remote objects as DTOs. A DTO is merely just a set of attributes and if you have big objects you would always transfer all the attributes even if you do not need or use them. In the latter case prefer using a Proxy pattern.
The intention of a Data Transfer Object is to store data from different sources and then transfer it into a database (or Remote Facade) at once.
However, the DTO pattern violates the Single Responsibility Principle, since the DTO not only stores data, but also transfers it from or to the database/facade.
The need to separate data objects from business objects is not an antipattern, since it is probably required to separate the database layer anyway.
Instead of DTOs you should use the Aggregate and Repository Patterns, which separates the collection of objects (Aggregate) and the data transfer (Repository).
To transfer a group of objects you can use the Unit Of Work pattern, that holds a set of Repositories and a transaction context; in order to transfer each object in the aggregate separately within the transaction.