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".
Related
I've seen some DDD projects with value object representations of entities.
They usually appear like EmployeeDetail, EmployeeDescriptor, EmployeeRecord, etc. Sometimes it holds the entity ID, sometimes not.
Is that a pattern? If yes, does it have a name?
What are the use cases?
Are they value objects, parameter objects, or anything else?
Are they referenced in the domain model (as property) or are they "floating" just as parameters and returns of methods?
Going beyond...
I wonder if I can define any aggregate as an ID + BODY (detail, descriptor, etc) + METHODS (behavior).
public class Employee {
private EmployeeID id;
private EmployeeDetail detail; //the "body"
}
Could I design my aggregates like this to avoid code duplication when using this kind of object?
The immediate advantage of doing this is to avoid those methods with too many parameters in the aggregate factory method.
public class Employee {
...
public static Employee from(EmployeeID id, EmployeeDetail detail){...};
}
instead of
public class Employee {
...
public static Employee from(EmployeeID id, + 10 Value Objects here){...};
}
What do you think?
What you're proposing is the idiomatic (via case classes) approach to modeling an aggregate in Scala: you have an ID essentially pointing to a mutable container of an immutable object graph representing the state (and likely some static functions for defining the state transitions). You are moving away from the more traditional OOP conceptions of domain-driven design to the more FP conceptions (come to the dark side... ;) ).
If doing this, you'll typically want to partition the state so that operations on the aggregate will [as] rarely [as possible] change multiple branches of the state, which enables reuse of as much of the previous object graph as possible.
Could I design my aggregates like this to avoid code duplication when using this kind of object?
What you are proposing is representing the entire entity except its id as a 'bulky' value object. A concept or object's place in your domain (finding that involves defining your bounded contexts and their ubiquitous languages) dictates whether it is treated as a value object or an entity, not coding convenience.
However, if you go with your scheme as a general principle, you risk tangling unrelated data into a single value object. That leads to many conceptual and technical difficulties. Take updating an entity for example. Entities are designed to evolve in their lifecycle in response to operations performed on it. Each operation updates only the relevant properties of an entity. With your solution, for any operations, you have to construct a new value object (as value objects are defined to be immutable) as replacement, potentially copying many irrelevant data.
The examples you are citing are most likely entities with only one value object attribute.
OK - great question...
DDD Question Answered
The difference between an entity object and a value object comes down to perspective - and needs for the given situation.
Let's take a simple example...
A airplane flight to your favourite destination has...
Seats 1A, 10B, 21C available for you too book (entities)
3 of 22 Seats available (value object).
The first reflects individually identifiable seat entities that could be filled.
The second reflects that there are 3 seats available (value object).
With value object you are not concerned with which individual entities (seats) are available - just the total number.
It's not difficult to understand that it depends on who's asking and how much it matters.
Some flights you book a seat and others you book a (any) seat on a plane.
General
Ask yourself a question! Do I care about the individual element or the totality?
NB. An entity (plane) can consider seats, identity and / or value object - depending on use case. Also worth noting, it has multiple depends - Cockpit seats are more likely to be entity seats; and passenger seats value objects.
I'm pretty sure I want the pilot seat to have a qualified pilot; and qualified co-pilot; but I don't really care that much where the passengers seats. Well except I want to make sure the emergency exit seats are suitable passengers to help exit the plane in an emergency.
No simple answer, but a complex set of a pieces to thing about, and to consider for each situation and domain complexity.
Hope that explains some bits, happy to answer follow-up questions...
Have seen some similar questions:
What is the difference between a JavaBean and a POJO?
What is the Difference Between POJO (Plain Old Java Object) and DTO (Data Transfer Object)?
Can you also please tell me the contexts in which they are used? Or the purpose of them?
JavaBeans
A JavaBean is a class that follows the JavaBeans conventions as defined by Sun. Wikipedia has a pretty good summary of what JavaBeans are:
JavaBeans are reusable software components for Java that can be manipulated visually in a builder tool. Practically, they are classes written in the Java programming language conforming to a particular convention. They are used to encapsulate many objects into a single object (the bean), so that they can be passed around as a single bean object instead of as multiple individual objects. A JavaBean is a Java Object that is serializable, has a nullary constructor, and allows access to properties using getter and setter methods.
In order to function as a JavaBean class, an object class must obey certain conventions about method naming, construction, and behavior. These conventions make it possible to have tools that can use, reuse, replace, and connect JavaBeans.
The required conventions are:
The class must have a public default constructor. This allows easy instantiation within editing and activation frameworks.
The class properties must be accessible using get, set, and other methods (so-called accessor methods and mutator methods), following a standard naming convention. This allows easy automated inspection and updating of bean state within frameworks, many of which include custom editors for various types of properties.
The class should be serializable. This allows applications and frameworks to reliably save, store, and restore the bean's state in a fashion that is independent of the VM and platform.
Because these requirements are largely expressed as conventions rather than by implementing interfaces, some developers view JavaBeans as Plain Old Java Objects that follow specific naming conventions.
POJO
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, as opposed to heavyweight EJB 2.x (especially Entity Beans, Stateless Session Beans are not that bad IMO). Today, the term is used for any simple object with no extra stuff. Again, Wikipedia does a good job at defining POJO:
POJO is an acronym for Plain Old Java
Object. The name is used to emphasize
that the object in question is an
ordinary Java Object, not a special
object, and in particular not an
Enterprise JavaBean (especially before
EJB 3). The term was coined by Martin
Fowler, Rebecca Parsons and Josh
MacKenzie in September 2000:
"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."
The term continues the pattern of
older terms for technologies that do
not use fancy new features, such as
POTS (Plain Old Telephone Service) in
telephony, and PODS (Plain Old Data
Structures) that are defined in C++
but use only C language features, and
POD (Plain Old Documentation) in Perl.
The term has most likely gained
widespread acceptance because of the
need for a common and easily
understood term that contrasts with
complicated object frameworks. A
JavaBean is a POJO that is
serializable, has a no-argument
constructor, and allows access to
properties using getter and setter
methods. An Enterprise JavaBean is not
a single class but an entire component
model (again, EJB 3 reduces the
complexity of Enterprise JavaBeans).
As designs using POJOs have become
more commonly-used, systems have
arisen that give POJOs some of the
functionality used in frameworks and
more choice about which areas of
functionality are actually needed.
Hibernate and Spring are examples.
Value Object
A Value Object or VO is an object such as java.lang.Integer that hold values (hence value objects). For a more formal definition, I often refer to Martin Fowler's description of Value Object:
In Patterns of Enterprise Application Architecture I described Value Object as a small object such as a Money or date range object. Their key property is that they follow value semantics rather than reference semantics.
You can usually tell them because their notion of equality isn't based on identity, instead two value objects are equal if all their fields are equal. Although all fields are equal, you don't need to compare all fields if a subset is unique - for example currency codes for currency objects are enough to test equality.
A general heuristic is that value objects should be entirely immutable. If you want to change a value object you should replace the object with a new one and not be allowed to update the values of the value object itself - updatable value objects lead to aliasing problems.
Early J2EE literature used the term value object to describe a different notion, what I call a Data Transfer Object. They have since changed their usage and use the term Transfer Object instead.
You can find some more good material on value objects on the wiki and by Dirk Riehle.
Data Transfer Object
Data Transfer Object or DTO is a (anti) pattern introduced with EJB. Instead of performing many remote calls on EJBs, the idea was to encapsulate data in a value object that could be transfered over the network: a Data Transfer Object. Wikipedia has a decent definition of Data Transfer Object:
Data transfer object (DTO), formerly known as value objects or VO, is a design pattern used to transfer data between software application subsystems. DTOs are often used in conjunction with data access objects to retrieve data from a database.
The difference between data transfer objects and business objects or data access objects is that a DTO does not have any behaviour except for storage and retrieval of its own data (accessors and mutators).
In a traditional EJB architecture, DTOs serve dual purposes: first, they work around the problem that entity beans are not serializable; second, they implicitly define an assembly phase where all data to be used by the view is fetched and marshalled into the DTOs before returning control to the presentation tier.
So, for many people, DTOs and VOs are the same thing (but Fowler uses VOs to mean something else as we saw). Most of time, they follow the JavaBeans conventions and are thus JavaBeans too. And all are POJOs.
DTO vs VO
DTO - Data transfer objects are just data containers which are used to transport data between layers and tiers.
It mainly contains attributes. You can even use public attributes without getters and setters.
Data transfer objects do not contain any business logic.
Analogy: Simple Registration form with attributes username,
password and email id.
When this form is submitted in RegistrationServlet file you will get all the attributes from view layer to business layer where you pass
the attributes to java beans and then to the DAO or the persistence layer.
DTO's helps in transporting the attributes from view layer to business layer and finally to the persistence layer.
DTO was mainly used to get data transported across the network efficiently, it may be even from JVM to another JVM.
DTOs are often java.io.Serializable - in order to transfer data across JVM.
VO - A Value Object [1][2] represents itself a fixed set of data and is similar to a Java enum. A Value Object's identity is based on their state rather than on their object identity and is immutable. A real world example would be Color.RED, Color.BLUE, SEX.FEMALE etc.
POJO vs JavaBeans
[1]
The Java-Beanness of a POJO is that its private attributes are all accessed via public getters and setters that conform to the JavaBeans conventions. e.g.
private String foo;
public String getFoo(){...}
public void setFoo(String foo){...};
[2]
JavaBeans must implement Serializable and have a no-argument constructor, whereas in POJO does not have these restrictions.
Basically,
DTO: "Data transfer objects " can travel between seperate layers in software architecture.
VO: "Value objects " hold a object such as Integer,Money etc.
POJO: Plain Old Java Object which is not a special object.
Java Beans: requires a Java Class to be serializable, have a no-arg constructor and a getter and setter for each field
Java Beans are not the same thing as EJBs.
The JavaBeans specification in Java 1.0 was Sun's attempt to allow Java objects to be manipulated in an IDE that looked like VB. There were rules laid down for objects that qualified as "Java Beans":
Default constructor
Getters and setters for private data members that followed the proper naming convention
Serializable
Maybe others that I'm forgetting.
EJBs came later. They combine distributed components and a transactional model, running in a container that manages threads, pooling, life cycle, and provides services. They are a far cry from Java Beans.
DTOs came about in the Java context because people found out that the EJB 1.0 spec was too "chatty" with the database. Rather than make a roundtrip for every data element, people would package them into Java Beans in bulk and ship them around.
POJOs were a reaction against EJBs.
POJO :
It is a java file(class) which doesn't extend or implement any other java file(class).
Bean:
It is a java file(class) in which all variables are private, methods are public and appropriate getters and setters are used for accessing variables.
Normal class:
It is a java file(class) which may consist of public/private/default/protected variables and which may or may not extend or implement another java file(class).
Value Object : Use when need to measure the objects' equality based on the objects' value.
Data Transfer Object : Pass data with multiple attributes in one shot from client to server across layer, to avoid multiple calls to remote server.
Plain Old Java Object : It's like simple class which properties, public no-arg constructor. As we declare for JPA entity.
difference-between-value-object-pattern-and-data-transfer-pattern
First Talk About
Normal Class - that's mean any class define that's a normally in java it's means you create different type of method properties etc.
Bean - Bean is nothing it's only a object of that particular class using this bean you can access your java class same as object..
and after that talk about last one POJO
POJO - POJO is that class which have no any services it's have only a default constructor and private property and those property for setting a value corresponding setter and getter methods.
It's short form of Plain Java Object.
In Which Cases it is a good coding practice to use implements serializable other than Writing & Reading object to/from file.In a project i went through code. A class using implements serializable even if in that class/project no any Writing/Reading objects to/from file?
If the object leaves the JVM it was created in, the class should implement Serializable.
Serialization is a method by which an object can be represented as a sequence of bytes that includes the object's data as well as information about the object's type and the types of data stored in the object.
After a serialized object has been written into a file, it can be read from the file and deserialized that is, the type information and bytes that represent the object and its data can be used to recreate the object in memory.
This is the main purpose of de-serialization. To get the object information, object type, variable type information from a written(loosely speaking) representation of an object. And hence serialization is required in the first place, to make this possible.
So, whenever, your object has a possibility of leaving the JVM, the program is being executed in, you should make the class, implement Serializable.
Reading/Writing objects into files (Memory), or passing an object over internet or any other type of connection. Whenever the object, leaves the JVM it was created in, it should implement Serializable, so that it can be serialized and deserialized for recognition once it enters back into another/same JVM.
Many good reads at :
1: Why Java needs Serializable interface?
2: What is the purpose of Serialization in Java?
Benefits of serialization:
To persist data for future use.
To send data to a remote computer using client/server Java technologies like RMI , socket programming etc.
To flatten an object into array of bytes in memory.
To send objects between the servers in a cluster.
To exchange data between applets and servlets.
To store user session in Web applications
To activate/passivate enterprise java beans.
You can refer to this article for more details.
If you ever expect your object to be used as data in a RMI setting, they should be serializable, as RMI either needs objects Serializable (if they are to be serialized and sent to the remote side) or to be a UnicastRemoteObject if you need a remote reference.
In earlier versions of java (before java 5) marker interfaces were good way to declare meta data but currently we having annotation which are more powerful to declare meta data for classes.
Annotation provides the very flexible and dynamic capability and we can provide the configuration for annotation meta deta that either we want to send that information in byte code or at run time.
Here If you are not willing to read & write object then there is one purpose left of serialization is, declare metadata for class and if you are goint to declare meta data for class then personally I suggest you don't use serialization just go for annotation.
Annotation is better choice than marker interface and JUnit is a perfect example of using Annotation e.g. #Test for specifying a Test Class. Same can also be achieved by using Test marker interface.
There is one more example which indicate that Annotations are better choice #ThreadSafe looks lot better than implementing ThraedSafe marker interface.
There are other cases in which you want to send an object by value instead of by reference:
Sending objects over the network.
Can't really send objects by reference here.
Multithreading, particularly in Android
Android uses Serializable/Parcelable to send information between Activities. It has something to do with memory mapping and multithreading. I don't really understand this though.
Along with Martin C's answer I want to add that - if you use Serializable then you can easily load your Object graph to memory. For example you have a Student class which have a Deportment. So if you serialize your Student then the Department also be saved. Moreover it also allow you -
1. to rename variables in a serialized class while maintaining backwards-compatibility.
2. to access data from deleted fields in a new version (in other words, change the internal representation of your data while maintaining backwards-compatibility).
Some frameworks/environments might depend upon data objects being serializable. For example in J2EE, the HttpSession attributes must be serializable in order to benefit from Session Persistence. Also RMI and other dark ages artifacts use serialization.
Therefore, though you might not immediately need your data objects to be serializable, it might make sense to declare Serializable just in case (It is almost free, unless you need to go through the pain of declaring readObject/writeObject methods)
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?
I also see other terms as well: Entity Object, Value Object, etc. Are there other terms out there that I should know, and what do these terms refer to?
Can the differences between them, if any, be identified by reading code?
A class is a template for creating objects. Not all OO languages use classes (see Self, Javascript). Typically classes are implemented as objects.
An object is a bundle of data that is packaged with functions that act on that data (called methods). Calling a class's constructor allocates memory for the object and initializes its member variables.
An entity is an object that represents something that has an identity that the system is interested in tracking. Typical examples are Customers and Accounts.
A value object is a value, it doesn't have an identity, and two instances with the same value are considered to be identical. Typical examples are monetary amounts, locations, payment types.
A data transfer object is used for passing a bunch of data around. Typically they're used in distributed systems to send data as a bundle in order to avoid repeated network calls. Data transfer objects have no identity (or there is no expectation they should have any), they are just containers for data.
Generally you can tell the difference between entities and value objects because entities have a recognizable identity, and the system is concerned with creating them, storing them, and changing them. In cases where objects map to some database, entities have primary keys that are either some kind of composite natural key or an artificial key, while value objects are compared by value.
In general a class is a construct which defines a set of properties and methods/functions while an Object is the actual instance of a class which is created at runtime.
Sample class definition:
public class Example{
...
}
The following will create an instance of the Example class as an Object at runtime;
new Example();
Class defines an entity, while an object is the actualentity
Class is a conceptual modelthat defines allthecharacteristics and actions required of an object, whilean object is a real model
Class is a prototype of an object
All objects belonging to the same class have the samecharacteristics and actions