Is "Facade design pattern" and Java interface conceptually same ? As both achieve abstraction by providing simple way of expressing a complex functionality.
Can we say by creating interface we followed Facade pattern ?
No, I's not the same.
For implementing Facade design pattern is not necessary to implement any interface. It just provide a new layer to communicate to your services, as an example.
class ServiceFacade() {
private Service1 service1;
private Service2 service2;
public void update(Entity entity){
service1.update(entity);
service2.update(entity);
}
}
So it will give you one point to communicate with all services that are related. Instead of calling service1.update() and service2.update() you just call facade.update(), so you are sure that both services update the entity.
One more example: maybe all the time service is updated you need to refresh() you cache. This also can be incapsulate in your Facade:
public void update(Entity entity) {
service.update(entity);
cache.refresh();
}
If your facade class has only one dependency, but you want to extend the functionality of this dependency, you can achieve that by implementing Decorator pattern. Here is where you do need to implement the interface.
Let's take a look at the following example. Suppose you have a service and you want to extend it with a simple cache
class CachedService implement Service {
private Service service;
CachedService(Service service, Cache cache){
......
}
......
private Entity get(Long id) {
Entity cachedEntity = cache.get(id);
if (cachedEntity != null){
return cachedEntity;
}
return service.get(id);
}
}
Façade is a conceptual design pattern. There's no specific interface nor implementation.
It's just an additional layer on which you implement operations that otherwise would force upper layers to understand and be coupled to irrelevant implementation details for themselves.
For example, a façade method could be a credit card payment process on which you need to send many requests and process their responses to store the billing result on some database.
Without a façade method, every part of your system on which you need to perform a payment using a credit card would need to repeat those 5-10 code lines all over again. At some point, you find that you need an extra step when performing those operations and you realize that you need to refactor thousands of code lines. That is, your solution could have a security hole and you won't be able to respond to the security treat in time. A façade can make the difference from being a poor software provider to being a serious and robust one!
In the opposite side, when you implement proper façades, when some layer needs to perform a credit card payment it just injects a billing façade and it stays agnostic about how the billing process is being made hence you don't repeat yourself in many parts of your system and refactoring isn't a tedious and time-consuming task.
While façade pattern doesn't define an interface per se, façades should be defined as interfaces, since they can also act as proxies: an implementation could make use of the service layer directly and other might call a RESTful API to perform the same task, depending on who needs the façade itself.
Anyway, this design pattern isn't specific to simplifying a given service layer. You might consider a façade method a method that may simplify a complex string manipulation involving many regular expressions, replacements and insertions.
Facade hides the complexity of the system and provides an interface to the client from where the client can access the system, meaning that Facade hides low-level functionality from client.
Will that interface be java interface it is up to you.
Good question. I was once asked in an interview what design pattern an Interface represents, and my best guess was Facade. I don't really know what the interviewer was trying to get at, though.
Related
In our project architecture we are using a classic MVC pattern including a classic service layer (opening the transaction and calling the DAO layer).
For each service we have an implementation and his interface. But to be honest, I'm pretty sure that for one service and his interface, we will never have more than one implementation. So ok maybe it's more clear to have the public method declared in the interface helping to know what the service does, but an interface is used to have multiple implementation and if we know that we won't have more than one implementation, should we keep them?
From the documentation:
Implementing an interface allows a class to become more formal about
the behavior it promises to provide. Interfaces form a contract
between the class and the outside world, and this contract is enforced
at build time by the compiler.
If you know that you will only have one implementation, the implementation itself will define the contract, so you can remove the interfaces.
But writing an interface could help you to better define the contract and also, you could need at a given point to write a mock for the service, in such a case you would benefit from the use of interfaces.
i think this is a good approach for keeping interfaces.
reasons:
1. say you want to write junits for the same with a different implementations ex. inspite of getting data from database you want to get data from a separate datasource then a different implementation will suffice.
I have a (DAL) Data Access Layer (but this question is relevant for DAOs as well) which is communicating with a restful web service in android (which is less relevant other than the fact that I am not wanting to include heavy restful libraries as the interaction isn't so complex).
I have a object which wraps a list which is populated by information from this data access layer, as the user scans down and reaches the bottom of this list, this object retrieves another set of information from the DAL.
I would like the calling class of this list wrapping object to only have to make calls to the the list wrapping object and not the DAL (or a DAO). I could then construct a single DAL and pass it to the constructors of these list wrapping objects, then the calling class can just keep making calls to this list wrapping object and that object can handle the retreival of new information.
So, does this sound like bad practice or just a really bad explanation?
Is it a bad idea to inject DALs and DAOs in the constructor of the domain object?
The answer depends on whether you feel strongly about "anemic domain models" and mixing object-oriented with functional programming.
One problem is that you'll create a cyclic dependency that way: model and persistence packages have to know about each other. If you use a more functional style, and don't give a DAO reference to a model object, then it's a one-way relationship.
I wouldn't like your design much. I fear that it's too coupled. I'm not bothered by mixing a functional style in.
Domain Objects are typically data carriers without any real logic. I would hence consider it bad design to tightly couple it with your DAO logic. The general logic might go something like:
public class DataService {
private DAO dao;
}
public class UserService {
private DataService svc;
public DomainObject createDomainObject() {
return new DomainObject(dao.getData());
}
}
You are introducing a circular dependency there, so it's not the best design.
If you are developing an android app, and you are scrolling a list, then SlowAdapter and EfficientAdapter are probably what you are looking for.
If I understood you correctly what you are implementing is pagination. And your solution for it is how I would (and have) implemented it myself.
Passing the DAL to the constructor is not bad per se. Best practise would be using a Dependency Injection framework (Spring is a prominent example) in-order to avoid "hard coded" dependencies between layers.
But since you mentioned Android I doubt that using such a framework is a good idea or even possible. (Maybe Android has some sort of DI build-in?)
To summarize you seem to have given some thought about your application architecture. I wouldn't worry about passing arguments to a constructor.
I have a domain object called VehicleRecord which is a hibernate entity. CRUD operations on these VehicleRecords are handled through an entity access object implemented as a stateless session bean interface
#Local
interface VehicleRecordEao {
void add(VehicleRecord record);
List<VehicleRecord> findAll();
...
}
#Stateless
class HibernateVehicleRecordEaoBean implements VehicleRecordEao { ... }
From a business layer perspective removing and adding these records is more than just a CRUD operation. There may be logging and security requirements for example. To provide these operations to clients sessions beans are created in a business layer.
#Local
interface VehicleRecordManager {
void createVehicleRecord(VehicleRecord record);
List<VehicleRecord> findAll(String make, String model);
...
}
#Stateless
class VehicleRecordManagerBean {
public void createVehicleRecordManager(VehicleRecord record) {
//business rules such as logging, security,
// perhaps a required web service interaction
//add the new record with the Eao bean
}
...
}
Controllers work between the presentation layer and the above business layer to get work done, converting between presentation objects (like forms) and entities as necessary.
Something here is not right (smells). I have a class called Manager which has got to be a red flag, but several examples in EJB books actually suggest this kind of high level class and tend to use the name manager themselves. I am new to EJB design but in OO design making high level classes called Manager or Handler or Utility screams of procedural and requires one to rethink their design.
Are these procedural-utility-class session beans the normal pattern, or is bad to organize your session by a bunch of methods related only to the entity they operate on? Are there any naming conventions for session beans? Should the Eao's and business session beans work at the same layer?
If there is a less smelly alternative to this pattern I would love to know, thanks.
Your approach is more-or-less standard. Yes, at a fundamental level this is a procedural approach and goes hand-in-hand with what has been dubbed the Anemic Domain Model "anti-pattern". The alternative is to incorporate your business logic into your domain model so as to create a more OO design where your operations are coupled with your DO's. If you were to go down this route you should be aware of the inherent pros and cons. If you feel that your approach makes the most sense, is easy to understand, test, scale, etc... Then role with it. I have worked on several n-tier projects where this exact "procedural" style is used -- rest assured it is quite standard in EE applications to do things this way.
It's an age old discussion. Should a bread bake itself, or does an oven bake it? Does a message sends itself, or does the postoffice do it? Do I send a message to my friend Pete by asking him to send a message to himself?
According to those who came up with the term ADM, it's more "natural" and more "OO" to let each object do all those things itself. Taken to the extreme (which no one advocates of course, but just as an example) class User would contain all logic for the entire application, since ultimately the user does everything.
If you use slim entities (entities containing only data) and service or DAO objects to work with them, then you're not necessarily not doing OO. There is still a lot of OO going around. Services implement interfaces, inherit from base classes, have encapsulated state (e.g. the EntityManager in JPA), have transparent interceptors via dynamic proxies (e.g. to check security and start/commit a transaction), etc etc.
The term "Anemic" has a definite negative association, but calling the same thing "Slim" actually sounds good. This isn't just a case of using different names to hide code smells, but represents a different thinking among different groups of people.
I think it's up to your taste, and depending on the complexity of your application - these things matter less / more. I think in design, you need to simply thing around the line of:
"If someone new came in without any prior knowledge of the system in question - is it intuitive enough for that person to follow and trace the codes, and straight forward to find where things are"
In your case - naming the EJB related to the Entity Object makes it straight forward and simple - what I dont get is why did you separate it to 2 classes EAO and Manager. Why dont you just combine them into one, so if the EJB/Bean class deals with VehicleRecord Entity, then it will be the "VehicleRecordEAO" or "VehicleRecordManager" or "VehicleRecordAccess" or anything really.
I think EAO / DAO / Access sounds more like getter / setter - or any other simple operations. I dont see anything wrong with "Manager" and make it consistent across the board that all business layer will be called "Manager".
Or if you feel better, think of it as the Facade Pattern - so you can call your business layer (the Manager) as VehicleRecordFacade and VehicleRecordFacadeBean.
That way you basically follow the name and concept of Facade pattern, where it becomes intermediary between application layer and the data layer.
Something here is not right (smells).
I have a class called Manager which
has got to be a red flag
My answer here is towards this concern of yours.
Yes. It is a red flag. Naming a class like "VehicleRecordManager" would be a code smell suggesting that Single responsibility principle would be violated sooner or later.
To elaborate, let me take a few use cases to deal with VehicleRecord
Buy a vehicle
Rent a vehicle
Search for a vehicle
Sell a vehicle
Find Dealers
In most Java applications when we write up a "VehicleService" ( or "VehicleManager") all the above operations would be placed in this class ! Well, this one is easy to do, but hard to maintain. And certainly this class has a lot of responsibilities, hence many reasons to change. (violating Single responsibility principle )
Would calling it VehicleDao eliminate some of the smelliness? A simple change, but indicates clearly it's concerned with data access concerns.
I've in mind an intelligent system which can choose among available OSGi services dynamically. That is, choose an implementation or another depending of some runtime parameter. For example, notify to a running algorithm that change an operator after several iterations, or depending of load balancing in a system or whatever.
while(stopCriterion){
operator.doSomething(); //There exist many operator implementations
}
My first approach is to use DS to expose the services and bind services with 0..n and dynamic policy. Then, from an external intelligent component, notify the algorithm which service use in every iteration (using EventAdmin, maybe?).
operator[selected].doSomething();
This could help me to reduce complexity when many experiments with a lot of different service implementations must be executed. Also, I am planning to use Remote Services specification with Eclipse Communication Framework to make research in distributed algorithms and that stuff, so dynamically appearing of new implementations in execution time also could be possible
However, I don't know if is this a good idea or there exist another better mechanism to dynamically select which implementation use. I think that using ServiceTracker instead DS is not a good option, but I'm open to suggestions :)
Thanks in advance.
This looks to me like a strategy pattern, which can very well be implemented using services. Assuming you have a type of service called Operator (and have an interface with the same name), this would work roughly like this:
Create a OperatorProvider service, which contains the necessary functionality, and some additional information (such as, when is this implementation suitable), and create a number of instances of that, one for each of your strategies.
Create a selector service, which implements the Operator interface, and channels all calls to the service to the most suitable OperatorProvider. The way in which this service selects the most suitable provider, is probably part of the intelligence.
The actual user of the service now only has a dependency on an Operator service, and does not have to worry about the provider selection.
I assume you can put the selection strategy in the selector service, but if it really is an external component, you can use any mechanism you like to handle the communication between the intelligent component and the selector: a service interface, events, etc.
I guess, some sort of dynamic strategy pattern or even dependency injection could fit your needs. Some class uses a strategy (you called it operator) which can change at runtime. I think, you have another service that can tell the which strategy to use (based on runtime parameters).
A rough implementation could look like that:
public Worker {
private Operator operator; // your actual operator strategy
public void setOperator(Operator actualOperator) {
this.operator = operator;
}
public doSomething() {
while(stopCriterion) {
Operator operatorForThisIteration = operator; // pick the injected service
operatorForThisIteration.doSomething;
}
}
}
And another service, the one that can inject dependencies to worker instances, would maintain a list of all worker instances, implement some logic to choose a new service and inject in into all (or some) workers.
I have an in-house enterprise application (EJB2) that works with a certain BPM vendor. The current implementation of the in-house application involves pulling in an object that is only exposed by the vendor's API and making changes to it through the exposed methods in the API.
I'm thinking that I need to somehow map an internal object to this external one, but that seems too simple and I'm not quite sure of the best strategy to go about doing this. Can anyone shed some light on how they have handled such a situation in the past?
I want to "black box" this vendor's software so I can replace it easily if needed. What would be the best approach from a design point of view to somehow map an internal object to this exposed API object? Keep in mind that my in-house app needs to talk to the API still, so there is going to be some dependency between the two, but I want to reduce it so I can also test in isolation from this software using junit.
Thanks,
Jason
Create an interface for the service layer, internally all your code can work with that. Then make a class that uses that interface and calls the third party api methods and as the api facade.
i.e.
interface IAPIEndpoint {
MyDomainDataEntity getData();
}
class MyAPIEndpoint : IAPIEndpoint {
public MyDomainDataEntity getData() {
MyDomainDataEntity dataEntity = new MyDomainDataEntity();
// Call the third party api and fill it
return dataEntity;
}
}
It is always a good idea to interface out third party apis so you don't get their funk invading your app domain, and you can swap out as needed. You could make another class implementation that uses a different service entirely.
To use it in code you just call
IAPIEndpoint endpoint = new MyAPIEndpoint(); // or get it specific to the lang you are using.
Making your stuff based on interfaces when it spans multiple implementations is the way to go. It works great for TDD as well so you can just swap out the interface to a local test one that can inspect your domain code entirely separate from the third party api.
Abstraction; implement a DAL which will provide the transition from internal to external and back.
Then if you switched vendors your internals would remain valuable and you could change out the vendor specific code; assuming the vendors provide the same functionality and the data types related to each other.
I will be the black sheep here and advocate for the YAGNI principle. The problem is that if you do an abstraction layer now, it will look so close to the third party API that it will just be a redundant layer. Since you don't know now what a hypothetical future second vendor's API will look like, you don't know what differences you need to account for, and any future port is likely to require a rework for those unforeseen differences anyway.
If you need a test framework, my recommendation is to make your own test implementation using the same API as the BPM vendor. Even better, almost all reputable API providers provide some sort of sandbox mode for testing. If they don't, you should ask for one.