I have been reading Clean Architecture by R. C. Martin.
I'm trying to make sense of it, by developing a small project where I'm trying to apply its concepts.
One core concept in the domain layer is to not use frameworks, 3rd party lib, and avoid #Annotations, simply make the classes in the domain pure POJOs.
I would like to know 2 things.
Is it conceptually right to do my "entities" validations inside the domain layer and if so, using Bean Validation would be a reasonable option since it is a specification by java itself?
The job of a POJO domain (business) object is to faithfully represent the values of the content, and maintain the integrity of the information it represents. Validating any data input is a key part of that. Protecting against faulty inputs is a main job of the domain POJO.
So, yes, it makes perfect sense to use the Bean Validation framework to assist in this effort to faithfully represent the domain data correctly.
The admonition against frameworks and libraries should not be misinterpreted as simply and literally no frameworks/libraries. The goal of that advice is to not intertwine the internals of the domain POJO with the outer world of the application’s complexities. The domain POJO should be unaware of how it is being used. So you should be able to pick up the class of a domain POJO from this app’s codebase and drop it into any other app’s code base with no further programming. The domain object should be agnostic and ignorant of the app within which it is being used.
Avoiding this kind of unnecessary messy intertwining is what is meant by “clean” versus “dirty” architecture. Every part of your app should focus on its own responsibility, to do a job that no other part of the app can do, with all little interference or entanglement from other parts of your app as is practical.
The Bean Validation implementation library is used internally by your domain POJO, without concern for the outer app, except for the configuration necessary to load a Bean Validation implementation. This scenario is entirely reasonable, and does not violate Martin’s advice.
For example, your Customer, Invoice, and PurchaseOrder classes should remain blissfully ignorant of your choice of a reactive/flow architecture, or some event bus coordinating parts of your app, or whether your app is a local desktop app built in JavaFX versus a web app built in Vaadin Flow.
I'm developing a small web app with Spring and java. And came up with a question about package names.
I have a layer separation app, having:
-the "web" layer with my controllers
-the "domain" layer with my model
-the "connector" layer, in charge of doing http communication with external web services
-the "service" layer, with contains my bussiness and app logic.
I have transformers, comparators used for sorting and other classes, all are used inside my service layer, because are part of the bussiness logic. My question is, should the transformers be inside the service package, something like "service.transformer", and same with "servcice.sorting", or should they be a completely separated package, outside the "service" package?
I'd like to hear your opinions
There are a couple of considerations to be made when packaging your classes. First, where are the classes used? And, second, how should they be packaged? Regarding the first, you say that your transformers, comparators and associated miscellaneous classes are only used by your service classes. If so, then it makes sense to put them inside your service package.
Regarding the second question, you should think about whether they would ever need to be used elsewhere in this project or another. If so, then you might want to package them at a higher independent level. That makes it easier to package them up into their own library for use elsewhere.
If your using a modern IDE, I wouldn't think too hard about it since it is trivial to refactor your code later as needs change. For example, in Eclipse adding and optimizing import statements, or moving classes from one package to another is a few keystrokes.
I've been reading a lot about package-by-feature naming convention. So I've decided to give it a try in a new project. However, I'm not sure how it should be named my packages that will be used by most of my classes, since I'm using a huge framework, such as Spring and Hibernate, for example.
This is how handle our Spring contexts classes:
And our database access class, the one that manages connections and so on.
I've a draft about this: using a common package for these frameworks, like:
com.company.project.common.spring
com.company.project.common.database
But I'm afraid that this still looks like package-by-layer a bit. :)
How the packages that will be accessed by my feature classes should be created ?
The common recommendation is "package by feature, not layer". What I often do is "package by feature, then layer". I also think that top-level packages should be "feature"-based (functional components, whatever). But I also like to have my layers separated into sub-packages.
From my point of view, framework-related code does not per se constitute "features" (as in "important, high-level aspects of the problem domain"), therefore package-by-feature is does not make much sense here. But still, this is important code and you need an approach to structure it.
I am normally use two approaches:
If I need to extend or augment libraries I'm using, I structure packages parallel to the package structure of the library. For instance if I'd need to implement some new number formatter for Spring, I'll probably name the package com.acme.foo.springframework.format.number, parallel to org.springframework.format.number.
However if I need to implement common base classes for layers of features, this would be probably something like com.acme.foo.common.<layer>. For instance if we have com.acme.foo.<feature>.dataaccess packages for data access layer of some feature, com.acme.foo.common.dataaccess could hold base classes for data access layers of all features.
Both approaches are used in parallel. You just have to decide whether some class is a framework or library extension (can you imagine using it outside this project?) or is it closer to the layers of your project.
My domain classes and persistance logic (Hibernate) are in one project called model. This jar is included within all of my apps.
Packaged com.company.model & com.company.persistance
Another Utils.jar - contains DateTime, String, Thread, etc general helper classes. This again is included within all of my apps.
Packaged com.company.utils
I have a CXF/Spring app that exposes services for manipulating my data. CRUD functionality, ALL other common functions. This is the 'way in' to my database for any app designed.
Packaged com.company.services and running on Glassfish app server
I have other apps that use the web services (Spring injected) to manipulate my data. Including a web app that will use YUI widgets and the XML/JSON from the web services for a nice smooth UI.
I understand its not really a question! I suppose Im looking for confirmation that this is how others are designing their software. If my architecture makes good, logical sense! Obviously there are security concerns - I will want some applications allowed to only access service x. I will address these later.
Sounds good.
It depends also of the type of application you're developing and the specific requirements for that ( it has to be deployed every week, it has to be deployed in several locations etc )
But so far sounds good enough.
Looks like you can formulate a question from here in the future for some specific scenario.
Since this is not a question, mine is not really an answer. CW
My only comment would be to put the persistence and Hibernate classes into a separate module; so that the model module can be purely beans/POJO/your domain classes.
Here's how I've organized a few multi-module projects before
project-data - contains domain classes and DAOs (interfaces only)
project-services - "Business logic" layer services, makes use of DAO interfaces.
Depends on project-data.
project-hibernate - Hibernate implementation of DAO interfaces.
Depends on project-data.
Conceivably if I were to use some other sort of data-access method I would just create a separate module for that. Client apps could then choose which modules to be dependent on.
Only suggestion I might have is that when you're creating service/models that you group them by subpackage name. ie
com.company.model.core
com.company.service.core
com.company.model.billing
com.company.service.billing
Also, be careful to ensure that no controller code (manipulating your UI) ends up in the services.
First of all, I know how to build a Java application. But I have always been puzzled about where to put my classes. There are proponents for organizing the packages in a strictly domain oriented fashion, others separate by tier.
I myself have always had problems with
naming,
placing
So,
Where do you put your domain specific constants (and what is the best name for such a class)?
Where do you put classes for stuff which is both infrastructural and domain specific (for instance I have a FileStorageStrategy class, which stores the files either in the database, or alternatively in database)?
Where to put Exceptions?
Are there any standards to which I can refer?
I've really come to like Maven's Standard Directory Layout.
One of the key ideas for me is to have two source roots - one for production code and one for test code like so:
MyProject/src/main/java/com/acme/Widget.java
MyProject/src/test/java/com/acme/WidgetTest.java
(here, both src/main/java and src/test/java are source roots).
Advantages:
Your tests have package (or "default") level access to your classes under test.
You can easily package only your production sources into a JAR by dropping src/test/java as a source root.
One rule of thumb about class placement and packages:
Generally speaking, well structured projects will be free of circular dependencies. Learn when they are bad (and when they are not), and consider a tool like JDepend or SonarJ that will help you eliminate them.
I'm a huge fan of organized sources, so I always create the following directory structure:
/src - for your packages & classes
/test - for unit tests
/docs - for documentation, generated and manually edited
/lib - 3rd party libraries
/etc - unrelated stuff
/bin (or /classes) - compiled classes, output of your compile
/dist - for distribution packages, hopefully auto generated by a build system
In /src I'm using the default Java patterns: Package names starting with your domain (org.yourdomain.yourprojectname) and class names reflecting the OOP aspect you're creating with the class (see the other commenters). Common package names like util, model, view, events are useful, too.
I tend to put constants for a specific topic in an own class, like SessionConstants or ServiceConstants in the same package of the domain classes.
Where I'm working, we're using Maven 2 and we have a pretty nice archetype for our projects. The goal was to obtain a good separation of concerns, thus we defined a project structure using multiple modules (one for each application 'layer'):
- common: common code used by the other layers (e.g., i18n)
- entities: the domain entities
- repositories: this module contains the daos interfaces and implementations
- services-intf: interfaces for the services (e.g, UserService, ...)
- services-impl: implementations of the services (e.g, UserServiceImpl)
- web: everything regarding the web content (e.g., css, jsps, jsf pages, ...)
- ws: web services
Each module has its own dependencies (e.g., repositories could have jpa) and some are project wide (thus they belong in the common module). Dependencies between the different project modules clearly separate things (e.g., the web layer depends on the service layer but doesn't know about the repository layer).
Each module has its own base package, for example if the application package is "com.foo.bar", then we have:
com.foo.bar.common
com.foo.bar.entities
com.foo.bar.repositories
com.foo.bar.services
com.foo.bar.services.impl
...
Each module respects the standard maven project structure:
src\
..main\java
...\resources
..test\java
...\resources
Unit tests for a given layer easily find their place under \src\test... Everything that is domain specific has it's place in the entities module. Now something like a FileStorageStrategy should go into the repositories module, since we don't need to know exactly what the implementation is. In the services layer, we only know the repository interface, we do not care what the specific implementation is (separation of concerns).
There are multiple advantages to this approach:
clear separation of concerns
each module is packageable as a jar (or a war in the case of the web module) and thus allows for easier code reuse (e.g., we could install the module in the maven repository and reuse it in another project)
maximum independence of each part of the project
I know this doesn't answer all your questions, but I think this could put you on the right path and could prove useful to others.
Class names should always be descriptive and self-explanatory. If you have multiple domains of responsibility for your classes then they should probably be refactored.
Likewise for you packages. They should be grouped by domain of responsibility. Every domain has it's own exceptions.
Generally don't sweat it until you get to a point where it is becoming overwhelming and bloated. Then sit down and don't code, just refactor the classes out, compiling regularly to make sure everything works. Then continue as you did before.
Use packages to group related functionality together.
Usually the top of your package tree is your domain name reversed (com.domain.subdomain) to guarantee uniqueness, and then usually there will be a package for your application. Then subdivide that by related area, so your FileStorageStrategy might go in, say, com.domain.subdomain.myapp.storage, and then there might be specific implementations/subclasses/whatever in com.domain.subdomain.myapp.storage.file and com.domain.subdomain.myapp.storage.database. These names can get pretty long, but import keeps them all at the top of files and IDEs can help to manage that as well.
Exceptions usually go in the same package as the classes that throw them, so if you had, say, FileStorageException it would go in the same package as FileStorageStrategy. Likewise an interface defining constants would be in the same package.
There's not really any standard as such, just use common sense, and if it all gets too messy, refactor!
One thing that I found very helpful for unit tests was to have a myApp/src/ and also myApp/test_src/ directories. This way, I can place unit tests in the same packages as the classes they test, and yet I can easily exclude the test cases when I prepare my production installation.
Short answer: draw your system architecture in terms of modules, drawn side-by-side, with each module sliced vertically into layers (e.g. view, model, persistence). Then use a structure like com.mycompany.myapp.somemodule.somelayer, e.g. com.mycompany.myapp.client.view or com.mycompany.myapp.server.model.
Using the top level of packages for application modules, in the old-fashioned computer-science sense of modular programming, ought to be obvious. However, on most of the projects I have worked on we end up forgetting to do that, and end up with a mess of packages without that top-level structure. This anti-pattern usually shows itself as a package for something like 'listeners' or 'actions' that groups otherwise unrelated classes simply because they happen to implement the same interface.
Within a module, or in a small application, use packages for the application layers. Likely packages include things like the following, depending on the architecture:
com.mycompany.myapp.view
com.mycompany.myapp.model
com.mycompany.myapp.services
com.mycompany.myapp.rules
com.mycompany.myapp.persistence (or 'dao' for data access layer)
com.mycompany.myapp.util (beware of this being used as if it were 'misc')
Within each of these layers, it is natural to group classes by type if there are a lot. A common anti-pattern here is to unnecessarily introduce too many packages and levels of sub-package so that there are only a few classes in each package.
I think keep it simple and don't over think it. Don't over abstract and layer too much. Just keep it neat, and as it grows, refactoring it is trivial. One of the best features of IDEs is refactoring, so why not make use of it and save you brain power for solving problems that are related to your app, rather then meta issues like code organisation.
One thing I've done in the past - if I'm extending a class I'll try and follow their conventions. For example, when working with the Spring Framework, I'll have my MVC Controller classes in a package called com.mydomain.myapp.web.servlet.mvc
If I'm not extending something I just go with what is simplest. com.mydomain.domain for Domain Objects (although if you have a ton of domain objects this package could get a bit unwieldy).
For domain specific constants, I actually put them as public constants in the most related class. For example, if I have a "Member" class and have a maximum member name length constant, I put it in the Member class. Some shops make a separate Constants class but I don't see the value in lumping unrelated numbers and strings into a single class. I've seen some other shops try to solve this problem by creating SEPARATE Constants classes, but that just seems like a waste of time and the result is too confusing. Using this setup, a large project with multiple developers will be duplicating constants all over the place.
I like break my classes down into packages that are related to each other.
For example:
Model For database related calls
View Classes that deal with what you see
Control Core functionality classes
Util Any misc. classes that are used (typically static functions)
etc.