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.
I'm writing some simple DAOs in Java using JDBC (no Spring, Hibernate or anything else).
Is it better to put the implementation DAOs in the same package as their interfaces or to put them in a sub-package?
Example:
com.mycompany.myproject.dao.MyDao
com.mycompany.myproject.dao.MyDaoImpl
OR
com.mycompany.myproject.dao.MyDao
com.mycompany.myproject.dao.impl.MyDaoImpl
If you suggest the sub-package structure, what would you suggest as a sub-package name? .impl? .sql? .jdbc?
Realistically, I'm not going to have multiple implementations. Am I over-engineering this?
When designing an application there is no standard way of structuring in packages, experience is what usually helps every one to decide what are the appropriate names for our packages.
About packaging implementations of your interfaces in the same package or in a different one just think about how Java itself is structured: usually an implementation class is packaged in the same package that its interface, but is not all the times.
If you were about to have several implementations of the same DAO's then it would make sense having them structured in .jdbc, .jpa or .jdo sub packages. If your are only going to have one implementation both of the options you enumerate make sense in some way (same package or a .impl sub package).
Regarding over-engineering I would recommend you this article. Even though you are going to have just one implementation of your DAO's, it would make sense to have them defined as an interface and implementation as that will help you in a potential future to rewrite your DAOs for other frameworks whilst the code that makes use of them keeps unchanged.
At the end it's up to you (or you and your peers) to reach a consensus and make the decision that makes more sense in your specific case.
EDIT
An application usually has one implementation per DAO interface and that isn't over-engineering at all, it simply doesn't make sense to have the same DAO interface implemented for JPA and for JDO. Some of the purposes of using the interface/implementation pattern is to ease re-factoring, testing by means of mock objects, etc..
P.S.: I usually rely on JDepend to distribute my application classes in packages avoiding cycles as most as I can.
I don't think either is better, but in this case I prefer the first alternative. It would be in line with having ArrayList, LinkedList, etc. , in the same package as List.
When using additional frameworks, such as hibernate I prefer the second option with MyDao and HibernateDao as the implementor.
I would go with your second option (although none is really better), because you can see immediately in your imports if an impl is imported and refactoring would be simpler if you want to move your impl in another project.
This is not over-engineering. There are multiple advantages to use DAO:
It improves the quality of your code by decoupling database access from other considerations
Testing your code is made easier and you can test it with a finer grain.
If some day, you find out that Hibernate is actually a lot easier for you, it won't impact the rest of your code.
Namespaces and packages only exist to prevent collisions. Neither is preferable as long as they are unique.
One purpose of packages is to improve readability for the other programmers on your team. Generally, I put the implementation in the same package as the interface, because that is the obvious place to look, and in most cases, the DAO implementation is simple. If your implementation is complex, then you should find a framework that is appropriate for your application.
Other reasons to consider a separate package include: if you are writing a library that will be used by other groups, or if you want to support multiple implementations.
I'm new to Java. I've discovered, while trying to structure my code, that Java intimately ties source file organisation (directory structure) to package structure and package structure to external visibility of classes (a class is either visible to all other packages, or none).
This makes it quite difficult to organise the internal implementation details of my public library into logical units of related functionality while maintaining good encapsulation. JSR 294 explains it best:
Today, an implementation can be partitioned into multiple packages.
Subparts of such an implementation need to be more tightly coupled to
each other than to the surrounding software environment. Today
designers are forced to declare elements of the program that are
needed by other subparts of the implementation as public - thereby
making them globally accessible, which is clearly suboptimal.
Alternately, the entire implementation can be placed in a single
package. This resolves the issue above, but is unwieldy, and exposes
all internals of all subparts to each other.
So my question is, what workarounds exist for this limitation, and what are the pros & cons? Two are mentioned in the JSR - use packages for logical grouping (violating encapsulation); place everything in a single package (unwieldy). Are there other pros/cons to these workarounds? Are there other solutions? (I've become vaguely aware of OSGi bundles, but I've found it hard to understand how they work and what the the pros/cons might be (perhaps that's a con). It appears to be very intrusive compared to vanilla packages, to development & deployment.
Note: I'll upvote any good answers, but the the best answer will be one that comprehensively folds in the pros & cons of others (plagiarise!).
Related (but not duplicate!) questions
Anticipating cries of 'Possible duplicate', here are similar questions that I've found on SO; I present them here for reference and also to explain why they don't answer my question.
Java : Expose only a single package in a jar file: asks how to do it, but given that it's not possible in current releases of Java, doesn't discuss workarounds. Has interesting pointers to forthcoming Modularization (Project Jigsaw) in Java 8.
Package and visibility - duplicate question of the above, basically.
Best practice for controlling access to a ".internal" package - question and answers seem to be specific to OSGi or Eclipse plug-ins.
Tools like ProGuard can be used to repackage a JAR, exposing only those classes you specify in the configuration file. (It does this in addition to optimizing, inlining, and obfuscating.) You might be able to set up ProGuard in e.g. a Maven or Ant build, so you write your library exposing methods as public, and then use ProGuard to eliminate them from the generated JAR.
I'll get the ball rolling. Steal this answer and add to it/correct it/elaborate please!
Use multiple packages for multiple logical groupings
Pros: effective logical grouping of related code.
Cons: when internal implementation detail classes in different packages need to use one another, they must be made public - even to the end user - violating encapsulation. (Work around this by using a standard naming convention for packages containing internal implementation details such as .internal or .impl).
Put everything in one package
Pros: effective encapsulation
Cons: unwieldy for development/maintenance of the library if it contains many classes
Use OSGi bundles
Pros: ? (do they fix the problem?)
Cons: appears to be very intrusive at development (for both library user and author) and deployment, compared to just deploying .jar files.
Wait for Jigsaw in Java 8
http://openjdk.java.net/projects/jigsaw/
Pros: fixes the problem for good?
Cons: doesn't exist yet, not specific release date known.
I've never found this to be a problem. The workaround (if you want to call it that) is called good API design.
If you design your library well, then you can almost always do the following:
Put the main public API in one package e.g. "my.package.core" or just "my.package"
Put helper modules in other packages (according to logical groupings), but give each one it's own public API subset (e.g. a factory class like "my.package.foobarimpl.FoobarFactory")
The main public API package uses only the public API of helper modules
Your tests should also run primarily against the public APIs (since this is what you care about in terms of regressions or functionality)
To me the "right level of encapsulation" for a package is therefore to expose enough public API that your package can be used effectively as a dependency. No more and no less. It shouldn't matter whether it is being used by another package in the same library or by an external user. If you design your packages around this principle, you increase the chance of effective re-use.
Making parts of a package "globally accessible" really doesn't do any harm as long as your API is reasonably well designed. Remember that packages aren't object instances and as a result encapsulation doesn't matter nearly as much: making elements of a package public is usually much less harmful than exposing internal implementation details of a class (which I agree should almost always be private/protected).
Consider java.lang.String for example. It has a big public API, but whatever you do with the public API can't interfere with other users of java.lang.String. It's perfectly safe to use as a dependency from multiple places at the same time. On the other hand, all hell would break loose if you allowed users of java.lang.String to directly access the internal character array (which would allow in-place mutation of immutable Strings.... nasty!!).
P.S. Honourable mention goes to OSGi because it is a pretty awesome technology and very useful in many circumstances. However its sweet spot is really around deployment and lifecycle management of modules (stopping / starting / loading etc.). You don't really need it for code organisation IMHO.
getting started with osgi, i wonder what's the conceptual diffence between bundles and components. And when to use which of them. Any pointers are welcome.
EDIT:
Components and Bundles provide different interfaces and therefore they are probably not interchangeable
ComponentContext
BundleContext
A component is:
an active participant in the system
aware of and adapt to its environment
environment = services provided by other components
environment = resources, devices, ...
may provide services to other components and use services from other components
have a lifecycle
In short:
Component provide services
Bundle manage the lifecycle
A bundle can have only one activator (needing a BundleContext), and can have as many active components as you want.
That means you may end up trying to fit in one activator several loosely-related concerns into a single class.
That is why it may be easier to manage those components by Declarative Services, through the SCR (the "Service Component Runtime" which is an "extender bundle" implementing the new and improved OSGi R4.2 DS - Declarative Service - specification).
This is especially true since OSGi 4.2 because it is now much easier to write DS components as POJOs: the activate and deactivate methods are no longer required to take a ComponentContext parameter. See also Lazy Declarative Service.
Note:
It can help to replace those terms in the context of OSGi and look at "how we got there" (excellent blog post by Neil Bartlett)
Here are some relevant extracts, where the "modules" end up being the OSGi Bundles (managing Components which declare Services):
Module Separation
Our first requirement is to cleanly separate modules so that classes from one module do not have the uncontrolled ability to see and obscure classes from other modules.
In traditional Java the so-called “classpath” is an enormous list of classes, and if multiple classes happen to have the same fully-qualified name then the first will always be found and the second and all others will be ignored.
The way to prevent uncontrolled visibility and obscuring of classes is to create a class loader for each module. A class loader is able to load only the classes it knows about directly, which in our system would be the contents of a single module.
Module Access level
If we stop here then modules will be completely isolated and unable to communicate with each other. To make the system practical we need to add back in the ability to see classes in other modules, but we do it in a careful and constrained way.
At this point we input another requirement: modules would like the ability to hide some of their implementation details.
We would like to have a “module” access level, but the problem today is that the javac compiler has no idea where the module boundaries lie.
The solution we choose in our module system is to allow modules to “export” only portions of their contents. If some part of a module is non-exported then it simply cannot be seen by other modules.
When importing, we should import what we actually need to use, irrespective of where it comes from and ignoring all the things that happen to be packaged alongside it.
Granularity of Exports and Imports
OSGi chooses packages.
The contents of a Java package are intended to be somewhat coherent, but it is not too onerous to list packages as imports and exports, and it doesn’t break anything to put some packages in one module and other packages in another module.
Code that is supposed to be internal to our module can be placed in one or more non-exported packages.
Package Wiring
Now that we have a model for how modules isolate themselves and then reconnect, we can imagine building a framework that constructs concrete runtime instances of these modules. It would be responsible for installing modules and constructing class loaders that know about the contents of their respective modules.
Then it would look at the imports of newly installed modules and trying to find matching exports.
An unexpected benefit from this is we can dynamically install, update and uninstall modules. Installing a new module has no effect on those modules that are already resolved, though it may enable some previously unresolvable modules to be resolved. When uninstalling or updating, the framework knows exactly which modules are affected and it will change their state if necessary.
Versions
Our module system is looking good, but we cannot yet handle the changes that inevitably occur in modules over time. We need to support versions.
How do we do this? First, an exporter can simply state some useful information about the packages it is exporting: “this is version 1.0.0 of the API”. An importer can now import only the version that is compatible with what it expects and has been compiled/tested against, and refuse to accept
Packaging Modules and Metadata
Our module system will need a way to package the contents of a module along with metadata describing the imports and exports into a deployable unit.
So the only question is, where should we put the metadata, i.e. the lists of imports and exports, versions and so on?
As it happens OSGi was designed before 2000, so it did choose either of these solutions. Instead it looked back at the JAR File Specification, where the answer is spelled out:
META-INF/MANIFEST.MF is the standard location for arbitrary application-specific metadata.
Late Binding
The final piece of modularity puzzle is late binding of implementations to interfaces. I would argue that it is a crucial feature of modularity, even though some module systems ignore it entirely, or at least consider it out of scope.
We should look for a decentralised approach.
Rather than being told what to do by the God Class, let us suppose that each module can simply create objects and publish them somewhere that the other modules can find them. We call these published objects “services”, and the place where they are published the “service registry”.
The most important information about a service is the interface (or interfaces) that it implements, so we can use that as the primary registration key.
Now a module needing to find instances of a particular interface can simply query the registry and find out what services are available at that time. The registry itself is still a central component existing outside of any module, but it is not “God”… rather, it is like a shared whiteboard.
In OSGi terminology a "component" is like a run-time service. Each component has an implementation class, and can optionally implement a public interface, effectively providing this "service". This aspect of OSGi is sometimes likened to a service registry pattern.
Components in OSGi are, by definition, provided by a bundle. A bundle may contain/provide multiple components. While by itself a bundle may not provide a service, components/declarative services are used to make OSGi more service oriented. You are under no obligation to use components/services.
Closed. This question is opinion-based. It is not currently accepting answers.
Want to improve this question? Update the question so it can be answered with facts and citations by editing this post.
Closed 6 years ago.
Improve this question
I thought about this awhile ago and it recently resurfaced as my shop is doing its first real Java web app.
As an intro, I see two main package naming strategies. (To be clear, I'm not referring to the whole 'domain.company.project' part of this, I'm talking about the package convention beneath that.) Anyway, the package naming conventions that I see are as follows:
Functional: Naming your packages according to their function architecturally rather than their identity according to the business domain. Another term for this might be naming according to 'layer'. So, you'd have a *.ui package and a *.domain package and a *.orm package. Your packages are horizontal slices rather than vertical.
This is much more common than logical naming. In fact, I don't believe I've ever seen or heard of a project that does this. This of course makes me leery (sort of like thinking that you've come up with a solution to an NP problem) as I'm not terribly smart and I assume everyone must have great reasons for doing it the way they do. On the other hand, I'm not opposed to people just missing the elephant in the room and I've never heard a an actual argument for doing package naming this way. It just seems to be the de facto standard.
Logical: Naming your packages according to their business domain identity and putting every class that has to do with that vertical slice of functionality into that package.
I have never seen or heard of this, as I mentioned before, but it makes a ton of sense to me.
I tend to approach systems vertically rather than horizontally. I want to go in and develop the Order Processing system, not the data access layer. Obviously, there's a good chance that I'll touch the data access layer in the development of that system, but the point is that I don't think of it that way. What this means, of course, is that when I receive a change order or want to implement some new feature, it'd be nice to not have to go fishing around in a bunch of packages in order to find all the related classes. Instead, I just look in the X package because what I'm doing has to do with X.
From a development standpoint, I see it as a major win to have your packages document your business domain rather than your architecture. I feel like the domain is almost always the part of the system that's harder to grok where as the system's architecture, especially at this point, is almost becoming mundane in its implementation. The fact that I can come to a system with this type of naming convention and instantly from the naming of the packages know that it deals with orders, customers, enterprises, products, etc. seems pretty darn handy.
It seems like this would allow you to take much better advantage of Java's access modifiers. This allows you to much more cleanly define interfaces into subsystems rather than into layers of the system. So if you have an orders subsystem that you want to be transparently persistent, you could in theory just never let anything else know that it's persistent by not having to create public interfaces to its persistence classes in the dao layer and instead packaging the dao class in with only the classes it deals with. Obviously, if you wanted to expose this functionality, you could provide an interface for it or make it public. It just seems like you lose a lot of this by having a vertical slice of your system's features split across multiple packages.
I suppose one disadvantage that I can see is that it does make ripping out layers a little bit more difficult. Instead of just deleting or renaming a package and then dropping a new one in place with an alternate technology, you have to go in and change all of the classes in all of the packages. However, I don't see this is a big deal. It may be from a lack of experience, but I have to imagine that the amount of times you swap out technologies pales in comparison to the amount of times you go in and edit vertical feature slices within your system.
So I guess the question then would go out to you, how do you name your packages and why? Please understand that I don't necessarily think that I've stumbled onto the golden goose or something here. I'm pretty new to all this with mostly academic experience. However, I can't spot the holes in my reasoning so I'm hoping you all can so that I can move on.
For package design, I first divide by layer, then by some other functionality.
There are some additional rules:
layers are stacked from most general (bottom) to most specific (top)
each layer has a public interface (abstraction)
a layer can only depend on the public interface of another layer (encapsulation)
a layer can only depend on more general layers (dependencies from top to bottom)
a layer preferably depends on the layer directly below it
So, for a web application for example, you could have the following layers in your application tier (from top to bottom):
presentation layer: generates the UI that will be shown in the client tier
application layer: contains logic that is specific to an application, stateful
service layer: groups functionality by domain, stateless
integration layer: provides access to the backend tier (db, jms, email, ...)
For the resulting package layout, these are some additional rules:
the root of every package name is <prefix.company>.<appname>.<layer>
the interface of a layer is further split up by functionality: <root>.<logic>
the private implementation of a layer is prefixed with private: <root>.private
Here is an example layout.
The presentation layer is divided by view technology, and optionally by (groups of) applications.
com.company.appname.presentation.internal
com.company.appname.presentation.springmvc.product
com.company.appname.presentation.servlet
...
The application layer is divided into use cases.
com.company.appname.application.lookupproduct
com.company.appname.application.internal.lookupproduct
com.company.appname.application.editclient
com.company.appname.application.internal.editclient
...
The service layer is divided into business domains, influenced by the domain logic in a backend tier.
com.company.appname.service.clientservice
com.company.appname.service.internal.jmsclientservice
com.company.appname.service.internal.xmlclientservice
com.company.appname.service.productservice
...
The integration layer is divided into 'technologies' and access objects.
com.company.appname.integration.jmsgateway
com.company.appname.integration.internal.mqjmsgateway
com.company.appname.integration.productdao
com.company.appname.integration.internal.dbproductdao
com.company.appname.integration.internal.mockproductdao
...
Advantages of separating packages like this is that it is easier to manage complexity, and it increases testability and reusability. While it seems like a lot of overhead, in my experience it actually comes very natural and everyone working on this structure (or similar) picks it up in a matter of days.
Why do I think the vertical approach is not so good?
In the layered model, several different high-level modules can use the same lower-level module. For example: you can build multiple views for the same application, multiple applications can use the same service, multiple services can use the same gateway. The trick here is that when moving through the layers, the level of functionality changes. Modules in more specific layers don't map 1-1 on modules from the more general layer, because the levels of functionality they express don't map 1-1.
When you use the vertical approach for package design, i.e. you divide by functionality first, then you force all building blocks with different levels of functionality into the same 'functionality jacket'. You might design your general modules for the more specific one. But this violates the important principle that the more general layer should not know about more specific layers. The service layer for example shouldn't be modeled after concepts from the application layer.
I find myself sticking with Uncle Bob's package design principles. In short, classes which are to be reused together and changed together (for the same reason, e.g. a dependency change or a framework change) should be put in the same package. IMO, the functional breakdown would have better chance of achieving these goals than the vertical/business-specific break-down in most applications.
For example, a horizontal slice of domain objects can be reused by different kinds of front-ends or even applications and a horizontal slice of the web front-end is likely to change together when the underlying web framework needs to be changed. On the other hand, it's easy to imagine the ripple effect of these changes across many packages if classes across different functional areas are grouped in those packages.
Obviously, not all kinds of software are the same and the vertical breakdown may make sense (in terms of achieving the goals of reusability and closeability-to-change) in certain projects.
There are usually both levels of division present. From the top, there are deployment units. These are named 'logically' (in your terms, think Eclipse features). Inside deployment unit, you have functional division of packages (think Eclipse plugins).
For example, feature is com.feature, and it consists of com.feature.client, com.feature.core and com.feature.ui plugins. Inside plugins, I have very little division to other packages, although that's not unusual too.
Update: Btw, there is great talk by Juergen Hoeller about code organization at InfoQ: http://www.infoq.com/presentations/code-organization-large-projects. Juergen is one of architects of Spring, and knows a lot about this stuff.
Most java projects I've worked on slice the java packages functionally first, then logically.
Usually parts are sufficiently large that they're broken up into separate build artifacts, where you might put core functionality into one jar, apis into another, web frontend stuff into a warfile, etc.
Both functional (architectural) and logical (feature) approaches to packaging have a place. Many example applications (those found in text books etc.) follow the functional approach of placing presentation, business services, data mapping, and other architectural layers into separate packages. In example applications, each package often has only a few or just one class.
This initial approach is fine since a contrived example often serves to: 1) conceptually map out the architecture of the framework being presented, 2) is done so with a single logical purpose (e.g. add/remove/update/delete pets from a clinic). The problem is that many readers take this as a standard that has no bounds.
As a "business" application expands to include more and more features, following the functional approach becomes a burden. Although I know where to look for types based on architecture layer (e.g. web controllers under a "web" or "ui" package, etc.), developing a single logical feature begins to require jumping back and forth between many packages. This is cumbersome, at the very least, but its worse than that.
Since logically related types are not packaged together, the API is overly publicized; the interaction between logically related types is forced to be 'public' so that types can import and interact with each other (the ability to minimize to default/package visibility is lost).
If I am building a framework library, by all means my packages will follow a functional/architectural packaging approach. My API consumers might even appreciate that their import statements contain intuitive package named after the architecture.
Conversely, when building a business application I will package by feature. I have no problem placing Widget, WidgetService, and WidgetController all in the same "com.myorg.widget." package and then taking advantage of default visibility (and having fewer import statements as well as inter-package dependencies).
There are, however, cross-over cases. If my WidgetService is used by many logical domains (features), I might create a "com.myorg.common.service." package. There is also a good chance that I create classes with intention to be re-usable across features and end up with packages such as "com.myorg.common.ui.helpers." and "com.myorg.common.util.". I may even end up moving all these later "common" classes in a separate project and include them in my business application as a myorg-commons.jar dependency.
Packages are to be compiled and distributed as a unit. When considering what classes belong in a package, one of the key criteria is its dependencies. What other packages (including third-party libraries) does this class depend on. A well-organized system will cluster classes with similar dependencies in a package. This limits the impact of a change in one library, since only a few well-defined packages will depend on it.
It sounds like your logical, vertical system might tend to "smear" dependencies across most packages. That is, if every feature is packaged as a vertical slice, every package will depend on every third party library that you use. Any change to a library is likely to ripple through your whole system.
I personally prefer grouping classes logically then within that include a subpackage for each functional participation.
Goals of packaging
Packages are after all about grouping things together - the idea being related classes live close to each other. If they live in the same package they can take advantage of package private to limit visibility. The problem is lumping all your view and persitance stuff into one package can lead to a lot of classes being mixed up into a single package. The next sensible thing to do is thus create view, persistance, util sub packages and refactor classes accordingly. Underfortunately protected and package private scoping does not support the concept of the current package and sub package as this would aide in enforcing such visibility rules.
I see now value in separation via functionality becase what value is there to group all the view related stuff. Things in this naming strategy become disconnected with some classes in the view whilst others are in persistance and so on.
An example of my logical packaging structure
For purposes of illustration lets name two modules - ill use the name module as a concept that groups classes under a particular branch of a pacckage tree.
apple.model
apple.store
banana.model
banana.store
Advantages
A client using the Banana.store.BananaStore is only exposed to the functionality we wish to make available. The hibernate version is an implementation detail which they do not need to be aware nor should they see these classes as they add clutter to storage operations.
Other Logical v Functional advantages
The further up towards the root the broader the scope becomes and things belonging to one package start to exhibit more and more dependencies on things belonging to toher modules. If one were to examine for example the "banana" module most of the dependencies would be limited to within that module. In fact most helpers under "banana" would not be referenced at all outside this package scope.
Why functionality ?
What value does one achieve by lumping things based on functionality. Most classes in such a case are independent of each other with little or no need to take advantage of package private methods or classes. Refactoring them so into their own subpackages gains little but does help reduce the clutter.
Developer changes to the system
When developers are tasked to make changes that are a bit more than trivial it seems silly that potentially they have changes that include files from all areas of the package tree. With the logical structured approach their changes are more local within the same part of the package tree which just seems right.
It depends on the granularity of your logical processes?
If they're standalone, you often have a new project for them in source control, rather than a new package.
The project I'm on at the moment is erring towards logical splitting, there's a package for the jython aspect, a package for a rule engine, packages for foo, bar, binglewozzle, etc. I'm looking at having the XML specific parsers/writers for each module within that package, rather than having an XML package (which I have done previously), although there will still be a core XML package where shared logic goes. One reason for this however is that it may be extensible (plugins) and thus each plugin will need to also define its XML (or database, etc) code, so centralising this could introduce problems later on.
In the end it seems to be how it seems most sensible for the particular project. I think it's easy to package along the lines of the typical project layered diagram however. You'll end up with a mix of logical and functional packaging.
What's needed is tagged namespaces. An XML parser for some Jython functionality could be tagged both Jython and XML, rather than having to choose one or the other.
Or maybe I'm wibbling.
I try to design package structures in such a way that if I were to draw a dependency graph, it would be easy to follow and use a consistent pattern, with as few circular references as possible.
For me, this is much easier to maintain and visualize in a vertical naming system rather than horizontal. if component1.display has a reference to component2.dataaccess, that throws off more warning bells than if display.component1 has a reference to dataaccess. component2.
Of course, components shared by both go in their own package.
I totally follow and propose the logical ("by-feature") organization! A package should follow the concept of a "module" as closely as possible. The functional organization may spread a module over a project, resulting in less encapsulation, and prone to changes in implementation details.
Let's take an Eclipse plugin for example: putting all the views or actions in one package would be a mess. Instead, each component of a feature should go to the feature's package, or if there are many, into subpackages (featureA.handlers, featureA.preferences etc.)
Of course, the problem lies in the hierarchical package system (which among others Java has), which makes the handling of orthogonal concerns impossible or at least very difficult - although they occur everywhere!
I would personally go for functional naming. The short reason: it avoids code duplication or dependency nightmare.
Let me elaborate a bit. What happens when you are using an external jar file, with its own package tree? You are effectively importing the (compiled) code into your project, and with it a (functionally separated) package tree. Would it make sense to use the two naming conventions at the same time? No, unless that was hidden from you. And it is, if your project is small enough and has a single component. But if you have several logical units, you probably don't want to re-implement, let's say, the data file loading module. You want to share it between logical units, not have artificial dependencies between logically unrelated units, and not have to choose which unit you are going to put that particular shared tool into.
I guess this is why functional naming is the most used in projects that reach, or are meant to reach, a certain size, and logical naming is used in class naming conventions to keep track of the specific role, if any of each class in a package.
I will try to respond more precisely to each of your points on logical naming.
If you have to go fishing in old classes to modify functionalities when you have a change of plans, it's a sign of bad abstraction: you should build classes that provide a well defined functionality, definable in one short sentence. Only a few, top-level classes should assemble all these to reflect your business intelligence. This way, you will be able to reuse more code, have easier maintenance, clearer documentation and less dependency issues.
That mainly depends on the way you grok your project. Definitely, logical and functional view are orthogonal. So if you use one naming convention, you need to apply the other one to class names in order to keep some order, or fork from one naming convention to an other at some depth.
Access modifiers are a good way to allow other classes that understand your processing to access the innards of your class. Logical relationship does not mean an understanding of algorithmic or concurrency constraints. Functional may, although it does not. I am very weary of access modifiers other than public and private, because they often hide a lack of proper architecturing and class abstraction.
In big, commercial projects, changing technologies happens more often than you would believe. For instance, I have had to change 3 times already of XML parser, 2 times of caching technology, and 2 times of geolocalisation software. Good thing I had hid all the gritty details in a dedicated package...
It is an interesting experiment not to use packages at all (except for the root package.)
The question that arises then, is, when and why it makes sense to introduce packages. Presumably, the answer will be different from what you would have answered at the beginning of the project.
I presume that your question arises at all, because packages are like categories and it's sometimes hard to decide for one or the other. Sometimes tags would be more appreciate to communicate that a class is usable in many contexts.
From a purely practical standpoint, java's visibility constructs allow classes in the same package to access methods and properties with protected and default visibility, as well as the public ones. Using non-public methods from a completely different layer of the code would definitely be a big code smell. So I tend to put classes from the same layer into the same package.
I don't often use these protected or default methods elsewhere - except possibly in the unit tests for the class - but when I do, it is always from a class at the same layer
It depends. In my line of work, we sometimes split packages by functions (data access, analytics) or by asset class (credit, equities, interest rates). Just select the structure which is most convenient for your team.
From my experience, re-usability creates more problems than solving. With the latest & cheap processors and memory, I would prefer duplication of code rather than tightly integrating in order to reuse.