Why should I prefer OSGi Services over exported packages?_问答_开发者

I am trying to get my head around OSGi Services. The main question I keep asking myself is: What's the benefit of using services instead of working with bundles and their exported packages?

As far as I know it seems the concept of Late Binding has something to do with it. Bundle dependencies are wired together at bundle start, so they are pretty fixed I guess. But with services it seems to be almost the same. A bundle starts and registers services or binds to services. Of course services can come and go whenever they want and you have to keep track of these chances. But the core idea doesn't seem that different to me.

Another aspect to this seems to be that services are more flexible. There could be many implementations for one specific Interface. On the other hand there can be a lot of different implementations for a specific exported package too.

In another text I read that the disadvantage of using exported packages is that they make the application more fragile than services. The author wrote that if you remove one bundle from the dependency graph other dependencies would no longer be met, thus possibly causing a domino effect on the whole graph. But couldn't the same happen if a service would go offline? To me it looks like service dependencies are no better than bundle dependencies.

So far I could not find a blog post, book or presentation that could clearly describe why services are better than just exposing functionality by exporting and importing packages.

To sum my questions up:

What are the key benefits of using OSGi Services that make them superior to exporting and importing packages?

Addition

I have tried to gather further information about this issue and come up with some kind of comparison between plain export/import of packages and services. Maybe this will help us to find a satisfying answer.

Start/Stop/Update

Both, bundles (hence packages) and services, can be started and stopped. In addition to that they can be kind of updated. Services are also tied to the bundle life cycle itself. But in this case I just mean if you can start and stop services or bundles (so that the exported packages "disappear").
Tracking of changes

ServiceTracker and BundleTracker make it possible to track and react to changes in the availability of bundles and services.
Specific dependencies to other bundles or services.

If you want to use an exported package you have to import it.
```
Import-Package: net.jens.helloworld
```
Would net.jens.helloworld provide a service I would also need to import the package in order to get the interface.

So in both cases their would be some sort of "tight coupling" to a more or less specific package.
Ability to have more than one implementation

Specific packages can be exported by more than one bundle. There could be a package net.jens.twitterclient which is exported by bundle A and bundle B. The same applies to services. The interface net.jens.twitterclient.TwitterService could be published by bundle A and B.

To sum this up here a short comparison (Exported packages/services):

YES/YES
YES/YES
YES/YES
YES/YES

So there is no difference.

Additionally it seems that services add more complexity and introduce another layer of dependencies (see image below).

alt text http://img688.imageshack.us/img688/4421/bundleservicecomparison.png

So if there is no real difference between exported packages and services what is the benefit of using services?

My explanation:

The use of services seems more complex. But services themselves seem to be more lightweight. It should be a difference (in terms of performance and resources) if you start/stop a whole bundle or if you just start and stop a specific service.

From a architectural standpoint I also guess that bundles could be viewed as foundation of the application. A foundation shouldn't change often in terms of starting and stopping bundles. The functionality is provided by services of this packages in some kind of dynamic layer above the "bundle layer". This "service layer开发者_运维技巧" could be subject to frequent changes. For example the service for querying a database is unregistered if the database is going offline.

What's your opinion? Am I starting to get the whole point of services or am I still thinking the wrong way? Are there things I am missing that would make services far more attractive over exported packages?

Its quite simple: Bundles are just providing classes you can use. Using Imports/Exports you can shield visibility and avoid (for example) versioning conflicts. Services are instances of classes that satisfy a certain contract (interfaces).

So, when using Services you don't have to care about the origin of a implementation nor of implementation details. They may even change while you are using a certain service.

When you just want to rely on the Bundle Layer of OSGi, you easily introduce crosscutting dependencies to concrete implementations which you usually never want. (read below about DI)

This is not an OSGi thing only - just good practice.

In non OSGi worlds you may use Dependency Injection (DI) frameworks like Guice, Spring or similar. OSGi has the Service Layer built into the framework and lets higher level frameworks (Spring, Guice) use this layer. - so in the end you usually dont use the OSGi Service API directly but DI adapters from user friendly frameworks (Spring-->Spring DM,Guice-->Peaberry etc).

HTH, Toni

I'd recommend purchasing this book. It does an excellent job explaining services and walking through the construction of a non-trivial application that makes use of OSGi Services.

http://equinoxosgi.org/

My company routinely builds 100+ bundle applications using services. The primary benefits we gain from using services are:

1) Loose coupling of producer/consumer implementation

2) Hot swappable service providers

3) Cleaner application architecture

When you start with OSGi, it is always easier to start with an export-package approach it certainly feels more java-like. But when your application starts growing and you need a bit of dynamicity, services are the way to go.

Export-package only does the resolution on startup, whereas services is an on-going resolution (which you may want or not). From a support point of view having services can be very scary (Is it deterministic? How can I replicate problems?), but it is also very powerful.

Peter Kriens explains why he thinks that Services are a paradigm shift in the same way OO was in its time. see µServices and Duct Tape.

In all my OSGi experience I haven't had yet the occasion to implement complex services (i.e. more than one layer), and certainly annotations seem the way to go. You can also use Spring dynamic module to ease the pain of dealing with service trackers. (and many other options like iPOJO, and Blueprint)

Lets consider the two following scenarios:

Bundle A offers a service which is an arithmetic addition add(x,y) return x+y. To achieve this, it exports "mathOpe package" with "IAddition interface", and registers a service within the service registry. Bundles B, C, D, ... consume this service.
Bundle A exports "mathOpe package", where we found a class Addition exposing an operation (x+y)<--add(x,y). Bundles B, C, D, ... import the package mathOpe.

Comparison of scenario 1 vs. scenario 2:

Just one implementation instance vs. many instances (Feel free to make it static!)
Dynamic service management start, stop, update vs. no management, the consumer is owning the implementation (the "service")
Flexible (we can imagine a remote service over a network) vs. not flexible

... among others.

PS: I am not an OSGI expert nor a Java one, this answer shows only my understanding of the phenomena :)

I think this excellent article could answer a lot of your questions:OSGi, and How It Got That Way.

The main advantage of using a service instead of the implementation class is that the bundle offering the service will do the initialization of the class itself.

The bundle that uses the service does not need to know anything about how the service is initialized.

If you do not use a service you will always have to call kind of a factory to create the service instance. This factory will leak details of the service that should remain private.