I am very new to OSGi.
I am developing a plugin A (osgi bundle), suppose A which depends on libraries, suppose B-1.0 and C-1.0. Now If the library C-1.0 depends on library B-2.0 (Note: the different version of library B). So my plugin has two different versions of the library B in its classpath. Now, How can I handle this situation ?
As I am studying from last 4-5 days about OSGi that it creates a classloader for each plugin in the JIRA application, so that dependency version mismatch do not occur between plugins. But What would a developer do If a plugin itself needs two different versions of a library jar ?
Can I create two different classloaders in a single osgi bundle through OSGi, say one for package X and another one for package Y ?
Please help me in any of the above scenarios or point me to the right direction.
Thanks in advance.
Remember that bundles do not depend on other bundles!!
Bundles import packages that are exported by other bundles. (unless you have used Require-Bundle, but you should not). So to rephrase the scenario from your example:
Bundle A imports package org.foo. Bundle C exports package org.foo, and OSGi wires the import to the export. So far so good.
Bundle C also imports package org.bar. Bundle B 1.0 exports package org.bar. Therefore OSGi wires these together and everything is still fine.
Now... bundle A also imports package org.wibble. Bundle B 2.0 exports package org.wibble. This is fine as well! Bundles B 1.0 and B 2.0 are simply different bundles as far as OSGi is concerned, they can both be installed at the same time.
So when you look at the dependencies the way they actually work, you find that it's perfectly possible for A to import code that comes from two different versions of B. However there is a limitation. Consider the following:
Bundle D imports packages org.foo and org.bar v1.0 (yes, packages are versioned).
Bundle E exports package org.foo, which satisfies the import in D. Bundle E also imports package org.bar v2.0.
Some other bundles (say F v1 and F v2) export the 2 versions of the org.bar packages.
Actually this scenario can still work. D can import version 1.0 of package org.bar from somewhere, and E can import version 2.0 of package org.bar from somewhere else, at the same time as D is importing package org.foo from E. I personally find this pretty incredible! But it does not work if org.foo "uses" org.bar, where "uses" means that some types in org.bar are visible in the API of org.foo. In this case, bundle D would be exposed to 2 different copies org.bar, which is not allowed, so OSGi will prevent bundle D from running by not allowing it to enter RESOLVED or ACTIVE states.
In osgi bundle or plugin you'll have meta-inf flie which will define which classes you import if you pass extra agrument being the version=2.0 then it will use the class from B-2.0 if you don't specify anything then it'll resolve to one that is loaded by classloader first.
i.e.
import-package(C 1.0):
b.some.package; version="2.0" or b.some.package; version="[2.0,4.0)"
import-package(A 1.0):
b.some.package; version="1.0" or b.some.package; version="1.0"
Hope this helps
Anup
Since each OSGi bundle has its own classloader, there will be 4 bundles in the runtime, and also 4 classloaders (A, B-1.0, B-2.0, C-1.0).
You may have two copies of the same class included in B (one from 1.0 and another from 2.0). If you run this, you may simply run into a ClassCastException in the A code because two versions of B classes are not the same.
OSGi provides a "uses" clause to detect this type of situations early. For example, C may have a uses clauses like the following:
Export-Package: c.some.package;uses="b.some.package";version="1.0"
Import-Package: b.some.package;version="2.0"
In this case, you will have an earlier failure (while resolving A), known as a uses conflict, because C places a constraint for its consumer on an acceptable version of B.
Conceptually, the only way to fix this problem is to have consumers of B (A and C in this case) agree on the version of B.
Related
I am looking for a way to ensure that all the features I deploy in Karaf require dependencies that are of the same version. The project is composed of more than 40 bundles which makes it difficult to verify manually.
I am thinking of developping a Maven plug-in that would make the check, but before I would like to be sure that such a solution do not exist yet.
If you want to be sure you use the same versions then create a parent project and define versions of dependencies only there. So you can be sure all your modules have the same dependencies. Of course this only makes sense if all these modules are very closely related (e.g. belong to the same application / release unit).
Why would you even want to do this? Each bundle should depend on the versions of the package it needs, and that dependency should be a range. So if you compile against and API package version 1.0.0, and you are a consumer of that API, then you should import with the range [1.0.0, 2.0.0). Refer to the OSGi Core Release 5 specification, section 3.7.3 ("Semantic Versioning") for details.
At runtime the OSGi Framework will ensure that your bundle is wired to a package version that is within its permitted range. Obviously if you have non-overlapping version ranges from different importers then the Framework will not be able to satisfy them with a single exporter.
Can a OSGi bundle have two dependencies, each on a different version of the same OSGi bundle?
Can a OSGi package have two dependencies, each on a different version of the same OSGi package?
(I am trying to learn OSGi from the ground up. This question is just intended to help me understand the basic concepts. From reading online articles about OSGi services, I gather that such dependencies certainly wouldn't be recommended practice. But are they possible at all?)
(Update: rephrased the two questions.)
No. OSGi provides a consistent class space for a bundle. This means that it is only exposed to a single class of a given name. So a bundle cannot simultaneously see more than one version of a package at a time.
This does not mean that ClassCastExceptions are impossible since code your bundle is directly dependent on, can expose objects from their dependencies to your bundle. The proper use of uses constraints on export packages is important to prevent this.
Can a OSGi bundle depend on two different versions of another OSGi bundle at the same time?
Can an OSGi package depend on two different versions of another OSGi package at the same time?
Sort of. You can depend on ranges or specific versions of another OSGI bundle or package like this:
Import-Package: org.osgi.framework;version="[1.3,2.0)"
Not sure if that applies in the first section because bundles should not depend on other bundles, only packages. This is what 'Require-Bundle' does but is suggested you don't use it. Require-Bundle takes versions as well so theoretically it should support version ranges.
Once your OSGi bundle is resolved within OSGi, it will find the package of any of those versions. However, it can't resolve a package (org.osgi.framework) to two separate bundles (one which provides version 1.9 and one which provides 1.8). It will choose the most recent version based on SemVer.
If you try to specify it twice in Import-Package, you will get a 'Duplicate Import' error.
I am learning OSGi framework. What i found that Bundles consists of .class files and manifest file. Now suppose I have two bundles A and B and bundle B depends on bundle A's service. So while compiling bundle B i have to keep Bundle A's service class in the classpath of bundle B or have to keep it(Bundle A's service) inside bundle B so that compiler can compile. Now if we are doing so then how modularity is achieved and how the two bundles are independent. Please help.
Thanks
Use interfaces!!
You have in interface I. Class A implements I and therefore depends on it, both at compile time and at runtime. Class B uses an instance of I, which it obtains as an OSGi Service. Therefore B depends on I at compile time and at runtime.
Therefore neither A nor B has direct knowledge of each other. Implementation code is hidden, only the interface is visible and shared. And that's how modularity is achieved.
Manually doing dependency management is error prone. I suggest you use maven to manage the dependencies. In addition, maven-bundle-pluginIn could help you check import/export bundle at compile time; while maven-eclipse-plugin help you generate eclipse projects according to the dependency configuration in pom.
I'm building an OSGi-based web application consisting of just two bundles for now. In one of them, I'm loading process instances from a process engine. Each process instance is supposed to correspond with a specific version of the other bundle, which it was initially assigned to.
For example, I would like to load one process instance in Bundle A and work with it using packages from bundle B in version 1.0. Afterwards, I would get hold of another process instance and work with it using packages from bundle B in version 2.0.
Do you see any way to achieve this functionality?
Thank you very much in advance!
Johannes
Bundle A can only be exposed to a single version of a package at any given point. So other than possibly just using reflection, Bundle A cannot use 2 versions of the same package exported by different versions of Bundle B.
Or, do you mean that the 2 versions of Bundle B implement that same package differently. Then Bundle A could see different implementation objects from the 2 versions of Bundle B because they appear to be the same interface type to Bundle A. If this is the case, then OSGi services are the best way for Bundle B to publish these objects for Bundle A to use.
1) Bundle A reexports package com.X, which it gets from bundle C
2) Bundle B exports package com.X
3) Now bunlde D has dependency on both A and B.
From where will the bundle D get the package com.X from?
The first question is why you have 2 bundles defining the same package - this is called split packages and isn't recommended because you can have problems with shadowing.
With Import-Package the runtime will pick either bundle A or B to resolve the package dependency and you can't control this directly (you can do various tricks like the Eclipse guys do by setting mandatory properties on the exports).
With Require-Bundle you'll end up with a merged com.X package, so you'll see the superset of types, but I'm not sure what happens if you have overlapping types.
The simplest thing is to avoid split packages in the first place.