This is one the best article for learning OSGI in terms of both theoretical and conceptual aspect. This article helped me in creation of OSGI project from scratch. The author covers two examples one is developed on eclipse IDE using MANIFEST.MF file, which is quite older, and the other one is developed on IntelliJ using MAVEN.
The second example is close to our IGB project in terms of project structure. However, the example is somewhat bigger and it might be difficult to understand in first go. So I recommend to create simple "Hello World" project to understand the basic workflow and configurations of OSGI and then comeback to this example to know how bundles interact with each other in order to deliver end result.

Tutorial update:

The tutorial mentions four OSGi framework implementations - Equinox, Knopflerfish, Felix, and Concierge. There is another implementation - Apache Karaf.

Note that the tutorial is relatively recent - 2021.

Github link for the Book Inventory (second) tutorial: https://github.com/nipunaupeksha/osgi-book-inventory.

Made a fork in my "aloraine" account on github: https://github.com/aloraine/osgi-book-inventory
Cloned it to my local.
Built jar files from the cloned project with:

mvn package

jar files:

local aloraine$ find . | grep jar
./org.wso2.carbon.book.provider/target/org.wso2.carbon.book.provider-1.0-SNAPSHOT.jar
./org.wso2.carbon.book.reader/target/org.wso2.carbon.book.reader-1.0-SNAPSHOT.jar
./.idea/jarRepositories.xml

Unpacked org.wso2.carbon.book.provider-1.0-SNAPSHOT.jar file.
Examined descriptor file OSGI-INF org.wso2.carbon.book.provider.internal.BookProviderServiceComponent.xml, with contents:

<?xml version="1.0" encoding="UTF-8"?>
<scr:component xmlns:scr="http://www.osgi.org/xmlns/scr/v1.1.0" immediate="true" name="org.wso2.carbon.book.provider" activate="activate" deactivate="deactivate">    
    <implementation class="org.wso2.carbon.book.provider.internal.BookProviderServiceComponent"/>
    </scr:component>

Examined the bundle's MANIFEST.MF file:

MANIFEST.MF 
Manifest-Version: 1.0
Bnd-LastModified: 1688057846042
Build-Jdk: 11.0.18
Built-By: aloraine
Bundle-Description: A book shelf for carbon
Bundle-ManifestVersion: 2
Bundle-Name: org.wso2.carbon.book.provider
Bundle-SymbolicName: org.wso2.carbon.book.provider
Bundle-Version: 1.0.0.SNAPSHOT
Created-By: Apache Maven Bundle Plugin
Export-Package: org.wso2.carbon.book.provider;uses:="org.wso2.carbon.boo
 k.provider.model";version="1.0.0.SNAPSHOT",org.wso2.carbon.book.provide
 r.model;version="1.0.0.SNAPSHOT"
Import-Package: org.osgi.framework;version="[1.7.0,2.0.0)",org.osgi.serv
 ice.component;version="[1.2.0,2.0.0)"
Service-Component: OSGI-INF/org.wso2.carbon.book.provider.internal.BookP
 roviderServiceComponent.xml
Tool: Bnd-2.1.0.20130426-122213

Decided to try load these bundles into karaf.
Downloaded and installed the latest version of karaf: 4.3.9
Unpacked karaf "tarball" to a local directory and adding its "bin" directory to my path.
Started karaf using JDK11 with:

karaf debug

This launches a command-line interface to the karaf environment.

Next, I attempted to start the project bundles with:

bundle:install -s mvn:org.wso2.carbon/org.wso2.carbon.book.provider/1.0-SNAPSHOT
karaf@root()> bundle:install -s mvn:org.wso2.carbon/org.wso2.carbon.book.provider/1.0-SNAPSHOT
Bundle ID: 52
Error executing command: Error installing bundles:
	Unable to start bundle mvn:org.wso2.carbon/org.wso2.carbon.book.provider/1.0-SNAPSHOT: org.osgi.framework.BundleException: Unable to resolve org.wso2.carbon.book.provider [52](R 52.0): missing requirement [org.wso2.carbon.book.provider [52](R 52.0)] osgi.wiring.package; (&(osgi.wiring.package=org.osgi.service.component)(version>=1.2.0)(!(version>=2.0.0))) Unresolved requirements: [[org.wso2.carbon.book.provider [52](R 52.0)] osgi.wiring.package; (&(osgi.wiring.package=org.osgi.service.component)(version>=1.2.0)(!(version>=2.0.0)))]

Note: the "-s" option also starts the bundle.

As shown in the preceding comment, the framework lacks the required package org.osgi.service.component.

Within karaf, I can check all the exported and available packages with:

karaf@root()> exports | grep org.osgi  

This lists 41 packages, including org.osgi.framework version 1.9.0, which satisfies the first import requirement listed in the bundle MANIFEST for org.wso2.carbon.book.provider.

I restart the karaf container, getting rid of any and all bundles and packages except the karaf framework's bundles and packages with:

karaf clean 

I check the number of packages with:

 
karaf@root()> exports | wc -l
807

and the number of bundles with:

karaf@root()> la | wc -l
55

and the number of user-installed (by me) bundles with:

karaf@root()> list

I then try to install the "provider" bundle again with:

karaf@root()> bundle:install -s mvn:org.wso2.carbon/org.wso2.carbon.book.provider/1.0-SNAPSHOT                                                     
Bundle ID: 52
Error executing command: Error installing bundles:
	Unable to start bundle mvn:org.wso2.carbon/org.wso2.carbon.book.provider/1.0-SNAPSHOT: org.osgi.framework.BundleException: Unable to resolve org.wso2.carbon.book.provider [52](R 52.0): missing requirement [org.wso2.carbon.book.provider [52](R 52.0)] osgi.wiring.package; (&(osgi.wiring.package=org.osgi.service.component)(version>=1.2.0)(!(version>=2.0.0))) Unresolved requirements: [[org.wso2.carbon.book.provider [52](R 52.0)] osgi.wiring.package; (&(osgi.wiring.package=org.osgi.service.component)(version>=1.2.0)(!(version>=2.0.0)))]

and check the exported packages count again:

karaf@root()> exports | wc -l
809

After trying to install this new bundle, I see there are two more packages being exported.

They are from our project: org.wso2.carbon.book.provider.model and org.wso2.carbon.book.provider.

However, the bundles are not started. I guess the packages are available for use? That's interesting.

Let's check the imports for the "provider" bundle:

karaf@root()> imports -b 52 --no-format
org.osgi.framework        	[1.7.0,2.0.0)	        	52	org.wso2.carbon.book.provider
org.osgi.service.component	[1.2.0,2.0.0)	        	52	org.wso2.carbon.book.provider

Let's use "diag" to assess the problem:

karaf@root()> diag
org.wso2.carbon.book.provider (52)
----------------------------------
Status: Installed
Unsatisfied Requirements:
[org.wso2.carbon.book.provider [52](R 52.0)] osgi.wiring.package; (&(osgi.wiring.package=org.osgi.service.component)(version>=1.2.0)(!(version>=2.0.0)))

I located this package / artifact in my ".m2" directory and looked at its POM:

local aloraine$ more org.osgi.service.component-1.5.1.pom
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>org.osgi</groupId>
  <artifactId>org.osgi.service.component</artifactId>
  <version>1.5.1</version>
  <description>OSGi Companion Code for org.osgi.service.component Version 1.5.1</description>
  <name>org.osgi:org.osgi.service.component</name>
  <url>https://docs.osgi.org/</url>
  <organization>
    <name>Eclipse Foundation</name>
  </organization>
...

Next, I installed this into my karaf session with:

karaf@root()> bundle:install mvn:org.osgi/org.osgi.service.component
Bundle ID: 55

Summarizing the findings thus far:

• karaf has many commands that help with detecting and solving errors, such as "import" and "export", which list packages being exported from or imported into various bundles
• unix commands such as "grep" and "wc -l" are supported, making debugging faster and easier
• the framework does not appear to contain service component infrastructure needed by this project

In the previous comment, I said "the framework does not appear to contain service component infrastructure needed by this project."

This seems very strange!

I found a Web page on karaf that mentioned some useful commands, include "feature:list".

There are 174 "features" listed! One of these is "scr", with Description "Declarative Service support." Its current "State" is "Uninstalled."

Maybe, if I somehow "install" this feature, I would not need to install the "org.osgi:org.osgi.service.component" bundle from my local "mvn" repository?

So, I tried to install it with:

karaf@root()> feature:install scr

After this, the "scr" feature State changed to "Started."

I then uninstalled the previously installed bundles. Freshly attempting to re-install and re-start bundle org.wso2.carbon/org.wso2.carbon.book.provider/1.0-SNAPSHOT succeeded without any error.

I then quit and restarted karaf with a fresh, clean environment with "karaf clean".
I confirmed that the "scr" feature was not active. I then activated it as before with "feature:install scr" and then tried to load the "provider" bundle.

This time, it loaded without any error.

Thus, karaf actually does contain support declarative services. You just have to install that feature within the framework to activate it, I guess.

Trying now to figure out which version of declarative services / scr is used in this tutorial.

I was able to build it using JDK 11 and run it in the latest version (4.3.9) of karaf, also running in JDK 11. This suggests that maybe the most up-to-date system for declarative services is being used?

Upon consultation with Nowlan Freese, we realized that the only way to be truly sure about this is to look at the package import statements in the code.

Investigating the code:

package org.wso2.carbon.book.provider.internal;

import org.osgi.framework.BundleContext;
import org.osgi.service.component.ComponentContext;
import org.osgi.service.component.annotations.Activate;
import org.osgi.service.component.annotations.Component;
import org.osgi.service.component.annotations.Deactivate;
import org.wso2.carbon.book.provider.BookProvider;
import org.wso2.carbon.book.provider.BookProviderImpl;

import java.util.logging.Logger;

@Component(
        name = "org.wso2.carbon.book.provider",
        immediate = true
)

Now, looking at Component imports in an IGB project (master branch, jdk 1.8 build):

• ./core/igb-services-api/src/main/java/org/lorainelab/igb/services/search/SearchModeRegistry.java

imports and Component annotation:

import aQute.bnd.annotation.component.Component;
import aQute.bnd.annotation.component.Reference;
...
@Component(name = SearchModeRegistry.COMPONENT_NAME, immediate = true, provide = SearchModeRegistry.class)

Our project (IGB) does not appear to be using felix scr but instead is using something perhaps even older - annotations from package aQute.bnd.annotation.component.