OSGI org.slf4j.impl dependency

I'm new to OSGI (sorry) and having a few issues trying to deploy my package and related dependencies.

This is my 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/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.felix.test</groupId>
    <artifactId>com.felix.test</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>bundle</packaging>

    <dependencies>
        <dependency>
            <groupId>org.apache.felix</groupId>
            <artifactId>org.osgi.core</artifactId>
            <version>1.0.0</version>
            <scope>provided</scope>
        </dependency>
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>servlet-api</artifactId>
            <version>2.5</version>
            <scope>provided</scope>
        </dependency>
        <dependency>
            <groupId>org.apache.felix</groupId>
            <artifactId>org.apache.felix.scr.annotations</artifactId>
            <version>1.9.6</version>
            <scope>provided</scope>
        </dependency>
        <dependency>
            <groupId>org.apache.httpcomponents</groupId>
            <artifactId>httpclient</artifactId>
            <version>4.5</version>
            <scope>provided</scope>
        </dependency>
        <dependency>
            <groupId>net.sf.ehcache</groupId>
            <artifactId>ehcache</artifactId>
            <version>2.10.0</version>
        </dependency>
        <dependency>
            <groupId>org.apache.commons</groupId>
            <artifactId>commons-lang3</artifactId>
            <version>3.4</version>
        </dependency>
    </dependencies>
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <configuration>
                    <source>1.5</source>
                    <target>1.5</target>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.felix</groupId>
                <artifactId>maven-bundle-plugin</artifactId>
                <version>2.5.4</version>
                <extensions>true</extensions>
                <configuration>
                    <instructions>
                        <Export-Package>
                            com.felix.test.search
                        </Export-Package>
                        <Bundle-SymbolicName>
                            ${project.artifactId}
                        </Bundle-SymbolicName>
                        <Bundle-Activator>
                            com.felix.test.Activator
                        </Bundle-Activator>
                    </instructions>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

Then I'm bundling this using the Maven command:

mvn org.apache.felix:maven-bundle-plugin:bundleall

This is successful and generates my bundle as well as 3 dependency bundles:

net.sf.ehcache_2.10.0.jar
org.apache.commons.lang3_3.4.0.jar
slf4j.api_1.7.7.jar

This seems OK and I can install and start the first two but when I try and start slf4j I get the following exception:

org.osgi.framework.BundleException: Unable to resolve slf4j.api [25](R 25.0): missing requirement [slf4j.api [25](R 25.0)] osgi.wiring.package; (&(osgi.wiring.package=org.slf4j.impl)(version>=1.6.0)) Unresolved requirements: [[slf4j.api [25](R 25.0)] osgi.wiring.package; (&(osgi.wiring.package=org.slf4j.impl)(version>=1.6.0))]

I'm pretty sure I'm missing something very simple but can't pin it down. Any help would be much appreciated!

Solution

Slf4j has an unusual design (some might say a bad design, ahem). It is an API bundle that depends on an implementation package, namely org.slf4j.impl.

You need to install an additional bundle that implements the Slf4j API. There are lots of choices here... for example slf4j-simple is a basic implementation, whereas slf4j-jdk14 uses the Java 1.4 java.util.logging back end, etc.

Logback also contains an implementation of the API.