Create jar A without dependencies and make dependent project C download the dependency B that this dependency A depends on

Question

I have successfully created jar A that does not contain dependencies. This jar A depends on Jar B. I don't want to create a fat jar. I just want dependent project (lets call it C) to add my jar A as dependency, and as soon as jar A is added, project should pull in jar B.

Is this possible?

Question ends here. Below is just what I have tried so far:

What I have done:

I created Fat Jars first with both maven and gradle. Now this has all the dependencies, but my own classes are buried somewhere inside. And dependent project cannot find my classes.

Then I created a jar that does not contain any dependency. I created them separately with maven and gradle. This resolved my classes not being found issue. But then I ran into another issue. As soon as I run the project, it complains that jar B is missing. Rightfully so, as I never included it.

I will just show you my pom.xml

<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>custom-spring-boot-starter</groupId>
<artifactId>custom-spring-boot-starter</artifactId>
<version>0.0.1-SNAPSHOT</version>
<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.1.3.RELEASE</version>
    <relativePath /> <!-- lookup parent from repository -->
</parent>
<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-thymeleaf</artifactId>
        <scope>provided</scope>
    </dependency>
    <dependency> //THIS IS WHAT DEPENDENT PROJECT WILL NOT HAVE.
        <groupId>com.microsoft.azure</groupId>
        <artifactId>azure-servicebus</artifactId>
        <version>0.9.7</version>
    </dependency>
</dependencies>
<build>
    <plugins>
        <plugin>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.8.0</version>
            <configuration>
                <release>10</release>
            </configuration>
        </plugin>
    </plugins>
</build>

This created a jar without dependencies. I added it to dependent project C. But it is complaining that it cannot find dependent B (azure-servicebus in my example). I was hoping that because pom.xml file is present in the jar file, dependent project will download another dependency automatically.

Then I rewrote my pom.xml:

<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>custom-spring-boot-starter</groupId>
<artifactId>custom-spring-boot-starter</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>jar</packaging>
<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.1.3.RELEASE</version>
    <relativePath /> <!-- lookup parent from repository -->
</parent>
<properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
    <java.version>10</java.version>
</properties>
<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
    <dependency>
        <groupId>com.microsoft.azure</groupId>
        <artifactId>azure-servicebus</artifactId>
        <version>0.9.7</version>
    </dependency>
</dependencies>
<build>
    <plugins>
        <plugin>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.8.0</version>
            <configuration>
                <release>10</release>
            </configuration>
        </plugin>
        <plugin>
            <groupId>pl.project13.maven</groupId>
            <artifactId>git-commit-id-plugin</artifactId>
            <executions>
                <execution>
                    <id></id>
                    <goals>
                        <goal>revision</goal>
                    </goals>
                    <phase>validate</phase>
                </execution>
            </executions>
            <configuration>
                <dateFormat>yyyy-MM-dd-HH:mm:ss</dateFormat>
                <dotGitDirectory>${project.basedir}/.git</dotGitDirectory>
                <prefix>git</prefix>
                <verbose>false</verbose>
                <generateGitPropertiesFile>true</generateGitPropertiesFile>
                <generateGitPropertiesFilename>${project.build.outputDirectory}/git.properties</generateGitPropertiesFilename>
                <format>json</format>
                <gitDescribe>
                    <skip>false</skip>
                    <always>false</always>
                    <dirty>-dirty</dirty>
                </gitDescribe>
            </configuration>
        </plugin>

        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-jar-plugin</artifactId>
            <configuration>
                <archive>
                    <manifestEntries>
                        <Build-Jdk>${java.version} (${java.vendor} ${java.vm.version})</Build-Jdk>
                        <Digital-Voltage-Library-Version>${project.version}</Digital-Voltage-Library-Version>
                        <Build-Timestamp>${git.build.time}</Build-Timestamp>
                        <Build-Revision>${git.commit.id}</Build-Revision>
                        <Build-OS>${os.name} ${os.arch} ${os.version}</Build-OS>
                    </manifestEntries>
                </archive>
            </configuration>
        </plugin>
    </plugins>
</build>

This is the stacktrace of the error:

Caused by: java.lang.ClassNotFoundException: com.microsoft.windowsazure.services.servicebus.ServiceBusContract at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:582) at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:190) at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:499) at java.base/java.lang.Class.forName0(Native Method) at java.base/java.lang.Class.forName(Class.java:374) at org.springframework.util.ClassUtils.forName(ClassUtils.java:275) at org.springframework.boot.autoconfigure.condition.OnBeanCondition$BeanSearchSpec.getReturnType(OnBeanCondition.java:505) at org.springframework.boot.autoconfigure.condition.OnBeanCondition$BeanSearchSpec.addDeducedBeanTypeForBeanMethod(OnBeanCondition.java:491) ... 22 common frames omitted

This error easily goes away if I add azure-servicebus dependency to target project. But I don't want to do that. I want the dependency to pull in azure-servicebus.

This is how I am pulling in my jar A into project C.

compile fileTree(dir: '/lib', include: 'custom-spring-boot-starter-0.0.1-SNAPSHOT.jar')

Answer 1

I asked this question without knowing something very important.

When jars are put into artifactory, a corresponding .pom file also has to be placed alongside it (outside of the directory, just look at below link to understand directory structure). This pom file is what tells the dependent project that the jar you are dependent upon, requires so and so dependencies itself.

If you do mvn clean install, it automatically installs jar file and pom file at the correct location for you in your local maven repository.

This answer helped me understand:

https://stackoverflow.com/a/50002072/4828463

Thanks to everyone who tried.

Answer 2

Yes, if A has a Maven dependency on B, then B is automatically pulled when C depends on A. This is the Maven transitive dependency resolution.

Note that this has nothing to do with fat jars. B is not included in A, it is just mentioned as dependency in the POM of A.