no main manifest in maven jar

Question

I've build this project with mvn clean install . Now I have a .jar file but I'm not able to use it with:

λ java -jar flyway-commandline-xxx.jar
no main manifest attribute, in flyway-commandline-xxx.jar

What am I missing?

Answer 1

An executable jar needs a "manifest file" - which is located at META-INF/MANIFEST.MF inside the jar.

You can use the Maven Assembly plugin to produce this manifest file in line with this:

...
<plugin>
    <artifactId>maven-assembly-plugin</artifactId>
    <version>2.5.5</version>
    <configuration>
        <archive>
            <manifest>
                <mainClass>dk.tbsalling.ais.cli.AisCli</mainClass>
            </manifest>
        </archive>
        <descriptorRefs>
            <descriptorRef>jar-with-dependencies</descriptorRef>
        </descriptorRefs>
    </configuration>
    <executions>
        <execution>
            <id>make-assembly</id>
            <phase>package</phase>
            <goals>
                <goal>single</goal>
            </goals>
        </execution>
    </executions>
</plugin>
...

The MainClass (referenced in the manifest file) is the class containing the main(...) -method called when you execute the jar.

You can have a look at https://github.com/tbsalling/aiscli/blob/master/pom.xml for a full, working example.

Answer 2

Use below 2 maven plugins in pom.xml:

1. maven-assembly-plugin with a goal to package all dependencies.
2. maven-jar-plugin along with this to add manifest and make jar with the dependencies included by assembly plugin.

Since, in assembly plugin below, appendAssemblyId is false, so final jar name will be with appname.jar only. You can test assembly using command: "mvn clean compile assembly:single". (or using Jenkins JPaC file then add the command "clean compile assembly:single" under mavenGoals).

See below:

<plugin>
            <artifactId>maven-assembly-plugin</artifactId>
            <configuration>
                <finalName>appname</finalName>
                <appendAssemblyId>false</appendAssemblyId>
                <archive>
                    <manifest>
                        <addClasspath>true</addClasspath>
                        <mainClass>com.myorg.appname.MainClass</mainClass>
                    </manifest>
                </archive>
                <descriptorRefs>
                    <descriptorRef>jar-with-dependencies</descriptorRef>
                </descriptorRefs>
            </configuration>
            <executions>
                <execution>
                    <id>make-assembly</id>
                    <phase>package</phase>
                    <goals>
                        <goal>single</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-jar-plugin</artifactId>
            <configuration>
                <archive>
                    <manifest>
                        <addClasspath>true</addClasspath>
                        <classpathPrefix>lib/</classpathPrefix>
                        <mainClass>com.myorg.appname.MainClass</mainClass>
                    </manifest>
                </archive>
            </configuration>
    </plugin>