jib maven plugin 3.0.0 does not impose mainClass property while 3.1.0 does; but I just want to run a jar file

I have defined this pom.xml:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xmlns="http://maven.apache.org/POM/4.0.0"
         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>
    <parent>
        <groupId>com.example</groupId>
        <artifactId>foo</artifactId>
        <version>0.0.1-SNAPSHOT</version>
    </parent>
    <artifactId>app</artifactId>
    <properties>
        <docker.registry>some-repo</docker.registry>
        <docker.base.image>${docker.registry}/base/openjdk11:latest</docker.base.image>
        <docker.image>app</docker.image>
        <https.port>8443</https.port>
        <work.dir>/home/wiremock</work.dir>
        <jar>wiremock-jre8-standalone-${wiremock.standalone.version}.jar</jar>
        <full.path>${work.dir}/${jar}</full.path>
    </properties>
    <dependencies>
        <dependency>
            <groupId>com.github.tomakehurst</groupId>
            <artifactId>wiremock-jre8-standalone</artifactId>
            <version>${wiremock.standalone.version}</version>
            <scope>runtime</scope>
        </dependency>
    </dependencies>
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-dependency-plugin</artifactId>
                <version>${maven.dependency.plugin.version}</version>
                <executions>
                    <execution>
                        <!-- make sure the phase here is before the phase to which jib build is attached to -->
                        <phase>generate-sources</phase>
                        <goals>
                            <goal>copy-dependencies</goal>
                        </goals>
                        <configuration>
                            <outputDirectory>${project.basedir}/libs</outputDirectory>
                            <includeArtifactIds>wiremock-jre8-standalone</includeArtifactIds>
                        </configuration>
                    </execution>
                </executions>

            </plugin>
            <plugin>
                <groupId>com.google.cloud.tools</groupId>
                <artifactId>jib-maven-plugin</artifactId>
                <version>${jib.plugin.version}</version> <!-- <---------- here we can define 3.0.0 and 3.1.0, which leads to build success/build error -->
                <executions>
                    <execution>
                        <phase>package</phase>
                        <goals>
                            <goal>build</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <from>
                        <image>${docker.base.image}</image>
                    </from>
                    <to>
                        <image>${docker.image}</image>
                        <tags>
                            <tag>latest</tag>
                        </tags>
                    </to>
                    <extraDirectories>
                        <paths>
                            <path>
                                <from>libs/*.jar</from>
                                <into>${work.dir}</into>
                            </path>
                            <path>
                                <from>src/main/resources/stubs/__files</from>
                                <into>${work.dir}/__files</into>
                            </path>
                            <path>
                                <from>src/main/resources/stubs/mappings</from>
                                <into>${work.dir}/mappings</into>
                            </path>
                        </paths>
                        <permissions>
                            <permission>
                                <file>${work.dir}/**/*.*</file>
                                <mode>744</mode>
                            </permission>
                        </permissions>
                    </extraDirectories>
                    <container>
                        <appRoot>${work.dir}</appRoot>
                        <workingDirectory>${work.dir}</workingDirectory>
                        <ports>
                            <port>8080</port>
                            <port>${https.port}</port>
                        </ports>
                        <entrypoint>java,-jar,${full.path},--https-port,${https.port},--verbose</entrypoint>
                    </container>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

What I want to do, is to run java -jar wiremock-xxx.jar in my image.

When the plugin version is 3.0.0, the build and run succeeds. But, when I change to 3.1.0, mvn jib:dockerBuild errors out:

[ERROR] Failed to execute goal com.google.cloud.tools:jib-maven-plugin:3.1.0:dockerBuild (default-cli) on project app: Main class was not found, perhaps you should add a `mainClass` configuration to jib-maven-plugin -> [Help 1]

I know running a jar is not the most recommended way of using jib, but I think jib should allows it. And 3.0.0 to 3.1.0 is a minor version upgrade, there should not be any breaking change.

[chanseokoh]

I looked at your pom.xml again, and here’s what is happening exactly.

The following is not working as you intended and is being ignored. <from> must be a directory and cannot be a glob pattern. You should remove this copy configuration. Believe me, it’s being ignored and thus unnecessary.

                            <path>
                                <from>libs/*.jar</from>
                                <into>${work.dir}</into>
                            </path>

Therefore, it’s also unnecessary to define maven-dependency-plugin like below, whose intention I believe was to prepare <project>/libs for the broken <extraDirectories> copy above.

                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-dependency-plugin</artifactId>
                <version>${maven.dependency.plugin.version}</version>
                <executions>
                    <execution>
                        <!-- make sure the phase here is before the phase to which jib build is attached to -->
                        <phase>generate-sources</phase>
                        <goals>
                            <goal>copy-dependencies</goal>
                        </goals>
                        <configuration>
                            <outputDirectory>${project.basedir}/libs</outputDirectory>
                            <includeArtifactIds>wiremock-jre8-standalone</includeArtifactIds>
                        </configuration>
                    </execution>
                </executions>

At the same time, Jib is copying all the project dependencies to <appRoot>/libs (including wiremock-jre8-standalone-<version>.jar), which in your case is /home/wiremock/libs. (If you haven’t known, <appRoot>/libs, <appRoot>/classes, and <appRoot>/resoureces are the hard-coded directories where Jib copies dependencies, .class files, and resources, respectively.) That is, the wiremock-jre8-standalone-<version>.jar is at /home/wiremock/libs, not /home/wiremock. I actually confirmed this using your pom.xml. Therefore, java -jar in the entrypoint is pointing to a wrong location:

<full.path>${work.dir}/${jar}</full.path>
...
<entrypoint>java,-jar,${full.path}

The <full.path> should be <full.path>${work.dir}/libs/${jar}</full.path>.

You can use the dive tool to examine the contents of your build image, so check it out. It’s very handy.

---

What I don’t know is if running with maven, can I just run my dependency jar with packaged mode or not, haven’t tried that.

By now, you should have figured it out, but the mode doesn’t matter. Jib always copies all the dependency JARs (including wiremock-jre8-standalone-<version>.jar) into <appRoot>/libs. The mode is only for your application JAR, not dependencies.

[chanseokoh]

Sorry for the trouble. It’s a regression introduced in 3.1.0. Use 3.1.1, you should be all good.

Closing as a dup of #3295.