Maven Java integration

Question

Here's a simple Java code with SL4J built with maven. Following commands have no issues. All of them work well mvn compile mvn package mvn clean install mvn exec:java

But when I try any of the following

java -cp target/MaventestApp-1.0-SNAPSHOT.jar com.maven.helloworld.App
jar tf ./target/MaventestApp-1.0-SNAPSHOT.jar

I get the following error

    exception in thread "main" java.lang.NoClassDefFoundError: org/slf4j/LoggerFactory
        at com.maven.helloworld.App.main(App.java:17)
Caused by: java.lang.ClassNotFoundException: org.slf4j.LoggerFactory
        at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:338)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:357)

Here is the relevant part of my pom.xml file:

<properties>
  <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  <maven.compiler.source>1.7</maven.compiler.source>
  <maven.compiler.target>1.7</maven.compiler.target>
  <slf4jVersion>1.6.1</slf4jVersion>
</properties>

<dependencies>
  <dependency>
    <groupId>junit</groupId>
    <artifactId>junit</artifactId>
    <version>4.11</version>
    <scope>test</scope>
  </dependency>
  <dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-simple</artifactId>
    <version>${slf4jVersion}</version>
  </dependency>
  <dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-api</artifactId>
    <version>${slf4jVersion}</version>
  </dependency>
</dependencies>

<build>
  <pluginManagement><!-- lock down plugins versions to avoid using Maven defaults (may be moved to parent pom) -->
    <plugins>
      <plugin>
        <artifactId>maven-clean-plugin</artifactId>
        <version>3.0.0</version>
      </plugin>
      <!-- see http://maven.apache.org/ref/current/maven-core/default-bindings.html#Plugin_bindings_for_jar_packaging -->
      <plugin>
        <artifactId>maven-resources-plugin</artifactId>
        <version>3.0.2</version>
      </plugin>
      <plugin>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.7.0</version>
      </plugin>
      <plugin>
        <artifactId>maven-surefire-plugin</artifactId>
        <version>2.20.1</version>
      </plugin>
      <plugin>
        <artifactId>maven-jar-plugin</artifactId>
        <version>3.0.2</version>
        <configuration>
          <archive>
            <manifest>
              <addClasspath>true</addClasspath>
              <mainClass>com.maven.helloworld.App</mainClass>
            </manifest>
          </archive>
        </configuration>
      </plugin>
      <plugin>
        <artifactId>maven-install-plugin</artifactId>
        <version>2.5.2</version>
      </plugin>
      <plugin>
        <artifactId>maven-deploy-plugin</artifactId>
        <version>2.8.2</version>
      </plugin>
      <plugin>
        <groupId>org.codehaus.mojo</groupId>
        <artifactId>exec-maven-plugin</artifactId>
        <version>1.2.1</version>
        <executions>
          <execution>
            <goals>
              <goal>java</goal>
            </goals>
          </execution>
        </executions>
        <configuration>
          <mainClass>com.maven.helloworld.App</mainClass>
        </configuration>
      </plugin>
    </plugins>
  </pluginManagement>
</build>

Code for this sample is here

Can someone help me understand maven and execution process better? Appreciate your time.

Does [this](https://stackoverflow.com/questions/12926899/java-lang-noclassdeffounderror-org-slf4j-loggerfactory) help — soufrk, Jun 26 '18 at 10:59
Well `jar tf ...` won't execute anything, and shouldn't cause an error, either. You either need a fat jar or a manifest file. You may want to take a step back and understand how the classpath works. — Dave Newton, Jun 26 '18 at 11:02
The plain java command has no idea what so ever about Maven so you need to tell it. The simplest way right now is to use the `mvn exec:java` command as suggested in the answers. — Thorbjørn Ravn Andersen, Jun 26 '18 at 11:23

M.F · Answer 1 · 2018-06-26T11:58:46.993

There is the Exec Maven Plugin that should do the job. Something like

mvn exec:java -Dexec.mainClass=com.maven.helloworld.App

The actual problem is that you only add your jar to the classpath and since you do not have the sl4j-classes inside your own archive but declared them as external dependency they are simple missing (aka NoClassDefFoundError).

The command above links all runtime dependencies to the classpath as configured in the pom.

acm · Accepted Answer · 2018-06-26T13:42:00.287

The problem is how you are building your JAR file (dependencies are no included in it). You could use maven-assembly-plugin instead (see the differences between assembly and jar plugin here).

Basically, replace your current build section by the following:

<build>
  <plugins>
    <plugin>
      <artifactId>maven-assembly-plugin</artifactId>
      <configuration>
        <archive>
          <manifest>
            <mainClass>com.maven.helloworld.App</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>
  </plugins>
</build>

Then execute the generated jar-with-dependencies using the following command:

java -jar target/MaventestApp-1.0-SNAPSHOT-jar-with-dependencies.jar

See more about creating executable JARs here.

Maven Java integration

2 Answers2