Slf4j NoClassDefFoundError when compiling SparkJava sample project

Problem:

When trying out the SparkJava demo for websockets (spark-websockets, "Using WebSockets and Spark to create a real-time chat app"), I get the following compilation error:

Exception in thread "main" java.lang.NoClassDefFoundError: org/slf4j/LoggerFactory
 at spark.Service.<clinit>(Service.java:56)
 at spark.Spark$SingletonHolder.<clinit>(Spark.java:51)
 at spark.Spark.getInstance(Spark.java:55)
 at spark.Spark.<clinit>(Spark.java:61)
 at Chat.main(Chat.java:19)
Caused by: java.lang.ClassNotFoundException: org.slf4j.LoggerFactory
 at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
 at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
 at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349)
 at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
 ... 5 more

Workaround:

The following workaround worked as a quick way to try out the demo code:

• Open the project's pom.xml file
• Add the following dependency:

  
  <dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-api</artifactId>
    <version>1.7.21</version>
    <scope>compile</scope>
  </dependency>
  
  

If this worked, the project should now compile properly.

Notes:

The above workaround is one of many solutions that can work towards a successful compile. Others include adding a JAR file for a logger, for example. This solution was chosen as it is the quickest for a beginner to do while learning how to use SparkJava or while learning programming in Java.

This how-to was verified to work in revision 8cc09e8cea9257a0df3449106a57ad0467faf39b (Sep 4, 2017) of the spark-websockets project. Your results may vary for other revisions.

References:

• https://github.com/tipsy/spark-websocket
• more about slf4j logging https://mvnrepository.com/artifact/org.slf4j