Neo4j may fail to start with a java.lang.NoSuchMethodError exception thrown in the logs.

An example logging event can be seen below (note that only the exception important and other information may be different):

Exception in thread "main" java.lang.NoSuchMethodError: org.neo4j.driver.internal.async.connection.EventLoopGroupFactory.newEventLoopGroup(I)Lio/netty/channel/EventLoopGroup;
at com.neo4j.fabric.driver.DriverPool.<init>(DriverPool.java:64)

Causes

The exception arises because the underlying method being called cannot be found, this means that Neo4j cannot execute the code it expects to. Therefore the DBMS process cannot start. 

The main causes are conflicting Java libraries present in the classpath. Most of the time these are introduced by incompatible or conflicting plugins. For example, an outdated APOC jar in the plugin directory, or two different versions of the same plugin.

Troubleshooting

Ensure the correct plugin version is used for the Neo4j version, and that any older versions of the same plugin are removed. It is important to check this when performing any update or migration, to ensure that you are using compatible plugins, and that any incompatible versions are removed. 

If it is not clear which plugin is causing the exception, this can be troubleshot further:

1. Remove all jars from the plugin directory. 
2. Start Neo4j, confirm this starts as expected.
3. Introduce plugins on by one until the problematic jar is found.