How to fix the "Found Netty's native epoll transport in the classpath, but epoll is not available. Using NIO instead" warning?

Question

I am using Cassandra and, during startup, Netty prints a warning with a stack trace:

Found Netty's native epoll transport in the classpath, but epoll is not available. Using NIO instead."

The application works normally, but is there a way to fix the warning?

Here is the full stack trace:

16:29:46 WARN  com.datastax.driver.core.NettyUtil - Found Netty's native epoll transport in the classpath, but epoll is not available. Using NIO instead.
java.lang.UnsatisfiedLinkError: no netty-transport-native-epoll in java.library.path
    at java.lang.ClassLoader.loadLibrary(ClassLoader.java:1867)
    at java.lang.Runtime.loadLibrary0(Runtime.java:870)
    at java.lang.System.loadLibrary(System.java:1122)
    at io.netty.util.internal.NativeLibraryLoader.load(NativeLibraryLoader.java:168)
    at io.netty.channel.epoll.Native.<clinit>(Native.java:49)
    at io.netty.channel.epoll.Epoll.<clinit>(Epoll.java:30)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:264)
    at com.datastax.driver.core.NettyUtil.<clinit>(NettyUtil.java:68)
    at com.datastax.driver.core.NettyOptions.eventLoopGroup(NettyOptions.java:101)
    at com.datastax.driver.core.Connection$Factory.<init>(Connection.java:709)
    at com.datastax.driver.core.Cluster$Manager.init(Cluster.java:1386)
    at com.datastax.driver.core.Cluster.init(Cluster.java:162)
    at com.datastax.driver.core.Cluster.connectAsync(Cluster.java:341)
    at com.datastax.driver.core.Cluster.connect(Cluster.java:286)
    at org.springframework.cassandra.config.CassandraCqlSessionFactoryBean.connect(CassandraCqlSessionFactoryBean.java:100)
    at org.springframework.cassandra.config.CassandraCqlSessionFactoryBean.afterPropertiesSet(CassandraCqlSessionFactoryBean.java:94)
    at org.springframework.data.cassandra.config.CassandraSessionFactoryBean.afterPropertiesSet(CassandraSessionFactoryBean.java:60)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1642)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1579)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:545)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:482)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
    at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:207)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1128)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1056)
    at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:835)
    at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:741)
    at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:467)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1128)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1023)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:510)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:482)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
    at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:351)
    at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:108)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyPropertyValues(AbstractAutowireCapableBeanFactory.java:1486)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1231)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:543)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:482)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
    at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:207)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1128)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1056)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:566)
    at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:88)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:349)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1219)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:543)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:482)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:751)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:861)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:541)
    at org.springframework.boot.context.embedded.EmbeddedWebApplicationContext.refresh(EmbeddedWebApplicationContext.java:122)
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:761)
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:371)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:315)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1186)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1175)
    ...

Answer 1

If you are running on Linux, you can use the native Linux driver. For instance, if you are using Maven, add a dependency like this:

<dependency>
  <groupId>io.netty</groupId>
  <artifactId>netty-transport-native-epoll</artifactId>
  <version>4.0.27.Final</version>
  <classifier>linux-x86_64</classifier>
</dependency>

An alternative, you can suppress the warning by setting -Dcom.datastax.driver.FORCE_NIO=true. As it is only a performance optimization, it is a viable option to ignore it.

For more background, refer to the Netty documentation about native transports.