How to fix exception: Failed to instantiate SLF4J LoggerFactory?

I am trying to write a java web crawler for semantic web using jena library. I have build project using Netbeans. I am receiving two errors:

1. Failed to instantiate SLF4J LoggerFactory Reported exception: java.lang.NoClassDefFoundError: org/apache/log4j/Level

2. Could not initialize class com.hp.hpl.jena.rdf.model.impl.ModelCom at org.slf4j.LoggerFactory.bind(LoggerFactory.java:128)

I have included slf4j-api and slf4j-log both libraries.

You should also include log4j library as slf4j-log4j is just a static binder.