Elastic Search: java.lang.NoClassDefFoundError: Could not initialize class org.apache.log4j.LogManager
<p>Got below error when upgrade elastic search from 1.7 to 2.3.3</p>

<pre><code>java.lang.NoClassDefFoundError: Could not initialize class org.apache.log4j.LogManager
        at org.apache.log4j.Logger.getLogger(Logger.java:104)
        at org.elasticsearch.common.logging.log4j.Log4jESLoggerFactory.newInstance(Log4jESLoggerFactory.java:38)
        at org.elasticsearch.common.logging.ESLoggerFactory.newInstance(ESLoggerFactory.java:82)
        at org.elasticsearch.common.logging.ESLoggerFactory.getLogger(ESLoggerFactory.java:66)
        at org.elasticsearch.common.logging.Loggers.getLogger(Loggers.java:121)
        at org.elasticsearch.common.settings.Settings.&lt;clinit&gt;(Settings.java:63)
</code></pre>