This is thrown when the Java Virtual Machine (JVM) tries to load a particular class and the specified class cannot be found in the classpath. Make sure poi-ooxml library jar is added to your project's build path and you are using the latest version.
You might've forgot to add a dependency or it's written wrong. Also, you might be using incompatible versions of Java and Hibernate. More on this here: https://goo.gl/SjSlRA
java.lang.ClassNotFoundException: org.postgresql.Driver at java.net.URLClassLoader$1.run(URLClassLoader.java:366) at java.net.URLClassLoader$1.run(URLClassLoader.java:355)