While doing the upgrade from 4.x to 5.6, we are getting below error when we try to Run/Debug the project:-

java.lang.NoClassDefFoundError: com/tibco/cep/runtime/model/element/impl/simple/PropertyAtomStringSimple
at java.base/java.lang.ClassLoader.defineClass1(Native Method)
at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1016)
at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:877)
at com.tibco.cep.runtime.service.loader.ClassManager.findClass(ClassManager.java:116)
at com.tibco.cep.runtime.service.loader.BEClassLoader.findClass(BEClassLoader.java:1421)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:588)
at com.tibco.cep.runtime.service.loader.ClassManager.loadClass(ClassManager.java:142)
at com.tibco.cep.runtime.service.loader.BEClassLoader.registerClasses(BEClassLoader.java:519)
at com.tibco.cep.runtime.service.loader.BEClassLoader.update(BEClassLoader.java:340)

It is required to update the import settings. The class is now defined under: "com.tibco.cep.runtime.model.element.impl.property.simple.PropertyAtomStringSimple".

2) Please check if be.jar has been accidently placed under the top directory of your project, if it's there please remove and rebuild the ear file.