I was getting this error today during writing a program
java.io.NotSerializableException: com.xxxx.xxx.xxx.thunderhead.Project
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1156)
at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:326)
at java.util.ArrayList.writeObject(ArrayList.java:570)
Later I realized that I forget to implement Serializable interface in my bean. That fixed issue.