什么原因导致Tomcat出现这种错误？
SEVERE: Exception loading sessions from persistent storage
java.io.WriteAbortedException: writing aborted; java.io.NotSerializableException:
bean.ProjectAreaBean
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1333)
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:351)
at java.util.ArrayList.readObject(ArrayList.java:593)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(
DelegatingMethodAccessorImpl.java:25)