title: java.io.WriteAbortedException: writing aborted; java.io.NotSerializableException
body: <p>What causes this kind of error in Tomcat?</p>
  
  <pre><code>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)
  </code></pre>