Why is mstor throwing a NotSerializableException on inbox.close() - spring-boot

Question

I'm using the basic mstor logic with version 1.0.0 of of the maven mstor library, but it's throwing an Exception on the inbox.close() method. Note: I am not doing any writing to the disk so this Exception is odd. I made, as an attempt, my class which calls this code to "implement Serializable", but that did not help.
This code is running from a SpringBoot REST Service.
If I don't do an inbox.close(), then on Windows the mbox file is still open (not released by this library) after the method below completes.
Here's the basic code:
Properties properties = new Properties();
properties.setProperty("mail.store.protocol", "mstor");
properties.setProperty("mstor.mbox.metadataStrategy", "none");
properties.setProperty("mstor.mbox.cacheBuffers", "disabled");
properties.setProperty("mstor.mbox.bufferStrategy", "mapped");
properties.setProperty("mstor.metadata", "disabled");
properties.setProperty("mstor.mozillaCompatibility", "enabled");
Session session = Session.getInstance(properties);
try {
store = session.getStore(new URLName("mstor:" + pathToMboxFile));
store.connect();
inbox = (MStorFolder) store.getDefaultFolder();
inbox.open(Folder.READ_ONLY);
Message[] messages = inbox.getMessages();
int bodyPartCount = 0;
// ***********************
// process all mbox data.
// *************************
for (int pos = 0; pos < messages.length; pos++)
{
// processing.
}
catch (NoSuchProviderException e)
{
log.debug("MboxController NoSuchProviderException Exception: " + e.getMessage());
}
catch (javax.mail.MessagingException e)
{
errors.append(e);
log.debug("MboxController MessagingException Exception: " + e.getMessage());
}
finally
{
// close the mbox store
try
{
inbox.close(false);
store.close();
}
catch (MessagingException e)
{
log.debug("MboxController Closing Store Exception: " + e.getMessage());
}
}
Now, although the code does work and returns the mbox text and closes the file, when the inbox.close(false) runs, I get this error stack (or something close to it) each time in the Tomcat log:
2019-03-31 08:06:09 - Disk Write of 191 failed:
"java.io.NotSerializableException: net.fortuna.mstor.data.MessageInputStream
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1184)
at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1548)
at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1509)
at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1548)
at java.io.ObjectOutputStream.defaultWriteObject(ObjectOutputStream.java:441)
at net.sf.ehcache.Element.writeObject(Element.java:791)
at sun.reflect.GeneratedMethodAccessor88.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:1140)
at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1496)
at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)
at net.sf.ehcache.util.MemoryEfficientByteArrayOutputStream.serialize(MemoryEfficientByteArrayOutputStream.java:97)
at net.sf.ehcache.store.disk.DiskStorageFactory.serializeElement(DiskStorageFactory.java:413)
at net.sf.ehcache.store.disk.DiskStorageFactory.write(DiskStorageFactory.java:392)
at net.sf.ehcache.store.disk.DiskStorageFactory$DiskWriteTask.call(DiskStorageFactory.java:493)
at net.sf.ehcache.store.disk.DiskStorageFactory$PersistentDiskWriteTask.call(DiskStorageFactory.java:1151)
at net.sf.ehcache.store.disk.DiskStorageFactory$PersistentDiskWriteTask.call(DiskStorageFactory.java:1135)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)