GAE session. java.io.NotSerializableException: sun.nio.cs.UTF_8 issue [closed]

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I am pretty new to Google App Engine and have encountered an issue when i deployed my new application for the first time. I am trying to store some objects in a session. Everything has been working fine on my localhost. The stack trace below points me to the class "Group" not being serializable - But it is. Hope someone can help. Thanks in advance.

/CreateGroup.action
java.lang.RuntimeException: java.io.NotSerializableException: sun.nio.cs.UTF_8
    at com.google.apphosting.runtime.jetty.SessionManager.serialize(SessionManager.java:358)
    at com.google.apphosting.runtime.jetty.DatastoreSessionStore.createEntityForSession(DatastoreSessionStore.java:71)
    at com.google.apphosting.runtime.jetty.DatastoreSessionStore.saveSession(DatastoreSessionStore.java:93)
    at com.google.apphosting.runtime.jetty.SessionManager$AppEngineSession.save(SessionManager.java:153)
    at com.google.apphosting.runtime.jetty.SaveSessionFilter.doFilter(SaveSessionFilter.java:41)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at com.google.apphosting.utils.servlet.TransactionCleanupFilter.doFilter(TransactionCleanupFilter.java:43)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
    at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
    at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
    at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
    at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418)
    at com.google.apphosting.runtime.jetty.AppVersionHandlerMap.handle(AppVersionHandlerMap.java:249)
    at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
    at org.mortbay.jetty.Server.handle(Server.java:326)
    at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
    at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:923)
    at com.google.apphosting.runtime.jetty.RpcRequestParser.parseAvailable(RpcRequestParser.java:76)
    at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
    at com.google.apphosting.runtime.jetty.JettyServletEngineAdapter.serviceRequest(JettyServletEngineAdapter.java:135)
    at com.google.apphosting.runtime.JavaRuntime$RequestRunnable.run(JavaRuntime.java:391)
    at com.google.apphosting.runtime.ThreadGroupPool$PoolEntry.run(ThreadGroupPool.java:160)
    at java.lang.Thread.run(Thread.java:679)
Caused by: java.io.NotSerializableException: sun.nio.cs.UTF_8
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1180)
    at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1528)
    at java.io.ObjectOutputStream.defaultWriteObject(ObjectOutputStream.java:438)
    at com.xxxxxxxxx.xxxxxxxxx.model.Group.writeObject(Group.java)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:616)
    at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:962)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1480)
    at java.io开发者_运维百科.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1416)
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1174)
    at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:346)
    at java.util.HashMap.writeObject(HashMap.java:1018)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:616)
    at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:962)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1480)
    at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1416)
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1174)
    at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:346)
    at com.google.apphosting.runtime.jetty.SessionManager.serialize(SessionManager.java:355)
    ... 23 more

---

Group.java has these fields (The type CommonPool is also implementing Serializable):

@PersistenceCapable(identityType = IdentityType.APPLICATION)
public class Group implements Serializable {

    private static final long serialVersionUID = -5274228818084728571L;

    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    private Key key;

    @Persistent
    private String name;

    @Persistent
    private int memberCount;

    @Persistent
    private int maxMemberCount;

    @Persistent
    private String currencySymbol;

    @Persistent
    private String url;

    @Persistent
    private Date createdOn;

    @Persistent
    private CommonPool pool;

---

It looks like you've "overridden" the magic writeObject method to customize how Group is Serialized, and somewhere in there is an instance of sun.nio.cs.UTF_8. You say CommonPool is Serializable, but just implementing the interface doesn't make it so. It has to actually be serializable, or it'll fail. There's also a "Key" there. That could be the source of your problem, too.

---

Ok, so I figured it out (and I do feel a bit embarrassed) There was a field the in Group.java that i had overlooked (not in the code stub above) and which was not serializable ... stupid me. Sorry for the inconvenience and thank you Ryan for your quick response.