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Java Object serialization API provides a framework for encoding objects as byte streams and reconstructing objects from their byte-stream encodings. 


MGNLUI-351 - Getting issue details... STATUS

As explained in the issue above, Magnolia is currently unable to serialise and deserialise its own Admicentral UI. This may be a problem especially in clustered environments where a user session may need to be replicated across multiple Java VMs.


In Magnolia's case the root object being serialised when a servlet container shuts down is VaadinSession which is a Serializable class. Most Vaadin classes are serializable, including UI from which our AdmincentralUI inherits. This sort of forces us to take care of serialization in our own classes.


We want to detect and remove all serialisation issues in our code.

Furthermore, we want to find a way to detect serialization issues during Magnolia build so that they are caught and tackled early.

Downsides of Serialisation

Before proceeding with a possible solution I came up during my investigations and trials I'd like to point out some of the not so negligible disadvantages Serialization entails.

The points are mostly taken from Effective Java, 2nd Edition by Joshua Bloch which begins his chapter on Serialization with this significant warning: implement Serializable judiciously. 

Implementing Serializable decreases the flexibility to change a class’s implementation once it has been released.

"When a class implements Serializable, its byte-stream encoding (or serialized form) becomes part of its exported API. Once you distribute a class widely, you are generally required to support the serialized form forever, just as you are required to support all other parts of the exported API. If you do not make the effort to design a custom serialised form, but merely accept the default, the serialised form will forever be tied to the class’s original internal representation."


Increases the testing burden associated with releasing a new version of a class 

"When a serializable class is revised, it is important to check that it is possible to serialise an instance in the new release and deserialise it in old releases, and vice versa. [...] These tests cannot be constructed automatically because, in addition to binary compatibility, you must test for semantic compatibility. In other words, you must ensure both that the serialization-deserialization process succeeds and that it results in a faithful replica of the original object." 


It can consume excessive space

Especially using the default serialization mechanism you might end up with a huge object graph of painstakingly and recursively mirror every field and/or entry in a Collection.

It can consume excessive time

The serialization logic has no knowledge of the topology of the object graph, so it must go through an expensive graph traversal.


Detecting and removing serialization issues

Suggested approach

After several attempts (which will be mentioned later on), this is the approach I would suggest

  • start tomcat with this JVM parameter
  • login into Magnolia
  • stop and restart Magnolia
  • in the logs something like the following stack trace will show up

Serialization stacktrace
Jun 16, 2016 3:41:20 PM org.apache.catalina.session.StandardManager doLoad 
SEVERE: IOException while loading persisted sessions: writing aborted; info.magnolia.event.ResettableEventBus$1 
- field (class "info.magnolia.ui.admincentral.shellapp.applauncher.AppLauncherShellApp$3", name: "val$systemRegistration", type: "interface info.magnolia.event.HandlerRegistration") 
- object (class "info.magnolia.ui.admincentral.shellapp.applauncher.AppLauncherShellApp$3", info.magnolia.ui.admincentral.shellapp.applauncher.AppLauncherShellApp$3@662420ac) 
- field (class "com.vaadin.event.ListenerMethod", name: "target", type: "class java.lang.Object") 
- custom writeObject data (class "com.vaadin.event.ListenerMethod") 
- object (class "com.vaadin.event.ListenerMethod", com.vaadin.event.ListenerMethod@4637e20e) 
- custom writeObject data (class "java.util.HashSet") 
- object (class "java.util.LinkedHashSet", [com.vaadin.event.ListenerMethod@4637e20e]) 
- field (class "com.vaadin.event.EventRouter", name: "listenerList", type: "class java.util.LinkedHashSet") 
- object (class "com.vaadin.event.EventRouter", com.vaadin.event.EventRouter@50986eaf) 
- field (class "com.vaadin.server.AbstractClientConnector", name: "eventRouter", type: "class com.vaadin.event.EventRouter") 
- object (class "info.magnolia.ui.vaadin.applauncher.AppLauncher", info.magnolia.ui.vaadin.applauncher.AppLauncher@7a66f618) 
- custom writeObject data (class "java.util.EnumMap") 
- object (class "java.util.EnumMap", {APPLAUNCHER=info.magnolia.ui.vaadin.applauncher.AppLauncher@7a66f618, PULSE=com.vaadin.ui.VerticalLayout@141b10e1, FAVORITE=com.vaadin.ui.VerticalLayout@22f80d16}) 
- field (class "", name: "shellApps", type: "interface java.util.Map") 
- object (class "", 
- field (class "com.vaadin.server.AbstractClientConnector", name: "sharedState", type: "class com.vaadin.shared.communication.SharedState") 
- object (class "info.magnolia.ui.vaadin.magnoliashell.MagnoliaShell", info.magnolia.ui.vaadin.magnoliashell.MagnoliaShell@6ceda10d) 
- field (class "com.vaadin.ui.AbstractSingleComponentContainer", name: "content", type: "interface com.vaadin.ui.Component") 
- object (class "info.magnolia.ui.admincentral.AdmincentralUI", info.magnolia.ui.admincentral.AdmincentralUI@619fbf56) 
- custom writeObject data (class "java.util.HashMap") 
- object (class "java.util.HashMap", {0=info.magnolia.ui.admincentral.AdmincentralUI@619fbf56}) 
- field (class "com.vaadin.server.VaadinSession", name: "uIs", type: "interface java.util.Map") 
- root object (class "com.vaadin.server.VaadinSession", com.vaadin.server.VaadinSession@7238b701)
  • With this information try to fix the "offending" class. 
  • In the case above, an inner class of ResettableEventBus seems to be not serialisable. It is interesting to notice how the root object being serialised is a VaadinSession. Thanks to the extendedDebugInfo you can follow up the whole serialisation path in the object graph until it throws a


Unfortunately the report provided by extendedDebugInfo is not a full one, meaning that it stops at the first error encountered. This means your only option is to fix and start over the process outlined above until no more serialization exceptions show up.

Failed attempts 

Before giving up to the laborious manual process above, I tried several options, including some fancy recursive scripts through the whole object graph using reflections and other magic. But to no avail. You can read about them in my comment to JIRA issue.

Use default serialization mechanism sparingly

In the first attempt I tried, I basically made each problematic class implement Serializable and let Java do the job. This soon turned out into having a gigantic object graph to be serialised where even Magnolia's core classes came into the picture. Consider the following example.

public class ShellImpl extends AbstractUIContext implements Shell, MessageEventHandler {

     * Provides the current location of shell apps.
    public interface ShellAppLocationProvider {
        Location getShellAppLocation(String name);

    private EventBus admincentralEventBus;
    private AppController appController;
	private MessagesManager messagesManager;

	private MagnoliaShell magnoliaShell;

	private EventHandlerCollection<FragmentChangedHandler> handlers = new EventHandlerCollection<FragmentChangedHandler>();

    private ShellAppLocationProvider shellAppLocationProvider;

By simply implementing Serializable you get rid of the NotSerializableException for ShellImpl, still you now need to do the same for all its fields and the fields they're made of internally and so on and so forth. AppControllerImpl, for one, has a reference to info.magnolia.module.ModuleRegistry  which brings in core classes and soon you get a big, unmanageable mess of classes throwing NotSerializableException.

The most reasonable way to proceed so far seems to use custom serialisation by making some or all fields transient and then reconstructing the object with the special method private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException

For instance, this could be a less painful way to make ShellImpl serializable


public class ShellImpl extends AbstractUIContext implements Shell, MessageEventHandler, Serializable {


    private transient EventBus admincentralEventBus;

    private transient AppController appController;

    private transient MessagesManager messagesManager;

    private transient MagnoliaShell magnoliaShell;

    private transient EventHandlerCollection<FragmentChangedHandler> handlers = new EventHandlerCollection<FragmentChangedHandler>();

    private transient ShellAppLocationProvider shellAppLocationProvider;

    public ShellImpl(@Named(AdmincentralEventBus.NAME) final EventBus admincentralEventBus, final AppController appController, final MessagesManager messagesManager) {
        this.messagesManager = messagesManager;
        this.admincentralEventBus = admincentralEventBus;
        this.appController = appController;

    private void initShell() {
        this.magnoliaShell = new MagnoliaShell();
        admincentralEventBus.addHandler(AppLifecycleEvent.class, new AppLifecycleEventHandler.Adapter() {

        this.admincentralEventBus.addHandler(MessageEvent.class, this);
        this.magnoliaShell.setListener(new MagnoliaShell.Listener() {


	[more class internals omitted]
     * Override default deserialization logic to account for transient fields.
    private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
        this.appController = Components.getComponent(AppController.class);
        this.messagesManager = Components.getComponent(MessagesManager.class);
        this.admincentralEventBus = Components.getComponentWithAnnotation(EventBus.class, Components.named(AdmincentralEventBus.NAME));


Finding a way to detect serialization issues during Magnolia build

In this case I came up with a very simple Groovy script 

import org.apache.commons.lang3.SerializationUtils import com.vaadin.server.VaadinSession 
* This script will attempt to serialise a VaadinSession, that is the root object which is 
* usually serialised by a Servlet container when shutting down. 
* It will throw a if something goes wrong in the serialization process at any point of the object graph. 
* The Servlet container should be run with the following JVM option 
* in order to have a useful debugging output in case of error. */ 
cl = VaadinSession.class.classLoader 
vaadinSession = cl.loadClass('com.vaadin.server.VaadinSession').getCurrent() 


This works fine when run through the Magnolia Groovy Console and will basically output the same stack trace by Tomcat. I thought it could be run as an integration test, like we do for our crawler.groovy. However this does not work, the Groovy Maven plugin being basically disconnected from the Magnolia test instance (two different threads).


Find a way to run the script above as an integration test against a real Magnolia instance. One idea could be registering a GroovyTestServlet which will get passed the script source and executes it. Then we assert that the output does not contain

What to serialize

While trying to remove all serialization errors, to my dismay I realised there are much more classes needing to be serialised than what I expected. Also Magnolia core classes come into play and I start wondering what should actually be serialised, what fields be made transient and how to effectively serialise and deserialise the VaadinSession without going and making everything  Serializable




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