Why do I get ClassNotPersistableException while running GWT App Engine application in hosted mode?

Question

I am randomly getting an org.datanucleus.exceptions.ClassNotPersistableException when I try to perform a query on the local JDO data store of my GWT/App Engine application. This only happens when I run the application on Hosted mode. When I deploy it to the Google App Engine everything works perfectly.

Stack Trace:

org.datanucleus.exceptions.ClassNotPersistableException: The class "com.wayd.server.beans.WinePost" is not persistable. This means that it either hasnt been enhanced, or that the enhanced version of the file is not in the CLASSPATH (or is hidden by an unenhanced version), or the Meta-Data/annotations for the class are not found.
    at org.datanucleus.jdo.NucleusJDOHelper.getJDOExceptionForNucleusException(NucleusJDOHelper.java:305)
    at org.datanucleus.ObjectManagerImpl.getExtent(ObjectManagerImpl.java:3700)
    at org.datanucleus.jdo.JDOPersistenceManager.getExtent(JDOPersistenceManager.java:1515)
    at com.wayd.server.WinePostServiceImpl.getPosts(WinePostServiceImpl.java:212)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:585)
    at com.google.gwt.user.server.rpc.RPC.invokeAndEncodeResponse(RPC.java:527)
    ... 25 more
Caused by: org.datanucleus.exceptions.ClassNotPersistableException: The class "com.wayd.server.beans.WinePost" is not persistable. This means that it either hasnt been enhanced, or that the enhanced version of the file is not in the CLASSPATH (or is hidden by an unenhanced version), or the Meta-Data/annotations for the class are not found.
    at org.datanucleus.ObjectManagerImpl.assertClassPersistable(ObjectManagerImpl.java:3830)
    at org.datanucleus.ObjectManagerImpl.getExtent(ObjectManagerImpl.java:3693)
    ... 32 more)

WinePost class is a very simple JDO persistence capable class:

@PersistenceCapable(identityType = IdentityType.APPLICATION) public class WinePost {

@PrimaryKey
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
private Long id;

@Persistent
private User author;

@Persistent
private String grape;

@Persistent
private String comment;

public WinePost(final User author, final String grape,
        final String comment) {
    super();
    this.grape = grape;
    this.comment = comment;
}

public User getAuthor() {
    return author;
}

public void setAuthor(final User author) {
    this.author = author;
}

public Long getId() {
    return id;
}

public void setId(final Long id) {
    this.id = id;
}

public String getGrape() {
    return grape;
}

public void setGrape(final String grape) {
    this.grape = grape;
}

public String getComment() {
    return comment;
}

public void setComment(final String comment) {
    this.comment = comment;
}

public String getUserNickname() {
    String retVal = null;
    if (author != null) {
        retVal = author.getNickname();
    }
    return retVal;
}

public WinePostModel getWinePostModel() {
    final WinePostModel winePostModel = new WinePostModel(grape, vintage, getUserNickName());
    return winePostModel;
}

}

The datastore query is performed by the following method:

public ArrayList<WinePostModel> getPosts() {
        final ArrayList<WinePostModel> posts = new ArrayList<WinePostModel>();
        final PersistenceManager persistenceManager = PMF.get()
        .getPersistenceManager();

        final Extent<WinePost> winePostExtent = persistenceManager.getExtent(
                WinePost.class, false);
        for (final WinePost winePost : winePostExtent) {
            posts.add(winePost.getWinePostModel());
        }
        winePostExtent.closeAll();

        return posts;
    }

Answer 1

I am pretty sure there's nothing wrong with your code. The reason you get this error is because of a problem with the Datanucleus enhancer.

If you see this error, quit Jetty and check the console inside Eclipse (you'll need to select the correct console in the little toolbar above the console window). It should say something like:

DataNucleus Enhancer (version 1.1.4) : Enhancement of classes DataNucleus Enhancer completed with success for X classes. Timings : input=547 ms, enhance=76 ms, total=623 ms. Consult the log for full details

... where X is the number of classes it has processed. The number should be equal to the number of 'entity' classed you have defined.

But sometimes, for some reason it will say 0 which is why you get the ClassNotPersistableException error.

To fix, open an entity class and change something (add a space or something) and save. Check the console until it says that it has enhanced all of your entity classes. Then restart the debugger and try again.