Java – ClassCastException in datanucleus Dao object when using JDO to persist / retrieve object

I created a simple webapp using spring Jetty, I'm using datanucleus & to create a hello world JDO test DB4O.

I can insist that there is no problem with a class, but when I query the class, I get a ClassCastException, which cannot convert a.b.c.myclass to a.b.c.myclass

When I check the classloader of the original object I created, it is [webappclassloader @ 1592226291]. Of course, it is the spring webapp classloader

I perform persist and query operations in the same servlet method. When I reread objects from the DB through a simple query, I get a group of abcmyclass objects from the DB, but the class loader is [sun. Misc. Launcher]$ AppClassLoader@5acac268 ], so it's an exception

Follow datauucleus docs http://www.datanucleus.org/extensions/classloader_resolver.html

I introduced the first two recorded options and verified that the class loader is the webappclassloader in the servlet and these debugging steps in the servlet:

Thread.currentThread().getContextClassLoader().toString()
((JDOPersistenceManagerFactory)pm.getPersistenceManagerFactory()).getPrimaryClassLoader().toString()

Both use [webappclassloader @ 1592226291] as the class loader

I don't know where I was wrong

Solution

As an answer to my previous comments:

This exception indicates that it is a classloader problem Compare the class loader of the object with the class you used to convert

ClassLoader loaderOfObject = theObject.getClass().getClassLoader();
ClassLoader loaderOfLocalClass = MyClass.getClassLoader();
// have to be the same.
assert loaderOfObject.equals(loaderOfLocalClass);

By the way: if db4o uses the wrong class loader You can change it explicitly by configuring class loader

EmbeddedConfiguration configuration = Db4oEmbedded.newConfiguration();
    JdkReflector reflector = new JdkReflector(Thread.currentThread().getContextClassLoader());
    configuration.common().reflectWith(reflector);
    ObjectContainer container = Db4oEmbedded.openFile(configuration,"database.db4o");

When a single classloader is not enough: you can also pass an instance of db4o interface jdkloader instead of a classloader There, you can implement any class lookup method For example, find multiple class loaders