java.lang.ClassCastException with the same class object

问题

This piece of code irritates me, sometimes it working and some other times it doesn't !

The NamedQuery : (name = "User.findByLogin", query = "SELECT u FROM User u WHERE u.login = :login")

public User findByLogin(String login) {
        Query query = em.createNamedQuery("User.findByLogin");
        query.setParameter("login", login);
        try {
            return (User) query.getSingleResult();
        } catch (javax.persistence.NoResultException ex) {
            return null;
        }
    }

The error make me crazy !

Avertissement: EJB5184:A system exception occurred during an invocation on EJB UserFacade, method: public dz.admin.entity.User dz.admin.service.UserFacade.findByLogin(java.lang.String) Avertissement: javax.ejb.EJBException .... Caused by: java.lang.ClassCastException: dz.elit.admin.entity.User cannot be cast to dz.elit.admin.entity.User at dz.elit.admin.service.UserFacade.findByLogin(UserFacade.java:45)

回答1:

My guess would be that you have different classloaders. The same class loaded in a different classloader is still considered different by the JVM.

To verify this, you could try to catch the exception and print/log the classloaders.

public User findByLogin(String login) {
    Query query = em.createNamedQuery("User.findByLogin");
    query.setParameter("login", login);
    Object result = null;
    try {
        result = query.getSingleResult();
        return (User) result ;
    } catch (javax.persistence.NoResultException ex) {
        return null;
    } catch (ClassCastException ex) {
        logger.info("Object classloader: " + result.getClass().getClassLoader());
        logger.info("Target class classloader: " + User.class.getClassLoader());
        if(result.getClass().getClassLoader() != User.class.getClassLoader()) {
            logger.warn("Different classloaders detected!");
        }
    }
}

As for the solution, that of course depends on the rest of your setup... To get you started, I can give you some pointers to related questions asked before. Maybe some of the answers there can be helpful to you:

Solution for the ClassCastException due to ClassLoader issue
ClassCastException because of classloaders?
cast across classloader?
ClassCastException when casting to the same class

Some of the suggested solutions include changing your classloader setup, using a common interface, or serializing/deserializing your object.

回答2:

I face same issue in my spring-boot application and waste my so much time to solve it.

Issue is generate because of classloaders.

Classloaders of Model class and Session class both are different.

System.out.println("ClassLoader : " + Employee.class.getClassLoader());
System.out.println(session.getClass().getClassLoader()); //Hibernate Session object

ClassLoader : org.springframework.boot.devtools.restart.classloader.RestartClassLoader@a3af3c
sun.misc.Launcher$AppClassLoader@1d16e93

I face this issue because of this dependency.

<dependency>
  <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-devtools</artifactId>
    <optional>true</optional>
</dependency>

When i comment this then my code working fine, so i suggest you to please check your jars.

System.out.println("ClassLoader : " + Employee.class.getClassLoader());
System.out.println(session.getClass().getClassLoader()); //Hibernate Session object

ClassLoader : sun.misc.Launcher$AppClassLoader@1d16e93
sun.misc.Launcher$AppClassLoader@1d16e93

Hope this answer help you.

回答3:

Query query = em.createNamedQuery("User.findByLogin");
...
return (User) query.getSingleResult();

Probably you will need to check whether User really exist (not null). sometime it will be null and you will need to re-refer it

sometimes it working and some other times it doesn't !

来源：https://stackoverflow.com/questions/43865259/java-lang-classcastexception-with-the-same-class-object

标签

ejb-3.0

jpql

classcastexception

stateless-session-bean