在Hibernate中调用getSession（）。get（）时得到ClassCastException [英] getting ClassCastException when calling getSession().get() in Hibernate

问题描述

所以这个...会抛出 ClassCastException

p>

  MyStatus object = Main.getSession（）。get（MyStatus.class，1）; 
  

但这不会...

  Object object = Main.getSession（）。get（MyStatus.class，1）; 
  

我还做了 instanceof <$ c上面返回了$ c> Object 。它显然是一个 MyStatus 对象，它具有从数据库获取的所有正确数据。我的 Main 类是由Intellij自动生成的。

  public class Main {
 private static final SessionFactory ourSessionFactory; 
 
 static {
 try {
 Configuration configuration = new Configuration（）; 
 configuration.configure（）; 
 
 ourSessionFactory = configuration.buildSessionFactory（）; 
} catch（Throwable ex）{
抛出新的ExceptionInInitializerError（ex）; 
 
 
 
 public static Session getSession（）throws HibernateException {
 return ourSessionFactory.openSession（）; 
 
 
 public static void main（final String [] args）throws Exception {
 final Session session = getSession（）; 
尝试{
 System.out.println（查询所有被管实体...）; 
 final Metamodel metamodel = session.getSessionFactory（）。getMetamodel（）; （EntityType<> entityType：metamodel.getEntities（））{
 final String entityName = entityType.getName（）; 
。 
 final Query query = session.createQuery（from+ entityName）; 
 System.out.println（execution：+ query.getQueryString（））; 
 for（Object o：query.list（））{
 System.out.println（+ o）; 
} 
} 
} finally {
 session.close（）; 
} 
} 
} 
  

这里是我的 hibernate.cfg.xml file

 <？xml version ='1.0'编码= 'UTF-8' >？; 
<！DOCTYPE hibernate-configuration PUBLIC 
 -  // Hibernate / Hibernate Configuration DTD // EN
http://www.hibernate.org/dtd/hibernate-configuration- 3.0.dtd> 
< hibernate-configuration> 
< session-factory> 
< property name =hibernate.connection.driver_class> org.postgresql.Driver< / property> 
< property name =hibernate.connection.url> jdbc：postgresql：// localhost：5432 / testdb< / property> 
< property name =hibernate.connection.username>根< / property> 
< property name =hibernate.connection.password> test< / property> 
< property name =dialect> org.hibernate.dialect.H2Dialect< / property> 
 
< mapping resource =/ com / test / app / MyStatus.hbm.xmlclass =com.test.app.models.MyStatus/> 
< / session-factory> 
< / hibernate-configuration> 
  

这里是异常抛出无法转换为com.test.MyStatus $ b。 $ b org.glassfish.jersey.servlet.WebComponent.serviceImpl（WebComponent.java:489）
org.glassfish.jersey.servlet.WebComponent.service（WebComponent.java:427）
org.glassfish。 jersey.servlet.ServletContainer.service（ServletContainer.java:388）
org.glassfish.jersey.servlet.ServletContainer.service（ServletContainer.java:341）
org.glassfish.jersey.servlet.ServletContainer。服务（ServletContainer.java:228）

解决方案

<1>您在同一个类（相同的名称和包）上有 ClassCastException ，这意味着您正在处理不同的 ClassLoaders 。
如果打印 MyStatus.class.getClassLoader（）和 object.getClass（）。getClassLoader（）你会看到它们是ClassLoader的不同实例。
当您的Glassfish类路径和您的战争中有相同的类时，可能会发生这种情况。

2）另一个奇怪的事情是，您已将 MyStatus 声明为 com.test。 hibernate.cfg.xml 中的app.models.MyStatus ，但例外说 com.test.MyStatus 。这是两个不同的类，但很可能与 ClassCastException 问题无关。

This is very weird to me...

so this...would throw ClassCastException

MyStatus object = Main.getSession().get(MyStatus.class, 1);

but this will not...

Object object = Main.getSession().get(MyStatus.class, 1);

I also did instanceof the Object returned above. It is clearly an MyStatus object and it has all correct data got from DB. My Main class is auto-generated by Intellij.

public class Main {
    private static final SessionFactory ourSessionFactory;

    static {
        try {
            Configuration configuration = new Configuration();
            configuration.configure();

            ourSessionFactory = configuration.buildSessionFactory();
        } catch (Throwable ex) {
            throw new ExceptionInInitializerError(ex);
        }
    }

    public static Session getSession() throws HibernateException {
        return ourSessionFactory.openSession();
    }

    public static void main(final String[] args) throws Exception {
        final Session session = getSession();
        try {
            System.out.println("querying all the managed entities...");
            final Metamodel metamodel = session.getSessionFactory().getMetamodel();
            for (EntityType<?> entityType : metamodel.getEntities()) {
                final String entityName = entityType.getName();
                final Query query = session.createQuery("from " + entityName);
                System.out.println("executing: " + query.getQueryString());
                for (Object o : query.list()) {
                    System.out.println("  " + o);
                }
            }
        } finally {
            session.close();
        }
    }
}

here is my hibernate.cfg.xml file

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD//EN"
        "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <property name="hibernate.connection.driver_class">org.postgresql.Driver</property>
        <property name="hibernate.connection.url">jdbc:postgresql://localhost:5432/testdb</property>
        <property name="hibernate.connection.username">root</property>
        <property name="hibernate.connection.password">test</property>
        <property name="dialect">org.hibernate.dialect.H2Dialect</property>

        <mapping resource="/com/test/app/MyStatus.hbm.xml" class="com.test.app.models.MyStatus" />
    </session-factory>
</hibernate-configuration>

and here is the Exception thrown

javax.servlet.ServletException: java.lang.ClassCastException: com.test.MyStatus cannot be cast to com.test.MyStatus
    org.glassfish.jersey.servlet.WebComponent.serviceImpl(WebComponent.java:489)
    org.glassfish.jersey.servlet.WebComponent.service(WebComponent.java:427)
    org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:388)
    org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:341)
    org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:228)

解决方案

1) When you have ClassCastException on the same class (same name and package) it means that you are dealing with different ClassLoaders. If you print MyStatus.class.getClassLoader() and object.getClass().getClassLoader() you will see that they are different instances of ClassLoader. This situation can happen when you have the same class in your Glassfish classpath and in your war.

2) Another weird thing is that you have MyStatus declared as com.test.app.models.MyStatus in your hibernate.cfg.xml but the exception says com.test.MyStatus. These are two different classes but most likely are not related to the ClassCastException issue.

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