Question

我长期坚持这个问题。我曾经搜索过这个问题，但没有一个解决方案有效。

结构：

public interface GenericDAO<T extends Serializable, ID extends Serializable>

@Repository
public class AbstractGenericDAO<T extends Serializable, ID extends Serializable> 
    implements GenericDAO<T, ID> {

   private Class<T> persistentClass;

   @Autowired
   private SessionFactory sessionFactory;

   static Logger LOGGER = Logger.getLogger(AbstractGenericDAO.class);


   @SuppressWarnings("unchecked")
   public AbstractGenericDAO() {
       this.persistentClass = (Class<T>) ((ParameterizedType) getClass().getGenericSuperclass()).getActualTypeArguments()[0];
   }

   /**
    * @param entity
    * @return T
    * @throws DBException
    */
   @SuppressWarnings("unchecked")
   public T saveEntity(T entity) throws DBException {
       return saveEntity(entity, false);
   }

   /**
    * @param entity
    * @param explicitFlush
    * @return T
    * @throws DBException
    */
   @SuppressWarnings("unchecked")
   public T saveEntity(T entity, boolean explicitFlush) throws DBException {
       Session session = getSessionFactory().getCurrentSession();

       try {
           session.save(entity);
           if(explicitFlush) {
               session.flush();
               session.refresh(entity);
           }
       } catch (HibernateException he) {
           String errorMsg = "Could not save entity. Reason: " + he.getMessage();
           LOGGER.error(errorMsg, he);
           throw new DBException(errorMsg, he);
       }

       return entity;
   }

   /* (non-Javadoc)
    * @see com.amazon.fc.receive.dbaccess.dao.GenericDAO#getPersistentClass()
    */
   @SuppressWarnings("unchecked")
   public Class<T> getPersistentClass() {
       return persistentClass;
   }

   /**
    * @return the sessionFactory
    */
   public SessionFactory getSessionFactory() {
       return this.sessionFactory;
   }

   /**
    * @param sessionFactory the sessionFactory to set
    */
   @Autowired
   public void setSessionFactory(SessionFactory sessionFactory) {
       this.sessionFactory = sessionFactory;
   }
}

public interface ShlkActiveWorkflowDAO 
    extends GenericDAO<ShlkActiveWorkflow, Serializable> 

@Repository
public class ShlkActiveWorkflowDAOImpl 
    extends AbstractGenericDAO<ShlkActiveWorkflow, Serializable>
    implements ShlkActiveWorkflowDAO

我也在<context:component-scan>中使用application-config.xml 我+ <tx:annotation-driven />中的application-config.xml。

请提供有关如何解决此问题的一些信息。

Exception:

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'abstractGenericDAO' 

Constructor threw exception; nested exception is java.lang.ClassCastException: java.lang.Class cannot be cast to java.lang.reflect.ParameterizedType
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:946)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:890)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:479)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:450)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:290)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:287)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:189)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:557)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:842)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:416)
    at com.coral.spring.Launcher.<init>(Launcher.java:95)
    at com.coral.spring.Launcher.main(Launcher.java:56)
Caused by: org.springframework.beans.BeanInstantiationException: Could not instantiate bean class [com.workflow.dao.AbstractGenericDAO]: Constructor threw exception; nested exception is     
java.lang.ClassCastException: java.lang.Class cannot be cast to java.lang.reflect.ParameterizedType
    at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:141)
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:72)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:938)
    ... 12 more
Caused by: java.lang.ClassCastException: java.lang.Class cannot be cast to java.lang.reflect.ParameterizedType
    at com.workflow.dao.AbstractGenericDAO.<init>(AbstractGenericDAO.java:43)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
    at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:126)
    ... 14 more

Answer 1

从@Repository移除AbstractGenericDAO注释并将其设为abstract：

public abstract class AbstractGenericDAO<T extends Serializable, ID extends Serializable> 
   implements GenericDAO<T, ID>

出现问题是因为@Repository是@Component的特化，这意味着Spring会尝试创建AbstractGenericDAO个实例进行注入。由于AbstractGenericDAO超类（Object）不是通用的，因此您无法将其Type向下转换为ParameterizedType，因此这行代码将失败（在同一行中）如果您尝试使用new AbstractGenericDAO()）手动实例化它的方式：

this.persistentClass = (Class<T>) ((ParameterizedType) getClass().getGenericSuperclass()).getActualTypeArguments()[0];

专业课ShlkActiveWorkflowDAOImpl仍应使用@Repository进行注释。当spring尝试创建此类的实例时，将发生对AbstractGenericDAO构造函数的隐式调用，但这次上面提到的代码行将按预期运行。发生这种情况是因为getClass()返回ShlkActiveWorkflowDAOImpl.class，它是通用AbstractGenericDAO的子类（因此向下转换为ParameterizedType有效）。

自ShlkActiveWorkflowDAOImpl extends AbstractGenericDAO<ShlkActiveWorkflow, Serializable>以来，实际类型ShlkActiveWorkflow将在运行时正确反映出来。这是一种已知的解决方法，可避免将Class<T>引用传递给AbstractGenericDAO构造函数。

如果您担心@Autowired的{{1}}注释，请不要这样做。当你注入一个子类的实例时，Spring会正确连接所有内容。

Answer 2

Type genericSuperClass = getClass().getGenericSuperclass();

ParameterizedType parametrizedType = null;
while (parametrizedType == null) {
   if ((genericSuperClass instanceof ParameterizedType)) {
       parametrizedType = (ParameterizedType) genericSuperClass;
   } else {
       genericSuperClass = ((Class<?>) genericSuperClass).getGenericSuperclass();
   }
}

this.itemClass = (Class<T>) parametrizedType.getActualTypeArguments()[0];

java.lang.Class无法强制转换为java.lang.reflect.ParameterizedType

2 个答案: