在Spring中实例化泛型类bean的问题 [英] Problem instantiating generic class bean in Spring

问题描述

我试图在Spring中实例化一个泛型类，但是我得到以下异常：

bean的初始化失败;嵌套的异常是
org.springframework.aop.framework.AopConfigException：无法生成类[class football.dao.jpa.GenericJpaDAO]的CGLIB子类：此问题的常见原因包括使用最终类或不可见类;嵌套异常是java.lang.IllegalArgumentException：超类没有null构造函数，但没有给出参数：

这是Spring的XML配置容器：

 < bean id =clubDAOclass =football.dao.jpa.GenericJpaDAO> 
< constructor-arg type =EntityManagerFactoryref =entityManagerFactory/> 
< constructor-arg type =Classvalue =football.model.entities.ClubEntity/> 
< constructor-arg type =Stringvalue =ClubEntity/> 
< / bean> 
  

这是泛型类：

  public class GenericJpaDAO< T extends HavingID> {
 
 private EntityManager em; 
 private class entityClass; 
 private String entityName; 
 
 public GenericJpaDAO（Class entityClass，String entityName，
 EntityManagerFactory emFactory）{
 this.entityClass = entityClass; 
 this.entityName = entityName; 
 em = emFactory.createEntityManager（）; 
} 
 
 @Transactional 
 public void create（T entity）{
 em.persist（entity）; 
} 
 //更多方法
 
} 
  

我不确定可能会造成这种情况。我会很感激任何想法。

解决方案

这个问题与泛型无关，它是Spring AOP的限制。

如果使用CGLIB代理将方面（包括 @Transactional 方面）应用于类（如果目标类未实现任何接口或如果AOP配置为 proxy-target-class =true），则不需要参数构造函数：

  public class GenericJpaDAO< T extends HavingID> {
 ... 
 
 public GenericJpaDAO（）{} 
 $ b $ public GenericJpaDAO（Class entityClass，String entityName，
 EntityManagerFactory emFactory）{... } 
 ... 
} 
  

另请参阅： strong>

• 7.6代理机制

I'm trying to instantiate a generic class in Spring, but I get following exception:

Initialization of bean failed; nested exception is org.springframework.aop.framework.AopConfigException: Could not generate CGLIB subclass of class [class football.dao.jpa.GenericJpaDAO]: Common causes of this problem include using a final class or a non-visible class; nested exception is java.lang.IllegalArgumentException: Superclass has no null constructors but no arguments were given:

This is the XML configuration for Spring container:

<bean id="clubDAO" class="football.dao.jpa.GenericJpaDAO">
    <constructor-arg type="EntityManagerFactory" ref="entityManagerFactory"/>
    <constructor-arg type="Class" value="football.model.entities.ClubEntity"/>
    <constructor-arg type="String" value="ClubEntity"/>
</bean>

This is the generic class:

public class GenericJpaDAO <T extends HavingID> {

  private EntityManager em;
  private Class entityClass;
  private String entityName;

  public GenericJpaDAO( Class entityClass, String entityName,
        EntityManagerFactory emFactory ) {
    this.entityClass = entityClass;
    this.entityName = entityName;
    em = emFactory.createEntityManager();
  }

  @Transactional
  public void create( T entity ) {
      em.persist( entity );
  }
  // more methods

}

I'm not really sure what could be causing this. I would appreciate any ideas.

解决方案

This problem is not related to generics, it's a limitation of Spring AOP.

If aspects (including @Transactional aspect) are applied to the class using CGLIB proxy (this happens if target class doesn't implement any interfaces or if AOP is configured with proxy-target-class = "true"), no-argument constructor is required:

public class GenericJpaDAO <T extends HavingID> { 
  ...

  public GenericJpaDAO() {}

  public GenericJpaDAO( Class entityClass, String entityName, 
        EntityManagerFactory emFactory ) { ... } 
  ...
}

See also:

• 7.6 Proxying mechanisms

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