Question

我一直在努力解决这个问题并浏览了很多文章，但无法找出解决方案。感谢您对以下问题的帮助。

我需要能够在服务类中自动装配EntityManager，但会引发异常。由于类型擦除，默认构造函数可能存在问题，因此我尝试使用带参数的构造函数来设置Type。如何自动装配用户类型的EntityManager？

public interface IEntityManager<T extends IDomain<ID>, ID extends Serializable> {

    T findById(ID id, boolean lock);

    T save(T entity);

    void delete(T entity);
}

public class EntityManager<T extends IDomain<ID>, ID extends Serializable> 
        implements IEntityManager<T, ID> {

    private Class<T> entity;

    @Autowired(required=true)
    private SessionFactory sessionFactory;

    /*
    @SuppressWarnings("unchecked")
    public EntityManager() {
        this.entity = (Class<T>) ((ParameterizedType) getClass()
                            .getGenericSuperclass()).getActualTypeArguments()[0];

    } */

    @Autowired(required=true)
    public EntityManager(Class<T> entity) {
        this.entity = entity;
    }
}

@Service("UserService")
@Transactional
public class UserServiceImpl implements IUserService {

    @Autowired  
    EntityManager<User, Integer> entityManager;
}

以下是例外：

Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'UserService': Injection of autowired dependencies failed; 
nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire field: com.test.dummy.persistence.manager.EntityManager com.test.dummy.service.UserServiceImpl.entityManager; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [com.test.dummy.persistence.manager.EntityManager] found for dependency: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:292)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1185)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:537)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:475)

Answer 1

Spring 无法推断需要在User.class的构造函数中注入EntityManager以满足UserServiceImpl中的depadenency。

如果您使用的是 Spring 4 ，则可以定义类型为EntityManager<User, Integer>的bean：

@Configuration
public class Config {
      @Bean 
      public EntityManager<User, Integer> userEntityManager() {
            new EntityManager(User.class);
      } 
}

修改或者定义一个BeanDefinitionRegistryPostProcessor，它将检查需要EntityManager作为依赖关系的bean，识别所需的类型，为所需类型构造新的EntityManager bean并将其添加到bean注册表中。

@Component public class MyEntityManagerBeanDefinitionRegistryPostProcessor implements BeanDefinitionRegistryPostProcessor { @Override public void postProcessBeanFactory(final ConfigurableListableBeanFactory beanFactory) throws BeansException { for (String beanName : beanFactory.getBeanDefinitionNames()) { final BeanDefinition beanDefinition = getOriginatingBeanDefinition( beanFactory.getBeanDefinition(beanName)); final Class<?> beanClass = getClass(beanDefinition.getBeanClassName()); if (beanClass != null) { ReflectionUtils.doWithFields(beanClass, new ReflectionUtils.FieldCallback() { public void doWith(Field field) throws IllegalArgumentException, IllegalAccessException { if (field.isAnnotationPresent(Autowired.class) || field.isAnnotationPresent( Inject.class)) { if (field.getGenericType() instanceof ParameterizedType) { final ParameterizedType parameterizedType = (ParameterizedType) field.getGenericType(); final Class<?> rawType = (Class) parameterizedType.getRawType(); if (rawType.equals(EntityManager.class)) { final Class<?> typeArgument = (Class<?>) parameterizedType.getActualTypeArguments()[0]; beanFactory.registerSingleton(field.getName(), new EntityManager(typeArgument)); } } } } }); } } } private Class<?> getClass(String className) throws BeansException { if (className != null) { try { return Class.forName(className); } catch (ClassNotFoundException e) { throw new BeanInitializationException("beanClass not found", e); } } return null; } private BeanDefinition getOriginatingBeanDefinition(BeanDefinition beanDefinition) { while(beanDefinition.getOriginatingBeanDefinition() != null) { beanDefinition = beanDefinition.getOriginatingBeanDefinition(); } return beanDefinition; } @Override public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException { } }

最后，看起来你实际需要的是使用Spring Data Jpa，它本质上允许你通过定义这样的接口来定义参数化的DAO：

package com.myapp.repository; @Repository public interface UserRepository extends JpaRepository<User, Long> {}

启用它：

@Configuration @EnableJpaRepositories(basePackages = "com.myapp.repository") public class Config { }

然后注入UserRepository：

@Autowired private UserRepository userRepository;

Spring将注入一个基本的DAO实现。

Answer 2

您的EntityManager类可能缺少 @Component 注释。

自动装配的构造函数需要一个未注入的类实体。

@Autowired(required=true)
public EntityManager(Class<T> entity) {
    this.entity = entity;
}

尝试

@Autowired(required=true)
    public EntityManager() {

    }

参数化类的Spring自动装配问题

2 个答案: