Autowiring two beans implementing same interface - how to set default bean to autowire

Question

Background   I have a Spring 2 5 Java Tomcat application  There is the following bean  which is used throughout the application in many places  public class HibernateDeviceDao implements DeviceDao   and the following bean which is new   public class JdbcDeviceDao implements DeviceDao   The first bean is configured so  all beans in the package are included    lt context component-scan base-package  com initech service dao hibernate    gt    The second  new  bean is configured separately   lt bean id  jdbcDeviceDao  class  com initech service dao jdbc JdbcDeviceDao  gt       lt property name  dataSource  ref  jdbcDataSource  gt   lt  bean gt    This results  of course  in an exception when starting the server      nested exception is org springframework beans factory NoSuchBeanDefinitionException  No unique bean of type  com sevenp mobile samplemgmt service dao DeviceDao  is defined  expected single matching bean but found 2   deviceDao  jdbcDeviceDao    from a class trying to autowire the bean like this   Autowired private DeviceDao hibernateDevicDao    because there are two beans implementing the same interface    The question   Is it possible to configure the beans so that   1  I don t have to make changes to existing classes  which already have the HibernateDeviceDao autowired  2  still being able to use the second  new  bean like this    Autowired  Qualifier  jdbcDeviceDao     I e  i would need a way to configure the HibernateDeviceDao bean as the default bean to be autowired  simultaneously allowing the usage of a the JdbcDeviceDao when explicitly specifying so with the  Qualifier annotation    What I ve already tried   I tried setting the property  autowire-candidate  false    in the bean configuration for JdbcDeviceDao    lt bean id  jdbcDeviceDao  class  com initech service dao jdbc JdbcDeviceDao  autowire-candidate  false  gt       lt property name  dataSource  ref  jdbcDataSource   gt   lt  bean gt    because the Spring documentation says that     Indicates whether or not this bean should be considered when     looking for matching candidates to satisfy another bean s     autowiring requirements  Note that this does not affect explicit     references by name  which will get resolved even if the specified     bean is not marked as an autowire candidate     which I interpreted to mean that I could still autowire JdbcDeviceDao using the  Qualifier annotation and have the HibernateDeviceDao as default bean  Apparently my interpretation was not correct  though  as this results in the following error message when starting the server      Unsatisfied dependency of type  class com sevenp mobile samplemgmt service dao jdbc JdbcDeviceDao   expected at least 1 matching bean   coming from the class where I ve tried autowiring the bean with a qualifier     Autowired  Qualifier  jdbcDeviceDao     Solution   skaffman s suggestion to try the  Resource annotation worked  So the configuration has autowire-candidate set to false for jdbcDeviceDao and when using the jdbcDeviceDao I refer to it using the  Resource annotation  instead of  Qualifier     Resource name    jdbcDeviceDao   private JdbcDeviceListItemDao jdbcDeviceDao

User · Answer

What about  Primary      Indicates that a bean should be given preference when multiple candidates are qualified to autowire a single-valued dependency  If exactly one  primary  bean exists among the candidates  it will be the autowired value  This annotation is semantically equivalent to the  lt bean gt  element s primary attribute in Spring XML     Primary public class HibernateDeviceDao implements DeviceDao   Or if you want your Jdbc version to be used by default    lt bean id  jdbcDeviceDao  primary  true  class  com initech service dao jdbc JdbcDeviceDao  gt     Primary is also great for integration testing when you can easily replace production bean with stubbed version by annotating it

User · Answer

The reason why  Resource name     your child class name    works but  Autowired sometimes don t work is because of the difference of their Matching sequence  Matching sequence of  Autowire  Type  Qualifier  Name  Matching sequence of  Resource Name  Type  Qualifier   The more detail explanation can be found here  Inject and Resource and Autowired annotations  In this case   different child class inherited from the parent class or interface confuses  Autowire  because they are from same type  As  Resource use Name as first matching priority   it works

User · Answer

I d suggest marking the Hibernate DAO class with  Primary  i e   assuming you used  Repository on HibernateDeviceDao     Primary  Repository public class HibernateDeviceDao implements DeviceDao   This way it will be selected as the default autowire candididate  with no need to autowire-candidate on the other bean   Also  rather than using  Autowired  Qualifier  I find it more elegant to use  Resource for picking specific beans  i e    Resource name  jdbcDeviceDao   DeviceDao deviceDao

User · Answer

The use of  Qualifier will solve the issue  Explained as below example    public interface PersonType       MasterInterface   Component value  1 2    public class Person implements  PersonType     Bean implementing the interface  Qualifier  1 2       public void setPerson PersonType person            this person   person            Component value  1 5   public class NewPerson implements  PersonType     Qualifier  1 5       public void setNewPerson PersonType newPerson            this newPerson   newPerson           Now get the application context object in any component class    Object obj  BeanFactoryAnnotationUtils qualifiedBeanOfType  ctx  getAutowireCapableBeanFactory    PersonType class  type    type is the qualifier id  you can the object of class of which qualifier id is passed

User · Answer

For Spring 2 5  there s no  Primary  The only way is to use  Qualifier

[java] Autowiring two beans implementing same interface - how to set default bean to autowire?

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