Spring Why do we autowire the interface and not the implemented class

Question

Example  interface IA     public void someFunction        Resource name  b   class B implements IA     public void someFunction             busy code block       public void someBfunc              doing b things         Resource name  c   class C implements IA     public void someFunction             busy code block       public void someCfunc              doing C things        class MyRunner       Autowire    Qualifier  b      IA worker     worker someFunction        Can someone explain this to me     How does spring know which polymorphic type to use   Do I need  Qualifier or  Resource   Why do we autowire the interface and not the implemented class

User · Answer

Also it may cause some warnigs in logs like a Cglib2AopProxy Unable to proxy method  And many other reasons for this are described here Why always have single implementaion interfaces in service and dao layers

User · Answer

How does spring know which polymorphic type to use    As long as there is only a single implementation of the interface and that implementation is annotated with  Component with Spring s component scan enabled  Spring framework can find out the  interface  implementation  pair  If component scan is not enabled  then you have to define the bean explicitly in your application-config xml  or equivalent spring configuration file       Do I need  Qualifier or  Resource    Once you have more than one implementation  then you need to qualify each of them and during auto-wiring  you would need to use the  Qualifier annotation to inject the right implementation  along with  Autowired annotation  If you are using  Resource  J2EE semantics   then you should specify the bean name using the name attribute of this annotation      Why do we autowire the interface and not the implemented class    Firstly  it is always a good practice to code to interfaces in general  Secondly  in case of spring  you can inject any implementation at runtime  A typical use case is to inject mock implementation during testing stage   interface IA     public void someFunction        class B implements IA     public void someFunction             busy code block       public void someBfunc              doing b things         class C implements IA     public void someFunction             busy code block       public void someCfunc              doing C things        class MyRunner         Autowire       Qualifier  b         IA worker                  worker someFunction        Your bean configuration should look like this    lt bean id  b  class  B    gt   lt bean id  c  class  C    gt   lt bean id  runner  class  MyRunner    gt    Alternatively  if you enabled component scan on the package where these are present  then you should qualify each class with  Component as follows   interface IA     public void someFunction        Component value  b   class B implements IA     public void someFunction             busy code block       public void someBfunc              doing b things          Component value  c   class C implements IA     public void someFunction             busy code block       public void someCfunc              doing C things         Component     class MyRunner         Autowire       Qualifier  b         IA worker                  worker someFunction        Then worker in MyRunner will be injected with an instance of type B

[java] Spring: Why do we autowire the interface and not the implemented class?

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