Spring Autowired and Qualifier

Question

Is it auto detected with  Autowired  Is it dependency injection by name when  Qualifier is used  How can we do setter and constructor injection using these annotations

User · Answer

Autowired to autowire or search  by-type  Qualifier to autowire or search  by-name Other alternate option for  Qualifier is  Primary   Component  Qualifier  beanname   public class A    public class B     Constructor  Autowired   public B  Qualifier  beanname  A a           you need to add  autowire also     property  Autowired  Qualifier  beanname   private A a           If you don t want to add the two annotations  we can use  Resource public class B     property  Resource name  beanname   private A a     Importing properties is very similar  Value    property name        Value know how to interpret     private String name      more about  value

User · Answer

The  Qualifier annotation is used to resolve the autowiring conflict  when there are multiple beans of same type   The  Qualifier annotation can be used on any class annotated with  Component or on methods annotated with  Bean  This annotation can also be applied on constructor arguments or method parameters   Ex -       public interface Vehicle        public void start         public void stop        There are two beans  Car and Bike implements Vehicle interface     Component value  car   public class Car implements Vehicle          Override      public void start                System out println  Car started                  Override      public void stop                System out println  Car stopped                Component value  bike   public class Bike implements Vehicle          Override      public void start               System out println  Bike started                  Override      public void stop               System out println  Bike stopped               Injecting Bike bean in VehicleService using  Autowired with  Qualifier annotation  If you didn t use  Qualifier  it will throw NoUniqueBeanDefinitionException      Component public class VehicleService         Autowired      Qualifier  bike       private Vehicle vehicle       public void service              vehicle start             vehicle stop              Reference -  Qualifier annotation example

User · Answer

You can use  Qualifier along with  Autowired  In fact spring will ask you explicitly select the bean if ambiguous bean type are found  in which case you should provide the qualifier  For Example in following case it is necessary provide a qualifier     Component  Qualifier  staff    public Staff implements Person      Component  Qualifier  employee    public Manager implements Person       Component public Payroll        private Person person        Autowired     public Payroll  Qualifier  employee   Person person             this person   person             EDIT   In Lombok 1 18 4 it is finally possible to avoid the boilerplate on constructor injection when you have  Qualifier  so now it is possible to do the following    Component  Qualifier  staff    public Staff implements Person      Component  Qualifier  employee    public Manager implements Person       Component  RequiredArgsConstructor public Payroll       Qualifier  employee   private final Person person      provided you are using the new lombok config rule copyableAnnotations  by placing the following in lombok config in the root of your project      Copy the Qualifier annotation from the instance variables to the constructor   see https   github com rzwitserloot lombok issues 745 lombok copyableAnnotations    org springframework beans factory annotation Qualifier   This was recently introduced in latest lombok 1 18 4    The blog post where the issue is discussed in detail The original issue on github And a small github project to see it in action   NOTE  If you are using field or setter injection then you have to place the  Autowired and  Qualifier on top of the field or setter function like below any one of them will work   public Payroll       Autowired  Qualifier  employee   private final Person person      or  public Payroll      private final Person person      Autowired     Qualifier  employee      public void setPerson Person person         this person   person            If you are using constructor injection then the annotations should be placed on constructor  else the code would not work  Use it like below -  public Payroll        private Person person        Autowired     public Payroll  Qualifier  employee   Person person             this person   person

[java] Spring @Autowired and @Qualifier

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