使用@Qualifier批注和在Spring中直接注入bean有什么区别？

Question

Suppose I have a program

@Component
public interface Coach{
  public String giveCoaching();
}

@Component
public TennisCoach implements Coach{
  @Override
  public String giveCoaching(){
    return "Teaching forhand";
  }
}

I have two Demo classes in which I have injected the bean in different ways. what is the difference in both the injections

public class AppDemo{
  @AutoWired
  @Qualifier("tennisCoach")
  private Coach theCoach;
}

public class AppDemo{
  @AutoWired
  private TennisCoach tennisCoach;
  }
}

Answer 1

When you have more than 1 implementation for you interface, you will get an exception when Autowiring the bean. At that time @Qualifier will be used to choose the required implementation

@Component
public interface Coach{
  public String giveCoaching();
}

@Component
public TennisCoach implements Coach{
  @Override
  public String giveCoaching(){
    return "Teaching forhand";
  }
}

@Component
public CricketCoach implements Coach{
  @Override
  public String giveCoaching(){
    return "Teaching forbat";
  }
}

Now the ambiguity will occur when you autowire the Coach Interface like below

public class AppDemo{
  @AutoWired      
  private Coach theCoach;
}

So you have to qualify the right bean for the Coach Interface like below.

public class AppDemo{
  @AutoWired
  @Qualifier("tennisCoach")      
  private Coach theCoach;
}

Alternatively you can use @Primary annotation on top of any one of the implementation so that the Spring Container will by default choose the bean in case of more than 1 implementation for an interface.

But in the code below, you are directly creating the object for the implementation rather than interface.

public class AppDemo{
  @AutoWired
  private TennisCoach tennisCoach;
  }
}

Answer 2

当接口具有多个实现类时，将使用@Qualifier批注。您应该选择要在Spring上下文中作为bean注入的类。