Spring @Primary Annotation

@Primary annotation in Spring is used to indicate the primary bean when multiple beans of the same type are present for auto wiring. When multiple beans are eligible for auto wiring the @Primary annotation will help to determine which bean should be given preference. 

By applying @Primary annotation to a specific bean we are specifying that it is a primary bean for its respective type. When auto-wiring the dependencies if we are having multiple beans then the bean which is annotated with @Primary annotation will be given high preference compared to other beans. 

Examples for @Primary Annotation

Example 1:

Java

// on the below line creating a class for Dog.
@Component
public class Dog {

    // on the below line creating a string variable for dog
    // breed.
    private String dogBreed;

    // on the below line creating a constructor.
    public Dog(String breed) { this.dogBreed = breed; }
    // on the below line creating a getter method.
    public String getdogBreed() { return dogBreed; }
}
// on the below line creating a class for German Sheapered
// and annotating it with Primary annotation.
@Component
@Primary
public class GermanShepared extends Dog {
    // on the below line creting a constructor and passing
    // breed type.
    public GermanShepared() { super("GermanShepared"); }
}

// on the below line creating a class for LabraDog.
@Component
public class LabraDog extends Dog {
    // on the below line creting a constructor and passing
    // breed type.
    public LabraDog() { super("LabraDog"); }
}

Explanation:

In the above example, we are creating a class for Dog in which we are creating variables for dog breed names and creating a constructor to initialize it. Also, we are creating a getter method for the same. Then we are creating two classes for the German Shepherd Dog breed and the Labra Dog breed. Both of these classes extend the Dog class. We are annotating both of these classes with @Component annotation which tells Spring to create a bean for each class. To indicate the GermanShepared class as a primary bean we are annotating it with @Primary annotation. This tells Spring that GermanShepared bean should be used when a bean of type Dog is needed. 

Example 2:

Java

// on the below line creating a class for Movie.
@Component
public class Movie {

    // on the below line creating a string variable for
    // movie Type.
    private String movieType;

    // on the below line creating a constructor.
    public Movie(String movieType)
    {
        this.movieType = movieType;
    }
    // on the below line creating a getter method.
    public String getmovieType() { return movieType; }
}

// on the below line creating a class for Comedy movie and
// annotating it with Primary annotation.
@Component
@Primary
public class Comedy extends Movie {
    // on the below line creting a constructor and passing
    // movie type.
    public Comedy() { super("Comedy"); }
}

// on the below line creating a class for Horror movie.
@Component
public class Horror extends Movie {
    // on the below line creting a constructor and passing
    // movie type.
    public Horror() { super("Horror"); }
}

Explanation:

In the above example we are creating a class for Movie in which we are creating variables for movie type and creating a constructor to initialise it. Also we are creating a getter method for the same. Then we are creating two classes for Comedy movie and Horror movie. Both of this class extends the Movie class. We are annotating both of these classes with @Component annotation which tells spring to create a bean for each class. To indicate Comedy class as a primary bean we are annotating it with @Primary annotation. This tells Spring that Comedy movie bean should be used when a bean of type Movie is needed.