Very often, when using Spring’s dependency injection, you will simply use the @Autowired annotation on your fields, constructor or setter methods, since you know there will be one and only one instance of the required bean at runtime.

However, seldom you might end up wanting to have a specific bean of a certain instance if someone has configured it in the application context, but having the possibility to use a default value otherwise. If you’re using constructor dependency injection, you can use this trick to do it:

interface MyBean {
}

class MyDefaultBean implements MyBean {
}

@Component
class MyTarget {
    private final MyBean myBean;

    /**
     * The constructor called by Spring when an object of type {@link MyBean}
     * has been explicitly configured.
     */
    @Autowired(required = false)
    public MyTarget(MyBean myBean) {
        this.myBean = myBean;
    }

    /**
     * The constructor called by Spring when no explicit {@link MyBean}
     * is configured in the application context.
     */
    private MyTarget() {
        this(new MyDefaultBean());
    }
}

Here we have two constructors: one which takes a MyBean instance and is marked with @Autowired. Notice that we have marked the @Autowired annotation as not required, using required = false; otherwise start-up of the application context would fail. We can also create another default constructor, not annotated and private.

The combination of these two works as we wanted: if a specific instance of MyBean exists in the application context, the first constructor will be called and thus that instance will be used; otherwise, the second constructor will be called and we will use an instance of MyDefaultBean.