弹簧：@Autowired 没有@Component

Question

In a real project, I found out that @Component may be omitted in the following code:

// no @Component !!!!!
public class MovieRecommender {

    private final CustomerPreference customerPreference;

    @Autowired
    public MovieRecommender(CustomerPreference customerPreference) {
        this.customerPreference = customerPreference;
    }

    // ...
}

@Component
public class CustomerPreference {...}

(The example is taken from the official Spring docs https://docs.spring.io/spring-framework/docs/4.3.x/spring-framework-reference/htmlsingle/#beans-autowired-annotation , and the docs show no @Component at all, which may mean either that it is not needed, or that it is just not shown.)

The project where I work does not use any XML bean declarations, but it uses frameworks other than just Spring, so it is possible that something declares the class as a bean. Or it may be a feature of the version of Spring that we use, and if that feature is not documented, it may be dropped later.

Question: Must the class that uses @Autowired be annotated with @Component (well, be a bean)? Is there any official documentation about that?

UPD Folks, there is no @Configuration and no XML configs in the project, I know that such declarations make a bean from a class, but the question is not about them. I even wrote "(well, be a bean)" in the question above to cover that. Does @Autowired work in a class that is not a bean? Or maybe it declares the class that uses it as a bean?

Answer 1

there are several ways to instantiate a bean in Spring.
One of them indeed is with the @Component annotations, with that, Spring will scan all the packages defined for component-scan and initialize all annotated classes (either with @Component or one of the annotations that uses it - Controller, Service, etc.).

Other way to initialise beans is using a configuration class (annotated with @Configuration ) that includes methods annotated with @Bean . each of these methods will create a bean.

There's also an option to create the beans using xml configurations, but this is becoming less and less common, as the annotation-based approach is more convinient

Answer 2

According to https://stackoverflow.com/a/3813725/755804 , with autowireBean() it is possible to autowire a bean from a class not declared as a bean.

@Autowired
private AutowireCapableBeanFactory beanFactory;

public void sayHello(){
    System.out.println("Hello World");
    Bar bar = new Bar();
    beanFactory.autowireBean(bar);
    bar.sayHello();
}

and

package com.example.demo;

import org.springframework.beans.factory.annotation.Autowired;

public class Bar {
    @Autowired
    private Foo foo;

    public void sayHello(){
        System.out.println("Bar: Hello World! foo="+foo);
    }
}

On the other hand, by default the latest Spring does not assume that classes that use @Autowire are @Component -s.

UPD As to the mentioned real project, the stack trace shows that the constructor is called from createBean() . That is, the framework creates beans from classes declared in the framework's configs.