Spring boot Bean annotation and instance variable flow in Configuration

I am new to the spring boot framework and I have some confusion regarding the @Bean annotation and how the instance variable gets changed by the Bean. I have a below example and if someone can answer my questions will be really helpful:

1. If I am defining restTemplate in my instance variable will userRestTemplate template bean will be able to change its value?
2. If userRestTemplate is changing its value then userDetail will have the updated value by userRestTemplate bean?
3. If not what will be userdetail bean will be setting in setTemplate method?

@Configuration
public class UserConfiguration{
    RestTemplate restTemplate;

    @Bean
    @Named("userRestTemplate")
    public RestTemplate userRestTemplate(RestTemplateBuilder restTemplateBuilder) {
        RestTemplate restTemplate = restTemplateBuilder.build();
        //restTemplate.getMessageConverters().add(0, createMappingJacksonHttpMessageConverter());
        this.restTemplate = restTemplate;
        return restTemplate;
    }



    @Bean
    public UserDetail userDetail() {
        UserDetail user = new UserDetail();
        user.setTemplate(restTemplate);
        return user;
    }
}

Answer

Actually, I don’t think this is the standard way of creation.

As when you annotate the method with @Bean, you mean you want to ask Spring to help you to manage the bean. Therefore it makes no sense to store the bean instance by yourself as a local variable.

Instead of storing the instance in the class field, you should ask Spring to give you the instance instead.

There are multiple ways to do that.

1. Specify RestTemplate as a parameter in @Bean method. In case you have asked Spring to manage multiple RestTemplate instance, which is possible. Then you should specify the parameter name same as @Named which is userRestTemplate, so that Spring can find the correct restTemplate properly.

@Configuration
public class UserConfiguration{
    @Bean
    @Named("userRestTemplate")
    public RestTemplate userRestTemplate(RestTemplateBuilder restTemplateBuilder) {
        RestTemplate restTemplate = restTemplateBuilder.build();
        // this.restTemplate = restTemplate; // no this restTemplate instance setting;
        return restTemplate;
    }


    @Bean
    public UserDetail userDetail(RestTemplate userRestTemplate) {
        UserDetail user = new UserDetail();
        user.setTemplate(userRestTemplate);
        return user;
    }
}

2. Use @Autowired Annotation.

@Configuration
public class UserConfiguration{
    @Autowired
    private RestTemplate restTemplate;

    @Bean
    @Named("userRestTemplate")
    public RestTemplate userRestTemplate(RestTemplateBuilder restTemplateBuilder) {
        RestTemplate restTemplate = restTemplateBuilder.build();
        //restTemplate.getMessageConverters().add(0, createMappingJacksonHttpMessageConverter());
        // this.restTemplate = restTemplate; // again no setting this, okay.
        return restTemplate;
    }

    @Bean
    public UserDetail userDetail() {
        UserDetail user = new UserDetail();
        user.setTemplate(restTemplate);
        return user;
    }
}

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