Beans creation using annotation

[Puspender Tanwar]

I have three confusions while bean creation using annotations:
1. How is bean injected when a member variable is @Autowired ?
2. When I Autowired all three ways possible (i.e Member variable, setter & constructor), how the bean is injected(in my code the customerRepo bean)
3. I ran the code and go logs as below:
constructor injection
setter injection
why is constructor injection called first?

@Service("customerService")
public class CustomerServiceImpl implements CustomerService {
	
	@Autowired
	private CustomerRepo customerRepo;
		
	@Autowired
	public void setCustomerRepo(CustomerRepo customerRepo){
		System.out.println("setter injection");
		this.customerRepo = customerRepo;
	}
	
	@Autowired
	public CustomerServiceImpl(CustomerRepo customerRepo){
		System.out.println("constructor injection");
		this.customerRepo = customerRepo;
	}
	
	public List<Customer> findAll(){
		return customerRepo.findAll();
		
	}
	
}

[Jeanne Boyarsky]

3) A constructor is run when an object is instantiated. This means the only opportunity for a constructor to run is before the object is ready for use by other methods. Which means the constructor injection needs to happen before the setter.

2) Try it? The intent is to do all three on the same field

1) I don't understand the question. Spring injects it for you.

[Puspender Tanwar]

Jeanne Boyarsky wrote:3) A constructor is run when an object is instantiated. This means the only opportunity for a constructor to run is before the object is ready for use by other methods. Which means the constructor injection needs to happen before the setter.

Thanks, lots of things were going throw my mind and I skipped this simple concept.

Jeanne Boyarsky wrote:2) Try it? The intent is to do all three on the same field

Yes I tried it and intent is to do all three on the same field, actually just for better understanding of bean injection, not in general programming.

Jeanne Boyarsky wrote:1) I don't understand the question. Spring injects it for you.

My question was: like setter autowire inject bean using a setter method, constructor autowire uses a constructor then how the bean is injected in case of member variable autowiring? I hope now my question makes sense.

[Puspender Tanwar]

if someone having answers to my doubts, please respond.

[Stephan van Hulst]

Spring injects dependencies into fields that are annotated with @Autowired using reflection, likely using the methods of the java.lang.reflect.Field class.

[Peter Rooke]

Just to add (in IMHO) its best not to use field injection as it makes writing test cases awkward.    

[Stephan van Hulst]

I agree with Peter. You should pretty much always prefer constructor injection.

[Puspender Tanwar]

Stephan van Hulst wrote:I agree with Peter. You should pretty much always prefer constructor injection.

but there are some posts that advise using setter injection over constructor injection.

[Marco Behler]

I agree with Peter. you should pretty much always prefer constructor injection.

Just to add (in IMHO) its best not to use field injection as it makes writing test cases awkward.  

How so, regarding tests?

To be honest with you, I'm actually seeing this as a completely religious issue. I've worked with several injection styles over the years, and from a pragmatic viewpoint.......i really don't care

[Stephan van Hulst]

Because it's difficult to unit test your services if you can't inject mock dependencies into them. There aren't many nice mocking frameworks that allow you to inject dependencies straight into private fields.

[Stephan van Hulst]

Puspender Tanwar wrote:but there are some posts that advise using setter injection over constructor injection.

Please show us where you read that.

[Peter Rooke]

Using field injection you have tightly coupled your POJO code to the (Spring) DI framework.  Unit testing should really not involve the DI container, that's the purpose of integration tests.
So when you write unit tests, in the absence of the framework, your going to have to monkey around with reflection to inject the values.  

Injection
Always use constructor based dependency injection in your beans. Always use assertions for mandatory dependencies. For more background on why field based injection is evil, you can read this article by Oliver Gierke.
As always there is one exception to the rule, it’s fine to use field-based injection in tests when you’re using the SpringJUnit4ClassRunner.
.....
Testing
The golden rule for unit testing in Spring is keep Spring out of your unit testing! It should be possible to unit test the majority of your beans without needing to spin up an application context.

How not to hate Spring 2016

Feild DI considered harmful  

[Puspender Tanwar]

Stephan van Hulst wrote:

Puspender Tanwar wrote:but there are some posts that advise using setter injection over constructor injection.

Please show us where you read that.

I actually don't remember where I read that article. If I found that, definitely I will share that here.

Anyways, from this thread, I get to know that field injection are evil and we shouldn't use them.