Overview
I’ve been asked several times to explain the difference between injecting Spring beans with ‘@Resource’, ‘@Autowired’, and ‘@Inject’. While I received a few opinions from colleagues and read a couple of posts on this topic I didn’t feel like I had a complete picture.
Annotations
| Annotation |
Package |
Source |
| @Resource |
javax.annotation |
Java |
| @Inject |
javax.inject |
Java |
| @Qualifier |
javax.inject |
Java |
| @Autowired |
org.springframework.bean.factory |
Spring |
In order to explore the behavior of each annotation I fired up
Spring Tool Suite and started debugging the code. I used Spring 3.0.5.RELEASE in my research. The following is a summary of my findings.
The Code
I wanted to know how ‘@Resource’, ‘@Autowired’, and ‘@Inject’ resolved dependencies. I created an interface called ‘Party’ and created two implementations classes. This allowed me to inject beans without using the concrete type. This provided the flexibility I needed to determine how Spring resolves beans when there are multiple type matches.
public interface Party {
}
|
‘Person’ is a component and it implements ‘Party’.
package com.sourceallies.person;
...
@Component
public class Person implements Party {
}
|
‘Organization’ is a component and it implements ‘Party’.
package com.sourceallies.organization;
...
@Component
public class Organization implements Party {
}
|
I setup a Spring context that scans both of these packages for beans marked with ‘@Component’.
<context:component-scan base-package="com.sourceallies.organization"/>
<context:component-scan base-package="com.sourceallies.person"/>
|
Tests
Test 1: Ambiguous Beans
In this test I injected a ‘Party’ bean that has multiple implementations in the Spring context.
@Resource
private Party party;
|
@Autowired
private Party party;
|
@Inject
private Party party;
|
In all three cases a ‘NoSuchBeanDefinitionException’ is thrown. While this exception’s name implies that no beans were found, the message explains that two beans were found. All of these annotations result in the same exception.
org.springframework.beans.factory.NoSuchBeanDefinitionException:
No unique bean of type [com.sourceallies.Party] is defined:
expected single matching bean but found 2: [organization, person]
|
Test 2: Field Name
In this test I named the Party field person. By default beans marked with ‘@Component’ will have the same name as the class. Therefore the name of the class ‘Person’ is person.
@Resource
private Party person;
|
@Autowired
private Party person;
|
@Inject
private Party person;
|
‘@Resource’ can also take an optional ‘name’ attribute. This is equivalent to the ‘@Resource’ code above. In this case the field variable name remains ‘party’. There is no equivalent syntax for ‘@Autowired’ or ‘@Inject’. Instead you would have to use a ‘@Qualifier’. This syntax will be covered later.
@Resource(name="person")
private Party party;
|
All four of these styles inject the ‘Person’ bean.
Test 3: Field Type
In this test I changed the type to be a ‘Person’.
@Resource
private Person party;
|
@Autowired
private Person party;
|
@Inject
private Person party;
|
All of these annotations inject the ‘Person’ bean.
Test 4: Default Name Qualifier
In this test I use a ‘@Qualifier’ annotation to point to the default name of the ‘Person’ component.
@Resource
@Qualifier("person")
private Party party;
|
@Autowired
@Qualifier("person")
private Party party;
|
@Inject
@Qualifier("person")
private Party party;
|
All of these annotations inject the ‘Person’ bean.
Test 5: Qualified Name
I added a ‘@Qualifier’ annotation to the ‘Person’ class
package com.sourceallies.person;
...
@Component
@Qualifier("personBean")
public class Person implements Party {
}
|
In this test I use a ‘@Qualifier’ annotation to point to the qualified name of the ‘Person’ component.
@Resource
@Qualifier("personBean")
private Party party;
|
@Autowired
@Qualifier("personBean")
private Party party;
|
@Inject
@Qualifier("personBean")
private Party party;
|
All of these annotations inject the ‘Person’ bean.
Test 6: List of Beans
In this test I inject a list of beans.
@Resource
private List<Party> parties;
|
@Autowired
private List<Party> parties;
|
@Inject
private List<Party> parties;
|
All of these annotations inject 2 beans into the list. This can also be accomplished with a ‘@Qualifier’. Each bean marked with a specific qualifier will be added to the list.
Test 7: Conflicting messages
In this test I add a bad ‘@Qualifier’ and a matching field name.
@Resource
@Qualifier("bad")
private Party person;
|
@Autowired
@Qualifier("bad")
private Party person;
|
@Inject
@Qualifier("bad")
private Party person;
|
In this case the field marked with ‘@Resource’ uses the field name and ignores the ‘@Qualifier’. As a result the ‘Person’ bean is injected.
However the ‘@Autowired’ and ‘@Inject’ field throw a ‘NoSuchBeanDefinitionException’ error because it can not find a bean that matches the ‘@Qualifier’.
org.springframework.beans.factory.NoSuchBeanDefinitionException:
No matching bean of type [com.sourceallies.Party] found for dependency:
expected at least 1 bean which qualifies as autowire candidate for this dependency.
Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true),
@org.springframework.beans.factory.annotation.Qualifier(value=bad)}
|
Conclusions
With the exception of test 2 & 7 the configuration and outcomes were identical. When I looked under the hood I determined that the ‘@Autowired’ and ‘@Inject’ annotation behave identically. Both of these annotations use the
‘AutowiredAnnotationBeanPostProcessor’ to inject dependencies. ‘@Autowired’ and ‘@Inject’ can be used interchangeable to inject Spring beans. However the ‘@Resource’ annotation uses the
‘CommonAnnotationBeanPostProcessor’ to inject dependencies. Even though they use different post processor classes they all behave nearly identically. Below is a summary of their execution paths.
@Autowired and @Inject
- Matches by Type
- Restricts by Qualifiers
- Matches by Name
@Resource
- Matches by Name
- Matches by Type
- Restricts by Qualifiers (ignored if match is found by name)
While it could be argued that ‘@Resource’ will perform faster by name than ‘@Autowired’ and ‘@Inject’ it would be negligible. This isn’t a sufficient reason to favor one syntax over the others. I do however favor the ‘@Resource’ annotation for it’s concise notation style.
@Autowired
@Qualifier("person")
|
@Inject
@Qualifier("person")
|
You may argue that they can be equal concise if you use the field name to identify the bean name.
@Resource
private Party person;
|
@Autowired
private Party person;
|
@Inject
private Party person;
|
True enough, but what happens if you want to
refactor your code? By simply renaming the field name you’re no longer referring to the same bean. I recommend the following practices when wiring beans with annotations.
Spring Annotation Style Best Practices
- Explicitly name your component [@Component(“beanName”)]
- Use ‘@Resource’ with the ‘name’ attribute [@Resource(name=”beanName”)]
- Avoid ‘@Qualifier’ annotations unless you want to create a list of similar beans. For example you may want to mark a set of rules with a specific ‘@Qualifier’ annotation. This approach makes it simple to inject a group of rule classes into a list that can be used for processing data.
- Scan specific packages for components [context:component-scan base-package=”com.sourceallies.person”]. While this will result in more component-scan configurations it reduces the chance that you’ll add unnecessary components to your Spring context.
Following these guidelines will increase the readability and stability of your Spring annotation configurations.
Source:
http://blogs.sourceallies.com/2011/08/spring-injection-with-resource-and-autowired/comment-page-2/#comment-55380
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