& Continuous Learning

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;</pre>

```java
@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

1. Matches by Type
2. Restricts by Qualifiers
3. Matches by Name

@Resource

1. Matches by Name
2. Matches by Type
3. 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.

@Resource(name="person")

@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

1. Explicitly name your component [@Component("beanName")]
2. Use '@Resource' with the 'name' attribute [@Resource(name="beanName")]
3. 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.
4. 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.

Related Links

Testing Spring Wiring Beanoh Spring Wiring Test Framework