Spring Injection with @Resource, @Autowired and @Inject

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

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