Spring Injection with @Resource, @Autowired and @Inject » Source Allies Blog

Spring Injection with @Resource, @Autowired and @Inject » Source Allies Blog

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.

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