Understanding spring @Configuration class

autowired, configuration, java, spring

Solution

Migrating XML to `@Configuration`

It is possible to migrate the xml to a `@Configuration` in a few steps:

Create a `@Configuration` annotated class:

@Configuration
public class MyApplicationContext {

}

For each `<bean>` tag create a method annotated with `@Bean`:

@Configuration
public class MyApplicationContext {

  @Bean(name = "someBean")
  public SomeClass getSomeClass() {
    return new SomeClassImpl(someInterestingProperty); // We still need to inject someInterestingProperty
  }

  @Bean(name = "anotherBean")
  public AnotherClass getAnotherClass() {
    return new AnotherClassImpl(getSomeClass(), beanFromSomewhereElse); // We still need to inject beanFromSomewhereElse
  }
}

In order to import `beanFromSomewhereElse` we need to import it's definition. It can be defined in an XML and the we'll use `@ImportResource`:

@ImportResource("another-application-context.xml")
@Configuration
public class MyApplicationContext {
  ...  
}

If the bean is defined in another `@Configuration` class we can use the `@Import` annotation:

@Import(OtherConfiguration.class)
@Configuration
public class MyApplicationContext {
  ...
}

After we imported other XMLs or `@Configuration` classes, we can use the beans they declare in our context by declaring a private member to the `@Configuration` class as follows:

@Autowired
@Qualifier(value = "beanFromSomewhereElse")
private final StrangeBean beanFromSomewhereElse;

Or use it directly as parameter in the method which defines the bean that depends on this `beanFromSomewhereElse` using `@Qualifier` as follows:

@Bean(name = "anotherBean")
public AnotherClass getAnotherClass(@Qualifier (value = "beanFromSomewhereElse") final StrangeBean beanFromSomewhereElse) {
  return new AnotherClassImpl(getSomeClass(), beanFromSomewhereElse);
}

Importing properties is very similar to importing bean from another xml or `@Configuration` class. Instead of using `@Qualifier` we'll use `@Value` with properties as follows:

@Autowired
@Value("${some.interesting.property}")
private final String someInterestingProperty;

This can be used with SpEL expressions as well.

In order to allow spring to treat such classes as beans containers we need to mark this in our main xml by putting this tag in the context:

<context:annotation-config/>

You can now import `@Configuration` classes exactly the same as you would create a simple bean:

<bean class="some.package.MyApplicationContext"/>

There are ways to avoid spring XMLs altogether but they are not in the scope of this answer. You can find out one of these options in my blog post on which I'm basing my answer.

The advantages and disadvantages of using this method

Basically I find this method of declaring beans much more comfortable than using XMLs due to a few advantages I see:

- Typos - `@Configuration` classes are compiled and typos just won't allow compilations

- Fail fast (compile time) - If you forget to inject a bean you'll fail on compile time and not on run-time as with XMLs

- Easier to navigate in IDE - between constructors of beans to understand the dependency tree.

- Possible to easily debug configuration startup

The disadvantages are not many as I see them but there are a few which I could think of:

- Abuse - Code is easier to abuse than XMLs

- With XMLs you can define dependencies based on classes that are not available during compile time but are provided during run-time. With `@Configuration` classes you must have the classes available at compile time. Usually that's not an issue, but there are cases it may be.

Bottom line: It is perfectly fine to combine XMLs, `@Configuration` and annotations in your application context. Spring doesn't care about the method a bean was declared with.

Problem

Original source

Related problems

• Understanding Spring @Autowired usage