What Does @Configuration Do?

[Kevin Simonson]

Okay, I've made it to chapter 3 of Spring in Action, by Craig Walls, and on pages 66 and 67 he has two classes:

package com.myapp;
import javax.activation.DataSource;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Profile;
import org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseBuilder;
import org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseType;

@Configuration
@Profile("dev")
public class DevelopmentProfileConfig {

  @Bean(destroyMethod="shutdown")
  public DataSource dataSource() {
      return new EmbeddedDatabaseBuilder()
          .setType(EmbeddedDatabaseType.H2)
          .addScript("classpath:schema.sql")
          .addScript("classpath:test-data.sql")
          .build();
  }

}

and

package com.myapp;
import javax.activation.DataSource;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configure;
import org.springframework.context.annotation.Profile;
import org.springframework.jndi.JndiObjectFactoryBean;

@Configuration
@Profile("prod")
public class ProductionProfileConfig {

  @Bean
  public DataSource dataSource() {
    JndiObjectFactoryBean jndiObjectFactoryBean =
        new JndiObjectFactoryBean();
    jndiObjectFactoryBean.setJndiName("jdbc/myDS");
    jndiObjectFactoryBean.setResourceRef(true);
    jndiObjectFactoryBean.setProxyInterface(
        javax.sql.DataSource.class);
    return (DataSource) jndiObjectFactoryBean.getObject();
  }

}

As I understand it, each @Bean annotation creates and registers a bean. I think I also understand what @Profile does. My question is, what does @Configuration do? The book may have explained this somewhere in the previous two chapters, but I'm at a loss right now as to what it does. If someone could tell me what it does, I'd really appreciate it.

[L Foster]

Here is an excerpt from not one but 2 books on Spring (identical!).

Starting with Spring 3.0, you can configure Spring metadata purely inside Java classes, completely avoiding any XML configuration while enhancing annotation-based metadata. You annotate any Java class with @Configuration annotation at the class level and have methods annotated as @Configuration annotation on a factory method that instantiates an @Component annotation, or any other specialized bean, to define your application context. Let's see a simple example:

from Spring Essentials, and also from Spring MVC: Designing Real World Web Applications.

I am omitting the example, but trying very hard to mention all the "diverse" sources I have read. ;-)

I have not used Spring since before annotations were introduced, but it looks like @Configuration is used to declare that something can later be used as configuration.

Here's a third excerpt:

The @Configuration annotation hints that the contained class declares one or more @Bean definitions. The @Configuration annotation is meta-annotated with @Component; therefore, it is a candidate for component scanning.

from Spring Microservices that seems a bit more succinct.

I hope this helps.

[Kevin Simonson]

L Foster wrote:Here is an excerpt from not one but 2 books on Spring (identical!).

Starting with Spring 3.0, you can configure Spring metadata purely inside Java classes, completely avoiding any XML configuration while enhancing annotation-based metadata. You annotate any Java class with @Configuration annotation at the class level and have methods annotated as @Configuration annotation on a factory method that instantiates an @Component annotation, or any other specialized bean, to define your application context. Let's see a simple example:

from Spring Essentials, and also from Spring MVC: Designing Real World Web Applications.

I am omitting the example, but trying very hard to mention all the "diverse" sources I have read. ;-)

I have not used Spring since before annotations were introduced, but it looks like @Configuration is used to declare that something can later be used as configuration.

Here's a third excerpt:

The @Configuration annotation hints that the contained class declares one or more @Bean definitions. The @Configuration annotation is meta-annotated with @Component; therefore, it is a candidate for component scanning.

from Spring Microservices that seems a bit more succinct.

I hope this helps.

Thanks! So the @Configuration annotation simply lets the compiler know the code "is a candidate for component scanning"? And that's all it does?

[William Barnes]

That confuses me.   I always considered having all the configurations in xml was one of Springs main themes.  So having @Configuration hard code your configurations is really confusing.  

[Kevin Simonson]

William Barnes wrote:That confuses me.   I always considered having all the configurations in xml was one of Springs main themes.  So having @Configuration hard code your configurations is really confusing.  

One of the themes the book Spring in Action keeps hammering home is that now there are multiple ways to do it; one can still do the configurations in XML if one wants to, or one can use the @Configuration annotation, or one can do a mix.