What difference does @EnableConfigurationProperties make if a bean is already annotated with @ConfigurationProperties?

Question

The Spring Boot documentation says that to use the @ConfigurationProperties annotation

You also need to list the properties classes to register in the @EnableConfigurationProperties annotation, as shown in the following example:

and gives this code:

@Configuration @EnableConfigurationProperties(AcmeProperties.class) public class MyConfiguration { } 

But in the very next paragraph says:

Even if the preceding configuration creates a regular bean for AcmeProperties, we recommend that @ConfigurationProperties only deal with the environment and, in particular, does not inject other beans from the context. Having said that, the @EnableConfigurationProperties annotation is also automatically applied to your project so that any existing bean annotated with @ConfigurationProperties is configured from the Environment.

Suggesting that listing a @ConfigurationProperties bean under an @EnableConfigurationProperties annotation is not necessary.

So which is it? Experimentally, I've seen that if I annotate a bean with @ConfigurationProperties it gets properties injected to it as expected without needing to list it in @EnableConfigurationProperties, but if this is the case then why list anything that has a @ConfigurationProperties annotation under @EnableConfigurationProperties, as is shown in the documentation? Does it make any kind of difference?

Answer 1

As M. Deinum referred @EnableConfigurationProperties Is for enabling support of @ConfigurationProperties. If you take a look to the annotation Java Doc you can see:

Enable support for ConfigurationProperties annotated beans. ConfigurationProperties beans can be registered in the standard way (for example using Bean @Bean methods) or, for convenience, can be specified directly on this annotation. [...]

For example, let's say you have a class whose responsibility is to read and store information from your application.yml / application.properties that is required to make a connection to different databases. You annotate it with @ConfigurationProperties.

Then, you typically have a @Configuration annotated class that provides a DataSource @Bean to your application. You can use the @EnableConfigurationProperties to link it to the @ConfigurationProperties class and init your data sources accordingly.

Here is a small example:

application.yml

data-sources:   db1:     url: "jdbc:postgresql://localhost:5432}/db1"     username: test     password: test   db2:     url: "jdbc:postgresql://localhost:5432}/db2"     username: test     password: test 

DataSourcesConfiguration

@ConfigurationProperties public class DataSourcesConfiguration {      private Map<String, BasicDataSource> dataSources;      public void setDataSources(Map<String, BasicDataSource> dataSources) {         this.dataSources = dataSources;     }      Map<String, BasicDataSource > getDataSources() {         return dataSources;     } } 

DataSourceConnectionConfiguration

@Configuration @EnableConfigurationProperties(DataSourcesConfiguration.class) public class DatabaseConnectionConfiguration implements Provider<Connection> {      private DataSourcesConfiguration dataSourcesConfiguration;      public DatabaseConnectionConfiguration(DataSourcesConfiguration dataSourcesConfiguration) {         this.dataSourcesConfiguration = dataSourcesConfiguration;     }      @Bean     public DataSource dataSource() {         // Use dataSourcesConfiguration to create application data source. E.g., a AbstractRoutingDataSource..     }  }