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

@EnableConfigurationProperties imports EnableConfigurationPropertiesRegistrar which enables support for @ConfigurationProperties annotated beans.

@ConfigurationProperties is an annotation for externalized configuration, it is to be applied to a bean configuration class or method annotated with @Bean eg

   @ConfigurationProperties(prefix = "some-prefix")
   public SomePrefix prefixBean() { 
      return new SomePrefix(); 
}

To load the properties and bind them to properties within the method or the class that match the prefix. ps: some-prefix binds to SomePrefix because of spring's support for Relaxed binding.

Before springboot 2.2, You could do either of the following:

@Configuration
@ConfigurationProperties(prefix = "some-prefix")
public class ConfigProperties {
//...some code
}

or

@Configuration
@EnableConfigurationProperties(SomeClassToBeBounded.class)
public class ConfigProperties {

along with

@ConfigurationProperties(prefix = "some-prefix")
public class SomeClassToBeBounded{
//...some code
}

From springboot 2.2

You can do it in a much easier way:

@ConfigurationProperties(prefix = "some-prefix") 
@ConfigurationPropertiesScan 
public class ConfigProperties { 

//...some code
}

With this, the classpath scanner enabled by @SpringBootApplication finds the ConfigProperties class, even though we didn't annotate this class with @Component.

Answer 2

It took me a while to reach to this post but would like to add here so that others may get benefited.

@ConfigurationProperties - Used to bind a class with an externalized property file. Very powerful and must be used to separate out bean classes with configuration entity class.

@Configuration - Creates a Spring bean of configuration stereotype.

@EnableConfigurationProperties - Creates a binding between a configuration entity class and Spring configuration stereotype so that after injection within a service properties can be retrieved easily.

Answer 3

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..
    }

}

Answer 4

If we look at the code below:

@Configuration @EnableConfigurationProperties @ConfigurationProperties(prefix="ar1") public class ar1Settings { }

• @Configuration tells Spring to treat this as a configuration class and register it as a Bean

• @EnableConfigurationProperties tells Spring to treat this class as a consumer of application.yml/properties values

• @ConfigurationProperties tells Spring what section this class represents.

My understanding is that if you don't need to specify the section of the property file, then @ConfigurationProperties can be omitted.