As you can see, the amount of information that is printed in the debug mode can be somewhat overwhelming, so I've selected only one example of positive and negative matches each.

For each line of the report, Spring Boot tells us why certain configurations have been selected to be included, what they have been positively matched on, or, for the negative matches, what was missing that prevented a particular configuration being included in the mix. Let's look at the positive match for DataSourceAutoConfiguration:

• The @ConditionalOnClass classes found tells us that Spring Boot has detected the presence of a particular class, specifically two classes in our case: javax.sql.DataSource and org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseType.
• The OnClassCondition indicates the kind of matching that was used. This is supported by the @ConditionalOnClass and @ConditionalOnMissingClass annotations.

While OnClassCondition is the most common kind of detection, Spring Boot also uses many other conditions. For example, OnBeanCondition is used to check the presence or absence of specific bean instances, OnPropertyCondition is used to check the presence, absence, or specific value of a property, as well as any number of the custom conditions that can be defined using the @Conditional annotation and Condition interface implementations.

The negative matches show us a list of configurations that Spring Boot has evaluated, which means that they do exist in the classpath and were scanned by Spring Boot but didn't pass the conditions required for their inclusion. GsonAutoConfiguration, while available in the classpath as it is a part of the imported spring-boot-autoconfigure artifact, was not included because the required com.google.gson.Gson class was not detected as present in the classpath, thus failing the OnClassCondition.

The implementation of the GsonAutoConfiguration file looks as follows:

@Configuration 
@ConditionalOnClass(Gson.class) 
public class GsonAutoConfiguration { 
 
  @Bean 
  @ConditionalOnMissingBean 
  public Gson gson() { 
    return new Gson(); 
  } 
 
} 

After looking at the code, it is very easy to make the connection between the conditional annotations and report information that is provided by Spring Boot at the start time.