14.4. Boosting

Lucene uses boosting to attach more importance to specific fields or documents over others. Lucene differentiates between index and search-time boosting.

14.4.1. Static Index Time Boosting

The @Boost annotation is used to define a static boost value for an indexed class or property. This annotation can be used within @Field, or can be specified directly on the method or class level.
In the following example:
  • the probability of Essay reaching the top of the search list will be multiplied by 1.7.
  • @Field.boost and @Boost on a property are cumulative, therefore the summary field will be 3.0 (2 x 1.5), and more important than the ISBN field.
  • The text field is 1.2 times more important than the ISBN field.

Example 14.7. Different ways of using @Boost

@Indexed
@Boost(1.7f)
public class Essay {

    @Field(name = "Abstract", store=Store.YES, boost = @Boost(2f))
    @Boost(1.5f)
    public String getSummary() { return summary; }

    @Field(boost = @Boost(1.2f))
    public String getText() { return text; }

    @Field
    public String getISBN() { return isbn; }

}

14.4.2. Dynamic Index Time Boosting

The @Boost annotation defines a static boost factor that is independent of the state of the indexed entity at runtime. However, in some cases the boost factor may depend on the actual state of the entity. In this case, use the @DynamicBoost annotation together with an accompanying custom BoostStrategy.
@Boost and @DynamicBoost annotations can both be used in relation to an entity, and all defined boost factors are cumulative. The @DynamicBoost can be placed at either class or field level.
In the following example, a dynamic boost is defined on class level specifying VIPBoostStrategy as implementation of the BoostStrategy interface used at indexing time. Depending on the annotation placement, either the whole entity is passed to the defineBoost method or only the annotated field/property value. The passed object must be cast to the correct type.

Example 14.8. Dynamic boost example

public enum PersonType {
    NORMAL,
    VIP
}

@Indexed
@DynamicBoost(impl = VIPBoostStrategy.class)
public class Person {
    private PersonType type;   
}

public class VIPBoostStrategy implements BoostStrategy {
    public float defineBoost(Object value) {
        Person person = (Person) value;
        if (person.getType().equals(PersonType.VIP)) {
            return 2.0f;
        }
        else {
            return 1.0f;
        }
    }
}
In the provided example all indexed values of a VIP would be twice the importance of the values of a non-VIP.

Note

The specified BoostStrategy implementation must define a public no argument constructor.