In Java development, mapping between objects is a common and important task. As a powerful object mapping framework, MapStruct improves development efficiency by simplifying the writing of mapping code. However, in practical applications, we often face the situation where there is a Null value in the source or target object.
This article will explore in-depth some of the features of @Mapping annotation in MapStruct, especially how to handle Null values gracefully during mapping.
1. Introduction to @Mapping annotation
@Mapping annotation is an important tool in MapStruct for customizing field mapping relationships. Through the @Mapping annotation, we can specify the field mapping relationship between the source object and the target object, including field name, conversion method and other information.
When dealing with Null values, the @Mapping annotation provides some powerful options to make the mapping process more flexible and controllable.
2. Process the source object Null value
During mapping, the Null value in the source object may cause the corresponding field in the mapping result to be Null as well.
To avoid this, you can use the source attribute of the @Mapping annotation to specify a default value. When the source object field is Null, the specified default value will be mapped.
@Mapper
public interface MyMapper {
@Mapping(source = "sourceField", target = "targetField", defaultValue = "defaultValue")
TargetObject map(SourceObject source);
}3. Process the Null value of the target object
Similarly, Null values in the target object may also need to be processed specifically.
Through the target attribute annotated by @Mapping, we can specify that the target object field is set to a specific value when the source object field is Null.
@Mapper
public interface MyMapper {
@Mapping(source = "sourceField", target = "targetField", nullValuePropertyMappingStrategy = NullValuePropertyMappingStrategy.SET_TO_DEFAULT)
TargetObject map(SourceObject source);
}4. Customize Null value processing strategy
In addition to using default values and setting the target object field to default values, MapStruct also supports custom Null value processing policies.
By configuring the nullValueCheckStrategy property in the @Mapper annotation, we can specify a custom policy class to decide how to handle Null values during mapping.
@Mapper(nullValueCheckStrategy = )
public interface MyMapper {
@Mapping(source = "sourceField", target = "targetField")
TargetObject map(SourceObject source);
}5. Check Null value in implicit conversion
MapStruct also provides one of the options for nullValueCheckStrategy: NullValueCheckStrategy.ON_IMPLICIT_CONVERSION.
This option means checking the null value when performing an implicit conversion of the map, ensuring the robustness of the mapping results.
In MapStruct, implicit conversion refers to the fact that when performing field mapping, MapStruct will try to automatically perform type conversion to suit the type of the target object field.
For example, if the field type of the source object isIntegerThe field type of the target object isString, MapStruct will try toIntegerThe value of type is implicitly converted toStringtype.
When configurationnullValueCheckStrategyforNullValueCheckStrategy.ON_IMPLICIT_CONVERSIONWhen MapStruct checks whether the source object field is null when performing an implicit conversion.
If the source object field is null, MapStruct will ensure that no implicit conversion is performed and process the mapping results accordingly to prevent NullPointerException exceptions during the conversion process.
For example:
@Mapper(nullValueCheckStrategy = NullValueCheckStrategy.ON_IMPLICIT_CONVERSION)
public interface MyMapper {
@Mapping(source = "sourceIntegerField", target = "targetStringField")
TargetObject map(SourceObject source);
}In the above example, ifsourceIntegerFieldTo null, MapStruct checks when executing the mapping and makes sure that no attempt is made to implicitly convert the null value totargetStringField, thereby avoiding potential NullPointerException.
6. Summary
MapStruct's @Mapping annotation provides developers with flexible ways to handle Null values during mapping. Whether it is by setting the default value, setting the target object field to the default value, or customizing the Null value processing strategy, or checking the Null value in an implicit conversion, it can meet the needs of different scenarios.
In practical applications, choosing the appropriate processing method according to specific circumstances can make the mapping process more controllable and improve the robustness and maintainability of the code. By rationally utilizing the functionality of MapStruct, we can handle Null value issues in object mapping elegantly and efficiently.
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