MapStruct 1.4.2.Final Reference Guide

For Maven based projects add the following to your POM file in order to use MapStruct:

Add the following to your Gradle build file in order to enable MapStruct:

You can find a complete example in the mapstruct-examples project on GitHub.

Add the javac task configured as follows to your build.xml file in order to enable MapStruct in your Ant-based project. Adjust the paths as required for your project layout.

You can find a complete example in the mapstruct-examples project on GitHub.

The MapStruct code generator can be configured using annotation processor options.

When invoking javac directly, these options are passed to the compiler in the form -Akey=value. When using MapStruct via Maven, any processor options can be passed using an options element within the configuration of the Maven processor plug-in like this:

The following options exist:

MapStruct can be used with Java 9 (JPMS), support for it is experimental.

A core theme of Java 9 is the modularization of the JDK. One effect of this is that a specific module needs to be enabled for a project in order to use the javax.annotation.Generated annotation. @Generated is added by MapStruct to generated mapper classes to tag them as generated code, stating the date of generation, the generator version etc.

To allow usage of the @Generated annotation the module java.xml.ws.annotation must be enabled. When using Maven, this can be done like this:

If the @Generated annotation is not available, MapStruct will detect this situation and not add it to generated mappers.

To create a mapper simply define a Java interface with the required mapping method(s) and annotate it with the org.mapstruct.Mapper annotation:

The @Mapper annotation causes the MapStruct code generator to create an implementation of the CarMapper interface during build-time.

In the generated method implementations all readable properties from the source type (e.g. Car) will be copied into the corresponding property in the target type (e.g. CarDto):

To get a better understanding of what MapStruct does have a look at the following implementation of the carToCarDto() method as generated by MapStruct:

The general philosophy of MapStruct is to generate code which looks as much as possible as if you had written it yourself from hand. In particular this means that the values are copied from source to target by plain getter/setter invocations instead of reflection or similar.

As the example shows the generated code takes into account any name mappings specified via @Mapping. If the type of a mapped attribute is different in source and target entity, MapStruct will either apply an automatic conversion (as e.g. for the price property, see also Implicit type conversions) or optionally invoke / create another mapping method (as e.g. for the driver / engine property, see also Mapping object references). MapStruct will only create a new mapping method if and only if the source and target property are properties of a Bean and they themselves are Beans or simple properties. i.e. they are not Collection or Map type properties.

Collection-typed attributes with the same element type will be copied by creating a new instance of the target collection type containing the elements from the source property. For collection-typed attributes with different element types each element will be mapped individually and added to the target collection (see Mapping collections).

MapStruct takes all public properties of the source and target types into account. This includes properties declared on super-types.

MapStruct supports the use of meta annotations. The @Mapping annotation supports now @Target with ElementType#ANNOTATION_TYPE in addition to ElementType#METHOD. This allows @Mapping to be used on other (user defined) annotations for re-use purposes. For example:

Can be used to characterise an Entity without the need to have a common base type. For instance, ShelveEntity and BoxEntity do not share a common base type in the StorageMapper below.

Still, they do have some properties in common. The @ToEntity assumes both target beans ShelveEntity and BoxEntity have properties: "id", "creationDate" and "name". It furthermore assumes that the source beans ShelveDto and BoxDto always have a property "groupName". This concept is also known as "duck-typing". In other words, if it quacks like duck, walks like a duck its probably a duck.

This feature is still experimental. Error messages are not mature yet: the method on which the problem occurs is displayed, as well as the concerned values in the @Mapping annotation. However, the composition aspect is not visible. The messages are "as if" the @Mapping would be present on the concerned method directly. Therefore, the user should use this feature with care, especially when uncertain when a property is always present.

A more typesafe (but also more verbose) way would be to define base classes / interfaces on the target bean and the source bean and use @InheritConfiguration to achieve the same result (see Mapping configuration inheritance).

In some cases it can be required to manually implement a specific mapping from one type to another which can’t be generated by MapStruct. One way to handle this is to implement the custom method on another class which then is used by mappers generated by MapStruct (see Invoking other mappers).

Alternatively, when using Java 8 or later, you can implement custom methods directly in a mapper interface as default methods. The generated code will invoke the default methods if the argument and return types match.

As an example let’s assume the mapping from Person to PersonDto requires some special logic which can’t be generated by MapStruct. You could then define the mapper from the previous example like this:

The class generated by MapStruct implements the method carToCarDto(). The generated code in carToCarDto() will invoke the manually implemented personToPersonDto() method when mapping the driver attribute.

A mapper could also be defined in the form of an abstract class instead of an interface and implement the custom methods directly in the mapper class. In this case MapStruct will generate an extension of the abstract class with implementations of all abstract methods. An advantage of this approach over declaring default methods is that additional fields could be declared in the mapper class.

The previous example where the mapping from Person to PersonDto requires some special logic could then be defined like this:

MapStruct will generate a sub-class of CarMapper with an implementation of the carToCarDto() method as it is declared abstract. The generated code in carToCarDto() will invoke the manually implemented personToPersonDto() method when mapping the driver attribute.

MapStruct also supports mapping methods with several source parameters. This is useful e.g. in order to combine several entities into one data transfer object. The following shows an example:

The shown mapping method takes two source parameters and returns a combined target object. As with single-parameter mapping methods properties are mapped by name.

In case several source objects define a property with the same name, the source parameter from which to retrieve the property must be specified using the @Mapping annotation as shown for the description property in the example. An error will be raised when such an ambiguity is not resolved. For properties which only exist once in the given source objects it is optional to specify the source parameter’s name as it can be determined automatically.

MapStruct also offers the possibility to directly refer to a source parameter.

In this case the source parameter is directly mapped into the target as the example above demonstrates. The parameter hn, a non bean type (in this case java.lang.Integer) is mapped to houseNumber.

If you don’t want explicitly name all properties from nested source bean, you can use . as target. This will tell MapStruct to map every property from source bean to target object. The following shows an example:

The generated code will map every property from CustomerDto.record to Customer directly, without need to manually name any of them. The same goes for Customer.account.

When there are conflicts, these can be resolved by explicitely defining the mapping. For instance in the example above. name occurs in CustomerDto.record and in CustomerDto.account. The mapping @Mapping( target = "name", source = "record.name" ) resolves this conflict.

This "target this" notation can be very useful when mapping hierarchical objects to flat objects and vice versa (@InheritInverseConfiguration).

In some cases you need mappings which don’t create a new instance of the target type but instead update an existing instance of that type. This sort of mapping can be realized by adding a parameter for the target object and marking this parameter with @MappingTarget. The following shows an example:

The generated code of the updateCarFromDto() method will update the passed Car instance with the properties from the given CarDto object. There may be only one parameter marked as mapping target. Instead of void you may also set the method’s return type to the type of the target parameter, which will cause the generated implementation to update the passed mapping target and return it as well. This allows for fluent invocations of mapping methods.

For CollectionMappingStrategy.ACCESSOR_ONLY Collection- or map-typed properties of the target bean to be updated will be cleared and then populated with the values from the corresponding source collection or map. Otherwise, For CollectionMappingStrategy.ADDER_PREFERRED or CollectionMappingStrategy.TARGET_IMMUTABLE the target will not be cleared and the values will be populated immediately.

MapStruct also supports mappings of public fields that have no getters/setters. MapStruct will use the fields as read/write accessor if it cannot find suitable getter/setter methods for the property.

A field is considered as a read accessor if it is public or public final. If a field is static it is not considered as a read accessor.

A field is considered as a write accessor only if it is public. If a field is final and/or static it is not considered as a write accessor.

Small example:

For the configuration from above, the generated mapper looks like:

You can find the complete example in the mapstruct-examples-field-mapping project on GitHub.

MapStruct also supports mapping of immutable types via builders. When performing a mapping MapStruct checks if there is a builder for the type being mapped. This is done via the BuilderProvider SPI. If a Builder exists for a certain type, then that builder will be used for the mappings.

The default implementation of the BuilderProvider assumes the following:

If such type is found then MapStruct will use that type to perform the mapping to (i.e. it will look for setters into that type). To finish the mapping MapStruct generates code that will invoke the build method of the builder.

Supported builder frameworks:

MapStruct supports using constructors for mapping target types. When doing a mapping MapStruct checks if there is a builder for the type being mapped. If there is no builder, then MapStruct looks for a single accessible constructor. When there are multiple constructors then the following is done to pick the one which should be used:

When using a constructor then the names of the parameters of the constructor will be used and matched to the target properties. When the constructor has an annotation named @ConstructorProperties (from any package, see Non-shipped annotations) then this annotation will be used to get the names of the parameters.

When not using a DI framework, Mapper instances can be retrieved via the org.mapstruct.factory.Mappers class. Just invoke the getMapper() method, passing the interface type of the mapper to return:

By convention, a mapper interface should define a member called INSTANCE which holds a single instance of the mapper type:

This pattern makes it very easy for clients to use mapper objects without repeatedly instantiating new instances:

Note that mappers generated by MapStruct are stateless and thread-safe and thus can safely be accessed from several threads at the same time.

If you’re working with a dependency injection framework such as CDI (Contexts and Dependency Injection for Java^TM EE) or the Spring Framework, it is recommended to obtain mapper objects via dependency injection and not via the Mappers class as described above. For that purpose you can specify the component model which generated mapper classes should be based on either via @Mapper#componentModel or using a processor option as described in Configuration options.

Currently there is support for CDI and Spring (the latter either via its custom annotations or using the JSR 330 annotations). See Configuration options for the allowed values of the componentModel attribute which are the same as for the mapstruct.defaultComponentModel processor option. In both cases the required annotations will be added to the generated mapper implementations classes in order to make the same subject to dependency injection. The following shows an example using CDI:

The generated mapper implementation will be marked with the @ApplicationScoped annotation and thus can be injected into fields, constructor arguments etc. using the @Inject annotation:

A mapper which uses other mapper classes (see Invoking other mappers) will obtain these mappers using the configured component model. So if CarMapper from the previous example was using another mapper, this other mapper would have to be an injectable CDI bean as well.

When using dependency injection, you can choose between field and constructor injection. This can be done by either providing the injection strategy via @Mapper or @MapperConfig annotation.

The generated mapper will inject all classes defined in the uses attribute. When InjectionStrategy#CONSTRUCTOR is used, the constructor will have the appropriate annotation and the fields won’t. When InjectionStrategy#FIELD is used, the annotation is on the field itself. For now, the default injection strategy is field injection, but it can be configured with Configuration options. It is recommended to use constructor injection to simplify testing.

MapStruct takes care of type conversions automatically in many cases. If for instance an attribute is of type int in the source bean but of type String in the target bean, the generated code will transparently perform a conversion by calling String#valueOf(int) and Integer#parseInt(String), respectively.

Currently the following conversions are applied automatically:

Typically an object has not only primitive attributes but also references other objects. E.g. the Car class could contain a reference to a Person object (representing the car’s driver) which should be mapped to a PersonDto object referenced by the CarDto class.

In this case just define a mapping method for the referenced object type as well:

The generated code for the carToCarDto() method will invoke the personToPersonDto() method for mapping the driver attribute, while the generated implementation for personToPersonDto() performs the mapping of person objects.

That way it is possible to map arbitrary deep object graphs. When mapping from entities into data transfer objects it is often useful to cut references to other entities at a certain point. To do so, implement a custom mapping method (see the next section) which e.g. maps a referenced entity to its id in the target object.

When generating the implementation of a mapping method, MapStruct will apply the following routine for each attribute pair in the source and target object:

A mapping control (MappingControl) can be defined on all levels (@MapperConfig, @Mapper, @BeanMapping, @Mapping), the latter taking precedence over the former. For example: @Mapper( mappingControl = NoComplexMapping.class ) takes precedence over @MapperConfig( mappingControl = DeepClone.class ). @IterableMapping and @MapMapping work similar as @Mapping. MappingControl is experimental from MapStruct 1.4. MappingControl has an enum that corresponds to the first 4 options above: MappingControl.Use#DIRECT, MappingControl.Use#MAPPING_METHOD, MappingControl.Use#BUILT_IN_CONVERSION and MappingControl.Use#COMPLEX_MAPPING the presence of which allows the user to switch on a option. The absence of an enum switches off a mapping option. Default they are all present enabling all mapping options.

As explained above, MapStruct will generate a method based on the name of the source and target property. Unfortunately, in many occasions these names do not match.

The ‘.’ notation in an @Mapping source or target type can be used to control how properties should be mapped when names do not match. There is an elaborate example in our examples repository to explain how this problem can be overcome.

In the simplest scenario there’s a property on a nested level that needs to be corrected. Take for instance a property fish which has an identical name in FishTankDto and FishTank. For this property MapStruct automatically generates a mapping: FishDto fishToFishDto(Fish fish). MapStruct cannot possibly be aware of the deviating properties kind and type. Therefore this can be addressed in a mapping rule: @Mapping(target="fish.kind", source="fish.type"). This tells MapStruct to deviate from looking for a name kind at this level and map it to type.

The same constructs can be used to ignore certain properties at a nesting level, as is demonstrated in the second @Mapping rule.

MapStruct can even be used to “cherry pick” properties when source and target do not share the same nesting level (the same number of properties). This can be done in the source – and in the target type. This is demonstrated in the next 2 rules: @Mapping(target="ornament", source="interior.ornament") and @Mapping(target="material.materialType", source="material").

The latter can even be done when mappings first share a common base. For example: all properties that share the same name of Quality are mapped to QualityDto. Likewise, all properties of Report are mapped to ReportDto, with one exception: organisation in OrganisationDto is left empty (since there is no organization at the source level). Only the name is populated with the organisationName from Report. This is demonstrated in @Mapping(target="quality.report.organisation.name", source="quality.report.organisationName")

Coming back to the original example: what if kind and type would be beans themselves? In that case MapStruct would again generate a method continuing to map. Such is demonstrated in the next example:

Note what happens in @Mapping(target="quality.document", source="quality.report"). DocumentDto does not exist as such on the target side. It is mapped from Report. MapStruct continues to generate mapping code here. That mapping itself can be guided towards another name. This even works for constants and expression. Which is shown in the final example: @Mapping(target="quality.document.organisation.name", constant="NoIdeaInc").

MapStruct will perform a null check on each nested property in the source.

Sometimes mappings are not straightforward and some fields require custom logic.

The example below demonstrates how the properties length, width and height in FishTank can be mapped to the VolumeDto bean, which is a member of FishTankWithVolumeDto. VolumeDto contains the properties volume and description. Custom logic is achieved by defining a method which takes FishTank instance as a parameter and returns a VolumeDto. MapStruct will take the entire parameter source and generate code to call the custom method mapVolume in order to map the FishTank object to the target property volume.

The remainder of the fields could be mapped the regular way: using mappings defined defined by means of @Mapping annotations.

Note the @Mapping annotation where source field is equal to "source", indicating the parameter name source itself in the method map(FishTank source) instead of a (target) property in FishTank.

In addition to methods defined on the same mapper type MapStruct can also invoke mapping methods defined in other classes, be it mappers generated by MapStruct or hand-written mapping methods. This can be useful to structure your mapping code in several classes (e.g. with one mapper type per application module) or if you want to provide custom mapping logic which can’t be generated by MapStruct.

For instance the Car class might contain an attribute manufacturingDate while the corresponding DTO attribute is of type String. In order to map this attribute, you could implement a mapper class like this:

In the @Mapper annotation at the CarMapper interface reference the DateMapper class like this:

When generating code for the implementation of the carToCarDto() method, MapStruct will look for a method which maps a Date object into a String, find it on the DateMapper class and generate an invocation of asString() for mapping the manufacturingDate attribute.

Generated mappers retrieve referenced mappers using the component model configured for them. If e.g. CDI was used as component model for CarMapper, DateMapper would have to be a CDI bean as well. When using the default component model, any hand-written mapper classes to be referenced by MapStruct generated mappers must declare a public no-args constructor in order to be instantiable.

When having a custom mapper hooked into the generated mapper with @Mapper#uses(), an additional parameter of type Class (or a super-type of it) can be defined in the custom mapping method in order to perform general mapping tasks for specific target object types. That attribute must be annotated with @TargetType for MapStruct to generate calls that pass the Class instance representing the corresponding property type of the target bean.

For instance, the CarDto could have a property owner of type Reference that contains the primary key of a Person entity. You could now create a generic custom mapper that resolves any Reference objects to their corresponding managed JPA entity instances.

MapStruct will then generate something like this:

Additional context or state information can be passed through generated mapping methods to custom methods with @Context parameters. Such parameters are passed to other mapping methods, @ObjectFactory methods (see Object factories) or @BeforeMapping / @AfterMapping methods (see Mapping customization with before-mapping and after-mapping methods) when applicable and can thus be used in custom code.

@Context parameters are searched for @ObjectFactory methods, which are called on the provided context parameter value if applicable.

@Context parameters are also searched for @BeforeMapping / @AfterMapping methods, which are called on the provided context parameter value if applicable.

Note: no null checks are performed before calling before/after mapping methods on context parameters. The caller needs to make sure that null is not passed in that case.

For generated code to call a method that is declared with @Context parameters, the declaration of the mapping method being generated needs to contain at least those (or assignable) @Context parameters as well. The generated code will not create new instances of missing @Context parameters nor will it pass a literal null instead.

MapStruct will then generate something like this:

When mapping a property from one type to another, MapStruct looks for the most specific method which maps the source type into the target type. The method may either be declared on the same mapper interface or on another mapper which is registered via @Mapper#uses(). The same applies for factory methods (see Object factories).

The algorithm for finding a mapping or factory method resembles Java’s method resolution algorithm as much as possible. In particular, methods with a more specific source type will take precedence (e.g. if there are two methods, one which maps the searched source type, and another one which maps a super-type of the same). In case more than one most-specific method is found, an error will be raised.

In many occasions one requires mapping methods with the same method signature (apart from the name) that have different behavior. MapStruct has a handy mechanism to deal with such situations: @Qualifier (org.mapstruct.Qualifier). A ‘qualifier’ is a custom annotation that the user can write, ‘stick onto’ a mapping method which is included as used mapper and can be referred to in a bean property mapping, iterable mapping or map mapping. Multiple qualifiers can be ‘stuck onto’ a method and mapping.

So, let’s say there is a hand-written method to map titles with a String return type and String argument amongst many other referenced mappers with the same String return type - String argument signature:

And a mapper using this handwritten mapper, in which source and target have a property 'title' that should be mapped:

Without the use of qualifiers, this would result in an ambiguous mapping method error, because 2 qualifying methods are found (translateTitleEG, translateTitleGE) and MapStruct would not have a hint which one to choose.

Enter the qualifier approach:

And, some qualifiers to indicate which translator to use to map from source language to target language:

Please take note of the target TitleTranslator on type level, EnglishToGerman, GermanToEnglish on method level!

Then, using the qualifiers, the mapping could look like this:

In many occasions, declaring a new annotation to aid the selection process can be too much for what you try to achieve. For those situations, MapStruct has the @Named annotation. This annotation is a pre-defined qualifier (annotated with @Qualifier itself) and can be used to name a Mapper or, more directly a mapping method by means of its value. The same example above would look like:

Also map-based mapping methods are supported. The following shows an example:

Similar to iterable mappings, the generated code will iterate through the source map, convert each value and key (either by means of an implicit conversion or by invoking another mapping method) and put them into the target map:

MapStruct has a CollectionMappingStrategy, with the possible values: ACCESSOR_ONLY, SETTER_PREFERRED, ADDER_PREFERRED and TARGET_IMMUTABLE.

In the table below, the dash - indicates a property name. Next, the trailing s indicates the plural form. The table explains the options and how they are applied to the presence/absence of a set-s, add- and / or get-s method on the target object:

Some background: An adder method is typically used in case of generated (JPA) entities, to add a single element (entity) to an underlying collection. Invoking the adder establishes a parent-child relation between parent - the bean (entity) on which the adder is invoked - and its child(ren), the elements (entities) in the collection. To find the appropriate adder, MapStruct will try to make a match between the generic parameter type of the underlying collection and the single argument of a candidate adder. When there are more candidates, the plural setter / getter name is converted to singular and will be used in addition to make a match.

The option DEFAULT should not be used explicitly. It is used to distinguish between an explicit user desire to override the default in a @MapperConfig from the implicit Mapstruct choice in a @Mapper. The option DEFAULT is synonymous to ACCESSOR_ONLY.

When an iterable or map mapping method declares an interface type as return type, one of its implementation types will be instantiated in the generated code. The following table shows the supported interface types and their corresponding implementation types as instantiated in the generated code:

MapStruct supports the generation of methods which map one Java enum type into another.

By default, each constant from the source enum is mapped to a constant with the same name in the target enum type. If required, a constant from the source enum may be mapped to a constant with another name with help of the @ValueMapping annotation. Several constants from the source enum can be mapped to the same constant in the target type.

The following shows an example:

By default an error will be raised by MapStruct in case a constant of the source enum type does not have a corresponding constant with the same name in the target type and also is not mapped to another constant via @ValueMapping. This ensures that all constants are mapped in a safe and predictable manner. The generated mapping method will throw an IllegalStateException if for some reason an unrecognized source value occurs.

MapStruct also has a mechanism for mapping any remaining (unspecified) mappings to a default. This can be used only once in a set of value mappings and only applies to the source. It comes in two flavors: <ANY_REMAINING> and <ANY_UNMAPPED>. They cannot be used at the same time.

In case of source <ANY_REMAINING> MapStruct will continue to map a source enum constant to a target enum constant with the same name. The remainder of the source enum constants will be mapped to the target specified in the @ValueMapping with <ANY_REMAINING> source.

MapStruct will not attempt such name based mapping for <ANY_UNMAPPED> and directly apply the target specified in the @ValueMapping with <ANY_UNMAPPED> source to the remainder.

MapStruct is able to handle null sources and null targets by means of the <NULL> keyword.

Finally @InheritInverseConfiguration and @InheritConfiguration can be used in combination with @ValueMappings. <ANY_REMAINING> and <ANY_UNMAPPED> will be ignored in that case.

Note: MapStruct would have refrained from mapping the RETAIL and B2B when <ANY_UNMAPPED> was used instead of <ANY_REMAINING>.

MapStruct supports enum to a String mapping along the same lines as is described in enum-to-enum types. There are similarities and differences:

enum to String

String to enum

When no @ValueMapping(s) are defined then each constant from the source enum is mapped to a constant with the same name in the target enum type. However, there are cases where the source enum needs to be transformed before doing the mapping. E.g. a suffix needs to be applied to map from the source into the target enum.

MapStruct provides the following out of the box enum name transformation strategies:

It is also possible to register custom strategies. For more information on how to do that have a look at Custom Enum Transformation Strategy

Default values can be specified to set a predefined value to a target property if the corresponding source property is null. Constants can be specified to set such a predefined value in any case. Default values and constants are specified as String values. When the target type is a primitive or a boxed type, the String value is taken literal. Bit / octal / decimal / hex patterns are allowed in such a case as long as they are a valid literal. In all other cases, constant or default values are subject to type conversion either via built-in conversions or the invocation of other mapping methods in order to match the type required by the target property.

A mapping with a constant must not include a reference to a source property. The following example shows some mappings using default values and constants:

If s.getStringProp() == null, then the target property stringProperty will be set to "undefined" instead of applying the value from s.getStringProp(). If s.getLongProperty() == null, then the target property longProperty will be set to -1. The String "Constant Value" is set as is to the target property stringConstant. The value "3001" is type-converted to the Long (wrapper) class of target property longWrapperConstant. Date properties also require a date format. The constant "jack-jill-tom" demonstrates how the hand-written class StringListMapper is invoked to map the dash-separated list into a List<String>.

By means of Expressions it will be possible to include constructs from a number of languages.

Currently only Java is supported as a language. This feature is e.g. useful to invoke constructors. The entire source object is available for usage in the expression. Care should be taken to insert only valid Java code: MapStruct will not validate the expression at generation-time, but errors will show up in the generated classes during compilation.

The example below demonstrates how two source properties can be mapped to one target:

The example demonstrates how the source properties time and format are composed into one target property TimeAndFormat. Please note that the fully qualified package name is specified because MapStruct does not take care of the import of the TimeAndFormat class (unless it’s used otherwise explicitly in the SourceTargetMapper). This can be resolved by defining imports on the @Mapper annotation.

Default expressions are a combination of default values and expressions. They will only be used when the source attribute is null.

The same warnings and restrictions apply to default expressions that apply to expressions. Only Java is supported, and MapStruct will not validate the expression at generation-time.

The example below demonstrates how two source properties can be mapped to one target:

The example demonstrates how to use defaultExpression to set an ID field if the source field is null, this could be used to take the existing sourceId from the source object if it is set, or create a new Id if it isn’t. Please note that the fully qualified package name is specified because MapStruct does not take care of the import of the UUID class (unless it’s used otherwise explicitly in the SourceTargetMapper). This can be resolved by defining imports on the @Mapper annotation (see Expressions).

When result types have an inheritance relation, selecting either mapping method (@Mapping) or a factory method (@BeanMapping) can become ambiguous. Suppose an Apple and a Banana, which are both specializations of Fruit.

So, which Fruit must be factorized in the mapping method Fruit map(FruitDto source);? A Banana or an Apple? Here’s where the @BeanMapping#resultType comes in handy. It controls the factory method to select, or in absence of a factory method, the return type to create.

MapStruct offers control over the object to create when the source argument of the mapping method equals null. By default null will be returned.

However, by specifying nullValueMappingStrategy = NullValueMappingStrategy.RETURN_DEFAULT on @BeanMapping, @IterableMapping, @MapMapping, or globally on @Mapper or @MapperConfig, the mapping result can be altered to return empty default values. This means for:

The strategy works in a hierarchical fashion. Setting nullValueMappingStrategy on mapping method level will override @Mapper#nullValueMappingStrategy, and @Mapper#nullValueMappingStrategy will override @MapperConfig#nullValueMappingStrategy.

MapStruct offers control over the property to set in an @MappingTarget annotated target bean when the source property equals null or the presence check method results in 'absent'.

By default the target property will be set to null.

However:

The strategy works in a hierarchical fashion. Setting nullValuePropertyMappingStrategy on mapping method level will override @Mapper#nullValuePropertyMappingStrategy, and @Mapper#nullValuePropertyMappingStrategy will override @MapperConfig#nullValuePropertyMappingStrategy.

MapStruct offers control over when to generate a null check. By default (nullValueCheckStrategy = NullValueCheckStrategy.ON_IMPLICIT_CONVERSION) a null check will be generated for:

First calling a mapping method on the source property is not protected by a null check. Therefore generated mapping methods will do a null check prior to carrying out mapping on a source property. Handwritten mapping methods must take care of null value checking. They have the possibility to add 'meaning' to null. For instance: mapping null to a default value.

The option nullValueCheckStrategy = NullValueCheckStrategy.ALWAYS will always include a null check when source is non primitive, unless a source presence checker is defined on the source bean.

The strategy works in a hierarchical fashion. @Mapping#nullValueCheckStrategy will override @BeanMapping#nullValueCheckStrategy, @BeanMapping#nullValueCheckStrategy will override @Mapper#nullValueCheckStrategy and @Mapper#nullValueCheckStrategy will override @MaperConfig#nullValueCheckStrategy.

Some frameworks generate bean properties that have a source presence checker. Often this is in the form of a method hasXYZ, XYZ being a property on the source bean in a bean mapping method. MapStruct will call this hasXYZ instead of performing a null check when it finds such hasXYZ method.

Calling applications may require handling of exceptions when calling a mapping method. These exceptions could be thrown by hand-written logic and by the generated built-in mapping methods or type-conversions of MapStruct. When the calling application requires handling of exceptions, a throws clause can be defined in the mapping method:

The hand written logic might look like this:

MapStruct now, wraps the FatalException in a try-catch block and rethrows an unchecked RuntimeException. MapStruct delegates handling of the GearException to the application logic because it is defined as throws clause in the carToCarDto method:

Some notes on null checks. MapStruct does provide null checking only when required: when applying type-conversions or constructing a new type by invoking its constructor. This means that the user is responsible in hand-written code for returning valid non-null objects. Also null objects can be handed to hand-written code, since MapStruct does not want to make assumptions on the meaning assigned by the user to a null object. Hand-written code has to deal with this.

Method-level configuration annotations such as @Mapping, @BeanMapping, @IterableMapping, etc., can be inherited from one mapping method to a similar method using the annotation @InheritConfiguration:

The example above declares a mapping method carDtoToCar() with a configuration to define how the property numberOfSeats in the type Car shall be mapped. The update method that performs the mapping on an existing instance of Car needs the same configuration to successfully map all properties. Declaring @InheritConfiguration on the method lets MapStruct search for inheritance candidates to apply the annotations of the method that is inherited from.

One method A can inherit the configuration from another method B if all types of A (source types and result type) are assignable to the corresponding types of B.

Methods that are considered for inheritance need to be defined in the current mapper, a super class/interface, or in the shared configuration interface (as described in Shared configurations).

In case more than one method is applicable as source for the inheritance, the method name must be specified within the annotation: @InheritConfiguration( name = "carDtoToCar" ).

A method can use @InheritConfiguration and override or amend the configuration by additionally applying @Mapping, @BeanMapping, etc.

In case of bi-directional mappings, e.g. from entity to DTO and from DTO to entity, the mapping rules for the forward method and the reverse method are often similar and can simply be inversed by switching source and target.

Use the annotation @InheritInverseConfiguration to indicate that a method shall inherit the inverse configuration of the corresponding reverse method.

Here the carDtoToCar() method is the reverse mapping method for carToDto(). Note that any attribute mappings from carToDto() will be applied to the corresponding reverse mapping method as well. They are automatically reversed and copied to the method with the @InheritInverseConfiguration annotation.

Specific mappings from the inversed method can (optionally) be overridden by ignore, expression or constant in the mapping, e.g. like this: @Mapping(target = "numberOfSeats", ignore=true).

A method A is considered a reverse method of a method B, if the result type of A is the same as the single source type of B and if the single source type of A is the same as the result type of B.

Methods that are considered for inverse inheritance need to be defined in the current mapper, a super class/interface.

If multiple methods qualify, the method from which to inherit the configuration needs to be specified using the name property like this: @InheritInverseConfiguration(name = "carToDto").

@InheritConfiguration takes, in case of conflict precedence over @InheritInverseConfiguration.

Configurations are inherited transitively. So if method C defines a mapping @Mapping( target = "x", ignore = true), B defines a mapping @Mapping( target = "y", ignore = true), then if A inherits from B inherits from C, A will inherit mappings for both property x and y.

@Mapping#expression, @Mapping#defaultExpression, @Mapping#defaultValue and @Mapping#constant are excluded (silently ignored) in @InheritInverseConfiguration.

@Mapping#ignore is only applied when @Mapping#source is also present in @InheritInverseConfiguration.

Reverse mapping of nested source properties is experimental as of the 1.1.0.Beta2 release. Reverse mapping will take place automatically when the source property name and target property name are identical. Otherwise, @Mapping should specify both the target name and source name. In all cases, a suitable mapping method needs to be in place for the reverse mapping.

MapStruct offers the possibility to define a shared configuration by pointing to a central interface annotated with @MapperConfig. For a mapper to use the shared configuration, the configuration interface needs to be defined in the @Mapper#config property.

The @MapperConfig annotation has the same attributes as the @Mapper annotation. Any attributes not given via @Mapper will be inherited from the shared configuration. Attributes specified in @Mapper take precedence over the attributes specified via the referenced configuration class. List properties such as uses are simply combined:

The interface holding the @MapperConfig annotation may also declare prototypes of mapping methods that can be used to inherit method-level mapping annotations from. Such prototype methods are not meant to be implemented or used as part of the mapper API.

The attributes @Mapper#mappingInheritanceStrategy() / @MapperConfig#mappingInheritanceStrategy() configure when the method-level mapping configuration annotations are inherited from prototype methods in the interface to methods in the mapper:

In certain cases it may be required to customize a generated mapping method, e.g. to set an additional property in the target object which can’t be set by a generated method implementation. MapStruct supports this requirement using decorators.

To apply a decorator to a mapper class, specify it using the @DecoratedWith annotation.

The decorator must be a sub-type of the decorated mapper type. You can make it an abstract class which allows to only implement those methods of the mapper interface which you want to customize. For all non-implemented methods, a simple delegation to the original mapper will be generated using the default generation routine.

The PersonMapperDecorator shown below customizes the personToPersonDto(). It sets an additional attribute which is not present in the source type of the mapping. The addressToAddressDto() method is not customized.

The example shows how you can optionally inject a delegate with the generated default implementation and use this delegate in your customized decorator methods.

For a mapper with componentModel = "default", define a constructor with a single parameter which accepts the type of the decorated mapper.

When working with the component models spring or jsr330, this needs to be handled differently.

Decorators may not always fit the needs when it comes to customizing mappers. For example, if you need to perform the customization not only for a few selected methods, but for all methods that map specific super-types: in that case, you can use callback methods that are invoked before the mapping starts or after the mapping finished.

Callback methods can be implemented in the abstract mapper itself, in a type reference in Mapper#uses, or in a type used as @Context parameter.

If the @BeforeMapping / @AfterMapping method has parameters, the method invocation is only generated if the return type of the method (if non-void) is assignable to the return type of the mapping method and all parameters can be assigned by the source or target parameters of the mapping method:

For non-void methods, the return value of the method invocation is returned as the result of the mapping method if it is not null.

As with mapping methods, it is possible to specify type parameters for before/after-mapping methods.

All before/after-mapping methods that can be applied to a mapping method will be used. Mapping method selection based on qualifiers can be used to further control which methods may be chosen and which not. For that, the qualifier annotation needs to be applied to the before/after-method and referenced in BeanMapping#qualifiedBy or IterableMapping#qualifiedBy.

The order of the method invocation is determined primarily by their variant:

Within those groups, the method invocations are ordered by their location of definition:

Important: the order of methods declared within one type can not be guaranteed, as it depends on the compiler and the processing environment implementation.

Important: when using a builder, the @AfterMapping annotated method must have the builder as @MappingTarget annotated parameter so that the method is able to modify the object going to be build. The build method is called when the @AfterMapping annotated method scope finishes. MapStruct will not call the @AfterMapping annotated method if the real target is used as @MappingTarget annotated parameter.

MapStruct offers the possibility to override the AccessorNamingStrategy via the Service Provider Interface (SPI). A nice example is the use of the fluent API on the source object GolfPlayer and GolfPlayerDto below.

We want GolfPlayer to be mapped to a target object GolfPlayerDto similar like we 'always' do this:

This can be achieved with implementing the SPI org.mapstruct.ap.spi.AccessorNamingStrategy as in the following example. Here’s an implemented org.mapstruct.ap.spi.AccessorNamingStrategy:

The CustomAccessorNamingStrategy makes use of the DefaultAccessorNamingStrategy (also available in mapstruct-processor) and relies on that class to leave most of the default behaviour unchanged.

To use a custom SPI implementation, it must be located in a separate JAR file together with the file META-INF/services/org.mapstruct.ap.spi.AccessorNamingStrategy with the fully qualified name of your custom implementation as content (e.g. org.mapstruct.example.CustomAccessorNamingStrategy). This JAR file needs to be added to the annotation processor classpath (i.e. add it next to the place where you added the mapstruct-processor jar).

MapStruct offers the possibility to override the MappingExclusionProvider via the Service Provider Interface (SPI). A nice example is to not allow MapStruct to create an automatic sub-mapping for a certain type, i.e. MapStruct will not try to generate an automatic sub-mapping method for an excluded type.

We want to exclude the NestedTarget from the automatic sub-mapping method generation.

To use a custom SPI implementation, it must be located in a separate JAR file together with the file META-INF/services/org.mapstruct.ap.spi.MappingExclusionProvider with the fully qualified name of your custom implementation as content (e.g. org.mapstruct.example.CustomMappingExclusionProvider). This JAR file needs to be added to the annotation processor classpath (i.e. add it next to the place where you added the mapstruct-processor jar).

MapStruct offers the possibility to override the DefaultProvider via the Service Provider Interface (SPI). A nice example is to provide support for a custom builder strategy.

MapStruct offers the possibility to override the EnumMappingStrategy via the Service Provider Interface (SPI). This can be used when you have certain enums that follow some conventions within your organization. For example all enums which implement an interface named CustomEnumMarker are prefixed with CUSTOM_ and the default value for them when mapping from null is UNSPECIFIED

We want CheeseType and CustomCheeseType to be mapped without the need to manually define the value mappings:

This can be achieved with implementing the SPI org.mapstruct.ap.spi.EnumMappingStrategy as in the following example. Here’s an implemented org.mapstruct.ap.spi.EnumMappingStrategy:

The generated code then for the CheeseMapper looks like:

MapStruct offers the possibility to other transformations strategies by implementing EnumTransformationStrategy via the Service Provider Interface (SPI). A nice example is to provide support for a custom transformation strategy.

There are various use-cases you must resolve ambiguity for MapStruct to use a correct piece of code. However, the primary goal of MapStruct is to focus on bean mapping without polluting the entity code. For that reason, MapStruct is flexible enough to interact with already defined annotations from third-party libraries. The requirement to enable this behavior is to match the name of such annotation. Hence, we say that annotation can be from any package.

The annotations named @ConstructorProperties and @Default are currently examples of this kind of annotation.

A very common case is that no third-party dependency imported to your project provides such annotation or is inappropriate for use as already described. In such cases create your own annotation, for example:

MapStruct works together with Project Lombok as of MapStruct 1.2.0.Beta1 and Lombok 1.16.14.

MapStruct takes advantage of generated getters, setters, and constructors and uses them to generate the mapper implementations.