When is expression used in mapStruct?

Question

I am getting started with MapStruct. I am unable to understand when do we use "expression" tag in MapStruct? Why do we have certain mappings where we use "target" tag and "expression" tag? Does it mean that expressions are used when you want to map two or more fields within a bean to a single property/field in the target as mentioned in the documentation " http://mapstruct.org/documentation/stable/reference/html/#expressions "

Answer 1

In general, MapStruct expressions are used when you simple cannot write a MapStruct mapper. They should be used as a fallback approach when the library doesn't apply to your use-case.

For example, -- as the documentation says -- when a mapping requires more than one source variable, an expression can be used to "inject" them to a mapper method.

Another use case is when the source variable you need to use -- say bar -- is not a part of the source class but a member of one of its variables (here, classVar ). You would map it to the target field foo using a custom myCustomMethod method with @Mapping(target="foo", expression="java(myCustomMethod(source.classVar.bar)))" .

Answer 2

Expressions are used when you can't map a source - to a target property or when a constant does not apply. MapStruct envisioned that several language could be used to address expressions. However, only plain java is implemented (hence "java(... )" ). EL was envisioned but not yet realised.

A typical use case that I use is generating a UUID. But even there you could try the new @Context to achieve that goal.

Remember, the stuff within the brackets is put directly in the generated code. The IDE can't check its correctness, and you will only spot problems during compilation.

Expressions are IMHO a fallback means / gap filler for stuff that is not yet implemented in MapStruct.

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Note: Mapping target-to-source by means of a custom method as suggested in the other answers can be done automatically. MapStruct will recognised the signature (return type, source type) and call your custom method. You can do this in the same interface (default method) or in a used mapper.