MapStruct for automatic mapping in Spring Boot - Step-by-Step Execution

Concept Flow - MapStruct for automatic mapping

Define Source Class

↓

Define Target Class

↓

Create Mapper Interface

↓

MapStruct Generates Implementation

↓

Use Mapper to Convert Objects

↓

Get Mapped Target Object

MapStruct automatically creates code to convert one object type to another using a mapper interface.

Execution Sample

Spring Boot

public interface CarMapper {
    CarDto carToCarDto(Car car);
}

Defines a mapper interface method to convert a Car object to a CarDto object.

Execution Table

Step	Action	Input Object	Mapper Method Called	Output Object
1	Create Car object	Car{make='Toyota', year=2020}	N/A	N/A
2	Call mapper method carToCarDto	Car{make='Toyota', year=2020}	carToCarDto	N/A
3	MapStruct generates CarDto	N/A	carToCarDto	CarDto{make='Toyota', year=2020}
4	Return mapped CarDto	N/A	carToCarDto	CarDto{make='Toyota', year=2020}
5	Use CarDto object	N/A	N/A	CarDto{make='Toyota', year=2020}
6	End of mapping process	N/A	N/A	N/A

💡 Mapping ends after CarDto object is returned from mapper method.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
car	null	Car{make='Toyota', year=2020}	Car{make='Toyota', year=2020}	Car{make='Toyota', year=2020}	Car{make='Toyota', year=2020}
carDto	null	null	CarDto{make='Toyota', year=2020}	CarDto{make='Toyota', year=2020}	CarDto{make='Toyota', year=2020}

Key Moments - 3 Insights

Why don't we see the implementation code of the mapper method?

MapStruct generates the implementation automatically at compile time, so you only write the interface method (see execution_table step 3).

How does MapStruct know which fields to map?

MapStruct matches fields by name and type between source and target classes, so fields with the same name are mapped automatically (refer to variable_tracker showing matching fields).

What happens if source and target fields have different names?

You can configure MapStruct with annotations to map fields with different names, but by default only matching names are mapped (not shown in this simple trace).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of carDto after step 3?

A CarDto{make='Toyota', year=2020}

B null

C Car{make='Toyota', year=2020}

D N/A

💡 Hint

Check the 'Output Object' column at step 3 in the execution_table.

At which step does MapStruct generate the implementation code?

A Step 1

B Step 3

C Step 2

D Step 5

💡 Hint

Look for the step mentioning 'MapStruct generates CarDto' in the execution_table.

If the Car object had a field not present in CarDto, what would happen by default?

A Mapping would fail with error

B MapStruct would map it anyway

C MapStruct would ignore that field

D The field would be set to null in CarDto

💡 Hint

Recall that MapStruct maps matching fields by name; unmatched fields are ignored by default.

Concept Snapshot

MapStruct automates object mapping in Spring Boot.
Define source and target classes.
Create a mapper interface with mapping methods.
MapStruct generates implementation at compile time.
Use mapper methods to convert objects easily.
Matches fields by name automatically.

Full Transcript

MapStruct is a tool used in Spring Boot to automatically convert one object type to another. You start by defining your source and target classes, for example, Car and CarDto. Then you create a mapper interface with a method like carToCarDto. MapStruct generates the actual code behind the scenes during compilation. When you call the mapper method with a Car object, it returns a CarDto with matching fields copied over. This process saves you from writing repetitive conversion code. Fields with the same name and type are mapped automatically. If fields differ, you can configure mappings with annotations. The execution table shows the step-by-step flow from creating the Car object, calling the mapper, MapStruct generating the CarDto, and returning it for use. Variables track how car and carDto change during the process. Common confusions include why you don't see the implementation code (it's generated), how fields are matched (by name), and what happens with unmatched fields (ignored by default). The visual quiz tests understanding of these steps and behaviors.