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1. Introduction

In a previous tutorial, we've seen how to map lists with ModelMapper.

In this tutorial, we're going to show how to map our data between differently structured objects in ModelMapper.

Although ModelMapper's default conversion works pretty well in typical cases, we'll primarily focus on how to match objects that aren't similar enough to handle using the default configuration.

Hence, we'll set our sights on property mappings and configuration changes this time.

2. Maven Dependency

To start using the ModelMapper library, we'll add the dependency to our pom.xml:

<dependency>
    <groupId>org.modelmapper</groupId>
    <artifactId>modelmapper</artifactId>
    <version>2.4.4</version>
</dependency>

3. Default Configuration

ModelMapper provides a drop-in solution when our source and destination objects are similar to each other.

Let's have a look at Game and GameDTO, our domain object and corresponding data transfer object, respectively:

public class Game {

    private Long id;
    private String name;
    private Long timestamp;

    private Player creator;
    private List<Player> players = new ArrayList<>();

    private GameSettings settings;

    // constructors, getters and setters
}

public class GameDTO {

    private Long id;
    private String name;

    // constructors, getters and setters
}

GameDTO contains only two fields, but the field types and names perfectly match the source.

In such a case, ModelMapper handles the conversion without additional configuration:

@BeforeEach
public void setup() {
    this.mapper = new ModelMapper();
}

@Test
public void whenMapGameWithExactMatch_thenConvertsToDTO() {
    // when similar source object is provided
    Game game = new Game(1L, "Game 1");
    GameDTO gameDTO = this.mapper.map(game, GameDTO.class);
    
    // then it maps by default
    assertEquals(game.getId(), gameDTO.getId());
    assertEquals(game.getName(), gameDTO.getName());
}

4. What Is Property Mapping in ModelMapper

In our projects, most of the time, we need to customize our DTOs. Of course, this will result in different fields, hierarchies, and their irregular mappings to each other. Sometimes, we also need more than one DTO for a single source and vice versa.

Therefore, property mapping gives us a powerful way to extend our mapping logic.

Let's customize our GameDTO by adding a new field, creationTime:

public class GameDTO {

    private Long id;
    private String name;
    private Long creationTime;

    // constructors, getters and setters
}

And, we'll map Game‘s timestamp field into GameDTO‘s creationTime field. As we notice, the source field name is different from the destination field name this time.

To define property mappings, we'll use ModelMapper's TypeMap. So, let's create a TypeMap object and add a property mapping via its addMapping method:

@Test
public void whenMapGameWithBasicPropertyMapping_thenConvertsToDTO() {
    // setup
    TypeMap<Game, GameDTO> propertyMapper = this.mapper.createTypeMap(Game.class, GameDTO.class);
    propertyMapper.addMapping(Game::getTimestamp, GameDTO::setCreationTime);
    
    // when field names are different
    Game game = new Game(1L, "Game 1");
    game.setTimestamp(Instant.now().getEpochSecond());
    GameDTO gameDTO = this.mapper.map(game, GameDTO.class);
    
    // then it maps via property mapper
    assertEquals(game.getId(), gameDTO.getId());
    assertEquals(game.getName(), gameDTO.getName());
    assertEquals(game.getTimestamp(), gameDTO.getCreationTime());
}

4.1. Deep Mappings

There are also different ways of mapping. For instance, ModelMapper can map hierarchies — fields at different levels can be mapped deeply.

Let's define a String field named creator in GameDTO. However, the source creator field on the Game domain isn't a simple type but an object — Player:

public class Player {

    private Long id;
    private String name;
    
    // constructors, getters and setters
}

public class Game {
    // ...
    
    private Player creator;
    
    // ...
}

public class GameDTO {
    // ...
    
    private String creator;
    
    // ...
}

Thus, we won't transfer the entire Player object's data, but only the name field, to GameDTO. In order to define the deep mapping, we use TypeMap‘s addMappings method and add an ExpressionMap:

@Test
public void whenMapGameWithDeepMapping_thenConvertsToDTO() {
    // setup
    TypeMap<Game, GameDTO> propertyMapper = this.mapper.createTypeMap(Game.class, GameDTO.class);
    // add deep mapping to flatten source's Player object into a single field in destination
    propertyMapper.addMappings(
      mapper -> mapper.map(src -> src.getCreator().getName(), GameDTO::setCreator)
    );
    
    // when map between different hierarchies
    Game game = new Game(1L, "Game 1");
    game.setCreator(new Player(1L, "John"));
    GameDTO gameDTO = this.mapper.map(game, GameDTO.class);
    
    // then
    assertEquals(game.getCreator().getName(), gameDTO.getCreator());
}

4.2. Skipping Properties

Sometimes, we don't want to expose all the data in our DTOs. Whether to keep our DTOs lighter or conceal some sensible data, those reasons can cause us to exclude some fields when we're transferring to DTOs.

Luckily, ModelMapper supports property exclusion via skipping.

Let's exclude the id field from transferring with the help of the skip method:

@Test
public void whenMapGameWithSkipIdProperty_thenConvertsToDTO() {
    // setup
    TypeMap<Game, GameDTO> propertyMapper = this.mapper.createTypeMap(Game.class, GameDTO.class);
    propertyMapper.addMappings(mapper -> mapper.skip(GameDTO::setId));
    
    // when id is skipped
    Game game = new Game(1L, "Game 1");
    GameDTO gameDTO = this.mapper.map(game, GameDTO.class);
    
    // then destination id is null
    assertNull(gameDTO.getId());
    assertEquals(game.getName(), gameDTO.getName());
}

Therefore, the id field of GameDTO is skipped and not set.

4.3. Converter

Another provision of ModelMapper is Converter. We can customize conversions for specific source to destination mappings.

Suppose we have a collection of Players in the Game domain. Let's transfer the count of Players to GameDTO.

As a first step, we define an integer field, totalPlayers, in GameDTO:

public class GameDTO {
    // ...

    private int totalPlayers;
  
    // constructors, getters and setters
}

Respectively, we create the collectionToSize Converter:

Converter<Collection, Integer> collectionToSize = c -> c.getSource().size();

Finally, we register our Converter via the using method while we're adding our ExpressionMap:

propertyMapper.addMappings(
  mapper -> mapper.using(collectionToSize).map(Game::getPlayers, GameDTO::setTotalPlayers)
);

As a result, we map Game‘s getPlayers().size() to GameDTO‘s totalPlayers field:

@Test
public void whenMapGameWithCustomConverter_thenConvertsToDTO() {
    // setup
    TypeMap<Game, GameDTO> propertyMapper = this.mapper.createTypeMap(Game.class, GameDTO.class);
    Converter<Collection, Integer> collectionToSize = c -> c.getSource().size();
    propertyMapper.addMappings(
      mapper -> mapper.using(collectionToSize).map(Game::getPlayers, GameDTO::setTotalPlayers)
    );
    
    // when collection to size converter is provided
    Game game = new Game();
    game.addPlayer(new Player(1L, "John"));
    game.addPlayer(new Player(2L, "Bob"));
    GameDTO gameDTO = this.mapper.map(game, GameDTO.class);
    
    // then it maps the size to a custom field
    assertEquals(2, gameDTO.getTotalPlayers());
}

4.4. Provider

In another use case, we sometimes need to provide an instance for the destination object instead of letting ModalMapper initialize it. This is where the Provider comes in handy.

Accordingly, ModelMapper's Provider is the built-in way to customize the instantiation of destination objects.

Let's make a conversion, not Game to DTO, but Game to Game this time.

So, in principle, we have a persisted Game domain, and we fetch it from its repository. After that, we update the Game instance by merging another Game object into it:

@Test
public void whenUsingProvider_thenMergesGameInstances() {
    // setup
    TypeMap<Game, Game> propertyMapper = this.mapper.createTypeMap(Game.class, Game.class);
    // a provider to fetch a Game instance from a repository
    Provider<Game> gameProvider = p -> this.gameRepository.findById(1L);
    propertyMapper.setProvider(gameProvider);
    
    // when a state for update is given
    Game update = new Game(1L, "Game Updated!");
    update.setCreator(new Player(1L, "John"));
    Game updatedGame = this.mapper.map(update, Game.class);
    
    // then it merges the updates over on the provided instance
    assertEquals(1L, updatedGame.getId().longValue());
    assertEquals("Game Updated!", updatedGame.getName());
    assertEquals("John", updatedGame.getCreator().getName());
}

4.5. Conditional Mapping

ModelMapper also supports conditional mapping. One of its built-in conditional methods we can use is Conditions.isNull().

Let's skip the id field in case it's null in our source Game object:

@Test
public void whenUsingConditionalIsNull_thenMergesGameInstancesWithoutOverridingId() {
    // setup
    TypeMap<Game, Game> propertyMapper = this.mapper.createTypeMap(Game.class, Game.class);
    propertyMapper.setProvider(p -> this.gameRepository.findById(2L));
    propertyMapper.addMappings(mapper -> mapper.when(Conditions.isNull()).skip(Game::getId, Game::setId));
    
    // when game has no id
    Game update = new Game(null, "Not Persisted Game!");
    Game updatedGame = this.mapper.map(update, Game.class);
    
    // then destination game id is not overwritten
    assertEquals(2L, updatedGame.getId().longValue());
    assertEquals("Not Persisted Game!", updatedGame.getName());
}

As we notice, by using the isNull conditional combined with the skip method, we guarded our destination id against overwriting with a null value.

Moreover, we can also define custom Conditions. Let's define a condition to check if the Game‘s timestamp field has a value:

Condition<Long, Long> hasTimestamp = ctx -> ctx.getSource() != null && ctx.getSource() > 0;

Next, we use it in our property mapper with when method:

TypeMap<Game, GameDTO> propertyMapper = this.mapper.createTypeMap(Game.class, GameDTO.class);
Condition<Long, Long> hasTimestamp = ctx -> ctx.getSource() != null && ctx.getSource() > 0;
propertyMapper.addMappings(
  mapper -> mapper.when(hasTimestamp).map(Game::getTimestamp, GameDTO::setCreationTime)
);

Finally, ModelMapper only updates the GameDTO's creationTime field if the timestamp has a value greater than zero:

@Test
public void whenUsingCustomConditional_thenConvertsDTOSkipsZeroTimestamp() {
    // setup
    TypeMap<Game, GameDTO> propertyMapper = this.mapper.createTypeMap(Game.class, GameDTO.class);
    Condition<Long, Long> hasTimestamp = ctx -> ctx.getSource() != null && ctx.getSource() > 0;
    propertyMapper.addMappings(
      mapper -> mapper.when(hasTimestamp).map(Game::getTimestamp, GameDTO::setCreationTime)
    );
    
    // when game has zero timestamp
    Game game = new Game(1L, "Game 1");
    game.setTimestamp(0L);
    GameDTO gameDTO = this.mapper.map(game, GameDTO.class);
    
    // then timestamp field is not mapped
    assertEquals(game.getId(), gameDTO.getId());
    assertEquals(game.getName(), gameDTO.getName());
    assertNotEquals(0L ,gameDTO.getCreationTime());
    
    // when game has timestamp greater than zero
    game.setTimestamp(Instant.now().getEpochSecond());
    gameDTO = this.mapper.map(game, GameDTO.class);
    
    // then timestamp field is mapped
    assertEquals(game.getId(), gameDTO.getId());
    assertEquals(game.getName(), gameDTO.getName());
    assertEquals(game.getTimestamp() ,gameDTO.getCreationTime());
}

5. Alternative Ways of Mapping

Property mapping is a good approach in most cases because it allows us to make explicit definitions and clearly see how the mapping flows.

However, for some objects, especially when they have different property hierarchies, we can use the LOOSE matching strategy instead of TypeMap.

5.1. Matching Strategy LOOSE

To demonstrate the benefits of loose matching, let's add two more properties into GameDTO:

public class GameDTO {
    //...
    
    private GameMode mode;
    private int maxPlayers;
    
    // constructors, getters and setters
}

We should notice that mode and maxPlayers correspond to the properties of GameSettings, which is an inner object in our Game source class:

public class GameSettings {

    private GameMode mode;
    private int maxPlayers;

    // constructors, getters and setters
}

Thus, we can perform a two-way mapping, both from Game to GameDTO and the other way around without defining any TypeMap:

@Test
public void whenUsingLooseMappingStrategy_thenConvertsToDomainAndDTO() {
    // setup
    this.mapper.getConfiguration().setMatchingStrategy(MatchingStrategies.LOOSE);
    
    // when dto has flat fields for GameSetting
    GameDTO gameDTO = new GameDTO();
    gameDTO.setMode(GameMode.TURBO);
    gameDTO.setMaxPlayers(8);
    Game game = this.mapper.map(gameDTO, Game.class);
    
    // then it converts to inner objects without property mapper
    assertEquals(gameDTO.getMode(), game.getSettings().getMode());
    assertEquals(gameDTO.getMaxPlayers(), game.getSettings().getMaxPlayers());
    
    // when the GameSetting's field names match
    game = new Game();
    game.setSettings(new GameSettings(GameMode.NORMAL, 6));
    gameDTO = this.mapper.map(game, GameDTO.class);
    
    // then it flattens the fields on dto
    assertEquals(game.getSettings().getMode(), gameDTO.getMode());
    assertEquals(game.getSettings().getMaxPlayers(), gameDTO.getMaxPlayers());
}

5.2. Auto-Skip Null Properties

Additionally, ModelMapper has some global configurations that can be helpful. One of them is the setSkipNullEnabled setting.

So, we can automatically skip the source properties if they're null without writing any conditional mapping:

@Test
public void whenConfigurationSkipNullEnabled_thenConvertsToDTO() {
    // setup
    this.mapper.getConfiguration().setSkipNullEnabled(true);
    TypeMap<Game, Game> propertyMap = this.mapper.createTypeMap(Game.class, Game.class);
    propertyMap.setProvider(p -> this.gameRepository.findById(2L));
    
    // when game has no id
    Game update = new Game(null, "Not Persisted Game!");
    Game updatedGame = this.mapper.map(update, Game.class);
    
    // then destination game id is not overwritten
    assertEquals(2L, updatedGame.getId().longValue());
    assertEquals("Not Persisted Game!", updatedGame.getName());
}

5.3. Circular Referenced Objects

Sometimes, we need to deal with objects that have references to themselves. Generally, this results in a circular dependency and causes the famous StackOverflowError:

org.modelmapper.MappingException: ModelMapper mapping errors:

1) Error mapping com.bealdung.domain.Game to com.bealdung.dto.GameDTO

1 error
	...
Caused by: java.lang.StackOverflowError
	...

So, another configuration, setPreferNestedProperties, will help us in this case:

@Test
public void whenConfigurationPreferNestedPropertiesDisabled_thenConvertsCircularReferencedToDTO() {
    // setup
    this.mapper.getConfiguration().setPreferNestedProperties(false);
    
    // when game has circular reference: Game -> Player -> Game
    Game game = new Game(1L, "Game 1");
    Player player = new Player(1L, "John");
    player.setCurrentGame(game);
    game.setCreator(player);
    GameDTO gameDTO = this.mapper.map(game, GameDTO.class);
    
    // then it resolves without any exception
    assertEquals(game.getId(), gameDTO.getId());
    assertEquals(game.getName(), gameDTO.getName());
}

Therefore, when we pass false into setPreferNestedProperties, the mapping works without any exception.

6. Conclusion

In this article, we explained how to customize class-to-class mappings with property mappers in ModelMapper. Also, we saw some detailed examples of alternative configurations.

Like always, all the source code for the examples is available over on GitHub.

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Get started with Spring 5 and Spring Boot 2, through the Learn Spring course:

>> CHECK OUT THE COURSE
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