Gson Integration

ModelMapper’s Gson integration allows you to map a Gson JsonElement to a JavaBean.

Setup

To get started, add the modelmapper-gson Maven dependency to your project:

<dependency>
  <groupId>org.modelmapper.extensions</groupId>
  <artifactId>modelmapper-gson</artifactId>
  <version>0.7.5</version>
</dependency>

Next, configure ModelMapper to support the JsonElementValueReader, which allows for values to be read and mapped from a Gson JsonElement:

modelMapper.getConfiguration().addValueReader(new JsonElementValueReader());

Example Usage

Consider the following JSON representing an order:

{
  "id": 456,
  "customer": {
    "id": 789,
    "street_address": "123 Main Street", 
    "address_city": "SF"
  }
}

We may need to map this to a different object model:

// Assume getters and setters are present

public class Order {
  private int id;
  private Customer customer;
}

public class Customer {
  private Address address;
}

public class Address {
  private String street;
  private String city;
}

Since the order JSON in this example uses an underscore naming convention, we’ll need to configure ModelMapper to tokenize source property names by underscore:

modelMapper.getConfiguration().setSourceNameTokenizer(NameTokenizers.UNDERSCORE);

With that set, mapping a JsonElement for the order JSON to an Order object is simple:

JsonElement orderElement = new JsonParser().parse(orderJson);
Order order = modelMapper.map(orderElement, Order.class);

And we can assert that values are mapped as expected:

assertEquals(order.getId(), 456);
assertEquals(order.getCustomer().getId(), 789);
assertEquals(order.getCustomer().getAddress().getStreet(), "123 Main Street");
assertEquals(order.getCustomer().getAddress().getCity(), "SF");

Explicit Mapping

While ModelMapper will do its best to implicitly match JsonElement values to destination properties, sometimes you may need to explicitly define how one property maps to another. A PropertyMap allows us to do this.

Let’s define how a JsonElement maps to an Order by creating a PropertyMap. Our PropertyMap will include a map() statement that maps a source JsonElement’s customer.street_address field hierarchy to a destination Order’s getCustomer().getAddress().setStreet() method hierarchy:

PropertyMap<JsonElement, Order> orderMap = new PropertyMap<JsonElement Order>() {
  protected void configure() {
    map().getCustomer().getAddress().setStreet(this.<String>source("customer.street_address"));
  }
};

To use our PropertyMap, we’ll create a TypeMap for our order JsonElement and add our PropertyMap to it:

modelMapper.createTypeMap(orderElement, Order.class).addMappings(orderMap)

We can then map JsonElements to Orders as usual, with properties being mapped according to the PropertyMap that we defined:

Order order = modelMapper.map(orderElement, Order.class);

Things to Note

ModelMapper maintains a TypeMap for each source and destination type, containing the mappings bewteen the two types. For “generic” types such as JsonElement this can be problematic since the structure of a JsonElement can vary. In order to distinguish structurally different JsonElements that map to the same destination type, we can provide a type map name to ModelMapper.

Continuing with the example above, let’s map another order JSON, this one with a different structure, to the same Order class:

{
  "id": 222,
  "customer_id": 333,
  "customer_street_address": "444 Main Street",
  "customer_address_city": "LA"
}

Mapping this JSON to an order is simple, but we’ll need to provide a type map name to distinguish this JsonElement to Order mapping from the previous unnamed mapping:

JsonElement orderElement = new JsonParser().parse(flatOrderJson);
Order order = modelMapper.map(orderElement, Order.class, "flat");