1. Overview
In this tutorial, we’ll see how to use both Jackson and Gson to map different JSON fields onto a single Java field.
2. Maven Dependencies
In order to use the Jackson and Gson libraries we need to add the following dependencies to our POM:
<dependency>
<groupId>com.google.code.gson</groupId>
<artifactId>gson</artifactId>
<version>2.10.1</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-databind</artifactId>
<version>2.13.0</version>
<scope>test</scope>
</dependency>
3. Sample JSON
Let’s imagine that we want to get the details of the weather for different locations into our Java application. We’ve found a couple of web sites that publish weather data as JSON documents. However, they use slightly different formats:
{
"location": "London",
"temp": 15,
"weather": "Cloudy"
}
And:
{
"place": "Lisbon",
"temperature": 35,
"outlook": "Sunny"
}
We want to deserialize both of these formats into the same Java class, named Weather:
public class Weather {
private String location;
private int temp;
private String outlook;
}
So let’s take a look at how we can achieve this using both Jackson and Gson libraries.
4. Using Jackson
To achieve this, we’ll make use of Jackson’s @JsonProperty and @JsonAlias annotations. These will allow us to map more than one JSON property onto the same Java field.
First, we’re going to use the @JsonProperty annotation, so that Jackson knows the name of the JSON field to map. The value in the @JsonProperty annotation is used for both deserializing and serializing.
Then we can use the @JsonAlias annotation. As a result, Jackson will know the names of other fields in the JSON document that are mapped to the Java field. The value in the @JsonAlias annotation is only used for deserializing:
@JsonProperty("location")
@JsonAlias("place")
private String location;
@JsonProperty("temp")
@JsonAlias("temperature")
private int temp;
@JsonProperty("outlook")
@JsonAlias("weather")
private String outlook;
Now that we’ve added the annotations, let’s use Jackson’s ObjectMapper to create Java objects using the Weather class:
@Test
public void givenTwoJsonFormats_whenDeserialized_thenWeatherObjectsCreated() throws Exception {
ObjectMapper mapper = new ObjectMapper();
Weather weather = mapper.readValue("{\n"
+ " \"location\": \"London\",\n"
+ " \"temp\": 15,\n"
+ " \"weather\": \"Cloudy\"\n"
+ "}", Weather.class);
assertEquals("London", weather.getLocation());
assertEquals("Cloudy", weather.getOutlook());
assertEquals(15, weather.getTemp());
weather = mapper.readValue("{\n"
+ " \"place\": \"Lisbon\",\n"
+ " \"temperature\": 35,\n"
+ " \"outlook\": \"Sunny\"\n"
+ "}", Weather.class);
assertEquals("Lisbon", weather.getLocation());
assertEquals("Sunny", weather.getOutlook());
assertEquals(35, weather.getTemp());
}
5. Using Gson
Now, let’s try the same with Gson. We’ll need to use the value and the alternate parameters in the @SerializedName annotation.
The first one will be used as a default, while the second will be used to indicate an alternate name of the JSON field that we want to map:
@SerializedName(value="location", alternate="place")
private String location;
@SerializedName(value="temp", alternate="temperature")
private int temp;
@SerializedName(value="outlook", alternate="weather")
private String outlook;
Now that we’ve added the annotations, let’s test our example:
@Test
public void givenTwoJsonFormats_whenDeserialized_thenWeatherObjectsCreated() throws Exception {
Gson gson = new GsonBuilder().create();
Weather weather = gson.fromJson("{\n"
+ " \"location\": \"London\",\n"
+ " \"temp\": 15,\n"
+ " \"weather\": \"Cloudy\"\n"
+ "}", Weather.class);
assertEquals("London", weather.getLocation());
assertEquals("Cloudy", weather.getOutlook());
assertEquals(15, weather.getTemp());
weather = gson.fromJson("{\n"
+ " \"place\": \"Lisbon\",\n"
+ " \"temperature\": 35,\n"
+ " \"outlook\": \"Sunny\"\n"
+ "}", Weather.class);
assertEquals("Lisbon", weather.getLocation());
assertEquals("Sunny", weather.getOutlook());
assertEquals(35, weather.getTemp());
}
6. Conclusion
We saw that by using either Jackson’s @JsonAlias or Gson’s alternate parameter, we can easily convert different JSON formats into the same Java object.
You’ll find the examples in the Jackson and the Gson projects on GitHub.
