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In this article we’re going to look at the most common scenarios of serialization using the Gson library.

Let’s start by introducing a simple entity that we’re going to use throughout the following examples:

public class SourceClass {
    private int intValue;
    private String stringValue;

    // standard getters and setters
}

1. Serialize an Array of Entities

First, let’s serialize an array of objects with Gson:

@Test
public void givenArrayOfObjects_whenSerializing_thenCorrect() {
    SourceClass[] sourceArray = {new SourceClass(1, "one"), new SourceClass(2, "two")};
    String jsonString = new Gson().toJson(sourceArray);

    String expectedResult = 
      "[{"intValue":1,"stringValue":"one"},{"intValue":2,"stringValue":"two"}]";
    assertEquals(expectedResult, jsonString);
}

2. Serialize a Collection of Entities

Next, let’s do the same for a Collection of objects:

@Test
public void givenCollection_whenSerializing_thenCorrect() {
    Collection<SourceClass> sourceCollection = 
      Lists.newArrayList(new SourceClass(1, "one"), new SourceClass(2, "two"));
    String jsonCollection = new Gson().toJson(sourceCollection);

    String expectedResult = 
      "[{"intValue":1,"stringValue":"one"},{"intValue":2,"stringValue":"two"}]";
    assertEquals(expectedResult, jsonCollection);
}

3. Change the Field Names of an Entity on Serialization

Next, let’s see how we can change the name of the field when we’re serializing an entity.

We’re going to serialize our entity, containing the fields intValue and stringValue to a json with otherIntValue and otherStringValue:

@Test
public void givenUsingCustomSerializer_whenChangingNameOfFieldOnSerializing_thenCorrect() {
    SourceClass sourceObject = new SourceClass(7, "seven");
    GsonBuilder gsonBuildr = new GsonBuilder();
    gsonBuildr.registerTypeAdapter(SourceClass.class, new DifferentNameSerializer());
    String jsonString = gsonBuildr.create().toJson(sourceObject);

    String expectedResult = "{"otherIntValue":7,"otherStringValue":"seven"}";
    assertEquals(expectedResult, jsonString);
}

Note that we’re using a custom serializer here to change the name of our fields:

public class DifferentNameSerializer implements JsonSerializer<SourceClass> {
    @Override
    public JsonElement serialize
      (SourceClass src, Type typeOfSrc, JsonSerializationContext context) {
        String otherIntValueName = "otherIntValue";
        String otherStringValueName = "otherStringValue";

        JsonObject jObject = new JsonObject();
        jObject.addProperty(otherIntValueName, src.getIntValue());
        jObject.addProperty(otherStringValueName, src.getStringValue());

        return jObject;
    }
}

4. Ignore a Field when Serializing an Entity

Let’s now ignore a field completely when performing the serialization:

@Test
public void givenIgnoringAField_whenSerializingWithCustomSerializer_thenFieldIgnored() {
    SourceClass sourceObject = new SourceClass(7, "seven");
    GsonBuilder gsonBuildr = new GsonBuilder();
    gsonBuildr.registerTypeAdapter(SourceClass.class, new IgnoringFieldsSerializer());
    String jsonString = gsonBuildr.create().toJson(sourceObject);

    String expectedResult = "{"intValue":7}";
    assertEquals(expectedResult, jsonString);
}

Similar to the previous example, we’re using a custom serializer here as well:

public class IgnoringFieldsSerializer implements JsonSerializer<SourceClass> {
    @Override
    public JsonElement serialize
      (SourceClass src, Type typeOfSrc, JsonSerializationContext context) {
        String intValue = "intValue";
        JsonObject jObject = new JsonObject();
        jObject.addProperty(intValue, src.getIntValue());

        return jObject;
    }
}

Also note that we most likely need to do this in cases where we cannot change the source code of the entity, or if the field should only be ignored very specific cases. Otherwise we can ignore the field easier with a direct annotation on the entity class.

5. Serialize a Field Only If It Passes a Custom Condition

Finally, let’s analyze a more advanced usecase – we want to only serialize a field if it passes a specific, custom condition.

For example, let’s only serialize the int value if it’s positive and simply skip it if it’s negative:

@Test
public void givenUsingCustomDeserializer_whenFieldNotMatchesCriteria_thenIgnored() {
    SourceClass sourceObject = new SourceClass(-1, "minus 1");
    GsonBuilder gsonBuildr = new GsonBuilder();
    gsonBuildr.registerTypeAdapter(SourceClass.class, 
      new IgnoringFieldsNotMatchingCriteriaSerializer());
    Gson gson = gsonBuildr.create();
    Type sourceObjectType = new TypeToken<SourceClass>() {}.getType();
    String jsonString = gson.toJson(sourceObject, sourceObjectType);
    
    String expectedResult = "{"stringValue":"minus 1"}";
    assertEquals(expectedResult, jsonString);
}

Of course we’re using a custom serializer here as well:

public class IgnoringFieldsNotMatchingCriteriaSerializer 
  implements JsonSerializer<SourceClass> {
    @Override
    public JsonElement serialize
      (SourceClass src, Type typeOfSrc, JsonSerializationContext context) {
        JsonObject jObject = new JsonObject();

        // Criteria: intValue >= 0
        if (src.getIntValue() >= 0) {
            String intValue = "intValue";
            jObject.addProperty(intValue, src.getIntValue());
        }

        String stringValue = "stringValue";
        jObject.addProperty(stringValue, src.getStringValue());

        return jObject;
    }
}

And that’s it – 5 common usecases of serialization using Gson.

I just announced the new Spring Boot 2 material, coming in REST With Spring:

>> CHECK OUT THE LESSONS