📜Jackson Module

How to serialize Result objects with Jackson

When using Result objects with Jackson we might run into some problems. The Jackson datatype module for Result solves them by making Jackson treat results as if they were ordinary objects.

Jackson is a Java library for JSON parsing and generation. It is widely used for converting Java objects to JSON and vice versa, making it essential for handling data in web services and RESTful APIs.

How to Use this Add-On

Add this Maven dependency to your build:

Group ID	Artifact ID	Latest Version
com.leakyabstractions	result-jackson

Maven Central provides snippets for different build tools to declare this dependency.

Test Scenario

Let's start by creating a class ApiResponse containing one ordinary and one Result field.

/** Represents an API response */
public class ApiResponse {

  @JsonProperty
  String version;

  @JsonProperty
  Result<String, String> result;

  // Constructors, getters and setters omitted
}

Problem Overview

Then we will take a look at what happens when we try to serialize and deserialize ApiResponse objects.

Serialization Problem

Now, let's instantiate an ApiResponse object.

ApiResponse response = new ApiResponse();
response.setVersion("v1");
response.setResult(success("Perfect"));

And finally, let's try serializing it using an object mapper.

ObjectMapper objectMapper = new ObjectMapper();
String json = objectMapper.writeValueAsString(response);

We'll see that now we get an InvalidDefinitionException.

Java 8 optional type `java.util.Optional<java.lang.String>`
 not supported by default:
 add Module "com.fasterxml.jackson.datatype:jackson-datatype-jdk8"
 to enable handling

While this may look strange, it's the expected behavior. When Jackson examined the result object, it invoked Result::getSuccess and received an optional string value. But Jackson will not handle JDK 8 datatypes like Optional unless you register the appropriate modules.

@Test
void testSerializationProblem() {
  // Given
  ApiResponse response = new ApiResponse("v1", success("Perfect"));
  // Then
  ObjectMapper objectMapper = new ObjectMapper();
  InvalidDefinitionException error = assertThrows(InvalidDefinitionException.class,
      () -> objectMapper.writeValueAsString(response));
  assertTrue(error.getMessage().startsWith(
      "Java 8 optional type `java.util.Optional<java.lang.String>` not supported"));
}

This is Jackson's default serialization behavior. But we'd like to serialize the result field like this:

{
  "version": "v1",
  "result": {
    "failure": null,
    "success": "Perfect"
  }
}

Deserialization Problem

Now, let's reverse our previous example, this time trying to deserialize a JSON object into an ApiResponse.

String json = "{\"version\":\"v2\",\"result\":{\"success\":\"OK\"}}";
ObjectMapper objectMapper = new ObjectMapper();
objectMapper.readValue(json, ApiResponse.class);

We'll see that we get another InvalidDefinitionException. Let's inspect the stack trace.

Cannot construct instance of `com.leakyabstractions.result.api.Result`
 (no Creators, like default constructor, exist):
 abstract types either need to be mapped to concrete types,
 have custom deserializer, or contain additional type information

This behavior again makes sense. Essentially, Jackson cannot create new result objects because Result is an interface, not a concrete type.

@Test
void testDeserializationProblem() {
  // Given
  String json = "{\"version\":\"v2\",\"result\":{\"success\":\"OK\"}}";
  // Then
  ObjectMapper objectMapper = new ObjectMapper();
  InvalidDefinitionException error = assertThrows(InvalidDefinitionException.class,
      () -> objectMapper.readValue(json, ApiResponse.class));
  assertTrue(error.getMessage().startsWith(
      "Cannot construct instance of `com.leakyabstractions.result.api.Result`"));
}

Solution Implementation

What we want, is for Jackson to treat Result values as JSON objects that contain either a success or a failure value. Fortunately, there's a Jackson module that can solve this problem.

Registering the Jackson Datatype Module for Result

Once we have added Result-Jackson as a dependency, all we need to do is register ResultModule with our object mapper.

ObjectMapper objectMapper = new ObjectMapper();
objectMapper.registerModule(new ResultModule());

Alternatively, you can also make Jackson auto-discover the module.

objectMapper.findAndRegisterModules();

Regardless of the chosen registration mechanism, once the module is registered all functionality is available for all normal Jackson operations.

Serializing Results

Now, let's try and serialize our ApiResponse object again:

@Test
void serializeSuccessfulResult() throws Exception {
  // Given
  ApiResponse response = new ApiResponse("v3", success("All good"));
  // When
  ObjectMapper objectMapper = new ObjectMapper();
  objectMapper.registerModule(new ResultModule());
  String json = objectMapper.writeValueAsString(response);
  // Then
  assertTrue(json.contains("v3"));
  assertTrue(json.contains("All good"));
}

If we look at the serialized response, we'll see that this time the result field contains a null failure value and a non-null success value:

{
  "version": "v3",
  "result": {
    "failure": null,
    "success": "All good"
  }
}

Next, we can try serializing a failed result.

@Test
void serializeFailedResult() throws Exception {
  // Given
  ApiResponse response = new ApiResponse("v4", failure("Oops"));
  // When
  ObjectMapper objectMapper = new ObjectMapper();
  objectMapper.findAndRegisterModules();
  String json = objectMapper.writeValueAsString(response);
  // Then
  assertTrue(json.contains("v4"));
  assertTrue(json.contains("Oops"));
} // End

We can verify that the serialized response contains a non-null failure value and a null success value.

{
  "version": "v4",
  "result": {
    "failure": "Oops",
    "success": null
  }
}

Deserializing Results

Now, let's repeat our tests for deserialization. If we read our ApiResponse again, we'll see that we no longer get an InvalidDefinitionException.

@Test
void deserializeSuccessfulResult() throws Exception {
  // Given
  String json = "{\"version\":\"v5\",\"result\":{\"success\":\"Yay\"}}";
  // When
  ObjectMapper objectMapper = new ObjectMapper().findAndRegisterModules();
  ApiResponse response = objectMapper.readValue(json, ApiResponse.class);
  // Then
  assertEquals("v5", response.getVersion());
  assertEquals("Yay", response.getResult().orElse(null));
}

Finally, let's repeat the test again, this time with a failed result. We'll see that yet again we don't get an exception, and in fact, have a failed result.

@Test
void deserializeFailedResult() throws Exception {
  // Given
  String json = "{\"version\":\"v6\",\"result\":{\"failure\":\"Nay\"}}";
  // When
  ObjectMapper objectMapper = new ObjectMapper().findAndRegisterModules();
  ApiResponse response = objectMapper.readValue(json, ApiResponse.class);
  // Then
  assertEquals("v6", response.getVersion());
  assertEquals("Nay", response.getResult().getFailure().orElse(null));
}

Conclusion

We learned how to serialize and deserialize Result objects using Jackson, demonstrating how the provided datatype module enables Jackson to treat Results as ordinary objects.

The full source code for the examples is available on GitHub.