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Once the early-adopter seats are all used, the price will go up and stay at $33/year.

Partner – Microsoft – NPI EA (cat = Baeldung)
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Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Partner – Microsoft – NPI EA (cat= Spring Boot)
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Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, you can get started over on the documentation page.

And, you can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Partner – Orkes – NPI EA (cat=Spring)
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Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

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Partner – Orkes – NPI EA (tag=Microservices)
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Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

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eBook – Guide Spring Cloud – NPI EA (cat=Spring Cloud)
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eBook – Mockito – NPI EA (tag = Mockito)
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Mocking is an essential part of unit testing, and the Mockito library makes it easy to write clean and intuitive unit tests for your Java code.

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eBook – Java Concurrency – NPI EA (cat=Java Concurrency)
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Handling concurrency in an application can be a tricky process with many potential pitfalls. A solid grasp of the fundamentals will go a long way to help minimize these issues.

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eBook – Reactive – NPI EA (cat=Reactive)
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Spring 5 added support for reactive programming with the Spring WebFlux module, which has been improved upon ever since. Get started with the Reactor project basics and reactive programming in Spring Boot:

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eBook – Java Streams – NPI EA (cat=Java Streams)
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Since its introduction in Java 8, the Stream API has become a staple of Java development. The basic operations like iterating, filtering, mapping sequences of elements are deceptively simple to use.

But these can also be overused and fall into some common pitfalls.

To get a better understanding on how Streams work and how to combine them with other language features, check out our guide to Java Streams:

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eBook – Jackson – NPI EA (cat=Jackson)
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Do JSON right with Jackson

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eBook – HTTP Client – NPI EA (cat=Http Client-Side)
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eBook – Persistence – NPI EA (cat=Persistence)
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Working on getting your persistence layer right with Spring?

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eBook – RwS – NPI EA (cat=Spring MVC)
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Building a REST API with Spring?

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Course – LS – NPI EA (cat=Jackson)
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Get started with Spring and Spring Boot, through the Learn Spring course:

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Course – RWSB – NPI EA (cat=REST)
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Explore Spring Boot 3 and Spring 6 in-depth through building a full REST API with the framework:

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Course – LSS – NPI EA (cat=Spring Security)
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Yes, Spring Security can be complex, from the more advanced functionality within the Core to the deep OAuth support in the framework.

I built the security material as two full courses - Core and OAuth, to get practical with these more complex scenarios. We explore when and how to use each feature and code through it on the backing project.

You can explore the course here:

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Course – All Access – NPI EA (cat= Spring)
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All Access is finally out, with all of my Spring courses. Learn JUnit is out as well, and Learn Maven is coming fast. And, of course, quite a bit more affordable. Finally.

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Course – LSD – NPI EA (tag=Spring Data JPA)
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Spring Data JPA is a great way to handle the complexity of JPA with the powerful simplicity of Spring Boot.

Get started with Spring Data JPA through the guided reference course:

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Partner – LambdaTest – NPI EA (cat=Testing)
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End-to-end testing is a very useful method to make sure that your application works as intended. This highlights issues in the overall functionality of the software, that the unit and integration test stages may miss.

Playwright is an easy-to-use, but powerful tool that automates end-to-end testing, and supports all modern browsers and platforms.

When coupled with LambdaTest (an AI-powered cloud-based test execution platform) it can be further scaled to run the Playwright scripts in parallel across 3000+ browser and device combinations:

>> Automated End-to-End Testing With Playwright

Course – Spring Sale 2025 – NPI EA (cat= Baeldung)
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Course – Spring Sale 2025 – NPI (cat=Baeldung)
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eBook – Jackson – NPI (cat=Jackson)
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1. Overview

Polymorphic deserialization is a feature of Jackson, a popular JSON serialization and deserialization library for Java. It allows us to deserialize a JSON into a hierarchy of Java objects, even when the specific type is not known at compile time. The utility comes when you have a parent class and multiple subclasses, and we want to determine the actual type of the object during deserialization not to lose any information about the polymorphic nature of the object.

In this tutorial, we’ll explore how we can achieve that in two ways: using type-handling annotations to indicate the base class’s subtypes or a Reflections-based approach to scan and register all subtypes.

2. Polymorphic Deserialization Using @JsonTypeInfo with @JsonSubTypes

One of the most straightforward options is Jackson Polymorphic Type Handling Annotations.

Let’s see an implementation example where we’re going to use @JsonTypeInfo and @JsonSubTypes to indicate subtypes of Vehicle entity and deserialize it based on an existing property:

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.EXISTING_PROPERTY, property = "type", visible = true)
@JsonSubTypes({
    @JsonSubTypes.Type(value = Vehicle.ElectricVehicle.class, name = "ELECTRIC_VEHICLE"),
    @JsonSubTypes.Type(value = Vehicle.FuelVehicle.class, name = "FUEL_VEHICLE")
})
public class Vehicle {

    public String type;

    // standard setters and getters

    public static class ElectricVehicle extends Vehicle {

        String autonomy;
        String chargingTime;

        // standard setters and getters
    }

    public static class FuelVehicle extends Vehicle {

        String fuelType;
        String transmissionType;

        // standard setters and getters
    }
}

Now, let’s see how deserializing a JSON input into a Vehicle subtype works:

@Test
public void whenDeserializingPolymorphic_thenCorrect() throws JsonProcessingException {
    String json = "{\"type\":\"ELECTRIC_VEHICLE\",\"autonomy\":\"500\",\"chargingTime\":\"200\"}";

    Vehicle vehicle = new ObjectMapper().readerFor(Vehicle.class).readValue(json);

    assertEquals(Vehicle.ElectricVehicle.class, vehicle.getClass());
}

3. Polymorphic Deserialization Using @JsonTypeInfo with Reflections for Registering Subtypes

Next, let’s explore how we can use a different approach by creating a custom annotation and using Reflections Library for scanning and registering all existing subtypes.

3.1. Reflection Introduction

Reflection is a powerful feature that allows a Java program to inspect or manipulate its structure and behavior at runtime. This is handy because we can create a custom annotation to indicate the type name of each subtype and use Reflections to identify and register them.

Our guide to the Reflections Library describes it in greater detail, as well as its use cases.

3.2. Maven Dependency

Firstly, to use Reflections, we need to add the reflections dependency:

<dependency>
    <groupId>org.reflections</groupId>
    <artifactId>reflections</artifactId>
    <version>0.10.2</version>
</dependency>

We can find its latest version on Maven Central Repository.

3.3. Creating Custom Annotation to Indicate Type Names of Subtypes

Java annotations are a form of metadata that provide additional information about classes, methods, fields, and other program elements at compile time or runtime. They don’t directly affect the code’s logic but provide instructions or details to the compiler or runtime environment.

More information about custom annotations can be found in our article on creating a custom annotation in Java.

To indicate the type name of each Vehicle subtype, we’re going to create the following annotation, with runtime visibility and applicable to types (classes):

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface VehicleSubType {
    String value();
}

Now, let’s see how we should update the existing code to specify the type for each subclass defined:

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.EXISTING_PROPERTY, property = "type", visible = true)
public class Vehicle {

    public String type;

    // standard setters and getters

    @VehicleSubType("ELECTRIC_VEHICLE")
    public static class ElectricVehicle extends Vehicle {

        String autonomy;
        String chargingTime;

        // standard setters and getters
    }

    @VehicleSubType("FUEL_VEHICLE")
    public static class FuelVehicle extends Vehicle {

        String fuelType;
        String transmissionType;

        // standard setters and getters
    }
}

Notice that we still need to use @JsonTypeInfo annotation on the parent class to specify the property used to store type information.

3.4. Registering Subtypes Using Reflection

Finally, we need to customize the Jackson ObjectMapper to register the annotated classes as subtypes.

We’ll start by identifying all classes annotated with our custom annotation @VehicleSubType. After that, for each class found, we can extract the value of the annotation and register the subtype with the associated type name:

private ObjectMapper getCustomObjectMapper() {

    ObjectMapper objectMapper = new ObjectMapper();

    Reflections reflections = new Reflections("com.baeldung.jackson.polymorphicdeserialization.reflection");
    Set<Class<?>> subtypes = reflections.getTypesAnnotatedWith(VehicleSubType.class);

    for (Class<?> subType : subtypes) {
        VehicleSubType annotation = subType.getAnnotation(VehicleSubType.class);
        if (annotation != null) {
            String typeName = annotation.value();
            objectMapper.registerSubtypes(new NamedType(subType, typeName));
        }
    }

    return objectMapper;
}

Now, let’s test our code with the same input as before:

@Test
public void whenDeserializingPolymorphic_thenCorrect() throws JsonProcessingException {
    String json = "{\"type\":\"ELECTRIC_VEHICLE\",\"autonomy\":\"500\",\"chargingTime\":\"200\"}";
    ObjectMapper objectMapper = getCustomObjectMapper();

    Vehicle vehicle = objectMapper.readValue(json, Vehicle.class);

    assertEquals(Vehicle.ElectricVehicle.class, vehicle.getClass());
}

4. Differences Between the Two Approaches

The @JsonSubTypes approach offers explicit configuration by defining subtypes and their type names through annotations. This provides a centralized and clear hierarchy, ensuring compile-time safety.

Reflections-based registration allows dynamic subtype discovery at runtime. While it reduces boilerplate code, it introduces runtime overhead and lacks compile-time safety, and requires external dependencies for classpath scanning. However, this approach may be suitable when dealing with many subtypes because adding a new subtype doesn’t affect already existing code.

5. Conclusion

In this article, we’ve looked over two distinct approaches, focusing on using a custom annotation and Reflections for identifying and registering subtypes.

In conclusion, the choice between them depends on the application’s specific requirements. If the project’s subtypes are already known and stable, @JsonSubTypes provides a robust and safer option. Conversely, Reflections-based registration might be the better choice for projects demanding flexibility and runtime adaptability.

The code backing this article is available on GitHub. Once you're logged in as a Baeldung Pro Member, start learning and coding on the project.
Baeldung Pro – NPI EA (cat = Baeldung)
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Baeldung Pro comes with both absolutely No-Ads as well as finally with Dark Mode, for a clean learning experience:

>> Explore a clean Baeldung

Once the early-adopter seats are all used, the price will go up and stay at $33/year.

Partner – Microsoft – NPI EA (cat = Spring Boot)
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Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Partner – Orkes – NPI EA (cat = Spring)
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Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

Partner – Orkes – NPI EA (tag = Microservices)
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Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

eBook – HTTP Client – NPI EA (cat=HTTP Client-Side)
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The Apache HTTP Client is a very robust library, suitable for both simple and advanced use cases when testing HTTP endpoints. Check out our guide covering basic request and response handling, as well as security, cookies, timeouts, and more:

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eBook – Java Concurrency – NPI EA (cat=Java Concurrency)
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Handling concurrency in an application can be a tricky process with many potential pitfalls. A solid grasp of the fundamentals will go a long way to help minimize these issues.

Get started with understanding multi-threaded applications with our Java Concurrency guide:

>> Download the eBook

eBook – Java Streams – NPI EA (cat=Java Streams)
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Since its introduction in Java 8, the Stream API has become a staple of Java development. The basic operations like iterating, filtering, mapping sequences of elements are deceptively simple to use.

But these can also be overused and fall into some common pitfalls.

To get a better understanding on how Streams work and how to combine them with other language features, check out our guide to Java Streams:

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eBook – Persistence – NPI EA (cat=Persistence)
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Working on getting your persistence layer right with Spring?

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Course – LS – NPI EA (cat=REST)

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

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Course – Spring Sale 2025 – NPI EA (cat= Baeldung)
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Yes, we're now running our Spring Sale. All Courses are 25% off until 26th May, 2025:

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Course – Spring Sale 2025 – NPI (All)
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eBook Jackson – NPI EA – 3 (cat = Jackson)