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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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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.

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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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Explore Spring Boot 3 and Spring 6 in-depth through building a full REST API with the framework:

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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.

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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.

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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.

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1. Introduction

In this tutorial, we’ll discuss how JPA entities and the Java Serializable interface blend. First, we’ll take a look at the java.io.Serializable interface and why we need it. After that, we’ll take a look at the JPA specification and Hibernate as its most popular implementation.

2. What Is the Serializable Interface?

Serializable is one of the few marker interfaces found in core Java. Marker interfaces are special case interfaces with no methods or constants.

Object serialization is the process of converting Java objects into byte streams. We can then transfer these byte streams over the wire or store them in persistent memory. Deserialization is the reverse process, where we take byte streams and convert them back into Java objects. To allow object serialization (or deserialization), a class must implement the Serializable interface. Otherwise, we’ll run into java.io.NotSerializableException. Serialization is widely used in technologies such as RMI, JPA, and EJB.

3. JPA and Serializable

Let’s see what the JPA specification says about Serializable and how it pertains to Hibernate.

3.1. JPA Specification

One of the core parts of JPA is an entity class. We mark such classes as entities (either with the @Entity annotation or an XML descriptor). There are several requirements that our entity class must fulfill, and the one we’re most concerned with, according to the JPA specification, is:

If an entity instance is to be passed by value as a detached object (e.g., through a remote interface), the entity class must implement the Serializable interface.

In practice, if our object is to leave the domain of the JVM, it’ll require serialization.

Each entity class consists of persistent fields and properties. The specification requires that fields of an entity may be Java primitives, Java serializable types, or user-defined serializable types.

An entity class must also have a primary key. Primary keys can be primitive (single persistent field) or composite. Multiple rules apply to a composite key, one of which is that a composite key is required to be serializable.

Let’s create a simple example using Hibernate, H2 in-memory database, and a User domain object with UserId as a composite key:

@Entity
public class User {
    @EmbeddedId UserId userId;
    String email;
    
    // constructors, getters and setters
}

@Embeddable
public class UserId implements Serializable{
    private String name;
    private String lastName;
    
    // getters and setters
}

We can test our domain definition using the integration test:

@Test
public void givenUser_whenPersisted_thenOperationSuccessful() {
    UserId userId = new UserId();
    userId.setName("John");
    userId.setLastName("Doe");
    User user = new User(userId, "[email protected]");

    entityManager.persist(user);

    User userDb = entityManager.find(User.class, userId);
    assertEquals(userDb.email, "[email protected]");
}

If our UserId class does not implement the Serializable interface, we’ll get a MappingException with a concrete message that our composite key must implement the interface.

3.2. Hibernate @JoinColumn Annotation

Hibernate official documentation, when describing mapping in Hibernate, notes that the referenced field must be serializable when we use referencedColumnName from the @JoinColumn annotation. Usually, this field is a primary key in another entity. In rare cases of complex entity classes, our reference must be serializable.

Let’s extend the previous User class where the email field is no longer a String but an independent entity. Also, we’ll add an Account class that will reference a user and has a field type. Each User can have multiple accounts of different types. We’ll map Account by email since it’s more natural to search by email address:

@Entity
public class User {
    @EmbeddedId private UserId userId;
    private Email email;
}

@Entity
public class Email implements Serializable {
    @Id
    private long id;
    private String name;
    private String domain;
}

@Entity
public class Account {
    @Id
    private long id;
    private String type;
    @ManyToOne
    @JoinColumn(referencedColumnName = "email")
    private User user;
}

To test our model, we’ll write a test where we create two accounts for a user and query by an email object:

@Test
public void givenAssociation_whenPersisted_thenMultipleAccountsWillBeFoundByEmail() {
    // object creation 

    entityManager.persist(user);
    entityManager.persist(account);
    entityManager.persist(account2);

    List<Account> userAccounts = entityManager.createQuery("select a from Account a join fetch a.user where a.user.email = :email", Account.class)
      .setParameter("email", email)
      .getResultList();
    
    assertEquals(userAccounts.size(), 2);
}

NOTE: user is a reserved word in the H2 database and cannot be used to the name of an entity.

If the Email class does not implement the Serializable interface, we’ll get MappingException again, but this time with a somewhat cryptic message: “Could not determine type”.

3.3. Exposing Entities to the Presentation Layer

When sending objects over the wire using HTTP, we usually create specific DTOs (data transfer objects) for this purpose. By creating DTOs, we decouple internal domain objects from external services. If we want to expose our entities directly to the presentation layer without DTOs, then entities must be serializable.

We use the HttpSession object to store relevant data that help us identify users across multiple page visits to our website. The web server can store session data on a disk when shutting down gracefully or transfer session data to another web server in clustered environments. If an entity is part of this process, then it must be serializable. Otherwise, we’ll run into NotSerializableException.

4. Conclusion

In this article, we covered the basics of Java serialization and saw how it comes into play in JPA. First, we went over the JPA specification’s requirements regarding Serializable. After that, we looked into Hibernate as the most popular implementation of JPA. In the end, we covered how JPA entities work with web servers.

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:

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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 – 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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Course – Spring Sale 2025 – NPI (All)
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Course – LSD – NPI (cat=JPA)
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Get started with Spring Data JPA through the reference Learn Spring Data JPA:

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eBook Jackson – NPI EA – 3 (cat = Jackson)