Demystifying readEntity() in Java: How to Actually Read That API Response

This content originally appeared on DEV Community and was authored by David

“The response is empty.”
“But it works in Postman…”
“I don’t get it—it’s a 200!”

If you’ve ever worked with REST APIs in Java, chances are you’ve had one of those “why is my response object null” moments. Welcome to the world of readEntity().

Let me walk you through what it is, how it works, where it bites, and how to use it the right way.

What is readEntity()?

In Java, particularly when using JAX-RS client libraries (like Jersey), you often get an API response wrapped in a Response object.

But a Response is generic—it’s like a sealed envelope. You still need to unwrap it.

That’s where readEntity() comes in.

Response response = client
    .target("https://api.example.com/users/123")
    .request(MediaType.APPLICATION_JSON)
    .get();

User user = response.readEntity(User.class);

Boom. The readEntity(User.class) method tries to deserialize the response body into a User object.

But Here’s the Gotcha
If:

• Your class doesn’t match the structure of the response
• You forgot to register the right JSON provider
• Or the content-type doesn’t match what the server sends

Then readEntity() will return null, or worse… it will silently swallow the problem.

Working Example

Let’s make this real. Suppose you’re calling an API that returns a list of products:

API Response (JSON):

{
  "items": [
    { "id": 1, "name": "Laptop" },
    { "id": 2, "name": "Keyboard" }
  ]
}

Java Models:

public class Product {
    public int id;
    public String name;
}

public class ProductResponse {
    public List<Product> items;
}

The Code:

Client client = ClientBuilder.newClient();
Response response = client
    .target("https://api.example.com/products")
    .request(MediaType.APPLICATION_JSON)
    .get();

ProductResponse productResponse = response.readEntity(ProductResponse.class);

for (Product p : productResponse.items) {
    System.out.println(p.id + " - " + p.name);
}

Important to note:

When readEntity() is called on a Response object to deserialize the response body into a specific Java type (e.g., response.readEntity(MyObject.class)), the JAX-RS (Java API for RESTful Web Services) framework needs to find a suitable MessageBodyReader capable of handling the content type of the response and converting it into the requested Java type.

These MessageBodyReader implementations are registered as providers within the JAX-RS runtime environment (e.g., Jersey, RESTEasy).

Examples of such providers include:

• JSON providers: Like Jackson or Gson, which handle the deserialization of JSON data into Java objects.
• XML providers: For handling XML data.
• Plain text providers: For simple text-based responses.

Without a registered provider that can handle the specific media type and target Java type, calling readEntity() would result in an error like MessageBodyProviderNotFoundException. So the presence and proper registration of these providers are crucial for readEntity() to function correctly.

client.register(JacksonJsonProvider.class);

Pro Tip from My Experience

If you’re using readEntity() and getting an empty object or list—even though Postman shows it perfectly—print the raw body to debug, chances are that the content-type in the response differs from the one the provider is supposed to consume (JSON vs x-msgpack for example).

Final Thoughts

The readEntity() method feels like it should “just work”—and most of the time it does. But in edge cases (encoding issues, media type mismatches, incorrect models), you need to dig deeper.

Register your JSON provider
Double-check your response structure
Check that the content-type is the same across the client and the server

This content originally appeared on DEV Community and was authored by David