MongoDB Java驱动3.11：@Discriminator缺失及与@BsonDiscriminator的疑问

Great question—this is a common point of confusion thanks to a bit of shorthand in the official docs! Let me break it down clearly:

The Short Answer #

In MongoDB Java Driver 3.11, there is no standalone @Discriminator annotation. The @Discriminator mentioned in the official docs is just a simplified reference to @BsonDiscriminator, which lives in the org.bson.codecs.pojo.annotations package (exactly the one you found in the Javadoc).

Why the Shorthand? #

MongoDB's docs often use abbreviated names for their POJO annotations to make explanations flow better, but all actual annotation classes in the 3.11 driver follow the @Bson[Name] naming convention (think @BsonId, @BsonProperty, etc.). So when the docs say "add @Discriminator to your abstract class", they mean @BsonDiscriminator.

How to Use @BsonDiscriminator #

Here's a quick example to enable discriminators for an inheritance hierarchy:

1. Annotate your abstract base class
   This tells the codec to handle discriminator logic for all subclasses:

   import org.bson.codecs.pojo.annotations.BsonDiscriminator;
   
   @BsonDiscriminator
   public abstract class Animal {
       private String name;
   
       // Constructors, getters, setters
   }
   

2. Customize discriminator values for subclasses (optional)
   By default, the discriminator field (named _t by default) will use the simple class name. You can override this with the value parameter:

   import org.bson.codecs.pojo.annotations.BsonDiscriminator;
   
   @BsonDiscriminator(value = "domestic_dog")
   public class Dog extends Animal {
       private String breed;
   
       // Constructors, getters, setters
   }
   

3. Ensure you have the right dependency
   Make sure your build includes the bson module (where the POJO annotations live). For Maven:

   <dependency>
       <groupId>org.mongodb</groupId>
       <artifactId>bson</artifactId>
       <version>3.11.0</version>
   </dependency>
   

When you serialize a Dog instance, MongoDB will store a _t field with the value domestic_dog (or the class name if you don't customize it), and when deserializing, the driver will use this field to instantiate the correct subclass.

内容的提问来源于stack exchange，提问作者P_M