Master Polymorphic Deserialization in Spring Boot 3 with Jackson

Introduction

When the JSON body format is unknown, defining a separate API for each possible structure leads to redundant endpoints. Polymorphic deserialization with Jackson provides a concise and extensible solution.

Case Study

Assume an online store needs to handle three order types: regular, discount, and promo, each with different fields and calculation logic.

1. Interface Definition

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "orderType", visible = true)
@JsonSubTypes({
    @JsonSubTypes.Type(value = RegularOrder.class, name = "regular"),
    @JsonSubTypes.Type(value = DiscountOrder.class, name = "discount"),
    @JsonSubTypes.Type(value = PromoOrder.class, name = "promo")
})
public interface Order {
}

2. POJO Implementations

public class DiscountOrder implements Order {
    private double price;
    private int quantity;
    private double discountRate;
}
public class PromoOrder implements Order {
    private double price;
    private int quantity;
    private String promoCode;
}
public class RegularOrder implements Order {
    private double price;
    private int quantity;
}

3. Controller

@RestController
@RequestMapping("/orders")
public class OrderController {
    private final OrderService orderService;
    public OrderController(OrderService orderService) {
        this.orderService = orderService;
    }
    @PostMapping("/calculate")
    public ResponseEntity<Double> calculateTotal(@RequestBody Order order) {
        double total = 0;
        switch (order) {
            case RegularOrder r -> total = orderService.calculateRegularTotal(r);
            case DiscountOrder d -> total = orderService.calculateDiscountTotal(d);
            case PromoOrder p -> total = orderService.calculatePromoTotal(p);
            default -> logger.error("Unknown order type: {}", order);
        }
        return ResponseEntity.ok(total);
    }
}

4. Service

@Service
public class OrderService {
    public double calculateRegularTotal(RegularOrder order) {
        return order.getPrice() * order.getQuantity();
    }
    public double calculateDiscountTotal(DiscountOrder order) {
        return order.getPrice() * order.getQuantity() * (1 - order.getDiscountRate());
    }
    public double calculatePromoTotal(PromoOrder order) {
        return order.getPrice() * order.getQuantity() * 0.75;
    }
}

5. Alternative: Dynamic Field Types

By annotating a field with @JsonTypeInfo and @JsonSubTypes, the field can hold different Java types based on a "type" property in the JSON.

public class Order {
    @JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.EXTERNAL_PROPERTY, property = "type")
    @JsonSubTypes({
        @JsonSubTypes.Type(value = String.class, name = "string"),
        @JsonSubTypes.Type(value = Long.class, name = "long"),
        @JsonSubTypes.Type(value = Integer.class, name = "int")
    })
    protected Object value;
}

When the request contains "type":"long", the value field is deserialized as a Long, etc.