Unravel › Java concepts › APIs and web

At a glance

1. 1mapper.readValue(json, Order.class) fills an Order by matching JSON property names to fields, setters or constructor parametersUnknown properties cause an error unless you configure it to ignore them.
2. 2Annotations tune the mapping: @JsonProperty("order_id") renames, @JsonIgnore hides, @JsonInclude(NON_NULL) skips nulls, @JsonFormat sets date patterns
3. 3Generic targets need a TypeReference: mapper.readValue(json, new TypeReference<List<Order>>() {}), since type erasure removes the element type
4. 4JsonNode lets you read JSON as a tree when you do not have a class

• ObjectMapper is thread safe and expensive to create: build one and reuse it (Spring Boot already provides one)
• Jackson inspects classes by reflection, builds and caches a serializer or deserializer per type, and streams tokens with a low-level JsonParser

ObjectMapper mapper = new ObjectMapper().registerModule(new JavaTimeModule());Order order = mapper.readValue(json, Order.class);List<Order> list = mapper.readValue(json, new TypeReference<List<Order>>() {});String back = mapper.writeValueAsString(order);

1. line 1 uses ObjectMapper, the core of Jackson JSON
2. line 2 uses readValue, the core of Jackson JSON
3. line 3 uses readValue, the core of Jackson JSON
4. line 4 uses writeValueAsString, the core of Jackson JSON

How Unravel recognises it

When you paste code, Unravel looks for these signs of jackson json. Spotting them yourself is a good habit when you read unfamiliar code:

• Annotations: @JsonProperty @JsonIgnore @JsonCreator @JsonInclude @JsonFormat @JsonIgnoreProperties @JsonTypeInfo @JsonSubTypes
• Types: ObjectMapper JsonNode TypeReference
• Method calls: readValue() writeValueAsString()

Part of a learning path

• How a Spring request really works · previous: Spring proxies (how annotations work) · next: Spring REST controllers

Related concepts

Builds on this