POJO to JSON Converter — Serialise Java Objects with Jackson, Gson & Moshi

Jackson — the default in Spring

Spring Boot ships Jackson by default. ObjectMapper is thread-safe — reuse a single instance.

import com.fasterxml.jackson.databind.ObjectMapper;

private static final ObjectMapper MAPPER = new ObjectMapper();

public static String toJson(Object pojo) throws JsonProcessingException {
    return MAPPER.writeValueAsString(pojo);
}

// Pretty-printed:
String pretty = MAPPER.writerWithDefaultPrettyPrinter().writeValueAsString(pojo);

Annotate fields with @JsonProperty("snake_case") when the JSON field name differs from the Java field. @JsonIgnore skips a field; @JsonInclude(NON_NULL) drops nulls.

Gson — minimal dependency, Android friendly

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

private static final Gson GSON = new GsonBuilder()
    .serializeNulls()              // include null fields
    .setPrettyPrinting()           // optional
    .create();

public static String toJson(Object pojo) {
    return GSON.toJson(pojo);
}

Use @SerializedName("json_field") on a Java field to override the JSON key. Mark fields transient to skip them.

Moshi — Kotlin-first, codegen option

import com.squareup.moshi.Moshi;
import com.squareup.moshi.JsonAdapter;

Moshi moshi = new Moshi.Builder().build();
JsonAdapter<User> adapter = moshi.adapter(User.class);

String json = adapter.toJson(user);
// Pretty:
String pretty = adapter.indent("  ").toJson(user);

How POJO to JSON compares to alternatives

Frequently asked questions

What is POJO to JSON?

POJO to JSON conversion takes a Java instance — a Plain Old Java Object with fields, getters, and setters — and serialises it into a JSON string. The standard libraries are Jackson (`ObjectMapper.writeValueAsString(obj)`), Gson (`new Gson().toJson(obj)`), and Moshi (`moshi.adapter(MyType.class).toJson(obj)`). Each library reflects over the object's fields, applies any annotations (e.g. `@JsonProperty`, `@SerializedName`), and emits valid JSON.

Features

Example

class User { Integer id = 42; String name = "Ada"; boolean active = true; }  //  new ObjectMapper().writeValueAsString(user)

{"id":42,"name":"Ada","active":true}

Common use cases

1. 1

   Serialize a Spring Boot response

   Use the Jackson recipe to turn a POJO into JSON inside a REST controller.

2. 2

   Rename JSON keys

   Apply the right annotation to output snake_case keys from camelCase Java fields.

3. 3

   Choose a JSON library

   Compare Jackson, Gson, and Moshi to pick the best fit for your stack.

References

• Jackson Databind — FasterXML
• Gson User Guide — Google
• Moshi — Square
• RFC 8259 — The JSON Data Interchange Format — IETF

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