How do I convert a Java class to JSON with Jackson?

Add jackson-databind to your project, create an ObjectMapper instance, and call mapper.writeValueAsString(object). For a pretty-printed output: mapper.writerWithDefaultPrettyPrinter().writeValueAsString(object). For a List: mapper.writeValueAsString(list). Use @JsonProperty on fields to customise the JSON key names.

How do I convert a Java object to JSON with Gson?

Add gson to your project, create a Gson instance, and call new Gson().toJson(object). For pretty output: new GsonBuilder().setPrettyPrinting().create().toJson(object). Use @SerializedName("key") on fields to customise key names. Gson works without any additional configuration for simple POJOs.

Java to JSON Converter — Mock Data, Jackson & Gson

Java to JSON — 3 modes 100% client-side

Java class input

Instances

Formatting

Ready

Three ways to convert Java to JSON

Each mode covers a different intent — choose the one that fits your situation.

Mode 1 — Mock JSON

Paste a Java class definition and get a realistic JSON object with example values for each field. Useful for test fixtures, Postman collections, API documentation and Storybook stories — without spinning up a Spring Boot server.

Input

private Integer id;
private String name;
private Boolean active;

Output

{"id":1,"name":"string_1",
 "active":true}

Mode 2 — Serialization snippets

Get ready-to-run Java code for serializing objects to JSON with Jackson, Gson, JSON-B or Moshi. Covers basic serialization, pretty-print, List serialization, file output and Spring Boot REST controller patterns.

Jackson output

ObjectMapper mapper =
  new ObjectMapper();
String json = mapper
  .writeValueAsString(obj);

Mode 3 — Library Guide

A concise comparison of Jackson, Gson, JSON-B and Moshi — covering Maven dependencies, key features, default behaviour differences and when to choose each library. Includes a quick-reference decision table.

Comparison covers

Jackson vs Gson vs JSON-B
Spring Boot default
Android recommendation
Performance comparison
Annotation requirements

When do you need Java to JSON?

The four most common workflows where Java objects become JSON.

Spring Boot REST APIs

Spring Boot uses Jackson by default — when you return an object from a @RestController method, it is automatically serialized to JSON. For manual serialization, logging or testing, get the exact Jackson snippet for your scenario from Mode 2. Jackson ObjectMapper is already on the classpath in any Spring Boot project.

Test fixtures and mocking

Unit tests and integration tests need sample JSON payloads that match your POJO structure. Mode 1 generates them from your class definition — without instantiating the class or running a server. The output is ready for use with MockMvc, Postman or WireMock.

Android development

Android apps typically use Gson or Moshi for JSON serialization. Mode 2 gives you the exact Gson or Moshi snippet for your scenario — including Retrofit integration patterns and the @SerializedName annotation for field name mapping.

API documentation

OpenAPI and Swagger documentation requires JSON examples for every request and response schema. Mode 1 generates a realistic JSON sample from your response POJO — ready to paste into your @ApiResponse annotation or OpenAPI YAML specification.

Java JSON library quick reference

The four main libraries and their one-liner serialization methods.

Library	Serialize	Deserialize	Best for
Jackson	`mapper.writeValueAsString(obj)`	`mapper.readValue(json, T.class)`	Spring Boot · enterprise Java · streaming large payloads
Gson	`new Gson().toJson(obj)`	`new Gson().fromJson(json, T.class)`	Android · standalone Java · no-config POJOs
JSON-B	`Jsonb.create().toJson(obj)`	`Jsonb.create().fromJson(json, T.class)`	Jakarta EE · Quarkus · standards-based
Moshi	`moshi.adapter(T.class).toJson(obj)`	`moshi.adapter(T.class).fromJson(json)`	Android (modern) · Kotlin-first · Retrofit

Related tools

All code to json tools

Java parsed in your
browser. No server needed.

The mock JSON mode uses a field-level regex parser — no need to compile or run Java. The parser extracts field names and types from class, record and Lombok definitions, mapping them to realistic JSON values. The serialization snippets are generated statically — no API call, no server round-trip.

The library guide covers the actual default behaviour differences between Jackson, Gson, JSON-B and Moshi — including null handling, transient field behaviour, date serialization and the key annotation for each library.

No Java compiler needed

The mock JSON parser works on the class definition text — no compilation, no classpath, no IDE required.

4 libraries covered

Jackson, Gson, JSON-B and Moshi — with complete Maven/Gradle dependency snippets and real-world usage patterns for each.

Spring Boot ready

The Spring Boot REST controller snippet works with the Jackson version already included in spring-boot-starter-web — no additional dependencies needed.

52 tools, always free

No watermarks, no account. Funded by non-intrusive display advertising only.

Frequently asked questions

Common questions about converting Java objects to JSON.

How do I convert a Java object to JSON?

The easiest way is Jackson: add jackson-databind to your project and call new ObjectMapper().writeValueAsString(myObject). For Spring Boot projects, Jackson is already on the classpath. Select the Serialization Snippets mode above for the complete ready-to-run code.

How do I serialize a Java object to JSON with Jackson?

Create an ObjectMapper instance and call writeValueAsString(object). For pretty output: mapper.writerWithDefaultPrettyPrinter().writeValueAsString(object). For writing to a file: mapper.writeValue(new File("output.json"), object). Use @JsonProperty("keyName") on fields to customise the JSON key names.

What is the difference between Jackson and Gson?

Jackson is the default in Spring Boot, faster for large payloads and more configurable. Gson is simpler — works with private fields without getters and requires no annotations for basic use cases. For new Spring Boot projects, Jackson is the natural choice. For Android or standalone Java without Spring, Gson is often simpler.

How do I generate a JSON sample from a Java class?

Select the Mock JSON mode, paste your Java class definition, and click Generate. The tool parses field names and types and produces a JSON object with realistic example values — ready for Postman, MockMvc tests or API documentation without running any Java code.

Is the Java to JSON converter free?

Yes, completely free. No account required. JSONSwitcher is funded by non-intrusive display advertising.

Does it support primitive fields and nested generics?

Yes. int, long, boolean and double fields, arrays, List, Set, Optional and nested generics such as Map<String, List<Item>> are all turned into sample JSON.

Convert Java to JSONMock data, snippetsor library guide.