How to Format and Validate JSON in Java (Jackson, Gson, and No-Library Options)

Java has no built-in JSON support in the standard library — unlike Python or JavaScript, you need a library. Here’s the fastest path with the two most common ones, and what to reach for when adding a dependency isn’t worth it for a one-off check.

Jackson: ObjectMapper with a pretty printer

Jackson is the de facto standard in most Spring-based projects (Spring Boot pulls it in transitively):

ObjectMapper mapper = new ObjectMapper();
Object json = mapper.readValue(rawJsonString, Object.class);
String pretty = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(json);
System.out.println(pretty);

readValue throws JsonProcessingException (specifically JsonParseException for malformed syntax, MismatchedInputException for type mismatches) if the input is invalid — the message includes a line/column, but it’s often buried in a long exception chain that’s easy to miss when scrolling a stack trace.

Gson: shorter, fewer options

Gson gson = new GsonBuilder().setPrettyPrinting().create();
JsonElement json = JsonParser.parseString(rawJsonString);
System.out.println(gson.toJson(json));

Gson throws JsonSyntaxException on malformed input. It’s lighter-weight than Jackson and a common choice on Android specifically, though Jackson has taken over most server-side Java in recent years.

Validating structure, not just syntax

Both libraries confirm the JSON parses, but neither validates it against a schema — checking required fields exist, types match, enums are constrained — out of the box. For that you’d add everit-org/json-schema or networknt/json-schema-validator on top, which is a real dependency-and-setup cost just to sanity-check a payload shape.

When you don’t want to touch the build file

Adding a Maven/Gradle dependency, waiting for it to resolve, and writing a throwaway main() method is a lot of overhead just to check whether a JSON blob a teammate pasted in Slack is even valid. For that case, JSON Validator checks syntax and schema conformance directly in the browser — no build step, and it’ll point at the exact line if something’s wrong, which beats digging a line number out of a Jackson stack trace.

If you’re consuming a JSON API response and want a starting point for a Java model class instead of hand-typing fields, JSON to TypeScript at least gives you the accurate field names and nesting to translate into Java DTOs by hand — Jackson’s @JsonProperty mapping is far less error-prone when you’re not guessing at the shape from a wall of minified text.

The quick decision

• Already in a Spring Boot / Jackson project → writerWithDefaultPrettyPrinter(), no new dependency needed
• Android or already using Gson → setPrettyPrinting()
• Just checking a pasted blob, no code involved → JSON Formatter for syntax, JSON Validator if you also care about schema/shape