# unit-test-json-serialization

A skill with patterns for unit testing JSON serialization and deserialization using Jackson and @JsonTest in Spring Boot.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 3800 installs · ★ 363
- Author: giuseppe-trisciuoglio
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/giuseppe-trisciuoglio/developer-kit/tree/HEAD/plugins/developer-kit-java/skills/unit-test-json-serialization/SKILL.md
- Updated: 2026-09-10
- HTML: https://affpapa.org/en/nevibecode/skills/unit-test-json-serialization

## What it does

The skill lays out test patterns for POJO mapping, custom serializers, field name mappings via `@JsonProperty`, `@JsonIgnore`, nested objects, lists, date and time formats, and polymorphic types. Everything runs through `JacksonTester`, so you assert JSON paths with `extractingJsonPath*` and parse strings back into objects.

The workflow is short: annotate the test class with `@JsonTest`, autowire `JacksonTester` for the target type, check serialization, then deserialization, then edge cases like nulls, empty collections and missing fields. It also tells you to confirm each test fails meaningfully when the data is wrong.

You need `spring-boot-starter-json` and `spring-boot-starter-test`; the README shows both Maven and Gradle blocks. This is Spring Boot territory. In a plain Java project without Spring, `@JsonTest` and the `JacksonTester` auto-configuration are not there, so the examples have to be rewritten against bare Jackson.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add giuseppe-trisciuoglio/developer-kit --skill unit-test-json-serialization -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/unit-test-json-serialization/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/unit-test-json-serialization/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add giuseppe-trisciuoglio/developer-kit --skill unit-test-json-serialization -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/unit-test-json-serialization/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add giuseppe-trisciuoglio/developer-kit --skill unit-test-json-serialization -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/unit-test-json-serialization/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install giuseppe-trisciuoglio/developer-kit unit-test-json-serialization
```

2. **Manually: put the files in place**
- File: `.github/skills/unit-test-json-serialization/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add giuseppe-trisciuoglio/developer-kit --skill unit-test-json-serialization -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/giuseppe-trisciuoglio/developer-kit --path plugins/developer-kit-java/skills/unit-test-json-serialization
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/unit-test-json-serialization/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add giuseppe-trisciuoglio/developer-kit --skill unit-test-json-serialization -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/giuseppe-trisciuoglio/developer-kit/tree/HEAD/plugins/developer-kit-java/skills/unit-test-json-serialization/SKILL.md

## When to use it
- check that a DTO emits the expected JSON field names
- catch a regression in a custom money serializer
- confirm `@JsonIgnore` keeps an internal field out of the payload
- deserialize a polymorphic payload into the right subclass
- verify the `LocalDateTime` format in a response

## Example prompts
- Write a `@JsonTest` for `UserDto` and assert serialization with `JacksonTester`
- Add a deserialization test for JSON with `order_id` and `total_amount` into `Order`
- Check that the custom serializer renders the amount as `$99.99`

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 296aa3aa50c79b51.

BEGIN THIRD-PARTY FILE 296aa3aa50c79b51
````text
---
name: unit-test-json-serialization
description: Provides patterns for unit testing JSON serialization/deserialization with Jackson and `@JsonTest`. Validates JSON mapping, custom serializers, date formats, and polymorphic types. Use when testing JSON serialization, validating custom serializers, or writing JSON unit tests in Spring Boot applications.
allowed-tools: Read, Write, Bash, Glob, Grep
---

# Unit Testing JSON Serialization with `@JsonTest`

## Overview

Provides patterns for unit testing JSON serialization and deserialization using Spring's `@JsonTest` and Jackson. Covers POJO mapping, custom serializers, field name mappings, nested objects, date/time formatting, and polymorphic types.

## When to Use

- Testing JSON serialization/deserialization of DTOs
- Verifying custom Jackson serializers/deserializers
- Validating `@JsonProperty`, `@JsonIgnore`, and field name mappings
- Testing date/time format handling (LocalDateTime, Date)
- Testing null handling and missing fields
- Testing polymorphic type deserialization

## Instructions

1. **Annotate test class with `@JsonTest`** → Enables JacksonTester auto-configuration
2. **Autowire JacksonTester for target type** → Provides type-safe JSON assertions
3. **Test serialization** → Call `json.write(object)` and assert JSON paths with `extractingJsonPath*`
4. **Test deserialization** → Call `json.parse(json)` or `json.parseObject(json)` and assert object state
5. **Validate round-trip** → Serialize, then deserialize, verify same data (if object is properly comparable)
6. **Test edge cases** → Null values, missing fields, empty collections, invalid JSON
7. **Add validation checkpoints**: After each assertion, verify the test fails meaningfully with wrong data

## Examples

### Maven Setup
```xml
<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-json</artifactId>
</dependency>
<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-test</artifactId>
  <scope>test</scope>
</dependency>
```

### Gradle Setup
```kotlin
dependencies {
  implementation("org.springframework.boot:spring-boot-starter-json")
  testImplementation("org.springframework.boot:spring-boot-starter-test")
}
```

### Basic Serialization and Deserialization

```java
@JsonTest
````
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First 2327 of 10272 bytes. © giuseppe-trisciuoglio · MIT · https://github.com/giuseppe-trisciuoglio/developer-kit/tree/HEAD/plugins/developer-kit-java/skills/unit-test-json-serialization/SKILL.md

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