Your First Mock Server

Create your first mock API server in 10 minutes. Step-by-step tutorial covering YAML configuration, HTTP endpoints, JSON responses, and testing. Perfect for beginners to start mocking APIs quickly.

In this tutorial, you'll create a simple but functional mock API server from scratch. By the end, you'll have a working mock server that responds to HTTP requests with JSON data.

What you'll learn

• How to write a basic moclojer configuration file

• How to define HTTP endpoints

• How to structure JSON responses

• How to start and test your mock server

What you'll build

A simple user API with these endpoints:

• GET /users - List all users

• GET /users/123 - Get a specific user

• GET /health - Health check endpoint

Prerequisites

• moclojer installed (see installation guide)

• A text editor

• Command line access

• curl or a tool to make HTTP requests

Step 1: Create your configuration file

Create a new file called moclojer.yml in an empty directory:

- endpoint:
    method: GET
    path: /health
    response:
      status: 200
      headers:
        Content-Type: application/json
      body: >
        {
          "status": "ok",
          "service": "user-api",
          "timestamp": "2024-01-15T10:30:00Z"
        }

What this does:

• Creates one endpoint that responds to GET /health

• Returns HTTP status 200 (success)

• Sets the response content type to JSON

• Returns a simple JSON object with status information

Step 2: Start your mock server

Open your terminal in the directory with moclojer.yml and run:

# Using Docker
docker run -it -p 8000:8000 -v $(pwd)/moclojer.yml:/app/moclojer.yml ghcr.io/moclojer/moclojer:latest

# Using JAR file
java -jar moclojer.jar --config moclojer.yml

# Using native binary
./moclojer --config moclojer.yml

You should see output similar to:

Starting moclojer server on port 8000...
Server started successfully!

Step 3: Test your first endpoint

Open a new terminal window and test your endpoint:

curl http://localhost:8000/health

You should get this response:

{
  "status": "ok",
  "service": "user-api",
  "timestamp": "2024-01-15T10:30:00Z"
}

🎉 Congratulations! You've created your first mock API endpoint.

Step 4: Add more endpoints

Now let's expand your API. Stop the server (Ctrl+C) and update your moclojer.yml file:

- endpoint:
    method: GET
    path: /health
    response:
      status: 200
      headers:
        Content-Type: application/json
      body: >
        {
          "status": "ok",
          "service": "user-api",
          "timestamp": "2024-01-15T10:30:00Z"
        }

- endpoint:
    method: GET
    path: /users
    response:
      status: 200
      headers:
        Content-Type: application/json
      body: >
        [
          {
            "id": 1,
            "name": "Alice Johnson",
            "email": "[email protected]",
            "role": "admin"
          },
          {
            "id": 2,
            "name": "Bob Smith",
            "email": "[email protected]",
            "role": "user"
          },
          {
            "id": 3,
            "name": "Carol Davis",
            "email": "[email protected]",
            "role": "user"
          }
        ]

- endpoint:
    method: GET
    path: /users/1
    response:
      status: 200
      headers:
        Content-Type: application/json
      body: >
        {
          "id": 1,
          "name": "Alice Johnson",
          "email": "[email protected]",
          "role": "admin",
          "created_at": "2024-01-01T00:00:00Z",
          "last_login": "2024-01-15T09:30:00Z"
        }

What's new:

• /users endpoint returns a list of users

• /users/1 endpoint returns details for a specific user

• Each response includes different data structures (array vs object)

Step 5: Test your expanded API

Restart your server and test the new endpoints:

# Test the users list
curl http://localhost:8000/users

# Test a specific user
curl http://localhost:8000/users/1

# Test the health check still works
curl http://localhost:8000/health

Step 6: Handle different HTTP methods

Let's add a POST endpoint to create users. Add this to your moclojer.yml:

- endpoint:
    method: POST
    path: /users
    response:
      status: 201
      headers:
        Content-Type: application/json
      body: >
        {
          "id": 4,
          "name": "New User",
          "email": "[email protected]",
          "role": "user",
          "created_at": "2024-01-15T10:30:00Z",
          "message": "User created successfully"
        }

Test it:

curl -X POST \
  -H "Content-Type: application/json" \
  -d '{"name": "John Doe", "email": "[email protected]"}' \
  http://localhost:8000/users

Step 7: Add error responses

Real APIs return errors sometimes. Let's add a 404 response:

- endpoint:
    method: GET
    path: /users/999
    response:
      status: 404
      headers:
        Content-Type: application/json
      body: >
        {
          "error": "User not found",
          "code": "USER_NOT_FOUND",
          "message": "User with ID 999 does not exist"
        }

Test it:

curl http://localhost:8000/users/999

Understanding the configuration

Let's break down what you've learned:

Endpoint structure

- endpoint:           # Start of endpoint definition
    method: GET       # HTTP method (GET, POST, PUT, DELETE, etc.)
    path: /users      # URL path
    response:         # What to return
      status: 200     # HTTP status code
      headers:        # HTTP headers
        Content-Type: application/json
      body: >         # Response content
        {"message": "Hello"}

Key concepts

• Method: What HTTP verb this endpoint responds to

• Path: The URL pattern to match

• Status: HTTP status code (200=success, 404=not found, etc.)

• Headers: HTTP response headers (like Content-Type)

• Body: The actual response content

YAML tips

• Use > for multi-line strings (like JSON)

• Indentation matters - use 2 spaces consistently

• Each endpoint starts with - endpoint:

📚 Learn more: See the complete YAML Format Guide for advanced syntax, best practices, and troubleshooting.

What you've accomplished

✅ Created a working mock API server ✅ Defined multiple endpoints with different HTTP methods ✅ Structured JSON responses ✅ Added error handling ✅ Tested everything with curl

Your complete configuration

Here's your final moclojer.yml:

- endpoint:
    method: GET
    path: /health
    response:
      status: 200
      headers:
        Content-Type: application/json
      body: >
        {
          "status": "ok",
          "service": "user-api",
          "timestamp": "2024-01-15T10:30:00Z"
        }

- endpoint:
    method: GET
    path: /users
    response:
      status: 200
      headers:
        Content-Type: application/json
      body: >
        [
          {
            "id": 1,
            "name": "Alice Johnson",
            "email": "[email protected]",
            "role": "admin"
          },
          {
            "id": 2,
            "name": "Bob Smith",
            "email": "[email protected]",
            "role": "user"
          },
          {
            "id": 3,
            "name": "Carol Davis",
            "email": "[email protected]",
            "role": "user"
          }
        ]

- endpoint:
    method: GET
    path: /users/1
    response:
      status: 200
      headers:
        Content-Type: application/json
      body: >
        {
          "id": 1,
          "name": "Alice Johnson",
          "email": "[email protected]",
          "role": "admin",
          "created_at": "2024-01-01T00:00:00Z",
          "last_login": "2024-01-15T09:30:00Z"
        }

- endpoint:
    method: POST
    path: /users
    response:
      status: 201
      headers:
        Content-Type: application/json
      body: >
        {
          "id": 4,
          "name": "New User",
          "email": "[email protected]",
          "role": "user",
          "created_at": "2024-01-15T10:30:00Z",
          "message": "User created successfully"
        }

- endpoint:
    method: GET
    path: /users/999
    response:
      status: 404
      headers:
        Content-Type: application/json
      body: >
        {
          "error": "User not found",
          "code": "USER_NOT_FOUND",
          "message": "User with ID 999 does not exist"
        }

Next steps

You now have a solid foundation! But your API is still static - it always returns the same responses. In the next tutorial, you'll learn how to make your responses dynamic based on the request data.

👉 Dynamic Responses Tutorial - Learn to use path parameters, query parameters, and templates

Need help?

• Stuck? Check the Troubleshooting Guide with 20+ common issues solved

• Want practical examples? See CRUD Operations guide

• Have questions? Join the community discussions

See Also

• YAML Format Guide - Complete YAML syntax and best practices

• HTTP Methods - Detailed guide on GET, POST, PUT, DELETE, etc.

• CLI Reference - All command-line options

• OpenAPI Format - Alternative: Use OpenAPI specs

• Postman Collections - Alternative: Export from Postman