mock_server.md

How to implement a Mock Server in Android using Json Server?

I recently had a requirement to create a mock server for testing purposes in Android. This means we would have to setup a mock server in our system which would simulate the backend response. A great advantage of this is that the app can be tested without any dependency on the backend.

We can implement a mock server using JsonServer. Let's see how to implement this.

• Step 1: Install JSON Server:

  npm install -g json-server
  

  Note: In order to run the npm command, we need to install node.js first. Use this link to download and install it.

• Step 2: Create a db.json file in the root folder of your app project. This file will contain all the mock responses we need to simulate from the server. Let's say you have a login screen. And the login response is something like this:

  {
      "status": 2,
      "message": "Login is successful"
  }
  

  Then we would need to add a mock response in the db.json file like this:

  {
    "login": {
        "status" : 1,
        "message": "User logged in successfully"
    }
  }
  

  We need to give a name to every request data we add. This is very important. We need to add For instance, if we need to add data for logout, then the db.json will look like this:

  {
    "login": {
        "status" : 1,
        "message": "User logged in successfully"
    },
    "logout" : {
      "status" : 1,
      message: "User logged out"
    }
  }
  

• Step 3: Create a routes.json file in the root folder of your app project. This file will map your custom path urls to the corresponding mock response in the db.json. So for the above login and logout response, the routes.json will be:

  {
    "/api/logout": "/logout",    // -> Change the "api/logout" to your relevant path url. 
    "/api/login\\?userId=:userId": "/login?userId=:userId" // -> Change the "api/login" to your relevant path url. If we need to pass a query parameter to the request, then we can do it like this.
  }
  

  You can create a whole bunch of routes for your servers, like adding path parameters, query parameters, request body etc. Checkout the entire list from here.

• Step 4: Create a middleware.js file in the root folder of your app project. An example would be:

  module.exports = function (req, res, next) {
    if (req.method === 'POST') {
      // Converts POST to GET and move payload to query params
      // This way it will make JSON Server that it's GET request
      req.method = 'GET'
      req.query = req.body
    }
    // Continue to JSON Server router
    next();
  }
  

• Step 5: Open the terminal inside your Android Studio and run the below command to start your local server: json-server --watch db.json --port 3004 --routes routes.json --middlewares middleware.js

• Step 6: Change the base url of your server in the app. I generally use ngrok to get a mock url of the local server. You can use this guide to get started with ngrok.