Table viewer using JQuery plugin and JEE

DataTables is a JQuery plugin, facilitating building Ajax table editors. In this example, I show how to connect it to JEE backend, which is a simple Servlet.

Backend exposes a table, stored as a Java List within Servlet instance. Data is served back in JSON format using Jackson library. Example is deployed on Google App Engine, datatablesjee.appspot.com. Code is available on GitHub github.com/rafalrusin/datatablesjee.

First, we need to instantiate DataTables plugin within a html page. This is extremely easy, by using code below: AjaxSource parameter refers to Servlet URI, which handles requests for data. ServerSide argument is set to true, which means that backend will do sorting, filtering and pagination. This allows us to use large tables (>1000 rows) without performance problems on client side.

Now, we need to implement backend. Servlet requires a doGet method, which needs to retrieve parameters sent from client as an Ajax request. Those parameters describe search keyword, starting row and page size of a table.Then, we need to do filtering and sorting. I used a simple toString + contains methods on a single row in order to do filtering.

Sorting is done via custom comparator, which sorts by given column number. Following code does the job:

Last, we need to send JSON response back to client. Here, we use Jackson, which is a very convenient library for manipulating JSON in Java.iTotalRecords is total number of records, without filtering. iTotalDisplayRecords is number of records after applying filter. aaData is two dimensional array of strings, representing visible table data.

Summing up, I like the idea of Ajax in this form, because client side is not rendered directly by backend (no jsp, etc.). This makes it a detached view, which could be served as static content, from Apache Web Server for example, which is very performant.

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How to use mocks in controller tests

Even since I started to write tests for my Grails application I couldn't find many articles on using mocks. Everyone is talking about tests and TDD but if you search for it there isn't many articles.

Today I want to share with you a test with mocks for a simple and complete scenario. I have a simple application that can fetch Twitter tweets and present it to user. I use REST service and I use GET to fetch tweets by id like this: http://api.twitter.com/1/statuses/show/236024636775735296.json. You can copy and paste it into your browser to see a result.

My application uses Grails 2.1 with spock-0.6 for tests. I have TwitterReaderService that fetches tweets by id, then I parse a response into my Tweet class.


class TwitterReaderService {
Tweet readTweet(String id) throws TwitterError {
try {
String jsonBody = callTwitter(id)
Tweet parsedTweet = parseBody(jsonBody)
return parsedTweet
} catch (Throwable t) {
throw new TwitterError(t)
}
}

private String callTwitter(String id) {
// TODO: implementation
}

private Tweet parseBody(String jsonBody) {
// TODO: implementation
}
}

class Tweet {
String id
String userId
String username
String text
Date createdAt
}

class TwitterError extends RuntimeException {}

TwitterController plays main part here. Users call show action along with id of a tweet. This action is my subject under test. I've implemented some basic functionality. It's easier to focus on it while writing tests.


class TwitterController {
def twitterReaderService

def index() {
}

def show() {
Tweet tweet = twitterReaderService.readTweet(params.id)
if (tweet == null) {
flash.message = 'Tweet not found'
redirect(action: 'index')
return
}

[tweet: tweet]
}
}

Let's start writing a test from scratch. Most important thing here is that I use mock for my TwitterReaderService. I do not construct new TwitterReaderService(), because in this test I test only TwitterController. I am not interested in injected service. I know how this service is supposed to work and I am not interested in internals. So before every test I inject a twitterReaderServiceMock into controller:


import grails.test.mixin.TestFor
import spock.lang.Specification

@TestFor(TwitterController)
class TwitterControllerSpec extends Specification {
TwitterReaderService twitterReaderServiceMock = Mock(TwitterReaderService)

def setup() {
controller.twitterReaderService = twitterReaderServiceMock
}
}

Now it's time to think what scenarios I need to test. This line from TwitterReaderService is the most important:


Tweet readTweet(String id) throws TwitterError

You must think of this method like a black box right now. You know nothing of internals from controller's point of view. You're only interested what can be returned for you:

  • a TwitterError can be thrown
  • null can be returned
  • Tweet instance can be returned

This list is your test blueprint. Now answer a simple question for each element: "What do I want my controller to do in this situation?" and you have plan test:

  • show action should redirect to index if TwitterError is thrown and inform about error
  • show action should redirect to index and inform if tweet is not found
  • show action should show found tweet

That was easy and straightforward! And now is the best part: we use twitterReaderServiceMock to mock each of these three scenarios!

In Spock there is a good documentation about interaction with mocks. You declare what methods are called, how many times, what parameters are given and what should be returned. Remember a black box? Mock is your black box with detailed instruction, e.g.: I expect you that if receive exactly one call to readTweet with parameter '1' then you should throw me a TwitterError. Rephrase this sentence out loud and look at this:


1 * twitterReaderServiceMock.readTweet('1') >> { throw new TwitterError() }

This is a valid interaction definition on mock! It's that easy! Here is a complete test that fails for now:


import grails.test.mixin.TestFor
import spock.lang.Specification

@TestFor(TwitterController)
class TwitterControllerSpec extends Specification {
TwitterReaderService twitterReaderServiceMock = Mock(TwitterReaderService)

def setup() {
controller.twitterReaderService = twitterReaderServiceMock
}

def "show should redirect to index if TwitterError is thrown"() {
given:
controller.params.id = '1'
when:
controller.show()
then:
1 * twitterReaderServiceMock.readTweet('1') >> { throw new TwitterError() }
0 * _._
flash.message == 'There was an error on fetching your tweet'
response.redirectUrl == '/twitter/index'
}
}

| Failure: show should redirect to index if TwitterError is thrown(pl.refaktor.twitter.TwitterControllerSpec)
| pl.refaktor.twitter.TwitterError
at pl.refaktor.twitter.TwitterControllerSpec.show should redirect to index if TwitterError is thrown_closure1(TwitterControllerSpec.groovy:29)

You may notice 0 * _._ notation. It says: I don't want any other mocks or any other methods called. Fail this test if something is called! It's a good practice to ensure that there are no more interactions than you want.

Ok, now I need to implement controller logic to handle TwitterError.


class TwitterController {

def twitterReaderService

def index() {
}

def show() {
Tweet tweet

try {
tweet = twitterReaderService.readTweet(params.id)
} catch (TwitterError e) {
log.error(e)
flash.message = 'There was an error on fetching your tweet'
redirect(action: 'index')
return
}

[tweet: tweet]
}
}

My tests passes! We have two scenarios left. Rule stays the same: TwitterReaderService returns something and we test against it. So this line is the heart of each test, change only returned values after >>:


1 * twitterReaderServiceMock.readTweet('1') >> { throw new TwitterError() }

Here is a complete test for three scenarios and controller that passes it.


import grails.test.mixin.TestFor
import spock.lang.Specification

@TestFor(TwitterController)
class TwitterControllerSpec extends Specification {

TwitterReaderService twitterReaderServiceMock = Mock(TwitterReaderService)

def setup() {
controller.twitterReaderService = twitterReaderServiceMock
}

def "show should redirect to index if TwitterError is thrown"() {
given:
controller.params.id = '1'
when:
controller.show()
then:
1 * twitterReaderServiceMock.readTweet('1') >> { throw new TwitterError() }
0 * _._
flash.message == 'There was an error on fetching your tweet'
response.redirectUrl == '/twitter/index'
}

def "show should inform about not found tweet"() {
given:
controller.params.id = '1'
when:
controller.show()
then:
1 * twitterReaderServiceMock.readTweet('1') >> null
0 * _._
flash.message == 'Tweet not found'
response.redirectUrl == '/twitter/index'
}


def "show should show found tweet"() {
given:
controller.params.id = '1'
when:
controller.show()
then:
1 * twitterReaderServiceMock.readTweet('1') >> new Tweet()
0 * _._
flash.message == null
response.status == 200
}
}

class TwitterController {

def twitterReaderService

def index() {
}

def show() {
Tweet tweet

try {
tweet = twitterReaderService.readTweet(params.id)
} catch (TwitterError e) {
log.error(e)
flash.message = 'There was an error on fetching your tweet'
redirect(action: 'index')
return
}

if (tweet == null) {
flash.message = 'Tweet not found'
redirect(action: 'index')
return
}

[tweet: tweet]
}
}

The most important thing here is that we've tested controller-service interaction without logic implementation in service! That's why mock technique is so useful. It decouples your dependencies and let you focus on exactly one subject under test. Happy testing!