Poll Application

• Purpose
  • to demonstrate the basic functionality of the Spring Framework
  • to demonstrate API testing via Postman

Part 1 - Domain Implementation

• Domain objects are the backbone for an application and contain the business logic.
• Create a sub package of io.zipcoder.tc_spring_poll_application named domain.

Part 1.1 - Create class Option

• Create an Option class in the domain sub-package.

• Option class signature is annotated with @Entity

• Option has an id instance variable of type Long

  • id should be annotated with
    • @Id
      • denotes primary key of this entity
    • @GeneratedValue
      • configures the way of increment of the specified column(field)
    • @Column(name = "OPTION_ID")
      • specifies mapped column for a persistent property or field
      • without @Column specified, the framework assumes the field's variable-name is the persistent property name.

• Option has a value instance variable of type String

  • value should be annotated with
    • @Column(name = "OPTION_VALUE")

• Create a getter and setter for each of the respective instance variables.

Part 1.2 - Create class Poll

• Create a Poll class in the domain sub-package.

• Poll class signature is annotated with @Entity

• Poll has an id instance variable of type Long

  • id should be annotated with
    • @Id
    • @GeneratedValue
    • Column(name = "POLL_ID")

• Poll has a question instance variable of type String

  • question should be annotated with
    • @Column(name = "QUESTION")

• Poll has an options instance variable of type Set of Option

  • options should be annotated with
    • @OneToMany(cascade = CascadeType.ALL)
    • @JoinColumn(name = "POLL_ID")
    • @OrderBy

• Create a getter and setter for each of the respective instance variables.

Part 1.3 - Create class Vote

• Create a Vote class in the domain sub-package.

• Vote class signature is annotated with @Entity

• Vote has an id instance variable of type Long

  • id should be annotated with
    • @Id
    • @GeneratedValue
    • Column(name = "VOTE_ID")

• Vote has a option instance variable of type Option

  • option should be annotated with
    • @ManyToOne
    • @JoinColumn(name = "OPTION_ID")

• Create a getter and setter for each of the respective instance variables.

Part 2 - Repository Implementation

• Repositories or Data Access Objects (DAO), provide an abstraction for interacting with datastores.
• Typically DAOs include an interface that provides a set of finder methods such as findById, findAll, for retrieving data, and methods to persist and delete data.
• It is customary to have one Repository per domain object.
• Create a sub-package of io.zipcoder.tc_spring_poll_application named repositories.

Part 2.1 - Create interface OptionRepository

• Create an OptionRepository interface in the repositories subpackage.
• OptionRepository is a subclass of CrudRepository<Option, Long>

Part 2.2 - Create interface PollRepository

• Create a PollRepository interface in the repositories subpackage.
• PollRepository is a subclass of CrudRepository<Poll, Long>

Part 2.3 - Create interface VoteRepository

• Create a VoteRepository interface in the repositories subpackage.
• VoteRepository is a subclass of CrudRepository<Vote, Long>

Part 3 - Controller Implementation

• Controllers provides all of the necessary endpoints to access and manipulate respective domain objects.
  • REST resources are identified using URI endpoints.
• Create a sub package of io.zipcoder.tc_spring_poll_application named controller.

Part 3.1 - Create class PollController

• Create a PollController class in the controller sub package.

  • PollController signature should be annotated with @RestController

• PollController has a pollRepository instance variable of type PollRepository

• Create a constructor that accepts a PollRepository argument and assigns its value to the pollRepository member variable.

  • Mark the constructor with the @Autowired annotation.

Part 3.1.1 - Create GET request method

• The method definition below supplies a GET request on the /polls endpoint which provides a collection of all of the polls available in the QuickPolls application. Copy and paste this into your PollController class.

@GetMapping("/polls")
    public ResponseEntity<Iterable<Poll>> getAllPolls() {
        Iterable<Poll> allPolls = pollRepository.findAll();
        return new ResponseEntity<>(allPolls, HttpStatus.OK);
    }

• The method above begins with reading all of the polls using the PollRepository.
• We then create an instance of ResponseEntity and pass in Poll data and the HttpStatus.OK status value.
• The Poll data becomes part of the response body and OK (code 200) becomes the response status code.

Part 3.1.2 - Testing via Postman

• Ensure that the start-class tag in your pom.xml encapsulates io.zipcoder.springdemo.QuickPollApplication
• Open a command line and navigate to the project's root directory and run this command:
  • mvn spring-boot:run
• Launch the Postman app and enter the URI http://localhost:8080/polls and hit Send.
• Because we don’t have any polls created yet, this command should result in an empty collection.
• If your application cannot run because something is occupying a port, use this command with the respective port number specified:
  • kill -kill `lsof -t -i tcp:8080`

Part 3.1.3 - Create POST request method

• We accomplish the capability to add new polls to the PollController by implementing the POST verb functionality in a createPoll method:

@PostMapping("/polls")
    public ResponseEntity<?> createPoll(@RequestBody Poll poll){
       poll = pollRepository.save(poll);
       return new ResponseEntity<>(null, HttpStatus.CREATED);
    }

• Take note that the method
  • has a parameter of type @RequestBody Poll poll
    • @RequestBody tells Spring that the entire request body needs to be converted to an instance of Poll
  • delegates the Poll persistence to PollRepository’s save method
    • poll = pollRepository.save(poll);

Part 3.1.4 - Modify createPoll

• Best practice is to convey the URI to the newly created resource using the Location HTTP header via Spring's ServletUriComponentsBuilder utility class. This will ensure that the client has some way of knowing the URI of the newly created Poll.

URI newPollUri = ServletUriComponentsBuilder
	.fromCurrentRequest()
	.path("/{id}")
	.buildAndExpand(poll.getId())
	.toUri();

• Modify the createPoll method so that it returns a ResponseEntity which takes an argument of a new HttpHeaders() whose location has been set to the above newPollUri via the setLocation method.

Part 3.1.5 - Create GET request method

• The code snippet below enables us to access an individual poll.
• The value attribute in the @RequestMapping takes a URI template /polls/{pollId}.
• The placeholder {pollId} along with @PathVarible annotation allows Spring to examine the request URI path and extract the pollId parameter value.
• Inside the method, we use the PollRepository’s findOne finder method to read the poll and pass it as part of a ResponseEntity.

@RequestMapping(value="/polls/{pollId}", method=RequestMethod.GET)
public ResponseEntity<?> getPoll(@PathVariable Long pollId) {
	Poll p = pollRepository.findOne(pollId);
	return new ResponseEntity<> (p, HttpStatus.OK);
}

Part 3.1.6 - Create UPDATE request method

• The code snippet below enables us to update a poll.

@RequestMapping(value="/polls/{pollId}", method=RequestMethod.PUT)
public ResponseEntity<?> updatePoll(@RequestBody Poll poll, @PathVariable Long pollId) {
        // Save the entity
        Poll p = pollRepository.save(poll);
        return new ResponseEntity<>(HttpStatus.OK);
}

Part 3.1.7 - Create DELETE request method.

• The code snippet below enables us to delete a poll.

@RequestMapping(value="/polls/{pollId}", method=RequestMethod.DELETE)
public ResponseEntity<?> deletePoll(@PathVariable Long pollId) {
        pollRepository.delete(pollId);
        return new ResponseEntity<>(HttpStatus.OK);
}

Part 3.1.8 - Test Poll Creating Capabilities

• Restart the QuickPoll application.
• Use Postman to execute a POST to http://localhost:8080/polls/ whose request body is the JSON object below.
• You can modify the request body in Postman by navigating to the Body tab, selecting the raw radio button, and selecting the JSON option from the text format dropdown.

{
	"id": 1,
	"question": "What's the best netflix original?",
	"options": [
	    { "value": "Black Mirror" },
	    { "value": "Stranger Things" },
	    { "value": "Orange is the New Black"},
	    { "value": "The Get Down" }
	]
}

• Ensure the the data has been persisted by executing a GET to http://localhost:8080/polls/1
• Upon execution, you should receive this message body.

{
    "id": 1,
    "question": "What's the best netflix original?",
    "options": [
        {
            "id": 1,
            "value": "Stranger Things"
        },
        {
            "id": 2,
            "value": "The Get Down"
        },
        {
            "id": 3,
            "value": "Black Mirror"
        },
        {
            "id": 4,
            "value": "Orange is the New Black"
        }
    ]
}

Part 3.2 - Create class VoteController

• Following the principles used to create PollController, we implement the VoteController class.
• Below is the code for the VoteController class along with the functionality to create a vote.
• The VoteController uses an injected instance of VoteRepository to perform CRUD operations on Vote instances.

@RestController
public class VoteController {

    private VoteRepository voteRepository;
    
    @Autowired
    public VoteController(VoteRepository voteRepository) {
    	this.voteRepository = voteRepository;
    }

    @RequestMapping(value = "/polls/{pollId}/votes", method = RequestMethod.POST)
    public ResponseEntity<?> createVote(@PathVariable Long pollId, @RequestBody Vote
            vote) {
        vote = voteRepository.save(vote);
        // Set the headers for the newly created resource
        HttpHeaders responseHeaders = new HttpHeaders();
        responseHeaders.setLocation(ServletUriComponentsBuilder.
                fromCurrentRequest().path("/{id}").buildAndExpand(vote.getId()).toUri());
        return new ResponseEntity<>(null, responseHeaders, HttpStatus.CREATED);
    }
}

Part 3.2.1 - Testing VoteController

• To test the voting capabilities, POST a new Vote to the /polls/1/votes endpoint with the option object expressed in JSON below.
• On successful request execution, you will see a Location response header with value http://localhost:8080/polls/1/votes/1.

{
    "option": { "id": 1, "value": "Black Mirror" }
}

Part 3.2.2 - Modify VoteRepository

• The method findAll in the VoteRepository retrieves all votes in a Database rather than a given poll.
• To ensure we can get votes for a given poll, we must add the code below to our VoteRepository.

public interface VoteRepository extends CrudRepository<Vote, Long> {
    @Query(value = "SELECT v.* " +
            "FROM Option o, Vote v " +
            "WHERE o.POLL_ID = ?1 " +
            "AND v.OPTION_ID = o.OPTION_ID", nativeQuery = true)
    public Iterable<Vote> findVotesByPoll(Long pollId);
}

• The custom finder method findVotesByPoll takes the ID of the Poll as its parameter.
• The @Query annotation on this method takes a native SQL query along with the nativeQuery flag set to true.
• At runtime, Spring Data JPA replaces the ?1 placeholder with the passed-in pollId parameter value.

Part 3.2.3 - Modify VoteController

• Create a getAllVotes method in the VoteController

@RequestMapping(value="/polls/votes", method=RequestMethod.GET)
public Iterable<Vote> getAllVotes() {
	return voteRepository.findAll();
}

• Create a getVote method in the VoteController

@RequestMapping(value="/polls/{pollId}/votes", method=RequestMethod.GET)
public Iterable<Vote> getVote(@PathVariable Long pollId) {
	return voteRepository.findById(pollId);
}

Part 4 - Data Transfer Object (DTO) Implementation

• The final piece remaining for us is the implementation of the ComputeResult resource.
• Because we don’t have any domain objects that can directly help generate this resource representation, we implement two Data Transfer Objects or DTOs—OptionCount and VoteResult
• Create a sub package of java named dtos

Part 4.1 - Create class OptionCount

• The OptionCount DTO contains the ID of the option and a count of votes casted for that option.

public class OptionCount {
    private Long optionId;
    private int count;

    public Long getOptionId() {
        return optionId;
    }

    public void setOptionId(Long optionId) {
        this.optionId = optionId;
    }

    public int getCount() {
        return count;
    }

    public void setCount(int count) {
        this.count = count;
    }
}

Part 4.2 - Create class VoteResult

• The VoteResult DTO contains the total votes cast and a collection of OptionCount instances.

import java.util.Collection;
public class VoteResult {
    private int totalVotes;
    private Collection<OptionCount> results;

    public int getTotalVotes() {
        return totalVotes;
    }

    public void setTotalVotes(int totalVotes) {
        this.totalVotes = totalVotes;
    }

    public Collection<OptionCount> getResults() {
        return results;
    }

    public void setResults(Collection<OptionCount> results) {
        this.results = results;
    }
}

Part 4.3 - Create class ComputeResultController

• Following the principles used in creating the PollController and VoteController, we create a new ComputeResultController class

@RestController
public class ComputeResultController {
    
    private VoteRepository voteRepository;
    
    @Autowired
    public ComputeResultController(VoteRepository voteRepository) {
    	this.voteRepository = voteRepository;
    }

    @RequestMapping(value = "/computeresult", method = RequestMethod.GET)
    public ResponseEntity<?> computeResult(@RequestParam Long pollId) {
        VoteResult voteResult = new VoteResult();
        Iterable<Vote> allVotes = voteRepository.findVotesByPoll(pollId);

        //TODO: Implement algorithm to count votes
        return new ResponseEntity<VoteResult>(voteResult, HttpStatus.OK);
    }

• We inject an instance of VoteRepository into the controller, which is used to retrieve votes for a given poll.
• The computeResult method takes pollId as its parameter.
• The @RequestParam annotation instructs Spring to retrieve the pollId value from a HTTP query parameter.
• The computed results are sent to the client using a newly created instance of ResponseEntity.

Part 4.4 - Test via Postman

• Start/restart the QuickPoll application.
• Using the earlier Postman requests, create a poll and cast votes on its options.
• Ensure a JSON file with a status of 200 is returned by executing a GET request of http://localhost:8080/computeresult?pollId=1 via Postman

Part 5 - Error Handling

Part 5.1 - Create ResourceNotFoundException

• Create a exception package inside of io.zipcoder.springdemo.QuickPollApplication
• Create a ResourceNotFoundException class that extends RuntimeException. We'll use this to signal when a requested resource is not found.
• Annotate the ResourceNotFoundException class with @ResponseStatus(HttpStatus.NOT_FOUND). This informs Spring that any request mapping that throws a ResourceNotFoundException should result in a 404 NOT FOUND http status.
• Implement three constructors
  • A no-arg constructor
  • A constructor that takes a String message and passes it to the superclass constructor
  • A constructor that takes String message and Throwable cause and passes both to the superclass constructor

Part 5.2 - Verify polls

Create a void method in PollController called verifyPoll that checks if a specific poll id exists and throws a ResourceNotFoundException if not. Use this in any method that searches for or updates an existing poll (eg: Get, Put, and Delete methods).

Note: This means that trying to submit a PUT request for a resource that doesn't exist will not implicitly create it; it should throw a 404 instead.

Part 5.3 - Create custom Error Responses

Spring provides some built-in exception handling and error response, but we'll customize it a bit here. Create an ErrorDetail class in a new io.zipcoder.tc_spring_poll_application.dto.error package to hold relevant information any time an error occurs.

Fields (Don't forget to provide getters and setters):

• String title: a brief title of the error condition, eg: "Validation Failure" or "Internal Server Error"
• int status: the HTTP status code for the current request; redundant but useful for client-side error handling
• String detail: A short, human-readable description of the error that may be presented to a user
• long timeStamp: the time in milliseconds when the error occurred
• String developerMessage: detailed information such as exception class name or a stack trace useful for developers to debug

Part 5.4 - Create a @ControllerAdvice

In this section we add custom handling for the exceptions we created before. A @ControllerAdvice is an AOP feature that wraps a controller and adds some functionality when needed. In this case we are adding functionality only when an exception is thrown.

• Create RestExceptionHandler class annotated with @ControllerAdvice
• Create a handler method with the header shown below
• Populate an ErrorDetail object in the method, and return a ResponseEntity containing the ErrorDetail and an HTTP NOT_FOUND status
  • Use java.util's new Date().getTime() for the timestamp
  • Provide the detail and developer messages from the ResourceNotFoundException

@ExceptionHandler(ResourceNotFoundException.class)
public ResponseEntity<?> handleResourceNotFoundException(ResourceNotFoundException rnfe, HttpServletRequest request) {...}

Part 5.4 - Validating domain entities

Now it's time to make sure that all objects persisted to the database actually contain valid values. Use the org.hibernate.validator.constraints.NotEmpty and javax.validation.constraints.Size and javax.validation.Valid annotations for validation.

• In the Poll class:
  • options should be @Size(min=2, max = 6)
  • question should be @NotEmpty
• To enforce these validations, add @Valid annotations to Poll objects in RequestMapping-annotated controller methods (there should be 2)

Part 5.5 - Customizing validation errors

In order to customize validation errors we'll need a class for error information. Create a ValidationError class in io.zipcoder.tc_spring_poll_application.dto.error with the following fields and appropriate getters and setters:

• String code
• String message

We also need a new field in the ErrorDetail class to hold errors. There may be multiple validation errors associated with a request, sometimes more than one of the same type, so this field will be a collection, specifically a Map<String, List<ValidationError>> errors field.

Part 5.6 - Create a validation error handler

• add below handler to RestExceptionHandler

@ExceptionHandler(MethodArgumentNotValidException.class)
public ResponseEntity<?>
handleValidationError(  MethodArgumentNotValidException manve,
						HttpServletRequest request){...}

In this handler we need to do the following:

• Create the ErrorDetail object (similar to before)
• Get the list of field validation errors
• For each field error, add it to the appropriate list in the ErrorDetail (see below)
• Return a ResponseEntity containing the error detail and the appropriate HTTP status code (400 Bad Request)

List<FieldError> fieldErrors =  manve.getBindingResult().getFieldErrors();
for(FieldError fe : fieldErrors) {

	List<ValidationError> validationErrorList = errorDetail.getErrors().get(fe.getField());
	if(validationErrorList == null) {
		validationErrorList = new ArrayList<>();
		errorDetail.getErrors().put(fe.getField(), validationErrorList);
	}
	ValidationError validationError = new ValidationError();
	validationError.setCode(fe.getCode());
	validationError.setMessage(messageSource.getMessage(fe, null));
	validationErrorList.add(validationError);
}

Part 5.7 - Externalize strings in a messages.properties file

Commonly used strings in your Java program can be removed from the source code and placed in a separate file. This is called externalizing, and is useful for allowing changes to text displayed without impacting actual program logic. One example of where this is done is in internationalization, the practice of providing multilingual support in an application, allowing users to use an application in their native language.

There are two steps needed here to externalize and standardize the validation error messages:

• Create a messages.properties file in the src/main/resources directory with the given properties below
  • messages.properties is a key-value file stored in plain text. Your IDE may have a table-based view or show the contents as text
  • .properties files are a common idiom in Java applications; they contain additional information the application uses that doesn't impact the actual source code.
• Use an autowired MessageSource object in the RestExceptionHandler to set the message on ValidationError objects (ie: setMessage(messageSource.getMessage(fe,null)); )
  • This object will be autowired (or injected) the same way your CRUDRepository instances are.

messages.properties content:

NotEmpty.poll.question=Question is a required field
Size.poll.options=Options must be greater than {2} and less than {1}

Part 6 - Pagination

• To optimize performance, it is important to limit the amount of data returned, especially in the case of a mobile client.
• REST services have the ability to give clients access large datasets in manageable chunks, by splitting the data into discrete pages or paging data.
• For this lab, we will approach this by implementing the page number pagination pattern.

Get Data From Page

• For example, a client wanting a blog post in page 3 of a hypothetical blog service can use a GET method resembling the following: http://blog.example.com/posts?page=3

Limit Data Retrieved From Page

• It is possible for the client to override the default page size by passing in a page-size parameter: http://blog.example.com/posts?page=3&size=20

Pagination Data

• Pagination-specific information includes
  • total number of records
  • total number of pages
  • current page number
  • page size
• In the above blog-scenario, one would expect a response body with pagination information closely resembling the JSON object below.

{
"data": [
         ... Blog Data
    ],
    "totalPages": 9,
    "currentPageNumber": 2,
    "pageSize": 10,
    "totalRecords": 90
}

• Read more about REST pagination in Spring by clicking here.

Part 6.1 - Load Dummy Poll Data

• Create a src/main/resource/import.sql file with DML statements for populating the database upon bootstrap. The import.sql should insert at least 15 polls, each with 3 or more options.

  • Below is an example of SQL statements for creating a single poll with only one option.

    • Poll Creation

       insert into poll (poll_id, question) values (1, 'What is your favorite color?');

    • Option Creation

       insert into option (option_id, option_value, poll_id) values (1, 'Red', 1);

• Restart your application.

• Use Postman to ensure database is populated by import.sql.

Part 6.2 - Spring's Built-in Pagination

• Make use of Spring's built-in page number pagination support by researching org.springframework.data.repository.PagingAndSortingRepository.
• Modify respective Controller methods to handle Pageable arguments.
• Send a GET request to http://localhost:8080/polls?page=0&size=2 via Postman.
• Ensure the response is a JSON object with pagination-specific information.