Spring in practice : REST service(EN)
Spring MVC is the Web Framework built in Spring;
It helps you write web applications and takes care of a lot of boilerplate code, so you just have to focus on your application features.

With Spring Web (Spring MVC) you can write screens with a template solution which are used to generate HTML.But we don’t use this solution in this course.We will see how to write REST services.However if you are interested you can read official documentation.

With Spring Web you can expose REST services to another app (web api, JS app, android app…).This is the purpose of this lesson.You will learn how to develop endpoints on a backend application.These REST endpoints will be used later by a JS app or an Android app.

Data Transfert Object
A DTO is an object that carries data between processes. Data need to be serializable to go across the HTTP connection

Serialization is the process of translating data structures or object into a format that can be transmitted
It’s often just a bunch of fields and the getters and setters for them.
public class WindowDto { private Long id; private String name; private WindowStatus windowStatus; private String roomName; private Long roomId; public WindowDto() { } public WindowDto(Window window) { this.id = window.getId(); this.name = window.getName(); this.windowStatus = window.getWindowStatus(); this.roomName = window.getRoom().getName(); this.roomId = window.getRoom().getId(); } public Long getId() { return id; } public void setId(Long id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public WindowStatus getWindowStatus() { return windowStatus; } public void setWindowStatus(WindowStatus windowStatus) { this.windowStatus = windowStatus; } public String getRoomName() { return roomName; } public void setRoomName(String roomName) { this.roomName = roomName; } public Long getRoomId() { return roomId; } public void setRoomId(Long roomId) { this.roomId = roomId; } }
DTO will be used to transfer and to receive data in our REST controllers (entry point in our Java webapp).
Very often we find a constructor with the entity allowing to build a new instance. But beware, a DTO must always have an empty constructor. Libraries used to serialize or deserialize an object use the Java reflection API. In our case we will have a constructor allowing to build a WindowDto from Window entity.

HTTP
The Hypertext Transfer Protocol (HTTP) is an application protocol used for data communication on the World Wide Web.
HTTP defines methods (sometimes referred to as verbs) to indicate the desired action to be performed on the identified resource
A resource can be an image, a video, an HTML page, a JSON document.
To receive a response you have to send a request with a verb in a client an application as Curl, Wget…. or with a website

Each HTTP response has a status identified by a code. This code is sent by the server, by your app
1XX : Wait… request in progress
2XX : Here ! I send you a resource
3XX : Go away !
4XX : You made a mistake
5XX : I made a mistake
REST
HTTP requests are handled by the methods of a REST service.In Spring’s approach a REST service is a controller.It is able to respond to HTTP requests
GET: read resource
POST: creates new record or executing a query
PUT: edit a resource (sometimes we use only a post request)
DELETE: delete a record
Controllers are the link between the web http clients (browsers, mobiles) and your application.They should be lightweight and call other components in your application to perform actual work (DAO for example).
These components are easily identified by the @RestController annotation.
Example of addressable resources
Retrieve a window list : GET
/api/windows
Retrieve a particular window : GET
/api/windows/{window_id}
Create or update a window : POST
/api/windows
Update a window and update its status : PUT
/api/windows/{window_id}/switch
Delete a window : DELETE
/api/windows/{window_id}
This WindowController handles GET requests for /api/windows
by returning a list of WindowDto.
A complete example to manage windows
@RestController // (1) @RequestMapping("/api/windows") // (2) @Transactional // (3) public class WindowController { private final WindowDao windowDao; private final RoomDao roomDao; public WindowController(WindowDao windowDao, RoomDao roomDao) { // (4) this.windowDao = windowDao; this.roomDao = roomDao; } @GetMapping // (5) public List<WindowDto> findAll() { return windowDao.findAll().stream().map(WindowDto::new).collect(Collectors.toList()); // (6) } @GetMapping(path = "/{id}") public WindowDto findById(@PathVariable Long id) { return windowDao.findById(id).map(WindowDto::new).orElse(null); // (7) } @PutMapping(path = "/{id}/switch") public WindowDto switchStatus(@PathVariable Long id) { Window window = windowDao.findById(id).orElseThrow(IllegalArgumentException::new); window.setWindowStatus(window.getWindowStatus() == WindowStatus.OPEN ? WindowStatus.CLOSED: WindowStatus.OPEN); return new WindowDto(window); } @PostMapping // (8) public WindowDto create(@RequestBody WindowDto dto) { // WindowDto must always contain the window room Room room = roomDao.getById(dto.getRoomId()); Window window = null; // On creation id is not defined if (dto.getId() == null) { window = windowDao.save(new Window(room, dto.getName(), dto.getWindowStatus())); } else { window = windowDao.getById(dto.getId()); // (9) window.setWindowStatus(dto.getWindowStatus()); } return new WindowDto(window); } @DeleteMapping(path = "/{id}") public void delete(@PathVariable Long id) { windowDao.deleteById(id); } }
(1) RestController is a Spring stereotype to mark a class as a rest service
(2) @RequestMapping is used to define a global URL prefix used to manage a resource (in our example all requests that start with
/api/windows
will be handle by this controller)(3) @Transactional is used to delegate a transaction opening to Spring.Spring will initiate a transaction for each entry point of this controller.This is important because with Hibernate you cannot execute a query outside of a transaction.
(4) DAOs used by this controller are injected via constructor
(5) @GetMapping indicates that the following method will respond to a GET request.This method will return a window list.We transform our entities
Window
inWindowDto
(6) (7) We use Java Stream API to manipulate our data
(8) @PostMapping indicates that the following method will respond to a POST request (for saving).
(9) For an update you don’t need to call the DAO save method. Method
getById
load the persisted data and all changes on this object (attached to a persistent context) will be updated when the transaction will be commited.
Lab : Create your rest services
A basic example
This is the time to create your first REST controller with Spring.
Create a new class HelloController in package com.emse.spring.faircorp.api
@RestController @RequestMapping("/api/hello") @Transactional public class HelloController { @GetMapping("/{name}") public MessageDto welcome(@PathVariable String name) { return new MessageDto("Hello " + name); } class MessageDto { String message; public MessageDto(String message) { this.message = message; } public String getMessage() { return message; } } }
Test your service
Browser for GET requests
If your REST service expose an handler for a GET HTTP request, this handler can be tested in a browser.
Launch your app and open the URL http://localhost:8080/api/hello/Guillaume in your browser
When you type an URL in the adress bar, your browser send a GET HTTP request. You should see a response as this one
{"message":"Hello Guillaume"}
Swagger for all requests
With a browser you are limited to GET requests. If you want to test PUT, POST or DELETE HTTP requests, you need another tool. We will use swagger.
The advantage of swagger is that it is very well integrated into the Spring world. Update your build.gradle
file and add these dependencies
implementation 'io.springfox:springfox-boot-starter:3.0.0'
And now you can relaunch your app and open swagger interface http://localhost:8080/swagger-ui/index.html
All your endpoints are available. You can click on one of them to test it
Add WindowController
Read the previous examples and create
a DTO WindowDto and the REST service WindowController
a rest service which is able to
Retrieve a window list via a GET
Retrieve a particular window via a GET
Create or update a window via a POST
Update a window and switch its status via a PUT
Delete a window via a DELETE
Use swagger to test your API
create a new window
list all the window
find the window with id
-8
switch its status
deletes this window
More Rest service
You can now create BuildingDto, RoomDtoo, HeaterDto and write services which follow this service
/api/heaters (GET) send heaters list /api/heaters (POST) add a heater /api/heaters/{heater_id} (GET) read a heater /api/heaters/{heater_id} (DELETE) delete a heater
/api/rooms (GET) send room list /api/rooms (POST) add a room /api/rooms/{room_id} (GET) read a room /api/rooms/{room_id} (DELETE) delete a room and all its windows and its heaters /api/rooms/{room_id}/switchWindow switch the room windows (OPEN to CLOSED or inverse) /api/rooms/{room_id}/switchHeaters switch the room heaters (ON to OFF or inverse)
/api/buildings (GET) send building list /api/buildings (POST) add a building /api/buildings/{building_id} (GET) read a building /api/buildings/{building_id} (DELETE) delete a building and all its rooms and all its windows and heaters
CORS
Today browsers forbid a website to access to resources served by another website defined on a different domain. If you want to call your API on http://localhost:8080 from a webapp you should have this error
Access to fetch at 'http://localhost:8080/api/rooms' from origin 'null' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request’s mode to 'no-cors' to fetch the resource with CORS disabled.
Cross-Origin Resource Sharing is a mechanism that allows this dialog
To resolve this problem you have to manage CORS headers.
Add annotation @CrossOrigin
to your @RestController
to open your API to all other apps
@CrossOrigin
If your Vue.js app is launched on http://localhost:3010 ou can open your API only for this app
@CrossOrigin(origins = { "http://localhost:3010" }, maxAge = 3600)