向 Spring Boot 项目添加数据类
This is the second part of the Getting started with Spring Boot and Kotlin tutorial. Before proceeding, make sure you've completed previous steps:
Create a Spring Boot project with Kotlin
Add a data class to the Spring Boot project
Add database support for Spring Boot project
Use Spring Data CrudRepository for database access
In this part of the tutorial, you'll add some more functionality to the application and discover more Kotlin language features, such as data classes.
It requires changing the MessageController
class to respond with a JSON document containing a collection of serialized objects.
Update your application
- In the same package, next to the
DemoApplication.kt
file, create aMessage.kt
file. In the
Message.kt
file, create a data class with two properties:id
andtext
:// Message.kt package com.example.demo data class Message(val id: String?, val text: String)
Message
class will be used for data transfer: a list of serializedMessage
objects will make up the JSON document that the controller is going to respond to the browser request.The main purpose of data classes in Kotlin is to hold data. Such classes are marked with the
data
keyword, and some standard functionality and some utility functions are often mechanically derivable from the class structure.In this example, you declared
Message
as a data class as its main purpose is to store the data.Properties in Kotlin classes can be declared either as:
- mutable, using the
var
keyword - read-only, using the
val
keyword
The
Message
class declares two properties usingval
keyword, theid
andtext
. The compiler will automatically generate the getters for both of these properties. It will not be possible to reassign the values of these properties after an instance of theMessage
class is created.Kotlin provides built-in support for nullable types. In Kotlin, the type system distinguishes between references that can hold
null
(nullable references) and those that cannot (non-nullable references).
For example, a regular variable of typeString
cannot holdnull
. To allow nulls, you can declare a variable as a nullable string by writingString?
.The
id
property of theMessage
class is declared as a nullable type this time. Hence, it is possible to create an instance ofMessage
class by passingnull
as a value forid
:Message(null, "Hello!") - mutable, using the
In the
MessageController.kt
file, instead of theindex()
function, create thelistMessages()
function returning a list ofMessage
objects:// MessageController.kt package com.example.demo import org.springframework.web.bind.annotation.GetMapping import org.springframework.web.bind.annotation.RequestMapping import org.springframework.web.bind.annotation.RestController @RestController @RequestMapping("/") class MessageController { @GetMapping fun listMessages() = listOf( Message("1", "Hello!"), Message("2", "Bonjour!"), Message("3", "Privet!"), ) }
The Kotlin Standard Library provides implementations for basic collection types: sets, lists, and maps.
Each collection type can be read-only or mutable:- A read-only collection comes with operations for accessing collection elements.
- A mutable collection comes also with write operations for adding, removing, and updating its elements.
The corresponding factory functions are also provided by the Kotlin Standard Library to create instances of such collections.
In this tutorial, you use the
listOf()
function to create a list ofMessage
objects. This is the factory function to create a read-only list of objects: you can't add or remove elements from the list.
If it is required to perform write operations on the list, call themutableListOf()
function to create a mutable list instance.A trailing comma is a comma symbol after the last item of a series of elements:
Message("3", "Privet!"), This is a convenient feature of Kotlin syntax and is entirely optional – your code will still work without them.
In the example above, creating a list of
Message
objects includes the trailing comma after the lastlistOf()
function argument.
The response from MessageController
will now be a JSON document containing a collection of Message
objects.
Any controller in the Spring application renders JSON response by default if Jackson library is on the classpath. As you specified the
spring-boot-starter-web
dependency in thebuild.gradle.kts
file, you received Jackson as a transitive dependency. Hence, the application responds with a JSON document if the endpoint returns a data structure that can be serialized to JSON.
Here is a complete code of the DemoApplication.kt
, MessageController.kt
, and Message.kt
files:
// DemoApplication.kt
package com.example.demo
import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.runApplication
@SpringBootApplication
class DemoApplication
fun main(args: Array) {
runApplication(*args)
}
// MessageController.kt
package com.example.demo
import org.springframework.web.bind.annotation.GetMapping
import org.springframework.web.bind.annotation.RequestMapping
import org.springframework.web.bind.annotation.RestController
@RestController
@RequestMapping("/")
class MessageController {
@GetMapping
fun listMessages() = listOf(
Message("1", "Hello!"),
Message("2", "Bonjour!"),
Message("3", "Privet!"),
)
}
// Message.kt
package com.example.demo
data class Message(val id: String?, val text: String)
Run the application
The Spring application is ready to run:
Run the application again.
Once the application starts, open the following URL:
http://localhost:8080
You will see a page with a collection of messages in JSON format:
Next step
In the next part of the tutorial, you'll add and configure a database to your project, and make HTTP requests.