Welcome to the final lesson on Spring Data JPA using Kotlin. In this lesson, we will delve into Pagination and Sorting. In previous lessons, we explored the CrudRepository
interface, entity classes, derived queries, custom query methods, and established entity relationships using Kotlin with Spring. Today, we'll enhance our Todo application by adding pagination and sorting capabilities. These features are crucial for managing large datasets and improving user interactions. By the end of this lesson, you'll be skilled at implementing pagination and sorting in your Kotlin-based Spring applications, optimizing efficiency and responsiveness.
Imagine your ToDo app becomes immensely popular, leading to thousands of ToDo items. When a user requests todos using GET /todos
, a couple of issues could occur:
- Your application will try to load all these items into memory at once for serialization and return, potentially causing memory-related issues and crashing the application.
- Even if your application's memory can handle all these objects, users might face long wait times for data transfer over the internet and display on the UI.
These issues can be mitigated with pagination — a technique that breaks data into manageable chunks. To implement this, controller endpoints can accept parameters like page
and pageSize
. For example, GET /todos?page=1&pageSize=10
retrieves the first 10 todos, and GET /todos?page=5&pageSize=10
retrieves todos from 41 to 50.
Another useful feature is specifying the order in which clients receive data. For example, GET /todos?sortBy=title
or GET /todos?sortBy=title&order=desc
lets users sort data by title in ascending or descending order.
Spring applications do not automatically process these query parameters for database requests; this needs to be implemented by the developer. However, Spring Data JPA supports pagination and sorting, and you'll soon discover how to implement these features using Kotlin.
To implement pagination and sorting, you primarily rely on methods already provided by the JpaRepository
interface, which extends the PagingAndSortingRepository
interface out of the box:
Modifier and Type | Method | Description |
---|---|---|
Page<T> | findAll(pageable: Pageable) | Returns a Page of entities adhering to the restrictions defined in the Pageable object. |
Iterable<T> | findAll(sort: Sort) | Returns all entities sorted according to the specified options in the Sort object. |
As you can see, the PagingAndSortingRepository
allows passing a Sort
object into the findAll
method to specify sorting criteria, or a Pageable
object (which can include both pagination and sorting information) to retrieve items in a paginated format.
It's also possible to pass Pageable
and Sort
parameters to derived methods. Here's an example:
Kotlin1package com.codesignal.repositories 2 3import com.codesignal.entities.TodoItem 4import org.springframework.data.domain.Page 5import org.springframework.data.domain.Pageable 6import org.springframework.data.domain.Sort 7import org.springframework.data.jpa.repository.JpaRepository 8 9interface TodoRepository : JpaRepository<TodoItem, Long> { 10 fun findByIsCompleted(isCompleted: Boolean, pageable: Pageable): Page<TodoItem> 11 fun findByTitleContaining(keyword: String, sort: Sort): List<TodoItem> 12}
In the code above, custom methods findByIsCompleted
and findByTitleContaining
accept Pageable
and Sort
objects to handle pagination and sorting, respectively.
Creating a Sort
object in Kotlin can be done as follows:
Kotlin1import org.springframework.data.domain.Sort 2 3val sortByTitleAsc = Sort.by("title").ascending() 4val sortByDateDesc = Sort.by("date").descending() 5val sortByMultipleFields = Sort.by("date").descending().and(Sort.by("title").ascending())
These Sort
objects can be used independently or as part of a Pageable
object.
To create a Pageable
object, you can use the PageRequest
class in various ways:
Kotlin1import org.springframework.data.domain.PageRequest 2import org.springframework.data.domain.Pageable 3 4val firstPageWithTwoElements: Pageable = PageRequest.of(0, 2) 5val secondPageWithFiveElements: Pageable = PageRequest.of(1, 5)
In PageRequest.of(0, 2)
, the first parameter (0
) indicates the page number (0-based index), and the second parameter (2
) specifies the number of items per page.
You can combine pagination with sorting like this:
Kotlin1val sortedByTitle = PageRequest.of(0, 10, Sort.by("title").ascending())
When you combine pagination and sorting, Spring Data JPA sorts the items based on the specified criteria and then divides the sorted result into pages.
Having discussed how to create Pageable
and Sort
objects, let's implement a method in our TodoController
to handle paginated and sorted requests:
Kotlin1package com.codesignal.controllers 2 3import com.codesignal.entities.TodoItem 4import com.codesignal.repositories.TodoRepository 5import org.springframework.beans.factory.annotation.Autowired 6import org.springframework.data.domain.PageRequest 7import org.springframework.data.domain.Sort 8import org.springframework.web.bind.annotation.* 9 10@RestController 11class TodoController { 12 13 @Autowired 14 private lateinit var todoRepository: TodoRepository 15 16 @GetMapping("/todos/paged") 17 fun getPagedTodos( 18 @RequestParam page: Int, 19 @RequestParam size: Int, 20 @RequestParam(required = false) sortBy: String? 21 ): List<TodoItem> { 22 val sort = Sort.by(sortBy ?: "title").ascending() 23 val pageable = PageRequest.of(page, size, sort) 24 return todoRepository.findByIsCompleted(false, pageable).content 25 } 26}
Let's break down the getPagedTodos
method:
@GetMapping("/todos/paged")
: Maps HTTP GET requests to/todos/paged
to this method.- The method accepts three query parameters:
page
andsize
are required for pagination, whilesortBy
is optional for sorting. val sort = Sort.by(sortBy ?: "title").ascending()
: Creates aSort
object that defaults to sorting bytitle
ifsortBy
is not provided.val pageable = PageRequest.of(page, size, sort)
: Combines pagination and sorting.- Returns non-completed Todo items from the repository, applying pagination and sorting.
This implementation allows users to request paginated and sorted results via URL query parameters, e.g., GET /todos/paged?page=0&size=10&sortBy=title
.
In this lesson, we examined the importance of pagination and sorting, updated our TodoRepository
for these features, and discussed creating Pageable
and Sort
objects using Kotlin. We implemented a controller method to handle paginated and sorted requests, enhancing our application's ability to manage large datasets effectively. Practice the concepts to consolidate your understanding of pagination and sorting with Kotlin and Spring Data JPA.