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:

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:

Kotlin
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1package 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:

Kotlin
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1import 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:

Kotlin
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1import 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:

Kotlin
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1val 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:

Kotlin
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1package 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 and size are required for pagination, while sortBy is optional for sorting.
• val sort = Sort.by(sortBy ?: "title").ascending(): Creates a Sort object that defaults to sorting by title if sortBy 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.