Welcome! Our focus today is on utilizing Kotlin with Spring Data JPA to write intricate custom queries using derived query methods. These methods enable complex data retrieval operations with effortless Kotlin integration. Kotlin’s concise syntax and powerful features can enhance your experience with Spring Data JPA by making the code more readable and expressive.
The idea behind derived query methods is simple. In Kotlin, you define methods in your JpaRepository using method names that represent the query. Spring Boot then generates the implementation. Here's an example of a Kotlin repository method to find all items by title:
Kotlin1package com.codesignal.repositories 2 3import com.codesignal.entities.TodoItem 4import org.springframework.data.jpa.repository.JpaRepository 5 6interface TodoItemRepository : JpaRepository<TodoItem, Long> { 7 fun findByTitleContaining(title: String): List<TodoItem> 8}
By analyzing the method name, Spring Data JPA derives and generates the suitable implementation for execution at runtime.
Derived query methods follow a specific structure: <introducer>By<criteria><and | or>...<criteria><and | or>. Spring Data JPA supports introducers such as find, read, query, count, and get. It's important to note that the keywords find, read, query, and get are interchangeable and perform the same function. For example, consider the following method definitions:
Kotlin1fun findByTitle(title: String): List<TodoItem> 2fun readByTitleContaining(title: String): List<TodoItem> 3fun countByIsCompleted(isCompleted: Boolean): Long
Each method uses an introducer, followed by a conditional criterion.
Different conditions help shape your queries, from matching exact values to patterns or ranges:
- Equality Conditions
- Similarity Conditions
- Comparison Conditions
These conditions enhance the flexibility of your queries.
Equality conditions are employed for precise value or state matches. Here are some Kotlin examples:
Kotlin1fun findByTitle(title: String): List<TodoItem> 2fun findByTitleIs(title: String): List<TodoItem> 3fun findByTitleEquals(title: String): List<TodoItem> 4fun findByTitleIsNot(title: String): List<TodoItem> 5fun findByTitleIsNull(): List<TodoItem> 6fun findByTitleIsNotNull(): List<TodoItem> 7fun findByIsCompletedTrue(): List<TodoItem> 8fun findByIsCompletedFalse(): List<TodoItem>
These methods allow searches based on exact matches, null checks, or boolean values.
For partial matches, similarity conditions are particularly helpful:
Kotlin1fun findByTitleStartingWith(prefix: String): List<TodoItem> 2fun findByTitleEndingWith(suffix: String): List<TodoItem> 3fun findByTitleContaining(substring: String): List<TodoItem> 4fun findByTitleLike(pattern: String): List<TodoItem>
These keywords facilitate pattern recognition and substring matches.
Comparison conditions suit range-based or comparative queries. Here are some examples:
Kotlin1fun findByPriorityLessThan(priority: Int): List<TodoItem> 2fun findByPriorityLessThanEqual(priority: Int): List<TodoItem> 3fun findByPriorityGreaterThan(priority: Int): List<TodoItem> 4fun findByPriorityGreaterThanEqual(priority: Int): List<TodoItem> 5fun findByPriorityBetween(startPriority: Int, endPriority: Int): List<TodoItem> 6fun findByPriorityIn(priorities: Collection<Int>): List<TodoItem> 7fun findByDueDateAfter(dueDate: LocalDateTime): List<TodoItem> 8fun findByDueDateBefore(dueDate: LocalDateTime): List<TodoItem>
These methods enable querying based on numerical and date comparisons.
Refine searches by combining conditions using and or or:
Kotlin1fun findByTitleOrPriority(title: String, priority: Int): List<TodoItem> 2fun findByTitleOrPriorityAndIsCompleted(title: String, priority: Int, isCompleted: Boolean): List<TodoItem>
Imagine findByTitleOrPriorityAndIsCompleted fetching TodoItems either matching the title or priority while being completed. Such combinations allow precise queries with multiple criteria.
Derived query methods support result sorting. For instance:
Kotlin1fun findByTitleOrderByTitle(title: String): List<TodoItem> 2fun findByTitleOrderByTitleAsc(title: String): List<TodoItem> 3fun findByTitleOrderByTitleDesc(title: String): List<TodoItem>
These methods facilitate retrieving sorted results for easier data processing and comprehension. By default, if no sorting direction is specified in the method name, Spring Data JPA sorts the results in ascending order.
In this lesson, you've explored derived query methods with Kotlin in Spring Data JPA. You learned about their structure, condition types, and sorting query results. Various condition keywords for equality, similarity, and comparison were examined to help you craft complex custom queries. Get ready for hands-on practice exercises next, empowering you to write effective and efficient JPA queries with Kotlin.