This lesson focuses on implementing pagination in a Spring Boot application, starting with writing a test to fetch CashCards one at a time, sorted from highest to lowest amount (descending order).

Key Steps and Concepts:

1. Writing the Pagination Test:

   • A new test method shouldReturnAPageOfCashCards is added to CashCardApplicationTests.
   • The test makes a GET request to /cashcards?page=0&size=1, expecting to retrieve one CashCard per page.
   • It asserts that the response contains exactly one CashCard.

2. Running the Tests and Initial Failure:

   • Upon running the tests, the assertion fails because all three CashCards are returned instead of just one.

     expected: 1
     but was: 3
     

   • This failure is expected since pagination has not been implemented yet in the controller.

3. Implementing Pagination in the Controller:

   • A new findAll(Pageable pageable) method is added to CashCardController.

     @GetMapping
     private ResponseEntity<List<CashCard>> findAll(Pageable pageable) {
         Page<CashCard> page = cashCardRepository.findAll(
                 PageRequest.of(
                         pageable.getPageNumber(),
                         pageable.getPageSize()
         ));
         return ResponseEntity.ok(page.getContent());
     }
     

   • The Pageable object automatically captures the page and size parameters from the query string.

4. Understanding the Pagination Code:

   • Pageable is provided by Spring Web to handle pagination parameters.
   • PageRequest.of() creates a PageRequest object using the page number and size.
   • PageRequest is a simple implementation of Pageable used for pagination.

5. Compiling and New Compilation Error:

   • Running the tests now results in a compilation error:

     error: method findAll in interface CrudRepository<T,ID> cannot be applied to given types;
     

   • This happens because CrudRepository's findAll() method does not support pagination parameters.

6. Extending PagingAndSortingRepository:

   • To fix the error, CashCardRepository is updated to extend PagingAndSortingRepository in addition to CrudRepository:

     import org.springframework.data.repository.PagingAndSortingRepository;
     ...
     interface CashCardRepository extends CrudRepository<CashCard, Long>, PagingAndSortingRepository<CashCard, Long> { ... }
     

   • This adds support for pagination and sorting in the repository.

7. Encountering a Runtime Error:

   • After making the repository changes, running the tests leads to a runtime error:

     Caused by: java.lang.IllegalStateException: Ambiguous mapping. Cannot map 'cashCardController' method
     

   • The error indicates that there are two methods mapped to the same endpoint /cashcards with the same HTTP method.

8. Resolving the Ambiguous Mapping:

   • The issue is caused by having two @GetMapping methods in CashCardController mapped to /cashcards:

     • The new findAll(Pageable pageable) method.
     • The old findAll() method without pagination.

   • To resolve the conflict, the old findAll() method is removed:

     // Delete this one:
     @GetMapping()
     private ResponseEntity<Iterable<CashCard>> findAll() {
         return ResponseEntity.ok(cashCardRepository.findAll());
     }
     

9. Rerunning the Tests:

   • With the conflict resolved, running the tests again results in all tests passing successfully.

Conclusion:

• Pagination is implemented by:
  • Writing a test that expects paginated results.
  • Updating the controller to handle pagination parameters using Pageable and PageRequest.
  • Extending the repository to support pagination by implementing PagingAndSortingRepository.
• It's important to ensure that there are no conflicting endpoint mappings in the controller.

한국어 버전:

페이징 구현

이번 강의에서는 Spring Boot 애플리케이션에서 페이징을 구현하는 방법에 대해 알아봅니다. 먼저 테스트를 작성하여 한 번에 하나의 CashCard를 가져오고, 금액을 높은 순서로 정렬하는 작업부터 시작합니다.

주요 단계 및 개념:

1. 페이징 테스트 작성:
   • CashCardApplicationTests에 새로운 테스트 메소드 shouldReturnAPageOfCashCards를 추가합니다.
   • /cashcards?page=0&size=1로 GET 요청을 보내 한 페이지에 하나의 CashCard만 가져옵니다.
   • 응답이 정확히 하나의 CashCard를 포함하는지 확인합니다.