How to limit query results (#5616)

* JPA/Hibernate
* Spring Data JPA

BAEL-2208

persistence-modules/spring-data-jpa/src/main/java/com/baeldung/limit/CustomPassengerRepository.java

@@ -0,0 +1,8 @@
+package com.baeldung.limit;
+
+import java.util.List;
+
+interface CustomPassengerRepository {
+
+    List<Passenger> findOrderedBySeatNumberLimitedTo(int limit);
+}

persistence-modules/spring-data-jpa/src/main/java/com/baeldung/limit/Passenger.java

@@ -0,0 +1,82 @@
+package com.baeldung.limit;
+
+import javax.persistence.Basic;
+import javax.persistence.Column;
+import javax.persistence.Entity;
+import javax.persistence.GeneratedValue;
+import javax.persistence.Id;
+import java.util.Objects;
+
+@Entity
+class Passenger {
+
+    @Id
+    @GeneratedValue
+    @Column(nullable = false)
+    private Long id;
+
+    @Basic(optional = false)
+    @Column(nullable = false)
+    private String fistName;
+
+    @Basic(optional = false)
+    @Column(nullable = false)
+    private String lastName;
+
+    @Basic(optional = false)
+    @Column(nullable = false)
+    private int seatNumber;
+
+    private Passenger(String fistName, String lastName, int seatNumber) {
+        this.fistName = fistName;
+        this.lastName = lastName;
+        this.seatNumber = seatNumber;
+    }
+
+    static Passenger from(String firstName, String lastName, int seatNumber) {
+        return new Passenger(firstName, lastName, seatNumber);
+    }
+
+    @Override
+    public boolean equals(Object object) {
+        if (this == object)
+            return true;
+        if (object == null || getClass() != object.getClass())
+            return false;
+        Passenger passenger = (Passenger) object;
+        return getSeatNumber() == passenger.getSeatNumber() && Objects.equals(getFistName(), passenger.getFistName())
+          && Objects.equals(getLastName(), passenger.getLastName());
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(getFistName(), getLastName(), getSeatNumber());
+    }
+
+    @Override
+    public String toString() {
+        final StringBuilder toStringBuilder = new StringBuilder(getClass().getSimpleName());
+        toStringBuilder.append("{ id=").append(id);
+        toStringBuilder.append(", fistName='").append(fistName).append('\'');
+        toStringBuilder.append(", lastName='").append(lastName).append('\'');
+        toStringBuilder.append(", seatNumber=").append(seatNumber);
+        toStringBuilder.append('}');
+        return toStringBuilder.toString();
+    }
+
+    Long getId() {
+        return id;
+    }
+
+    String getFistName() {
+        return fistName;
+    }
+
+    String getLastName() {
+        return lastName;
+    }
+
+    int getSeatNumber() {
+        return seatNumber;
+    }
+}

persistence-modules/spring-data-jpa/src/main/java/com/baeldung/limit/PassengerRepository.java

@@ -0,0 +1,8 @@
+package com.baeldung.limit;
+
+import org.springframework.data.jpa.repository.JpaRepository;
+
+interface PassengerRepository extends JpaRepository<Passenger, Long>, CustomPassengerRepository {
+
+    Passenger findFirstByOrderBySeatNumberAsc();
+}

persistence-modules/spring-data-jpa/src/main/java/com/baeldung/limit/PassengerRepositoryImpl.java

@@ -0,0 +1,20 @@
+package com.baeldung.limit;
+
+import org.springframework.stereotype.Repository;
+
+import javax.persistence.EntityManager;
+import javax.persistence.PersistenceContext;
+import java.util.List;
+
+@Repository
+class PassengerRepositoryImpl implements CustomPassengerRepository {
+
+    @PersistenceContext
+    private EntityManager entityManager;
+
+    @Override
+    public List<Passenger> findOrderedBySeatNumberLimitedTo(int limit) {
+        return entityManager.createQuery("SELECT p FROM Passenger p ORDER BY p.seatNumber",
+          Passenger.class).setMaxResults(limit).getResultList();
+    }
+}

persistence-modules/spring-data-jpa/src/test/java/com/baeldung/limit/LimitIntegrationTest.java

@@ -0,0 +1,69 @@
+package com.baeldung.limit;
+
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.autoconfigure.orm.jpa.DataJpaTest;
+import org.springframework.data.domain.Page;
+import org.springframework.data.domain.PageRequest;
+import org.springframework.data.domain.Sort;
+import org.springframework.test.context.junit4.SpringRunner;
+
+import javax.persistence.EntityManager;
+import javax.persistence.PersistenceContext;
+import java.util.List;
+
+import static org.junit.Assert.assertEquals;
+
+@DataJpaTest
+@RunWith(SpringRunner.class)
+public class LimitIntegrationTest {
+
+    @PersistenceContext
+    private EntityManager entityManager;
+    @Autowired
+    private PassengerRepository repository;
+
+    @Before
+    public void before() {
+        entityManager.persist(Passenger.from("Jill", "Smith", 50));
+        entityManager.persist(Passenger.from("Eve", "Jackson", 95));
+        entityManager.persist(Passenger.from("Fred", "Bloggs", 22));
+        entityManager.persist(Passenger.from("Ricki", "Bobbie", 36));
+        entityManager.persist(Passenger.from("Siya", "Kolisi", 85));
+    }
+
+    @Test
+    public void givenSeveralPassengersWhenOrderedBySeatNumberLimitedToThenThePassengerInTheFirstFilledSeatIsReturned() {
+        Passenger expected = Passenger.from("Fred", "Bloggs", 22);
+
+        List<Passenger> passengers = repository.findOrderedBySeatNumberLimitedTo(1);
+
+        assertEquals(1, passengers.size());
+
+        Passenger actual = passengers.get(0);
+        assertEquals(actual, expected);
+    }
+
+    @Test
+    public void givenSeveralPassengersWhenFindFirstByOrderBySeatNumberAscThenThePassengerInTheFirstFilledSeatIsReturned() {
+        Passenger expected = Passenger.from("Fred", "Bloggs", 22);
+
+        Passenger actual = repository.findFirstByOrderBySeatNumberAsc();
+
+        assertEquals(actual, expected);
+    }
+
+    @Test
+    public void givenSeveralPassengersWhenFindPageSortedByThenThePassengerInTheFirstFilledSeatIsReturned() {
+        Passenger expected = Passenger.from("Fred", "Bloggs", 22);
+
+        Page<Passenger> page = repository.findAll(PageRequest.of(0, 1, Sort.by(Sort.Direction.ASC, "seatNumber")));
+
+        assertEquals(page.getContent().size(), 1);
+
+        Passenger actual = page.getContent().get(0);
+        assertEquals(expected, actual);
+    }
+}