diff --git a/spring-rest-simple/src/main/java/com/baeldung/repository/BookRepository.java b/spring-rest-simple/src/main/java/com/baeldung/repository/BookRepository.java
new file mode 100644
index 0000000000..239d74b19c
--- /dev/null
+++ b/spring-rest-simple/src/main/java/com/baeldung/repository/BookRepository.java
@@ -0,0 +1,25 @@
+package com.baeldung.repository;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Optional;
+
+import org.springframework.stereotype.Repository;
+
+import com.baeldung.web.dto.Book;
+
+@Repository
+public class BookRepository {
+
+    private List<Book> books = new ArrayList<>();
+
+    public Optional<Book> findById(long id) {
+        return books.stream()
+            .filter(book -> book.getId() == id)
+            .findFirst();
+    }
+
+    public void add(Book book) {
+        books.add(book);
+    }
+}
diff --git a/spring-rest-simple/src/main/java/com/baeldung/web/controller/ApiExceptionHandler.java b/spring-rest-simple/src/main/java/com/baeldung/web/controller/ApiExceptionHandler.java
new file mode 100644
index 0000000000..e7f0ac3510
--- /dev/null
+++ b/spring-rest-simple/src/main/java/com/baeldung/web/controller/ApiExceptionHandler.java
@@ -0,0 +1,22 @@
+package com.baeldung.web.controller;
+
+import org.springframework.http.HttpStatus;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.ExceptionHandler;
+import org.springframework.web.bind.annotation.RestControllerAdvice;
+
+import com.baeldung.web.error.ApiErrorResponse;
+import com.baeldung.web.error.BookNotFoundException;
+
+@RestControllerAdvice
+public class ApiExceptionHandler {
+
+    @ExceptionHandler(BookNotFoundException.class)
+    public ResponseEntity<ApiErrorResponse> handleApiException(
+        BookNotFoundException ex) {
+        ApiErrorResponse response = 
+            new ApiErrorResponse("error-0001",
+                "No book found with ID " + ex.getId());
+        return new ResponseEntity<>(response, HttpStatus.NOT_FOUND);
+    }
+}
\ No newline at end of file
diff --git a/spring-rest-simple/src/main/java/com/baeldung/web/controller/BookController.java b/spring-rest-simple/src/main/java/com/baeldung/web/controller/BookController.java
new file mode 100644
index 0000000000..126b40517d
--- /dev/null
+++ b/spring-rest-simple/src/main/java/com/baeldung/web/controller/BookController.java
@@ -0,0 +1,25 @@
+package com.baeldung.web.controller;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.PathVariable;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+import com.baeldung.repository.BookRepository;
+import com.baeldung.web.dto.Book;
+import com.baeldung.web.error.BookNotFoundException;
+
+@RestController
+@RequestMapping("/api/book")
+public class BookController {
+
+    @Autowired
+    private BookRepository repository;
+
+    @GetMapping("/{id}")
+    public Book findById(@PathVariable long id) {
+        return repository.findById(id)
+            .orElseThrow(() -> new BookNotFoundException(id));
+    }
+}
\ No newline at end of file
diff --git a/spring-rest-simple/src/main/java/com/baeldung/web/dto/Book.java b/spring-rest-simple/src/main/java/com/baeldung/web/dto/Book.java
new file mode 100644
index 0000000000..9ccd640319
--- /dev/null
+++ b/spring-rest-simple/src/main/java/com/baeldung/web/dto/Book.java
@@ -0,0 +1,32 @@
+package com.baeldung.web.dto;
+
+public class Book {
+
+    private long id;
+    private String title;
+    private String author;
+
+    public long getId() {
+        return id;
+    }
+
+    public void setId(long id) {
+        this.id = id;
+    }
+
+    public String getTitle() {
+        return title;
+    }
+
+    public void setTitle(String title) {
+        this.title = title;
+    }
+
+    public String getAuthor() {
+        return author;
+    }
+
+    public void setAuthor(String author) {
+        this.author = author;
+    }    
+}
\ No newline at end of file
diff --git a/spring-rest-simple/src/main/java/com/baeldung/web/error/ApiErrorResponse.java b/spring-rest-simple/src/main/java/com/baeldung/web/error/ApiErrorResponse.java
new file mode 100644
index 0000000000..a67a7a3f2f
--- /dev/null
+++ b/spring-rest-simple/src/main/java/com/baeldung/web/error/ApiErrorResponse.java
@@ -0,0 +1,29 @@
+package com.baeldung.web.error;
+
+public class ApiErrorResponse {
+
+    private String error;
+    private String message;
+
+    public ApiErrorResponse(String error, String message) {
+        super();
+        this.error = error;
+        this.message = message;
+    }
+
+    public String getError() {
+        return error;
+    }
+
+    public void setError(String error) {
+        this.error = error;
+    }
+
+    public String getMessage() {
+        return message;
+    }
+
+    public void setMessage(String message) {
+        this.message = message;
+    }
+}
\ No newline at end of file
diff --git a/spring-rest-simple/src/main/java/com/baeldung/web/error/BookNotFoundException.java b/spring-rest-simple/src/main/java/com/baeldung/web/error/BookNotFoundException.java
new file mode 100644
index 0000000000..ad93ef7f20
--- /dev/null
+++ b/spring-rest-simple/src/main/java/com/baeldung/web/error/BookNotFoundException.java
@@ -0,0 +1,19 @@
+package com.baeldung.web.error;
+
+@SuppressWarnings("serial")
+public class BookNotFoundException extends RuntimeException {
+
+    private long id;
+
+    public BookNotFoundException(long id) {
+        this.id = id;
+    }
+
+    public long getId() {
+        return id;
+    }
+
+    public void setId(long id) {
+        this.id = id;
+    }
+}
\ No newline at end of file
diff --git a/spring-rest-simple/src/test/java/com/baeldung/repository/BookRepositoryUnitTest.java b/spring-rest-simple/src/test/java/com/baeldung/repository/BookRepositoryUnitTest.java
new file mode 100644
index 0000000000..dd64f302fe
--- /dev/null
+++ b/spring-rest-simple/src/test/java/com/baeldung/repository/BookRepositoryUnitTest.java
@@ -0,0 +1,60 @@
+package com.baeldung.repository;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+import java.util.Optional;
+
+import org.junit.Before;
+import org.junit.Test;
+
+import com.baeldung.web.dto.Book;
+
+public class BookRepositoryUnitTest {
+
+    private BookRepository repository;
+
+    @Before
+    public void setUp() {
+        repository = new BookRepository();
+    }
+
+    @Test
+    public void givenNoBooks_WhenFindById_ThenReturnEmptyOptional() {
+        assertFalse(repository.findById(1).isPresent());
+    }
+
+    @Test
+    public void givenOneMatchingBook_WhenFindById_ThenReturnEmptyOptional() {
+
+        long id = 1;
+        Book expected = bookWithId(id);
+
+        repository.add(expected);
+
+        Optional<Book> found = repository.findById(id);
+
+        assertTrue(found.isPresent());
+        assertEquals(expected, found.get());
+    }
+
+    private static Book bookWithId(long id) {
+        Book book = new Book();
+        book.setId(id);
+        return book;
+    }
+
+    @Test
+    public void givenOneNonMatchingBook_WhenFindById_ThenReturnEmptyOptional() {
+
+        long id = 1;
+        Book expected = bookWithId(id);
+
+        repository.add(expected);
+
+        Optional<Book> found = repository.findById(id + 1);
+
+        assertFalse(found.isPresent());
+    }
+}
\ No newline at end of file
diff --git a/spring-rest-simple/src/test/java/com/baeldung/web/controller/BookControllerIntegrationTest.java b/spring-rest-simple/src/test/java/com/baeldung/web/controller/BookControllerIntegrationTest.java
new file mode 100644
index 0000000000..3bf5ce3c3d
--- /dev/null
+++ b/spring-rest-simple/src/test/java/com/baeldung/web/controller/BookControllerIntegrationTest.java
@@ -0,0 +1,74 @@
+package com.baeldung.web.controller;
+
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.doReturn;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.content;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
+
+import java.util.Optional;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.autoconfigure.web.servlet.WebMvcTest;
+import org.springframework.boot.test.mock.mockito.MockBean;
+import org.springframework.context.annotation.ComponentScan;
+import org.springframework.test.context.junit4.SpringRunner;
+import org.springframework.test.web.servlet.MockMvc;
+
+import com.baeldung.Application;
+import com.baeldung.repository.BookRepository;
+import com.baeldung.web.dto.Book;
+import com.baeldung.web.error.ApiErrorResponse;
+import com.fasterxml.jackson.databind.ObjectMapper;
+
+@RunWith(SpringRunner.class)
+@WebMvcTest(BookController.class)
+@ComponentScan(basePackageClasses = Application.class)
+public class BookControllerIntegrationTest {
+
+    private static final ObjectMapper MAPPER = new ObjectMapper();
+
+    @Autowired
+    private MockMvc mvc;
+
+    @MockBean
+    private BookRepository repository;
+
+    @Test
+    public void givenNoExistingBooks_WhenGetBookWithId1_ThenErrorReturned() throws Exception {
+
+        long id = 1;
+
+        doReturn(Optional.empty()).when(repository).findById(eq(id));
+
+        mvc.perform(get("/api/book/{id}", id))
+            .andExpect(status().isNotFound())
+            .andExpect(content().json(MAPPER.writeValueAsString(new ApiErrorResponse("error-0001", "No book found with ID " + id))));
+    }
+
+    @Test
+    public void givenMatchingBookExists_WhenGetBookWithId1_ThenBookReturned() throws Exception {
+
+        long id = 1;
+        Book book = book(id, "foo", "bar");
+
+        doReturn(Optional.of(book)).when(repository).findById(eq(id));
+
+        mvc.perform(get("/api/book/{id}", id))
+            .andExpect(status().isOk())
+            .andExpect(content().json(MAPPER.writeValueAsString(book)));
+    }
+
+    private static Book book(long id, String title, String author) {
+
+        Book book = new Book();
+
+        book.setId(id);
+        book.setTitle(title);
+        book.setAuthor(author);
+
+        return book;
+    }
+}
\ No newline at end of file