1. Overview

Spring Boot is an opinionated, convention-over-configuration focused addition to the Spring platform – highly useful to get started with minimum effort and create stand-alone, production-grade applications.

This tutorial is a starting point for Boot – a way to get started in a simple manner, with a basic web application.

We'll go over some core configuration, a front-end, quick data manipulation, and exception handling.

Further reading:

How to Change the Default Port in Spring Boot

Have a look at how you can change the default port in a Spring Boot application.

Intro to Spring Boot Starters

A quick overview of the most common Spring Boot Starters, along with examples on how to use them in a real-world project.

2. Setup

First, let's use Spring Initializr to generate the base for our project.

The generated project relies on the Boot parent:

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.4.0</version>
    <relativePath />
</parent>

The initial dependencies are going to be quite simple:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>
<dependency>
    <groupId>com.h2database</groupId>
    <artifactId>h2</artifactId>
</dependency>

3. Application Configuration

Next, we'll configure a simple main class for our application:

@SpringBootApplication
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}

Notice how we're using @SpringBootApplication as our primary application configuration class; behind the scenes, that's equivalent to @Configuration, @EnableAutoConfiguration, and @ComponentScan together.

Finally, we'll define a simple application.properties file – which for now only has one property:

server.port=8081

server.port changes the server port from the default 8080 to 8081; there are of course many more Spring Boot properties available.

4. Simple MVC View

Let's now add a simple front end using Thymeleaf.

First, we need to add the spring-boot-starter-thymeleaf dependency to our pom.xml:

<dependency> 
    <groupId>org.springframework.boot</groupId> 
    <artifactId>spring-boot-starter-thymeleaf</artifactId> 
</dependency>

That enables Thymeleaf by default – no extra configuration is necessary.

We can now configure it in our application.properties:

spring.thymeleaf.cache=false
spring.thymeleaf.enabled=true 
spring.thymeleaf.prefix=classpath:/templates/
spring.thymeleaf.suffix=.html

spring.application.name=Bootstrap Spring Boot

Next, we'll define a simple controller and a basic home page – with a welcome message:

@Controller
public class SimpleController {
    @Value("${spring.application.name}")
    String appName;

    @GetMapping("/")
    public String homePage(Model model) {
        model.addAttribute("appName", appName);
        return "home";
    }
}

Finally, here is our home.html:

<html>
<head><title>Home Page</title></head>
<body>
<h1>Hello !</h1>
<p>Welcome to <span th:text="${appName}">Our App</span></p>
</body>
</html>

Note how we used a property we defined in our properties – and then injected that so that we can show it on our home page.

5. Security

Next, let's add security to our application – by first including the security starter:

<dependency> 
    <groupId>org.springframework.boot</groupId> 
    <artifactId>spring-boot-starter-security</artifactId> 
</dependency>

By now, you're hopefully noticing a pattern – most Spring libraries are easily imported into our project with the use of simple Boot starters.

Once the spring-boot-starter-security dependency on the classpath of the application – all endpoints are secured by default, using either httpBasic or formLogin based on Spring Security's content-negotiation strategy.

That's why, if we have the starter on the classpath, we should usually define our own custom Security configuration by extending the WebSecurityConfigurerAdapter class:

@Configuration
@EnableWebSecurity
public class SecurityConfig extends WebSecurityConfigurerAdapter {

    @Override
    protected void configure(HttpSecurity http) throws Exception {
        http.authorizeRequests()
            .anyRequest()
            .permitAll()
            .and().csrf().disable();
    }
}

In our example, we're allowing unrestricted access to all endpoints.

 

Of course, Spring Security is an extensive topic and one not easily covered in a couple of lines of configuration – so I definitely encourage you to go deeper into the topic.

6. Simple Persistence

Let's start by defining our data model – a simple Book entity:

@Entity
public class Book {
 
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    @Column(nullable = false, unique = true)
    private String title;

    @Column(nullable = false)
    private String author;
}

And its repository, making good use of Spring Data here:

public interface BookRepository extends CrudRepository<Book, Long> {
    List<Book> findByTitle(String title);
}

Finally, we need to of course configure our new persistence layer:

@EnableJpaRepositories("com.baeldung.persistence.repo") 
@EntityScan("com.baeldung.persistence.model")
@SpringBootApplication 
public class Application {
   ...
}

Note that we're using:

  • @EnableJpaRepositories to scan the specified package for repositories
  • @EntityScan to pick up our JPA entities

To keep things simple, we're using an H2 in-memory database here – so that we don't have any external dependencies when we run the project.

Once we include H2 dependency, Spring Boot auto-detects it and sets up our persistence with no need for extra configuration, other than the data source properties:

spring.datasource.driver-class-name=org.h2.Driver
spring.datasource.url=jdbc:h2:mem:bootapp;DB_CLOSE_DELAY=-1
spring.datasource.username=sa
spring.datasource.password=

Of course, like security, persistence is a broader topic than this basic set here, and one you should certainly explore further.

7. Web and the Controller

Next, let's have a look at a web tier – and we'll start that by setting up a simple controller – the BookController.

We'll implement basic CRUD operations exposing Book resources with some simple validation:

@RestController
@RequestMapping("/api/books")
public class BookController {

    @Autowired
    private BookRepository bookRepository;

    @GetMapping
    public Iterable findAll() {
        return bookRepository.findAll();
    }

    @GetMapping("/title/{bookTitle}")
    public List findByTitle(@PathVariable String bookTitle) {
        return bookRepository.findByTitle(bookTitle);
    }

    @GetMapping("/{id}")
    public Book findOne(@PathVariable Long id) {
        return bookRepository.findById(id)
          .orElseThrow(BookNotFoundException::new);
    }

    @PostMapping
    @ResponseStatus(HttpStatus.CREATED)
    public Book create(@RequestBody Book book) {
        return bookRepository.save(book);
    }

    @DeleteMapping("/{id}")
    public void delete(@PathVariable Long id) {
        bookRepository.findById(id)
          .orElseThrow(BookNotFoundException::new);
        bookRepository.deleteById(id);
    }

    @PutMapping("/{id}")
    public Book updateBook(@RequestBody Book book, @PathVariable Long id) {
        if (book.getId() != id) {
          throw new BookIdMismatchException();
        }
        bookRepository.findById(id)
          .orElseThrow(BookNotFoundException::new);
        return bookRepository.save(book);
    }
}

Given this aspect of the application is an API, we made use of the @RestController annotation here – which equivalent to a @Controller along with @ResponseBody – so that each method marshalls the returned resource right to the HTTP response.

Just one note worth pointing out – we're exposing our Book entity as our external resource here. That's fine for our simple application here, but in a real-world application, you will likely want to separate these two concepts.

8. Error Handling

Now that the core application is ready to go, let's focus on a simple centralized error handling mechanism using @ControllerAdvice:

@ControllerAdvice
public class RestExceptionHandler extends ResponseEntityExceptionHandler {

    @ExceptionHandler({ BookNotFoundException.class })
    protected ResponseEntity<Object> handleNotFound(
      Exception ex, WebRequest request) {
        return handleExceptionInternal(ex, "Book not found", 
          new HttpHeaders(), HttpStatus.NOT_FOUND, request);
    }

    @ExceptionHandler({ BookIdMismatchException.class, 
      ConstraintViolationException.class, 
      DataIntegrityViolationException.class })
    public ResponseEntity<Object> handleBadRequest(
      Exception ex, WebRequest request) {
        return handleExceptionInternal(ex, ex.getLocalizedMessage(), 
          new HttpHeaders(), HttpStatus.BAD_REQUEST, request);
    }
}

Beyond the standard exceptions we're handling here, we're also using a custom exception:

BookNotFoundException:

public class BookNotFoundException extends RuntimeException {

    public BookNotFoundException(String message, Throwable cause) {
        super(message, cause);
    }
    // ...
}

This should give you an idea of what's possible with this global exception handling mechanism. If you'd like to see a full implementation, have a look at the in-depth tutorial.

Note that Spring Boot also provides an /error mapping by default. We can customize its view by creating a simple error.html:

<html lang="en">
<head><title>Error Occurred</title></head>
<body>
    <h1>Error Occurred!</h1>    
    <b>[<span th:text="${status}">status</span>]
        <span th:text="${error}">error</span>
    </b>
    <p th:text="${message}">message</p>
</body>
</html>

Like most other aspects in Boot, we can control that with a simple property:

server.error.path=/error2

9. Testing

Finally, let's test our new Books API.

We can make use of @SpringBootTest to load the application context and verify there are no errors when running the app:

@RunWith(SpringRunner.class)
@SpringBootTest
public class SpringContextTest {

    @Test
    public void contextLoads() {
    }
}

Next, let's add a JUnit test that verifies the calls to the API we're written, using RestAssured:

public class SpringBootBootstrapLiveTest {

    private static final String API_ROOT
      = "http://localhost:8081/api/books";

    private Book createRandomBook() {
        Book book = new Book();
        book.setTitle(randomAlphabetic(10));
        book.setAuthor(randomAlphabetic(15));
        return book;
    }

    private String createBookAsUri(Book book) {
        Response response = RestAssured.given()
          .contentType(MediaType.APPLICATION_JSON_VALUE)
          .body(book)
          .post(API_ROOT);
        return API_ROOT + "/" + response.jsonPath().get("id");
    }
}

First, we can try to find books using variant methods:

@Test
public void whenGetAllBooks_thenOK() {
    Response response = RestAssured.get(API_ROOT);
 
    assertEquals(HttpStatus.OK.value(), response.getStatusCode());
}

@Test
public void whenGetBooksByTitle_thenOK() {
    Book book = createRandomBook();
    createBookAsUri(book);
    Response response = RestAssured.get(
      API_ROOT + "/title/" + book.getTitle());
    
    assertEquals(HttpStatus.OK.value(), response.getStatusCode());
    assertTrue(response.as(List.class)
      .size() > 0);
}
@Test
public void whenGetCreatedBookById_thenOK() {
    Book book = createRandomBook();
    String location = createBookAsUri(book);
    Response response = RestAssured.get(location);
    
    assertEquals(HttpStatus.OK.value(), response.getStatusCode());
    assertEquals(book.getTitle(), response.jsonPath()
      .get("title"));
}

@Test
public void whenGetNotExistBookById_thenNotFound() {
    Response response = RestAssured.get(API_ROOT + "/" + randomNumeric(4));
    
    assertEquals(HttpStatus.NOT_FOUND.value(), response.getStatusCode());
}

Next, we'll test creating a new book:

@Test
public void whenCreateNewBook_thenCreated() {
    Book book = createRandomBook();
    Response response = RestAssured.given()
      .contentType(MediaType.APPLICATION_JSON_VALUE)
      .body(book)
      .post(API_ROOT);
    
    assertEquals(HttpStatus.CREATED.value(), response.getStatusCode());
}

@Test
public void whenInvalidBook_thenError() {
    Book book = createRandomBook();
    book.setAuthor(null);
    Response response = RestAssured.given()
      .contentType(MediaType.APPLICATION_JSON_VALUE)
      .body(book)
      .post(API_ROOT);
    
    assertEquals(HttpStatus.BAD_REQUEST.value(), response.getStatusCode());
}

Update an existing book:

@Test
public void whenUpdateCreatedBook_thenUpdated() {
    Book book = createRandomBook();
    String location = createBookAsUri(book);
    book.setId(Long.parseLong(location.split("api/books/")[1]));
    book.setAuthor("newAuthor");
    Response response = RestAssured.given()
      .contentType(MediaType.APPLICATION_JSON_VALUE)
      .body(book)
      .put(location);
    
    assertEquals(HttpStatus.OK.value(), response.getStatusCode());

    response = RestAssured.get(location);
    
    assertEquals(HttpStatus.OK.value(), response.getStatusCode());
    assertEquals("newAuthor", response.jsonPath()
      .get("author"));
}

And delete a book:

@Test
public void whenDeleteCreatedBook_thenOk() {
    Book book = createRandomBook();
    String location = createBookAsUri(book);
    Response response = RestAssured.delete(location);
    
    assertEquals(HttpStatus.OK.value(), response.getStatusCode());

    response = RestAssured.get(location);
    assertEquals(HttpStatus.NOT_FOUND.value(), response.getStatusCode());
}

10. Conclusion

This was a quick but comprehensive intro to Spring Boot.

We of course barely scratched the surface here – there's a lot more to this framework that we can cover in a single intro article.

That's exactly why we don't just have a single article about Boot on the site.

The full source code of our examples here is, as always, over on GitHub.

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Get started with Spring 5 and Spring Boot 2, through the Learn Spring course:

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