1. Overview

Spring Boot is an opinionated addition to the Spring platform, focused on convention over configuration — highly useful for getting started with minimum effort and creating standalone, production-grade applications.

This tutorial is a starting point for Boot, in other words 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 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, we can notice 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 is 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 not easily covered in a couple of lines of configuration. So, we definitely encourage deeper reading 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 the following:

  • @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. This is 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 to certainly explore further.

7. Web and the Controller

Next, let's have a look at a web tier. And we'll start 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 is equivalent to a @Controller along with @ResponseBody — so that each method marshals the returned resource right to the HTTP response.

Note that we're exposing our Book entity as our external resource here. That's fine for this simple application, but in a real-world application, we'll probably 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 gives us an idea of what's possible with this global exception handling mechanism. 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 that 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've 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());
}

Then we'll 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 we can 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 introduction to Spring Boot.

Of course, we barely scratched the surface here. There's a lot more to this framework than we can cover in a single intro article.

That's exactly why we have more than just a single article covering Boot on the site.

As always, the full source code of our examples here is over on GitHub.

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

>> CHECK OUT THE COURSE
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