Spring is a popular Java application framework for creating enterprise applications. Spring Boot is the next step in evolution of Spring framework. It helps create stand-alone, production-grade Spring based applications with minimal effort. It does not use XML configurations anymore and implements the convention over configuration principle.
Annotation is a form of metadata which provides data about a program that is not part of the program itself. Annotations do not have direct effect on the operation of the code they annotate.
Spring Boot basic annotations
In the example application, we have these Spring Boot annotations:
- @Bean - indicates that a method produces a bean to be managed by Spring.
- @Service - indicates that an annotated class is a service class.
- @Repository - indicates that an annotated class is a repository, which is an abstraction of data access and storage.
- @Configuration - indicates that a class is a configuration class that may contain bean definitions.
- @Controller - marks the class as web controller, capable of handling the requests.
- @RequestMapping - maps HTTP request with a path to a controller method.
- @Autowired - marks a constructor, field, or setter method to be autowired by Spring dependency injection.
- @SpringBootApplication - enables Spring Boot autoconfiguration and component scanning.
@Component is a generic stereotype for a Spring managed component. It turns the class into a Spring bean at the auto-scan time. Classes decorated with this annotation are considered as candidates for auto-detection when using annotation-based configuration and classpath scanning. @Repository, @Service, and @Controller are specializations of @Component for more specific use cases.
There are also Hibernate @Entity, @Table, @Id, and @GeneratedValue annotations in the example.
Spring Boot basic annotations example
The following application is a Spring Boot application which returns data from an H2 database using Spring Data JPA. The application uses FreeMarker as a template engine.
pom.xml
src
├───main
│ ├───java
│ │ └───com
│ │ └───zetcode
│ │ │ Application.java
│ │ ├───controller
│ │ │ MyController.java
│ │ ├───model
│ │ │ City.java
│ │ ├───repository
│ │ │ CityRepository.java
│ │ └───service
│ │ CityService.java
│ │ ICityService.java
│ └───resources
│ │ application.yaml
│ │ import.sql
│ ├───static
│ │ └───css
│ │ style.css
│ └───templates
│ index.ftl
│ showCities.ftl
└───test
└───java
This is the project structure.
pom.xml
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>swam.com</groupId>
<artifactId>springbootbasicannotations</artifactId>
<version>1.0-SNAPSHOT</version>
<packaging>jar</packaging>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<maven.compiler.source>11</maven.compiler.source>
<maven.compiler.target>11</maven.compiler.target>
</properties>
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>2.1.1.RELEASE</version>
</parent>
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-freemarker</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>
<dependency>
<groupId>com.h2database</groupId>
<artifactId>h2</artifactId>
<scope>runtime</scope>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
</plugin>
</plugins>
</build>
</project>
This is the Maven build file. It contains dependencies for Freemaker, Spring Data JPA, and H2 database. When Spring Boot finds Freemaker and H2 in the pom.xml, it automatically configures them. We can use them right away.
resources/application.yml
server:
servlet:
context-path: /myapp
spring:
main:
banner-mode: "off"
datasource:
platform: h2
logging:
level:
org:
springframework: ERROR
In the application.yml file we write various configuration settings of a Spring Boot application.
com/zetcode/model/City.java
package swam.com.model;
import java.util.Objects;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;
@Entity
@Table(name = "cities")
public class City {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String name;
private int population;
public City() {
}
public City(String name, int population) {
this.name = name;
this.population = population;
}
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getPopulation() {
return population;
}
public void setPopulation(int population) {
this.population = population;
}
@Override
public int hashCode() {
int hash = 7;
hash = 79 * hash + Objects.hashCode(this.id);
hash = 79 * hash + Objects.hashCode(this.name);
hash = 79 * hash + this.population;
return hash;
}
@Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (obj == null) {
return false;
}
if (getClass() != obj.getClass()) {
return false;
}
final City other = (City) obj;
if (this.population != other.population) {
return false;
}
if (!Objects.equals(this.name, other.name)) {
return false;
}
return Objects.equals(this.id, other.id);
}
@Override
public String toString() {
var builder = new StringBuilder();
builder.append("City{id=").append(id).append(", name=")
.append(name).append(", population=")
.append(population).append("}");
return builder.toString();
}
}
This is the City entity. Each entity must have at least two annotations defined: @Entity and @Id. The @Entity annotation specifies that the class is an entity and is mapped to a database table. The @Table annotation specifies the name of the database table to be used for mapping. The @Id annotation specifies the primary key of an entity and the @GeneratedValue provides for the specification of generation strategies for the values of primary keys.
resources/import.sql
INSERT INTO cities(name, population) VALUES('Bratislava', 432000);
INSERT INTO cities(name, population) VALUES('Budapest', 1759000);
INSERT INTO cities(name, population) VALUES('Prague', 1280000);
INSERT INTO cities(name, population) VALUES('Warsaw', 1748000);
INSERT INTO cities(name, population) VALUES('Los Angeles', 3971000);
INSERT INTO cities(name, population) VALUES('New York', 8550000);
INSERT INTO cities(name, population) VALUES('Edinburgh', 464000);
INSERT INTO cities(name, population) VALUES('Berlin', 3671000);
The schema is automatically created by Hibernate; later, the import.sql file is executed to fill the table with data.
com/zetcode/repository/CityRepository.java
package swam.com.repository;
import swam.com.model.City;
import org.springframework.data.repository.CrudRepository;
import org.springframework.stereotype.Repository;
@Repository
public interface CityRepository extends CrudRepository<City, Long> {
}
The @Repository annotation is used to define a repository.
com/zetcode/service/ICityService.java
package swam.com.service;
import swam.com.model.City;
import java.util.List;
public interface ICityService {
List<City> findAll();
}
ICityService provides a contract method to get all cities.
swam/com/service/CityService.java
package swam.com.service;
import swam.com.model.City;
import swam.com.repository.CityRepository;
import java.util.List;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
@Service
public class CityService implements ICityService {
@Autowired
private CityRepository cityRepository;
@Override
public List<City> findAll() {
return (List<City>) cityRepository.findAll();
}
}
The @Service annotation declares CityService to be a service class; a class that provides business services. The @Autowired annotation marks cityRepository field to be injected with CityRepository.
swam/com/controller/MyController.java
package swam.com.controller;
import swam.com.service.ICityService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.servlet.ModelAndView;
import java.util.HashMap;
import java.util.Map;
@Controller
public class MyController {
@Autowired
private ICityService cityService;
@RequestMapping("/")
public String index(Model model) {
return "index";
}
@RequestMapping("/cities")
public ModelAndView showCities() {
var cities = cityService.findAll();
Map<String, Object> params = new HashMap<>();
params.put("cities", cities);
return new ModelAndView("showCities", params);
}
}
The @Controller annotation marks a class as a web controller. The @RequestMapping maps HTTP request with a path to a controller method. In the second case, it maps the /cities URL to the showCities() method.
resources/templates/index.ftl
<!DOCTYPE html>
<html>
<head>
<title>Home page</title>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
<a href="cities">Show cities</a>
</body>
</html>
This is the index.ftl template file. It contains a link to create a request to show all cities.
resources/templates/showCities.ftl
<!DOCTYPE html>
<html>
<head>
<title>Cities</title>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<link rel="stylesheet" href="css/style.css">
</head>
<body>
<h2>List of cities</h2>
<table>
<tr>
<th>Id</th>
<th>Name</th>
<th>Population</th>
</tr>
<#list cities as city>
<tr>
<td>${city.id}</td>
<td>${city.name}</td>
<td>${city.population}</td>
</tr>
</#list>
</table>
</body>
</html>
This is the showCities.ftl template file. It uses FreeMarker #list macro to display all city objects.
resources/static/css/style.css
h2 {color: blue}
td:nth-child(3) {
text-align: right;
}
This is the style.css template file.
com/zetcode/Application.java
package swam.com;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
@SpringBootApplication
public class Application {
public static void main(String[] args) {
SpringApplication.run(Application.class, args);
}
}
The @SpringBootApplication enables auto-configuration and component scanning.
$ mvn spring-boot:run
We run the application and locate to the localhost:8080/myapp address.
In this tutorial, we have covered a few basic Spring Boot annotations.
