ZetCode

Spring Boot findById

last modified March 25, 2021

Spring Boot findById tutorial shows how to retrieve an entity by its Id using CrudRepository's findById method.

Spring is a popular Java/Kotlin application framework for creating enterprise applications. Spring Boot is an evolution of Spring framework which helps create stand-alone, production-grade Spring based applications with minimal effort.

CrudRepository

CrudRepository interface provides generic CRUD operations on a repository for a specific type. Its findById method retrieves an entity by its id. The return value is Optional<T>.

Optional<T> is a container object which may or may not contain a non-null value. If a value is present, isPresent returns true and get returns the value. The ifPresent invokes the specified method if the value is present; otherwise nothing is done.

Spring Boot findById example

The following application sets up a repository of City objects. In the console runner we retrieve city objects by their Ids.

build
build.gradle
gradle
gradlew
gradlew.bat
settings.gradle
src
├── main
│   ├── java
│   │   └── com
│   │       └── zetcode
│   │           ├── Application.java
│   │           ├── MyRunner.java
│   │           ├── model
│   │           │   └── City.java
│   │           ├── repository
│   │           │   └── CityRepository.java
│   │           └── service
│   │               ├── CityService.java
│   │               └── ICityService.java
│   └── resources
│       ├── application.properties
│       ├── data-h2.sql
│       └── schema-h2.sql
└── test
    ├── java
    └── resources

This is the project structure.

build.gradle
plugins {
    id 'org.springframework.boot' version '2.4.4'
    id 'io.spring.dependency-management' version '1.0.11.RELEASE'
    id 'java'
}

group = 'com.zetcode'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = '11'

repositories {
    mavenCentral()
}

dependencies {
    implementation 'org.springframework.boot:spring-boot-starter-data-jpa'
    runtimeOnly 'com.h2database:h2'
    testImplementation 'org.springframework.boot:spring-boot-starter-test'
}

test {
    useJUnitPlatform()
}

The build.gradle file contains dependencies for Spring Data JPA and H2 database.

resources/application.properties
spring.main.banner-mode=off
spring.datasource.platform=h2
spring.jpa.hibernate.ddl-auto=none

The application.properties is Spring Boot's main configuration file. The Spring Boot banner is turned off with the spring.main.banner-mode property.

The spring.datasource.platform sets the vendor name of the database. It is used in the initialization scripts. The spring.jpa.hibernate.ddl-auto disables the automatic creation of schemas from entities.

resources/schema-h2.sql
CREATE TABLE cities(id INT PRIMARY KEY AUTO_INCREMENT,
    name VARCHAR(255), population INT);

When the application is started, the schema-h2.sql script is executed. It creates a new database table.

resources/data-h2.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('Suzhou', 4327066);
INSERT INTO cities(name, population) VALUES('Zhengzhou', 4122087);
INSERT INTO cities(name, population) VALUES('Berlin', 3671000);

The table is filled with data from the data-h2.sql file.

com/zetcode/model/City.java
package com.zetcode.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.AUTO)
    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 boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        City city = (City) o;
        return population == city.population &&
               Objects.equals(name, city.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, population);
    }

    @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.

com/zetcode/repository/CityRepository.java
package com.zetcode.repository;

import com.zetcode.model.City;
import org.springframework.data.repository.CrudRepository;
import org.springframework.stereotype.Repository;

@Repository
public interface CityRepository extends CrudRepository<City, Long> {

}

By extending from the Spring CrudRepository, we have some basic methods for our data repository implemented.

com/zetcode/service/ICityService.java
package com.zetcode.service;

import com.zetcode.model.City;

import java.util.Optional;

public interface ICityService {

    Optional<City> findById(Long id);
}

ICityService provides a contract method to get a city by its Id.

com/zetcode/service/CityService.java
package com.zetcode.service;

import com.zetcode.model.City;
import com.zetcode.repository.CityRepository;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import java.util.Optional;

@Service
public class CityService implements ICityService {

    @Autowired
    private CityRepository cityRepository;

    @Override
    public Optional<City> findById(Long id) {

        return cityRepository.findById(id);
    }
}

CityService contains the implementation of the findById method. We use the repository to retrieve data from the database.

@Autowired
private CityRepository repository;

CityRepository is injected.

@Override
public Optional<City> findById(Long id) {

    return cityRepository.findById(id);
}

The findById returns Optional<City>.

com/zetcode/MyRunner.java
package com.zetcode;

import com.zetcode.service.ICityService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.CommandLineRunner;
import org.springframework.stereotype.Component;

@Component
public class MyRunner implements CommandLineRunner {

    @Autowired
    private ICityService cityService;

    @Override
    public void run(String... args) throws Exception {

        var id1 = 2L;
        cityService.findById(id1).ifPresent(System.out::println);

        var id2 = 24L;
        var val = cityService.findById(id2);

        if (val.isPresent()) {
            System.out.println(val.get());
        } else {
            System.out.printf("No city found with id %d%n", id2);
        }
    }
}

In the MyRunner, we look for two city entities.

var id1 = 2L;
cityService.findById(id1).ifPresent(System.out::println);

We find a city by its Id and if it is present, we print it to the console. Otherwise, nothing is printed.

var id2 = 24L;
var val = cityService.findById(id2);

if (val.isPresent()) {
    System.out.println(val.get());
} else {
    System.out.printf("No city found with id %d%n", id2);
}

In the second case, we check the presence of a value with isPresent. If a value is present, we print it. If not, we print a message that the city was not found.

com/zetcode/Application.java
package com.zetcode;

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 Application sets up the Spring Boot application. The @SpringBootApplication enables auto-configuration and component scanning.

$ ./gradlew bootRun
...
City{id=2, name=Budapest, population=1759000}
No city found with id 24
...

We run the application.

In this tutorial, we have showed how to find a specific entity with CrudRepository's findById method.

List all Spring Boot tutorials.