Java Backend Learn Files

I'm use for this course an IntelliJ IDEA.

For me, that use VSCode, its perfect install the VSCode Keymap:

1. File -> Settings -> Keymaps
2. Click on Get more keymaps in setting
3. Search VSCode and install
4. Choose VSCode on Keymap panel
5. Save

Maven Project

It's a Project Dependencies Manager.

To install some dependencies, you need to search the code part that makes maven add library.

A code part like this needs to be past on pom.xml:

<dependency>
    <groupId>org.hibernate</groupId>
    <artifactId>hibernate-core</artifactId>
    <version>5.5.5.Final</version>
</dependency>

Saving the file (Ctrl + S), maven will att your project.

Configure JPA with MySQL

To configure JPA, you need:

1. Create a META-INFfolder on src/main/resources
2. Inside META-INF, create a persistence.xml file

OBS: The file/folder names needs to be EXACTLY equals!

On persistence.xml, put:

<?xml version="1.0" encoding="UTF-8"?>
    <persistence xmlns="http://xmlns.jcp.org/xml/ns/persistence"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence
     http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd"
    version="2.1">
    <persistence-unit name="projectName" transaction-type="RESOURCE_LOCAL">
        <properties>
             <property name="javax.persistence.jdbc.url"
             value="jdbc:mysql://localhost/yourDatabase?useSSL=false&amp;serverTimezone=UTC" />
             <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver" />
             <property name="javax.persistence.jdbc.user" value="root" />
             <property name="javax.persistence.jdbc.password" value="" />
             <property name="hibernate.hbm2ddl.auto" value="update" />
             <property name="hibernate.dialect" value="org.hibernate.dialect.MySQL8Dialect" />
            </properties>
    </persistence-unit>
</persistence>

Maybe you need create a container: docker run --name=java_mysql -d mysql/mysql-server

Mapping Entities

For Map Entities, you need to put on Entities Class a decorator called @Entity. Should be imported by javax.persistence:

@Entity
public class User {
    private Integer id;
    private String username;
    //[...]
}

For Primary Keys, we use this decorators over declaration:

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
//[...]

Other columns naturally are created by Hibernate.

Operations on Database

To make create operations on Database, we need use a EntityManagerFactory and EntityManager from javax.persistence:

public static void main(String[] args) {
    EntityManagerFactory userEntityFactory = Persistence.createEntityManagerFactory("Course");

    EntityManager userEntity = userEntityFactory.createEntityManager();

    User user = new User(null, "Bruno Santana", "[email protected]", "123456789");

    //Quando o JPA faz uma operação no banco diferente de consulta, ele precisa de uma transação
    userEntity.getTransaction().begin();
    userEntity.persist(user);
    userEntity.getTransaction().commit();
    System.out.println("Data writed on Database");
    userEntity.close();
    userEntityFactory.close();

}

The projectName needs to be the exact name that contains in the name property at persistence-unit on persistence.xml. Finnaly we can close both the userEntity and userEntityFactory.

To make read operations, we can use find method:

User userFromDatabase = userEntity.find(User.class, 8);

To make remove operations you need to parse non-monitored Entities (Object that you inserted and not close entities):

userEntity.getTransaction().begin();
userEntity.remove(userFromDatabase);
userEntity.getTransaction().commit();

Spring

Framework that provides all configs and basic packages that should able to create an basic API. Management transactions and create a repository.

Create a Spring Project

1. Go to Spring Initializr Site
2. In Project, choose Maven
3. In Spring Boot, don't change anything
4. Fill Project metadata with the correct data
5. Choose a LTS java version
6. Click in GENERATE
7. Unzip files and open project

To running application, just type CTRL + Shift + F10 to Build and Run.

To change default port and other configs, you can change the application.properties in src/main/resources:

server.port = ${port:3333}

REST Controllers

In main package we need to create a subpackage called resourses. Create inside the new package a Category Class called CategoryResource:

import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;

@RestController
@RequestMapping(value="/categories")
public class CategoryResource {

    @RequestMapping(method = RequestMethod.GET)
    public String list(){
        return "List method working";
    }
}

1. Rest class controllers needs a decorator before declaration called @RestController.
2. We create another decorator @RequestMapping(value="/categories") parsing the endpoint
3. For this example, we create a GET Route, so we add a @RequestMapping(method = RequestMethod.GET) decorator before method.

Creating a DomainClass

Category

package com.brunosana.course.domain;

import java.io.Serializable;
import java.util.Objects;

public class Category implements Serializable {
    public static final Long serialVersionUID = 1L;

    private Integer id;
    private String name;

    public Category(){}

    public  Category(Integer id, String name){
        this.id = id;
        this.name = name;
    }

    public Integer getId(){
        return id;
    }

    public String getName(){
        return name;
    }

    public void setId(Integer id){
        this.id = id;
    }

    public void setName(String name){
        this.name = name;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Category category = (Category) o;
        return Objects.equals(id, category.id);
    }

    @Override
    public int hashCode() {
        return Objects.hash(id);
    }
}

New get Method on RestController:

    @RequestMapping(method = RequestMethod.GET)
    public List<Category> list(){
        Category category = new Category(1, "Info");
        Category categoryTwo = new Category(2, "Office");
        List<Category> list= new ArrayList<>();
        list.add(category);
        list.add(categoryTwo);
        return list;
    }

1. Needs a constructor
2. Needs getters and setters
3. Needs hashCode and equals methods to compare objects
4. Needs to implements Serializable (Talks java that the current class can be converted from a bit sequence)

Create a H2 Test Database

H2 it's a databases used for tests, running local and with low memory

To create a H2, we will use this dependencies:

<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>

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

In src/main/static/application.properties we add:

spring.h2.console.enabled=true
spring.h2.console.path=/h2
spring.datasource.url=jdbc:h2:mem:testdb
spring.datasource.username=sa
spring.datasource.password=
spring.datasource.driver-class-name=org.h2.Driver
spring.jpa.show-sql=true
spring.jpa.properties.hibernate.format_sql=true

So now we can run the application and go to the /h2 path. If we can connect, it's working (With decorators on Entity Domain Classes)!