JPA with hibernate 6

Docker for mysql

Start container

docker run -p 3308:3306 --name mysql_8.0.34 -e "MYSQL_ROOT_PASSWORD=root" -d mysql:8.0.34

docker exec -it mysql_8.0.34 /bin/bash

mysql -u root -p -A

docker ps -a

JPA

Configuration file

META-INF/Persistence.xml

EntityManager

    entityManagerFactory = Persistence.createEntityManagerFactory("Ecommerce-PU");
    entityManager = entityManagerFactory.createEntityManager();
    entityManager.close();
    entityManagerFactory.close();

Transaction Control

// begin a trasaction
entityManager.getTransaction().begin();
    
// database modification commands
entityManager.persist(product);
entityManager.merge(product);
entityManager.remove(product);

// return un object to a previous state
// exemple: 
Produto product = entityManager.find(Produto.class, 1);
// now product.getNome() value is "TV"
// we gonna change the name attribute
product.setNome("Microphone");
entityManager.refresh(product); // reset entity in the database
// after refresh name will return to the value "TV"
        
// this will transfer all objects in memory to a database
entityManager.flush(); // must be in a transaction control

// end transaction with a commit
entityManager.getTransaction().commit();


// clear the memory (cache) of EntityManager
entityManager.clear();
// this will allow us to "force" JPA to touch database, not just EntityManager's memory cache with the objects

Remove operations

ERROR: java.lang.IllegalArgumentException: Removing a detached instance ...

Produto product = new Produto();
product.setId(3);

entityManager.getTransaction().begin();

entityManager.remove(product);

entityManager.getTransaction().commit();

This error occurs because of JPA does not know the instance of object with id 3.

We need to recover it first from database using a find method in order to remove it.

Update objects

entityManager.merge(entity);

Merge not managed by EntityManager

With merge() we must inform all attributes of the entity, the attributes not informed will receive null value :

    Produto product = new Produto();
    product.setId(1);
    product.setNome("Novo Kindle Paperwhite");
    //product.setDescricao("Conheça o novo Kindle, agora com bla, bla bla");
    //product.setPreco(BigDecimal.valueOf(499.90d));

    entityManager.getTransaction().begin();
    entityManager.merge(product);
    entityManager.getTransaction().commit();

Result :

Merge with the knowledge of the EntityManager

Merge VS Persist

Persist

Persist is used just for INSERT operations. Guarantee for the creation of new records.

It gets the entity to save and transfer to the memory of the EntityManager.

entityManager.getTransaction().begin();

entityManager.persist(productPersist); // now productPersist is managed by the EntityManager of JPA
                                       // and it is in entityManager's memory
productPersist.setNome("New name"); // this will take effet in the database
                                    // we will see a UPDATE command and the name will be changed
entityManager.getTransaction().commit();

Merge

Merge is capable of INSERT and UPDATE operations.

It makes a copy of the entity to save and transfer to the memory of the EntityManager.

entityManager.getTransaction().begin();

entityManager.merge(productMerge); // now productMerge is copied to the EntityManager of JPA

productMerge.setNome("New name"); // this will not effet the record, because it's not managed 
                                    // by the EntityManager
                                    // no UPDATE command in this case
entityManager.getTransaction().commit();

In order to UPDATE the state of the entity, we need to explicitly refers to the object by setting the result of the merge into the same variable:

entityManager.getTransaction().begin();

productMerge = entityManager.merge(productMerge); // now we made productMerge managed by JPA

productMerge.setNome("New name"); // and this will take effect into the database

entityManager.getTransaction().commit();

Detach

In this case, we remove the product class from EntityManager supervision, which even after the commit does not change its state within the database.

@Test
public void detachEntityFromEntityManager() {
    Product product = entityManager.find(Product.class, 4);
    // product name is : Blu ray player
    entityManager.detach(product); // product will no longer be managed by JPAs EntityManager 

    entityManager.getTransaction().begin();
    product.setName("New IPad"); // this will not take effect
    entityManager.getTransaction().commit();

    entityManager.clear();

    Product productToAssertAfterClearCache = entityManager.find(Product.class, product.getId());
    Assertions.assertEquals("Blu ray player", productToAssertAfterClearCache.getName()); // test will pass 
                                                                                        // nothing was changed
}

Lombok

Dependency

...
<dependency>
    <groupId>org.projectlombok</groupId>
    <artifactId>lombok</artifactId>
    <version>1.18.30</version>
    <scope>provided</scope>
</dependency>
...

Annotation processor

...
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.11.0</version>
<configuration>
    <source>${maven.compiler.source}</source>
    <target>${maven.compiler.target}</target>
    <annotationProcessorPaths>
        <path>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.18.30</version>
        </path>
    </annotationProcessorPaths>
</configuration>
</plugin>
...

01-JPA.md 02-JPA-Relationship-Mappings.md

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