This article is a simple example of using Java Persistence API (JPA) to retrieve an entity from the persistence context. The JPA runtime needs to be configured with metadata that includes mapping information, such as which tables and columns should hold entities retrieved by this method.
The “entitymanager find by field” is a JPA method that allows users to retrieve or find an entity using the specified field.
Finding or Retrieving an Entity
- Student student = entintyManager.find(Student.class, 1L); once an entity in the database, this line will fetch student entity with identifier value of 1. This is the same as the SELECT * FROM STUDENT WHERE ID=1 query.
- What happens if the item is no longer there or if we provide the incorrect id by accident? Then the outcomes will just be null.
- It’s just one line, but it’s the Persistence transition or the State of Student entity that we need to comprehend.
Note: To fully understand the JPA entity life cycle, see JPA Entity lifecycle.
When EntityManager’s find() is used, a persistence transition or state change occurs.
- The student entity state becomes permanent in the persistence context after a successful call to entintyManager.find(Student.class, 1L).
- Persistence-context and transnational are always connected with persistent state entities.
- If you update any value of a student object using a student reference in this state, the change will be reflected in the persistence context.
In this case, the following technologies were used:
- JPA 2.1
- 5.2.6 Hibernate
- MySql 8.0 is a database management system.
- Maven 3
- STS 3.9.8 (Spring Tool Suite)
- Java 1.8
The Studen.java class is used for mapping.
@Id @GeneratedValue(strategy=GenerationType.AUTO, generator=”native”) @Entity(name=”STUDENT”) public class Student @GenericGenerator(“native”); @GenericGenerator(“native”); @GenericGenerator(“native”); @GenericGenerator( @Column(name = “ID”) private Long studentId; @Column(name = “FNAME”) private String firstName; @Column(name = “LNAME”) private String lastName; @Column(name = “CONTACT NO”) private String contact; @Column(name = “LNAME”) private String lastName; @Column(name = “CONTACT NO”) private String contact No, but there are setters and getters.
Example 1: Identifying an Entity Code
main public static void ( String[] args ) EntityManagerFactory emf = null; EntityManager entityManager = null; EntityTransaction transaction = null; EntityManagerFactory emf = null; EntityManager entityManager = null; emf = Persistence.createEntityManagerFactory(“jbd-pu”); entityManager = emf.createEntityManager(); transaction = entityManager.getTransaction(); transaction.begin(); /* When EntityManager’s find() calls, the state of the Student instance will be “Persitent” */ Student student = entityManager.find(Student.class, 2L); System.err.println(“Stu
Output :
INFO – HHH000397: Hibernate: Using ASTQueryTranslatorFactory student0_ should be chosen. student0 .CONTACT ID1 0 0 ID1 0 0 ID1 0 0 ID1 0 0 ID1 0 0_ NO, student0_, as CONTACT 2 0 0_. student0_, FNAME3 0 0_, FNAME3 0 0_, FNAME3 0 0_, FNAME3 0 0_ STUDENT student0_, LNAME4 0 0_, LNAME4 0 0_, LNAME4 0 0_, LNAME4 0 0_, LNAME4 0 0_, ID=? Student’s Initials: Arnold INFO – HHH000030: [jdbc:mysql:/localhost:3306/jpa JBD] Connection pool cleanup [jdbc:mysql:/localhost:3306/jpa JBD]
Example 2: Identifying an Entity Code
When you modify the value of the first name in the following example, you’ll see that the change is reflected in the persistence context.
EntityManagerFactory emf = null; EntityManager entityManager = null; EntityTransaction transaction = null; try public static void main(String[] args) public static void main(String[] args) public static void main(String[] args) public static void main(String[] args) public static void main(String[] args) public static void main(String[ entityManager = emf.createEntityManager(); transaction = entityManager.getTransaction(); transaction.begin(); emf = Persistence.createEntityManagerFactory(“jbd-pu”); entityManager = emf.createEntityManager(); transaction = entityManager.getTransaction(); transaction.begin(); / * The status of Student instance will be “Persitent” whenever EntityManager’s find() method is used. student = entityManager.find(Student.class, 2L); System.out.println(“Student First Name: “+student.getFirstName()); student.setFirstName(“Peter”); student = entityManager.find(Student.class, 2L); transaction.commit(); catch(Exception e) transaction.rollback(); finally entityManager.close(); emf.close();
Output :
In the console, you’ll see something like this.
Select student0_ in Hibernate. student0_, ID1 0 0_, ID1 0 0_, ID1 0 0_, ID1 0 0_ CONTACT NO is set to CONTACT 2 0 0_, and student0_ is set to CONTACT 2 0 0_. student0_, FNAME3 0 0_, FNAME3 0 0_, FNAME3 0 0_, FNAME3 0 0_ STUDENT student0_, LNAME4 0 0_, LNAME4 0 0_, LNAME4 0 0_, LNAME4 0 0_, LNAME4 0 0_, ID=? Arnold is the student’s first name. Peter is the student’s first name. Update STUDENT using CONTACT NO=?, FNAME=?, LNAME=? and ID=? in Hibernate.
JPA-Entity-Retrieve.zip is the application to download (10 KB)
this advertisement should be reported
The “jpa get list of entities” is a query that can be used to retrieve or find an Entity using JPA. The Entity can be retrieved by its identifier or it can be found in the database.
Frequently Asked Questions
How does JPA determine entity?
A: The Java Persistence API determines the entity based upon what type of object it is. For example, if you have a Person class with an ID property, that means JPA will consider your object to be a person and will create a persistent field for its unique identifier. If you have not defined any properties in your class yet, JPA assumes that all objects are instances of java.lang.Object (aka null).
Which method in JPA is used to retrieve the entity based on its primary keys?
A: The primary key is the first column of your database, typically an INTEGER or a BIGINT. It may also be either 0 or NULL, which represents absence and nothingness respectively.
How do I find my entity class?
A: There is no recommended class for you.
Related Tags
- jpa find by field
- entitymanager get list of entities
- entitymanager.find example
- jpa get by id
- get all records from table using jpa
