JPA EntityManager persist() and merge() method.

April 30, 2020

In this post, we will see the JPA EntityManager persist() and merge() method.

Table of Contents

Difference between persist() and merge() method.

1. The persist() method is used to create or save a new entity in the database. if we try to update an existing record using persist() method it will throw EntityExistsException. Using merge() method we can create/save a new record as well as we can update an existing record.

2. The return type of the persist() method is void whereas the merge() method returns a managed entity.

public void persist(Object entity);

public T merge(T entity);

3. Sample example of persist() and merge() method.

@Service("studentServiceImpl")
public class StudentServiceImpl implements StudentService {

	@PersistenceContext
	private EntityManager entityManger;

	@Transactional
	public Student saveStudent(Student student) {
		entityManger.persist(student);
		return student;
	}
	
	@Transactional
	public Student updateStudent(Student student) {
		student = entityManger.merge(student);
		return student;
	}

}

4. Since persist() method used to create new record only it will fire INSERT SQL statement whereas in case of merge() method INSERT SQL statement as well UPDATE SQL statement will get executed.

The query generated while saving a new Student entity using persist() method.

Hibernate:
insert
into
student
(name, roll_number, university, id)
values
(?, ?, ?, ?)

The query generated while saving a new Student entity using merge() method.

Hibernate:
insert
into
student
(name, roll_number, university, id)
values
(?, ?, ?, ?)

The query generated while updating an existing Student entity using merge() method.

Hibernate:
update
student
set
name=?,
roll_number=?,
university=?
where
id=?

Using persist() and merge() method ServiceImpl Example.

First, we need to create an EntityManger reference.

@PersistenceContext
private EntityManager entityManger;

Using this we can call persist() method.

package com.javatute.impl;

import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;

import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;

import com.javatute.entity.Student;
import com.javatute.service.StudentService;

@Service("studentServiceImpl")
public class StudentServiceImpl implements StudentService {

	@PersistenceContext
	private EntityManager entityManger;

	@Transactional
	public Student saveStudent(Student student) {
		entityManger.persist(student);
		return student;
	}

	@Transactional
	public Student updateStudent(Student student) {
		entityManger.merge(student);
		return student;
	}

}

Note – EntityManger persist() and merge() method Implementation in Hibernate.

@Override
public void persist(Object object) throws HibernateException {
checkOpen();
firePersist( new PersistEvent( null, object, this ) );
}

@Override
public Object merge(Object object) throws HibernateException {
checkOpen();
return fireMerge( new MergeEvent( null, object, this ));
}

Example of JPA EntityManager persist() and merge() methods using Spring Boot and Oracle.

Open eclipse and create maven project, Don’t forget to check ‘Create a simple project (skip)’ click on next. Fill all details(GroupId – entitymanager, ArtifactId – entitymanager and name – entitymanager) and click on finish. Keep packaging as the jar.

Modify pom.xml

<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>entitymanager</groupId>
	<artifactId>entitymanager</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<name>entitymanager</name>
	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>2.0.2.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-data-jpa</artifactId>
		</dependency>

		<dependency>
			<groupId>com.oracle</groupId>
			<artifactId>ojdbc6</artifactId>
			<version>11.2.0.3</version>
		</dependency>

	</dependencies>
	<build>
		<finalName>${project.artifactId}</finalName>
		<plugins>

			<plugin>
				<artifactId>maven-compiler-plugin</artifactId>
				<version>3.1</version>
				<configuration>
					<fork>true</fork>
					<executable>C:\Program Files\Java\jdk1.8.0_131\bin\javac.exe</executable>
				</configuration>
			</plugin>


		</plugins>
	</build>
</project>

Note – In pom.xml we have defined javac.exe path in configuration tag. You need to change accordingly i.e where you have installed JDK.

If you see any error for oracle dependency then follow these steps.

Let’s see the directory structure of JPA EntityManager persist() method example as below.

Directory structure –

Let’s see about JPA EntityManager find() method.

Student.java

package com.javatute.entity;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class Student {

	@Id
	@GeneratedValue(strategy = GenerationType.AUTO)
	private int id;

	@Column(name = "name")
	private String name;

	@Column(name = "roll_number")
	private String rollNumber;

	@Column(name = "university")
	String university;

	public int getId() {
		return id;
	}

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

	public String getName() {
		return name;
	}

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

	public String getRollNumber() {
		return rollNumber;
	}

	public void setRollNumber(String rollNumber) {
		this.rollNumber = rollNumber;
	}

	public String getUniversity() {
		return university;
	}

	public void setUniversity(String university) {
		this.university = university;
	}

}

StudentController.java

package com.javatute.controller;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.ResponseBody;
import org.springframework.web.bind.annotation.RestController;

import com.javatute.entity.Student;
import com.javatute.service.StudentService;

@RestController
@RequestMapping(value = "/student")
public class StudentController {

	@Autowired
	private StudentService studentService;

	@RequestMapping(value = "/save", method = RequestMethod.POST)
	@ResponseBody
	public Student saveStudent(@RequestBody Student student) {
		Student studentResponse = (Student) studentService.saveStudent(student);
		return studentResponse;
	}

	@RequestMapping(value = "/update", method = RequestMethod.PUT)
	@ResponseBody
	public Student updateStudent(@RequestBody Student student) {
		Student studentResponse = (Student) studentService.updateStudent(student);
		return studentResponse;
	}

	@RequestMapping(value = "/find/{id}", method = RequestMethod.GET)
	@ResponseBody
	public Student findByName(@PathVariable int id) {

		Student studentResponse = (Student) studentService.find(id);
		return studentResponse;
	}

}

We have seen controller and serviceimpl, interface for JPA EntityManager persist() and merge() method Example.

Note – See more details about @Controller and RestController here.

StudentService.java – interface

package com.javatute.service;

import org.springframework.stereotype.Component;

import com.javatute.entity.Student;

@Component
public interface StudentService {
	public Student saveStudent(Student student);
public Student updateStudent(Student student);
	public Student find(int id);

}

Note – See here more about @Component, @Controller, @Service and @Repository annotations here.

StudentServiceImpl.java

package com.javatute.impl;

import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;

import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;

import com.javatute.entity.Student;
import com.javatute.service.StudentService;

@Service("studentServiceImpl")
public class StudentServiceImpl implements StudentService {

	@PersistenceContext
	private EntityManager entityManger;

	@Transactional
	public Student saveStudent(Student student) {
		entityManger.persist(student);
		return student;
	}

	@Transactional
	public Student updateStudent(Student student) {
		entityManger.merge(student);
		return student;
	}

	@Transactional
	public Student find(int id) {
		Student response = (Student) entityManger.find(Student.class, id);
		return response;
	}

}

SpringMain.java

package com.javatute.main;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.autoconfigure.domain.EntityScan;
import org.springframework.context.annotation.ComponentScan;

@SpringBootApplication
@ComponentScan(basePackages = "com.*")
@EntityScan("com.javatute.entity")
public class SpringMain {
	public static void main(String[] args) {

		SpringApplication.run(SpringMain.class, args);
	}

}

Note – See more details about @ComponentScan here.

application.properties

# Connection url for the database
spring.datasource.url=jdbc:oracle:thin:@localhost:1521:XE
spring.datasource.username=SYSTEM
spring.datasource.password=oracle2
spring.datasource.driver-class-name=oracle.jdbc.driver.OracleDriver
# Show or not log for each sql query
spring.jpa.show-sql = true
spring.jpa.properties.hibernate.format_sql=true 
 
spring.jpa.hibernate.ddl-auto =create
spring.jpa.properties.hibernate.dialect = org.hibernate.dialect.Oracle10gDialect
 
server.port = 9091

#show sql values
logging.level.org.hibernate.type.descriptor.sql=trace

#hibernate.show_sql = true
#spring.jpa.hibernate.logging.level.sql =FINE
#show sql statement
#logging.level.org.hibernate.SQL=debug

Let’s run the SpringMain class(run as java application).

Testing of example using postman.

Perform save operation first using below REST API.

POST – http://localhost:9091/student/save

    {
        "name": "john",
        "rollNumber": "120",
        "university":"rgtu"
    }

Response Data.

    {
        "id": 1,
        "name": "john",
        "rollNumber": "120",
        "university": "rgtu"
    }

Perform update operation.

PUT – http://localhost:9091/student/update/

Request data.

{
    "id": 1,
    "name": "john",
    "rollNumber": "123",
    "university": "rgtu"
}

Response Data.

{
    "id": 1,
    "name": "john",
    "rollNumber": "123",
    "university": "rgtu"
}

GET – http://localhost:9091/student/find/1

That’s all about JPA EntityManager persist() and merge() methods.

Difference between persist() and merge() method.

Using persist() and merge() method ServiceImpl Example.

Example of JPA EntityManager persist() and merge() methods using Spring Boot and Oracle.

Testing of example using postman.

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