In hibernate; both merge() and update() methods updates an entity and can also affect the state of an object i.e. from detached to persistent. In this tutorial, we will explore the use of merge() method.
1. Introduction
- Object-Relational Mapping or ORM is the programming technique to map application domain model objects to the relational database tables
- Hibernate is a Java-based ORM tool that provides the framework for mapping application domain objects to the relational database tables and vice versa. It provides the reference implementation of Java Persistence API that makes it a great choice as an ORM tool with benefits of loose coupling
- A Framework that an option to map plain old Java objects to the traditional database tables with the use of JPA annotations as well as
XMLbased configuration
1.1 Hibernate Annotations
- Hibernate annotations are the newest way to define mappings without the use of a
XMLfile - Developers use annotations to provide metadata configuration along with the Java code. Thus, making the code easy to understand
- XML provides the ability to change the configuration without building the project. Thus, annotations are less powerful than the
XMLconfiguration and should only be used for table and column mappings - Annotations are preconfigured with sensible default values, which reduce the amount of coding required. For e.g., Class name defaults to Table name and Field names default to Column names
1.2 Hibernate merge() method
The merge() method in hibernate offers a considerable flexibility to save the entities in a database as developers need not worry about attaching the object to the session. When developers call the merge method, it first checks the same object in the cache. If exists then it will update the cache with the changes, if not it will load the values to the cache. So in either way, the merge() method will not throw an exception. The method signatures for this method are:
Object merge(Object object) Object merge(String entityName, Object object)
1.3 Download and Install Hibernate
You can read this tutorial in order to download and install Hibernate in the Eclipse IDE.
1.4 Download and Install MySQL
You can watch this video in order to download and install the MySQL database on your Windows operating system.
Now, open up the Eclipse IDE and let us see how to implement this method in the hibernate framework!
2. Hibernate Persist Example
Here is a systematic guide for implementing this tutorial in the hibernate framework.
2.1 Tools Used
We are using Eclipse Kepler SR2, JDK 8, MySQL database and Maven. Having said that, we have tested the code against JDK 1.7 and it works well.
2.2 Project Structure
Firstly, let us review the final project structure, in case you are confused about where you should create the corresponding files or folder later!
2.3 Project Creation
This section will demonstrate how to create a Java-based Maven project with Eclipse. In Eclipse IDE, go to File -> New -> Maven Project.
In the New Maven Project window, it will ask you to select the project location. By default, ‘Use default workspace location’ will be selected. Select the ‘Create a simple project (skip archetype selection)’ checkbox and just click on the next button to proceed.
It will ask you to ‘Enter the group and the artifact id for the project’. We will input the details as shown in the below image. The version number will be by default: 0.0.1-SNAPSHOT.
Click on Finish and the creation of a maven project is completed. If you observe, it has downloaded the maven dependencies and a pom.xml file will be created. It will have the following code:
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>com.hibernate</groupId> <artifactId>HibernateMerge</artifactId> <version>0.0.1-SNAPSHOT</version> <name>Hibernate Merge Method Example</name> <packaging>jar</packaging> </project>
We can start adding the dependencies that developers want like Hibernate, MySQL etc. Let us start building the application!
3. Application Building
Below are the steps involved in developing this application.
3.1 Database and Table Creation
The following script creates a database called mergequerydb with a table: employee. Open MySQL terminal and execute the script.
CREATE DATABASE IF NOT EXISTS mergequerydb; USE mergequerydb; CREATE TABLE employee ( id INT(50) NOT NULL AUTO_INCREMENT, name VARCHAR(200) DEFAULT NULL, designation VARCHAR(200) DEFAULT NULL, department VARCHAR(200) DEFAULT NULL, PRIMARY KEY (id) ); INSERT INTO employee (id, name, designation, department) VALUES (1, 'Mike', 'Software Developer', 'Software Development'); INSERT INTO employee (id, name, designation, department) VALUES (2, 'David', 'Team Lead', 'Software Development');
If everything goes well, the table will be created.
3.2 Maven Dependencies
Here, we specify the dependencies for the Hibernate framework and the MySQL connector. Maven will automatically resolve the rest dependencies such as Hibernate Core, MySQL etc. The updated file will have the following code:
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>com.hibernate</groupId>
<artifactId>HibernateMerge</artifactId>
<version>0.0.1-SNAPSHOT</version>
<name>Hibernate Merge Method Example</name>
<description>A project to demonstrate the merge() method in hibernate</description>
<packaging>jar</packaging>
<dependencies>
<!-- https://mvnrepository.com/artifact/org.hibernate/hibernate-core -->
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-core</artifactId>
<version>5.3.7.Final</version>
</dependency>
<!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>8.0.13</version>
</dependency>
</dependencies>
<build>
<finalName>${project.artifactId}</finalName>
</build>
</project>
3.3 Java Class Creation
Let us write the Java classes involved in this application.
3.3.1 Implementation of Model Class
Add the following code to the model definition to map the attributes with the column names.
Employee.java
package com.hibernate.model;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;
@Entity
@Table(name= "employee")
public class Employee {
@Id
@GeneratedValue(strategy= GenerationType.IDENTITY)
private int id;
private String name;
private String designation;
private String department;
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 getDesignation() {
return designation;
}
public void setDesignation(String designation) {
this.designation = designation;
}
public String getDepartment() {
return department;
}
public void setDepartment(String department) {
this.department = department;
}
@Override
public String toString() {
return "Employee [id=" + id + ", name=" + name + ", designation=" + designation + ", department=" + department
+ "]";
}
}
3.3.2 Implementation of Utility Class
Add the following code to the implementation class for testing the merge() method.
AppMain.java
package com.hibernate.util;
import org.hibernate.Session;
import org.hibernate.cfg.Configuration;
import com.hibernate.model.Employee;
public class AppMain {
public static void main(String[] args) {
// Creating the config instance & passing the hibernate config file.
Configuration config = new Configuration();
config.configure("hibernate.cfg.xml");
Session session = config.buildSessionFactory().openSession();
Employee emp1 = session.get(Employee.class, new Integer(1));
session.close();
// "emp1 is now a detached instance".
session = config.buildSessionFactory().openSession();
Employee emp2 = session.get(Employee.class, new Integer(1));
Employee emp3 = (Employee) session.merge(emp1);
if(emp2 == emp3) {
System.out.println("Emp2 and Emp3 are equal");
}
// Closing the session object.
session.close();
}
}
3.4. Hibernate Configuration File
In the configuration file, we will include the database and the mapping class details.
hibernate.cfg.xml
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd"> <hibernate-configuration> <session-factory> <!-- Database connection settings --> <property name="hibernate.connection.driver_class">com.mysql.cj.jdbc.Driver</property> <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/mergequerydb</property> <property name="hibernate.connection.username">root</property> <property name="hibernate.connection.password" /> <!-- Validation of the database schema --> <property name="hibernate.hbm2ddl.auto">validate</property> <!-- Sql dialect --> <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property> <!-- Printing the sql queries to the console --> <property name="show_sql">true</property> <!-- Model class --> <mapping class="com.hibernate.model.Employee" /> </session-factory> </hibernate-configuration>
Important points:
- Here, we instructed Hibernate to connect to a MySQL database named
mergequerydband the mapping class to be loaded - We have also instructed the Hibernate framework to use
MySQLDialecti.e. Hibernate will optimize the generated SQL statements for MySQL - This configuration will be used to create a hibernate
SessionFactoryobject show_sqltag will instruct the hibernate framework to log all the SQL statements on the console
4. Run the Application
To run the Hibernate application, Right-click on the AppMain class -> Run As -> Java Application. Developers can debug the example and see what happens after every step!
5. Project Demo
The code shows the following logs as the output of this tutorial.
Oct 27, 2018 11:36:29 AM org.hibernate.resource.transaction.backend.jdbc.internal.DdlTransactionIsolatorNonJtaImpl getIsolatedConnection INFO: HHH10001501: Connection obtained from JdbcConnectionAccess [org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator$ConnectionProviderJdbcConnectionAccess@2a3a299] for (non-JTA) DDL execution was not in auto-commit mode; the Connection 'local transaction' will be committed and the Connection will be set into auto-commit mode. Hibernate: select employee0_.id as id1_0_0_, employee0_.department as departme2_0_0_, employee0_.designation as designat3_0_0_, employee0_.name as name4_0_0_ from employee employee0_ where employee0_.id=? Emp2 and Emp3 are equal
That is all for this tutorial and I hope the article served you whatever you were looking for. Happy Learning and do not forget to share!
6. Conclusion
This post defines the implementation of the merge() method in the Hibernate framework and helps developers understand the basic configuration required to achieve this. Developers can download the sample application as an Eclipse project in the Downloads section.
7. Download the Eclipse Project
This was an example of the Hibernate merge method for beginners.
You can download the full source code of this example here: HibernateMerge
