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Introduction
Both Update and Merge operate on detached instances, that is, instances that have a corresponding item in the database but are not currently connected to a Session. These objects are put into a persistence state from a detached state using both techniques.
If another instance of the persistent object is currently attached to the Session and the update attempts to reconnect the instance, an error is thrown. For example, if the identical object already exists in the session, the update method cannot be utilized.
The merging creates a persistent instance for each value in the session. There is no modification to the input object. So, while merge may require more resources, it is more general than update. The merge method, then, will function properly if the identical object already exists in the session.
Lets look at session.merge() and session.update() and also the Difference between session.merge vs session.update in hibernate.
session.update()
When we use the update() function on any object, it first determines whether that object already exists in the session cache or not. If it does, then an exception known as the NonUniqueObjectException is thrown. In every other case, the object will be updated.
When the transaction is committed, this method adds an entity object to the persistent state and tracks and saves additional modifications. Update() does not produce any output. The Session interface offers the update() function, which can be found in the org.hibernate.session package.
The Session interface offers two update() methods, each with a unique set of parameters:
update(Object object)
update(String entityName, Object object)
How to use session.update()?
Employee employee = session.update(emp);
Example
In the following example, we save the object, evict (detach) it from the context, and then change its name and call update.
Person person = new Person();
person.setName("John");
session.save(person);
session.evict(person);
person.setName("Mary");
session.update(person);
session.merge()
Merge is used to move an object from a detached state to a persistent state, similar to the update() method.
When we use the merge() method, it checks to see if the identical object has already been stored in the session cache. The current modifications are copied into the cache if the object has already resided there; otherwise, it will load the values into the cache.
The org.hibernate.session package contains the merge() method, which is supplied by the Session interface.
The Session interface offers two merge() methods, each with a unique set of parameters:
merge(Object object)
merge(String entityName, Object object)
How to use session.merge()?
Employee employee = session.merge(emp);
Example
In the following example, we evict (detach) the saved entity from the context, change the name field, and then merge the detached entity:
Person person = new Person();
person.setName("John");
session.save(person);
session.evict(person);
person.setName("Mary");
Person mergedPerson = (Person) session.merge(person);
Difference between session.merge vs session.update in hibernate
merge()
update()
The database is updated using the merge() technique. If the object is already in the database, it will also update it.
Only the data is saved using the update() method. The object is not updated if it already exists.
Without any exceptions, it will update the database object.
Only when the session has a persistent object with the same primary key will creating a detached object into a persistent state produce an exception.
if you want to save your modifications at any time without knowing about the state of a session, then use merge() in hibernate.
if you are sure that the session does not contain an already persistent instance with the same identifier, then use the update to save the data in hibernate.
When we call the merge method on a detached instance, it will update it with an updated value.
When we call the update method on a detached instance, it will give an exception org.hibernate.NonUniqueObjectException
Frequently Asked Questions
What is the use of generator class in hibernate?
When creating an ID for an item that will be used as a primary key in the database, a generator class is utilized. The persistent class's objects have an ID as a special identification. According to our needs, we can use any generator classes in our program.
What is Association in Hibernate?
One of the main components of JPA and Hibernate is association mapping. In your domain model, they represent the connection between two database tables as attributes. This lets you explore your domain model's associations, and JPQL or Criteria queries easily.
Is dialect mandatory in Hibernate?
Hibernate's dialect property is not mandatory.
What distinguishes spring boot from hibernate?
While Hibernate framework is suitable for object-relational persistence, access data layers, and query retrieval services for enterprise-level applications, the Spring framework is helpful for transaction management, dependency injection, and aspect-oriented programming for applications.
What are ORM and JPA?
Java objects are changed into database tables through a technique known as object-relational mapping (ORM). In other words, this enables SQL-free interaction with a relational database. How to persist data in Java programs is specified by the Java Persistence API (JPA) specification.
Conclusion
In this article, we have extensively discussed the Difference between session.merge vs session.update in hibernate.
We hope this blog has helped you enhance your knowledge regarding the Difference between session.merge vs session.update in hibernate. If you want to learn more topics like Difference between session.merge vs session.update in hibernate, check out our articles on Hibernate Architecture, Hibernate Configuration, HB Generator classes.
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