Hello I'm new to Hibernate.
Can I use saveOrUpdate method with DynamicUpdate annotation on entity class?
Can someone who can explain to me when should I use saveOrUpdate, save and update methods? Or whaat are the advantages and disadvantages?
DynamicUpdate is used when you to have the UPDATE statement include only the columns that have been changed, as opposed to all entity columns.
If the entity is already associated to the current Session (like when loading it using Session.get() or Session.load()), the dirty checking mechanism will automatically detected any change and issue the UPDATE on flush time.
The save is used to change the entity state from TRANSIENT to PERSISTED and it's only useful when you are saving new objects into the database.
The 'update' method is used to re-associate a detached entity.
The saveOrUpdate combines both these methods, but again this is only meant when the entity in question is not yet associated to the current Session. This method makes the entity managed so that Hibernate can automatically synchronize the entity with the associated database row.
Related
I have this a class like bellow:
#Entity
#Table(name="work")
public class Work {
#Id
#Column(name="id")
private String id;
#OneToMany(orphanRemoval=true ,mappedBy="work", cascade=CascadeType.ALL , fetch=FetchType.EAGER)
private List<PersonRole> personRoleList;
}
As mine is an web application, when i update (comes from client) a personRoleList item and call :
session.update(work); //`work` is in detached state
It does not update the existing personRoleList item it actually add a new one.
Some other people also having the same problem. REF:
using-saveorupdate-in-hibernate-creates-new-records-instead-of-updating-existi
jpa-onetomany-not-deleting-child
I tried all suggested solution, but none of them work for me.
But then i just tried :
session.merge(work); //replacing session.update(work)
And it works as expected.!!
This is where I get confused. Because I can't find any explanation for this difference in behaviors in case of OneToMany relationship (or may be i missed ). I read some threads to understand the differences between update() and merge() and gone through the doc. REF:
what-are-the-differences-between-the-different-saving-methods-in-hibernate
differences-among-save-update-saveorupdate-merge-methods-in-session
But still it is not clear What are those behavioral pattern/logic/steps that creating this difference.?
Merge attempts to associate a currently transient object with a persistent object currently under management by the session by 'merging' them into one entity. Its intended use is when you have a detached object and an attached object and wish to resolve them.
In a merge(), Hibernate will read the entity from the database if there isn't already a managed instance in the session. In your example, this will result in Hibernate eagerly loading the collection (due to fetch=FetchType.EAGER). Then when your session ends, Hibernate will check for changes in the collection (due to cascade=CascadeType.ALL) and will perform the appropriate UPDATE in the database.
This differs from the update() scenario because in an update Hibernate always (by default) assumes the object is dirty and schedules an UPDATE. This update is likely what's causing creation of a new element in your collection - Hibernate hasn't looked in the database to bring the collection into session before issuing the UPDATE.
I'd bet you can get the desired behavior of update() by setting
select-before-update="true"
in your class mapping or by using the lock method to re-attach your object to the session before making changes.
From Chapter 9 of Java Persistence with Hibernate
It doesn’t matter if the item object is modified before or after it’s passed to
update(). The important thing here is that the call to update() is reattaching the detached instance to the new Session (and persistence context). Hibernate
always treats the object as dirty and schedules an SQL UPDATE, which will be executed during flush. You can see the same unit of work in figure 9.8.
You may be surprised and probably hoped that Hibernate could know that you
modified the detached item’s description (or that Hibernate should know you did
not modify anything). However, the new Session and its fresh persistence context
don’t have this information. Neither does the detached object contain some internal list of all the modifications you’ve made.
UDPATE in the database is needed. One way to avoid this UDPATE statement is to
configure the class mapping of Item with the select-before-update="true"
attribute. Hibernate then determines whether the object is dirty by executing a
SELECT statement and comparing the object’s current state to the current data-
base state.
I am trying to model a transient operations solution schema in Hibernate and I am unsure how to get the object graph and behavior I want from the model.
The table structure uses a correlation table (many-to-many) to create lists of users for the operation:
Operation OperationUsers Users
op_id op_id user_id
... user_id ...
In modeling the persistent class Operation.java using hibernate annotations, I created:
#ManyToMany(fetch=FetchType.LAZY)
#JoinColumn(name="op_id")
public List<User> users() { return userlist; }
So far, I have the following questions:
When a user is removed from the list, how do I avoid Hibernate
deleting the user from the Users table? It should just be removed
from the correlation table, not the Users table. I cannot see a valid
CascadeType to accomplish this.
Do I need to put anything more in the method body?
Do I need to add more annotation arguments?
I am expecting to do this without futzing with the User class.
Please tell me that I do not have to mess with User.java!
It's possible I'm overthinking this, but that's the nature of learning... Thanks in advance for any help you can offer!
From the documentation:
Hibernate defines and supports the following object states:
*Transient - an object is transient if it has just been instantiated using the new operator, and it is not associated with a Hibernate Session. It has no persistent representation in the database and no identifier value has been assigned. Transient instances will be destroyed by the garbage collector if the application does not hold a reference anymore. Use the Hibernate Session to make an object persistent (and let Hibernate take care of the SQL statements that need to be executed for this transition).
*Persistent - a persistent instance has a representation in the database and an identifier value. It might just have been saved or loaded, however, it is by definition in the scope of a Session. Hibernate will detect any changes made to an object in persistent state and synchronize the state with the database when the unit of work completes. Developers do not execute manual UPDATE statements, or DELETE statements when an object should be made transient.
*Detached - a detached instance is an object that has been persistent, but its Session has been closed. The reference to the object is still valid, of course, and the detached instance might even be modified in this state. A detached instance can be reattached to a new Session at a later point in time, making it (and all the modifications) persistent again. This feature enables a programming model for long running units of work that require user think-time. We call them application transactions, i.e., a unit of work from the point of view of the user.
As explained in this answer, you can detach your entity using Session.evict() to prevent hibernate from updating the database or simply clone it and make the needed changes on the copy.
It turns out that the specific answer to my primary question (#1 and the main topic) is: "Do not specify any CascadeType on the property."
The answer is mentioned sorta sideways in the answer to this question.
I was experimenting with eclipselink. I am trying to update an existing entity in database. The JPA entity only has public fields and fields are annotated with JPA annotation. The code loads an entity using EntityManager.find().
The code creates an instance of JPA entity, assigns value to public fields of the entity and invokes EntityManager.merge(entity) method. Eclipselink does not update database record. I enabled log to see whether Eclipselink issues SQL statement or not.
Eclipselink does not issue any update statement. Does this mean that even if I use field persistence, I can not assign value to public fields of the entity instead of using setter method?
Thanks,
Chir
Weaving introduces some optimizations such as lazy onetoone and manytones and change tracking. The only way these can work is if you use the accessor method on the entity, but if that isn't an option, you can turn them off as needed. See
http://eclipse.org/eclipselink/documentation/2.4/jpa/extensions/a_changetracking.htm
For details on change tracking.
This question already has answers here:
JPA EntityManager: Why use persist() over merge()?
(16 answers)
Closed 2 years ago.
So far, my preference has been to always use EntityManager's merge() take care of both insert and update. But I have also noticed that merge performs an additional select queries before update/insert to ensure record does not already exists in the database.
Now that I am working on a project requiring extensive (bulk) inserts to the database. From a performance point of view does it make sense to use persist instead of merge in a scenario where I absolutely know that I am always creating a new instance of objects to be persisted?
It's not a good idea using merge when a persist suffices - merge does quite a lot more of work. The topic has been discussed on StackOverflow before, and this article explains in detail the differences, with some nice flow diagrams to make things clear.
I would definitely go with persist persist() if, as you said:
(...) I absolutely know that I am always creating a new instance of objects to be persisted (...)
That's what this method is all about - it will protect you in cases where the Entity already exists (and will rollback your transaction).
If you're using the assigned generator, using merge instead of persist can cause a redundant SQL statement, therefore affecting performance.
Also, calling merge for managed entities is also a mistake since managed entities are automatically managed by Hibernate and their state is synchronized with the database record by the dirty checking mechanism upon flushing the Persistence Context.
To understand how all this works, you should first know that Hibernate shifts the developer mindset from SQL statements to entity state transitions.
Once an entity is actively managed by Hibernate, all changes are going to be automatically propagated to the database.
Hibernate monitors currently attached entities. But for an entity to become managed, it must be in the right entity state.
First, we must define all entity states:
New (Transient)
A newly created object that hasn’t ever been associated with a Hibernate Session (a.k.a Persistence Context) and is not mapped to any database table row is considered to be in the New (Transient) state.
To become persisted we need to either explicitly call the EntityManager#persist method or make use of the transitive persistence mechanism.
Persistent (Managed)
A persistent entity has been associated with a database table row and it’s being managed by the current running Persistence Context. Any change made to such entity is going to be detected and propagated to the database (during the Session flush-time).
With Hibernate, we no longer have to execute INSERT/UPDATE/DELETE statements. Hibernate employs a transactional write-behind working style and changes are synchronized at the very last responsible moment, during the current Session flush-time.
Detached
Once the current running Persistence Context is closed all the previously managed entities become detached. Successive changes will no longer be tracked and no automatic database synchronization is going to happen.
To associate a detached entity to an active Hibernate Session, you can choose one of the following options:
Reattaching
Hibernate (but not JPA 2.1) supports reattaching through the Session#update method.
A Hibernate Session can only associate one Entity object for a given database row. This is because the Persistence Context acts as an in-memory cache (first level cache) and only one value (entity) is associated with a given key (entity type and database identifier).
An entity can be reattached only if there is no other JVM object (matching the same database row) already associated to the current Hibernate Session.
Merging
The merge is going to copy the detached entity state (source) to a managed entity instance (destination). If the merging entity has no equivalent in the current Session, one will be fetched from the database.
The detached object instance will continue to remain detached even after the merge operation.
Removed
Although JPA demands that managed entities only are allowed to be removed, Hibernate can also delete detached entities (but only through a Session#delete method call).
A removed entity is only scheduled for deletion and the actual database DELETE statement will be executed during Session flush-time.
To understand the JPA state transitions better, you can visualize the following diagram:
Or if you use the Hibernate specific API:
I am currently testing hibernate envers and I am confused with its behavior. Sometimes when we do updates without doing any changes, it does not record an audit. But currently, it does. Just like to make sure.
This can occur if Hibernate does not have access to the old state of the entity to compare to the new state. This typically occurs with when persisting detached entities. When there is no old state to compare, Hibernate assumes that the audited property has changed and logs a revision.
You can get around this issue by re-attaching detached entities using the merge() method before persisting the entity.
From my experience, it only creates an entry in the revision table if you have made changes to the entity.