Jpa when is an object detached

However, while modifying the form, I had to be able to perform a classic undo using a hit on my cancel button and then retrieve the state of my country as it was before I began doing updates….

C The unsuccessful attempts and a first solution : I just have to say it was a pity for me to do that using JPA. D Another solution So although it works, it was a pain to code and maintain. No more clone here : you use directly the object from the database BUT you cut temporarily the connection between the database and Java.

Doing so, I can finally persist the changes… or not undo! Great but… I see 2 drawbacks using this method : — it is provider dependent Hibernate in this case, although I suppose the same method may exist for other JPA implementation — the code is quite ugly because you have to load all the collections depending on the object BEFORE you perform this detach : if not, be prepare to have some nice LazyInitializationException….

Fabrice Leray: I am not familiar with Seam and the way it manages the persistence context. From what I understand your problem is that the context is scoped too large. I always use a request scoped session and that makes the undo pretty easy: just throw an exception and the transaction gets rolled back immediately. Could you post an example? This is excelent article and it is very useful for me to understand the difference between merge and saveorupdate.

Before this i read so many articles regarding this topic but none of them are clear. I would like to exempt a managed object from the flush , but keep it managed after the flush, like this pseudo-code :. Of course, the methods entityManager. I can not see any reason for your pseudo code. To me it looks like the search for a workaround. If this is id zero or not already in database, the entity manager merge call will create a new record persist object in JPA lingo.

However, if that id is non-zero that already exists in database, merge will update the object. You should simply be able to use the synonym as you would a normal table. For the second question, there has to be some relationship to join the tables or you get a cartesian product.

The database tables do not have to have a relationship, you can create your own query if you have joining column s. Object added to persistence context as new entity […].

The problem with this wizardry comes when you need more complex or dynamic queries, reports, etc, or perhaps when you need performance. I enquired for that merge method, and I learnt here and there the main point: it copies the state of the passed object. These discussions makes me think twice about using Hibernate the next time…Everything is so boring complicated!

After you guys have fixed all your merge problems…have a look at the overload of SQL statements Hibernate produces! ORM should not be so complex nor SO automatic. They try to hide too much details from programmers. It seems like they pretend to make them forget the important facts when dealing with a persistent storage:. You cannot always assume you are the only one accessing or changing it. This maps really easily with Relational Databases and I guess also non relational ones :. Another problem not considered is when the new object A is associated to a detached object B.

But if you Merge A, it will first reattach merge B, and then persist insert A. EntityExistsException thrown or a PersistenceException at flush or commit time. According to JPA spec, this persist operation is ignored.

Your email address will not be published. Save my name, email, and website in this browser for the next time I comment. March 23rd, The two most common causes for this are: The EntityManager from which the object was retrieved has been closed. The object was received from outside of our application, e. Object added to persistence context as new entity 2. State copied to new entity. New entity added to persistence context 3. New entity returned 1.

Existing entity loaded. State copied from object to loaded entity 3. Loaded entity returned 1. Object added to persistence context 2.

EntityExistsException thrown or a PersistenceException at flush or commit time 1. State from object copied to loaded entity 2. NonUniqueObjectException thrown Looking at that table one may begin to understand why the saveOrUpdate method never became a part of the JPA specification and why the JSR members instead choose to go with the merge method. The problem with merge Before we continue, we need to discuss one disadvantage of the way EntityManager. The rule of thumb I stick to is this: When and only when and preferably where we create a new entity, invoke EntityManager.

This makes perfect sense when we view our domain access objects as collections. I call this the persist-on-new pattern.

When updating an existing entity, we do not invoke any EntityManager method; the JPA provider will automatically update the database at flush or commit time. When we receive an updated version of an existing simple entity an entity with no references to other entities from outside of our application and want to save the new state, we invoke EntityManager.

Because of the way merging works, we can also do this if we are unsure whether the object has been already persisted. When we need more control over the merging process, we use the DIY merge pattern. Learn more. Why need detached entities in JPA? Ask Question. Asked 7 years, 9 months ago. Active 2 years, 4 months ago.

Viewed 29k times. There are always so many questions related to issues with detached entities! Improve this question. Jaumzera 2, 23 23 silver badges 42 42 bronze badges. Alexey Andreev Alexey Andreev 1, 2 2 gold badges 17 17 silver badges 27 27 bronze badges. I tend to just use find to make entities attached again before I start to modify their state in the code and avoid merge altogether.

It just makes the code more dumb and thus easier to manage. But that's me. There's one context in which find must be used carefully. When retrieving many entities of the same class, calling find in a loop can be a performance killer. It's especially true when the entity is part of an class hierarchy left outer joins aren't free.

Another strategy should be used to deal with detached entities for these situations. Add a comment. Active Oldest Votes. I will explain why that scenario should not occur and why we need detached entities. Improve this answer. Andrei I Andrei I I can't understand both of your examples.

Why and from where do you receive these entities? If you receive them in a context of a transaction, why they come detached? If you receive them outside of a transaction, why do we get entities instead of DTOs? You receive them from the WEB Layer e. They come detached, because no one merged them. DTOs are actually a pattern to transfer data between different interfaces.

Consider, we have our client, which issues its request in terms of DTOs. Server receives client's request and calls several business methods, each acts in a separate transaction. But why don't we have a single transactions per request? I suggest, there are cases when we have an additional long-running processing that is not related to a database. I reformulated my first example. Also please understand, that JPA is something that gives freedom to the world, without forcing it to implement DTOs or to use another pattern.

Usually you have a SINGLE transaction per request, but of course there are scenarios when you need multiple transactions. Suppose you want to log every request, along with doing the business logic. These should be different transactions. Or suppose you should emit invoices for 10 users. If an invoice cannot be emitted, should not block other invoices from being generated.

Show 2 more comments. References Hibernate DOc Why? References Again Hibernate Doc I bet you haven't read through hibernate documentation itself, It has scenarios explaining them too : Simple Explanation:With reference to persistent objects.. DarkHorse DarkHorse 2, 17 17 silver badges 28 28 bronze badges. Once you've created a new entity instance using the common new operator it is in new state.

You can make it persistent by associating it to an entity manager:. If the DomesticCat entity type has a generated identifier, the value is associated to the instance when persist is called. If the identifier is not automatically generated, the application-assigned usually natural key value has to be set on the instance before persist is called. Load an entity instance by its identifier value with the entity manager's find method:.

In some cases, you don't really want to load the object state, but just having a reference to it ie a proxy. You can get this reference using the getReference method. This is especially useful to link a child to its parent without having to load the parent. You can reload an entity instance and it's collections at any time using the em. This is useful when database triggers are used to initialize some of the properties of the entity.

Note that only the entity instance and its collections are refreshed unless you specify REFRESH as a cascade style of any associations:. If you don't know the identifier values of the objects you are looking for, you need a query. Both query languages are portable across databases, the use entity and property names as identifiers instead of table and column names. You may also express your query in the native SQL of your database, with optional support from JPA for result set conversion into Java business objects.

This interface offers methods for parameter binding, result set handling, and for execution of the query. Queries are always created using the current entity manager:. A query is usually executed by invoking getResultList. This method loads the resulting instances of the query completely into memory.

Entity instances retrieved by a query are in persistent state. The getSingleResult method offers a shortcut if you know your query will only return a single object. JPA 2 provides more type-safe approaches to queries. But you can benefit form some type-safe convenience even when using JP-QL note that it's not as type-safe as the compiler has to trust you with the return type.

We highly recommend the Criteria API approach. While more verbose, it provides compiler-enforced safety including down to property names which will pay off when the application will move to maintenance mode. JPA queries can return tuples of objects if projection is used. Each result tuple is returned as an object array:. The criteria API provides a type-safe approach to projection results.

Check out Section 9. Queries may specify a particular property of an entity in the select clause, instead of an entity alias. You may call SQL aggregate functions as well. Returned non-transactional objects or aggregation results are considered "scalar" results and are not entities in persistent state in other words, they are considered "read only" :.

Both named and positional query parameters are supported, the Query API offers several methods to bind arguments. The JPA specification numbers positional parameters from one. Named parameters are identifiers of the form :paramname in the query string. Named parameters should be preferred, they are more robust and easier to read and understand:. Note that the actual program code is independent of the query language that is used, you may also define native SQL queries in metadata, or use Hibernate's native facilities by placing them in XML mapping files.

Use the SqlResultSetMapping please see the Hibernate Annotations reference documentation on how to map a SQL resultset mapping or the entity mapping if the column names of the query result are the same as the names declared in the entity mapping; remember that all entity columns have to be returned for this mechanism to work :.

For more information about scalar support in named queries, please refers to the Hibernate Annotations documentation. You can adjust the flush mode used when executing the query as well as define the lock mode used to load the entities. Adjusting the flush mode is interesting when one must guaranty that a query execution will not trigger a flush operation. Most of the time you don't need to care about this. Adjusting the lock mode is useful if you need to lock the objects returns by the query to a certain level.

If you want to use FlushMode. See below. Query hints for performance optimization, usually are implementation specific. Hints are declared using the query. The Hibernate EJB3 implementation offers the following query hints:. The value object accept both the native type or its string equivalent eg. Please refer to the Hibernate reference documentation for more information.

Transactional managed instances ie. There is no need to call a particular method to make your modifications persistent. A straightforward wayt to update the state of an entity instance is to find it, and then manipulate it directly, while the persistence context is open:. Therefore Hibernate offers an alternate approach, using detached instances. An object when loaded in the persistence context is managed by Hibernate. You can force an object to be detached ie. Many applications need to retrieve an object in one transaction, send it to the presentation layer for manipulation, and later save the changes in a new transaction.

There can be significant user think and waiting time between both transactions. Applications that use this kind of approach in a high-concurrency environment usually use versioned data to ensure isolation for the "long" unit of work.