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I wrote a DAL which abstracts the access to the data of a MySQL DB and I'm facing an annoying problem with the processes running in MySQL. As you can see, the test execution (after its termination of course) leaves a pending process in background in MySQL, which stays there until someone kills it.
If you were not calling close() or shutdown(), that would be the problem, but we of course always recommend running on the latest version of HikariCP.
Not the answer you're looking for?Browse other questions tagged java mysql hibernate jpa hikaricp or ask your own question. Why does Uncle Bob suggest that coding standards shouldn't be written down if you can avoid it? Persistence provides an ease-of-use abstraction on top of JDBC so that your code may be isolated from the database and vendor-specific peculiarities and optimizations. Much like EJB’s server-side types, however, entity beans gain powerful services when used within the context of the container. In the Java Persistence specification, the EntityManager is the central authority for all persistence actions.
You can use Java Persistence outside of an application server and in plain Java SE programs. This topic focuses on the details of the persistence service and how it can be accessed within Java EE. Allocated instances of the Customer class remain POJOs until you ask the EntityManager to create the entity in the database.
Before we can go any deeper into the entity manager service, we need to delve more deeply into the lifecycle of entity object instances.
When a persistence context is closed, all managed entity objects become detached and are unmanaged.
There are two types of persistence contexts: transaction-scoped and extended persistence contexts. Persistence contexts may live as long as a transaction and be closed when a transaction completes. Extended persistence contexts may be created and managed by application code, and we’ll see examples of this later in this topic. Entity instances become unmanaged and detached when a transaction scope or extended persistence context ends. This behavior is very different from the EJB 2.1 entity model, where entities are always managed by the container. This is exactly the kind of plumbing we earlier deemed evil when discussing the benefits of using a Container and Application Server. The persistence.xml deployment descriptor defines the identities and configuration properties of each persistence unit described within it.
The set of classes that belong to the persistence unit can be specified, or you can opt for the persistence provider to scan the JAR file automatically for the set of classes to deploy as entities. Scanning JAR files is guaranteed to work in Java EE environments but is not portable in Java SE applications. Now that you have packaged and deployed your persistence units, you need to obtain access to an EntityManager so that you can persist, update, remove, and query your entity beans within your databases. In EJB, an injected EntityManagerFactory is automatically closed by the EJB container when the instance is discarded. Using the EntityManagerFactory API is a bit verbose and can be awkward when you are making nested EJB calls, for instance.
An EXTENDED entity manager can only be injected into a stateful session bean; stateless session and message-driven beans are pooled, and there would be no way to close the persistence context and release any managed entity instances.
When this MyBean backing instance is created, a persistence context is also created for the injected manager field. Now that you have learned how to deploy and obtain a reference to an entity manager, you are ready to learn the semantics of interacting with it.
From our simple Employee entity defined earlier, we can use the EntityManager facilities to perform CRUD (Create, Read, Update, Delete) operations and build a simple persistent registry of employees.
Before we can take advantage of the EntityManager to flush and synchronize our changes with the database, we must set up a transactional context within which our code can run. When this method is called, the entity manager queues the Employee instances for insertion into the database, and the objects become managed. If the entity has any relationships with other entities, these entities may also be created within the database if you have the appropriate cascade policies set up. The persist() method throws an IllegalArgumentException if its parameter is not an entity type. Once you have located an entity bean by calling find(), calling getReference(), or creating and executing a query, the entity bean instance remains managed by the persistence context until the context is closed. The Java Persistence specification allows you to merge state changes made to a detached entity back into persistence storage using the entity manager’s merge() method. If the entity manager isn’t already managing an Employee instance with the same ID, a full copy of the parameter is made and returned from the merge() method. The merge() method will throw an IllegalArgumentException if its parameter is not an entity type.
Alternatively, we could have used EntityManager.find() to look up dave from the DB, and then directly changed the name upon that reference. To illustrate the difference between managed and unmanaged objects, here we manually detach dave from the EntityManager.
New to JPA2 is the addition of the Criteria API, a fluent expression to programmatically build queries. All object instances returned by find(), getResource(), or a query remain managed as long as the persistence context in which you accessed them remains active. After remove() is invoked, the rick instance will no longer be managed and will become detached. The remove() method throws an IllegalArgumentException if its parameter is not an entity type. If you are concerned that a current managed entity is not up-to-date with the database, then you can use the EntityManager.refresh() method. If the entity bean has any related entities, those entities may also be refreshed, depending on the cascade policy set up in the metadata of the entity mapping. The refresh() method throws an IllegalArgumentException if its parameter is not managed by the current entity manager instance. If you need to detach all managed entity instances from a persistence context, you can invoke the clear() method of the EntityManager.
When you call persist(), merge(), or remove(), these changes are not synchronized with the database until the entity manager decides to flush. The unwrap() method allows you to obtain a reference to the underlying persistence provider object that implements the EntityManager interface. And make sure your AnnotationContextConfiguration Class is on a package that is scanned by the spring framework like service and dao. Not the answer you're looking for?Browse other questions tagged java spring hibernate jpa spring-data or ask your own question.
The EntityManager API is probably the most important and interesting part of the Java Persistence API. In a sense, the EntityManager is the bridge between the OO and relational worlds, as depicted in figure 9.1. Despite all this under-the-hood power, the EntityManager is a small, simple, and intuitive interface, especially compared to the mapping steps we discussed in the previous topic and the query API.In fact, once we go over some basic concepts in the next few sections, the interface might seem almost trivial. Merges an entity to the EntityManager‘s persistence context and returns the merged entity. Synchronizes the state of entities in the EntityManager‘s persistence context with the database. As we mentioned (and will explore further in this and future topics), the reworked EJB 3 JPA entity model brings a whole host of features to the table: simplicity, OO support, and unit testability, to name a few.
However, the truth of the matter is that most layered applications designed using the EJB 2 CMP entity bean model never utilized container services directly anyway.
Even though JPA is not container-centric like session beans or MDBs, entities still have a lifecycle. Figure 9.2 An entity becomes managed when you persist, merge, refresh, or retrieve an entity. An entity can become attached to the EntityManager’s context when you pass the entity to the persist, merge, or refresh method. Unlike entities explicitly created using the new operator, an entity retrieved from the database using the EntityManager’s find method or a query is attached if retrieved within a transactional context. The merge and refresh methods are intended for entities that have been retrieved from the database and are in the detached state.
A detached entity is an entity that is no longer managed by the EntityManager and there is no guarantee that the state of the entity is in synch with the database.
In addition, if you call the clear method of EntityManager, it forces all entities in the persistence context to be detached.
The persistence context plays a vital role in the internal functionality of the Entity-Manager.
In a very simple sense, a persistence context is a self-contained collection of entities managed by an EntityManager during a given persistence scope.
The best way to understand this is to start by examining what the various persistence scopes are and what they do and then backtracking to the meaning of the term.
An EntityManager associated with a transaction-scoped persistence context is known as a transaction-scoped EntityManager. Figure 9.3 Transaction-scoped persistence contexts only keep entities attached within the boundaries of the enclosing transaction. Figure 9.4 For an extended persistence context, once an entity is attached in any given transaction, it is managed for all transactions in the lifetime of the persistence context. The term scope is used for persistence contexts in the same manner that it is used for Java variable scoping. At this point, we’ve covered the basic concepts needed to understand the functionality of the EntityManager.
We’ll explore the EJB 3 EntityManager interface by implementing an ActionBazaar component.
The updateitem method updates the Item entity data in the database using the merge method Q. Hibernate loads the object from the session or the cache or if it is not there from the database. Tries to create an update lock and throws an exception if this is not immediately possible.
If a version column exists and the object was changed, the version is incremented and verified during the update.I could not find a lock with no wait option as supported by the Hibernate Session API. Like I said, if I remove the NamedQuery the EntityManager is created successfully and I can persist, find and remove entries from the database without a problem.


The problem is I won't have direct access to the ID, only the token contained in the entry. Well it seems it was a stupid mistake after all, you can't see it here because I modified the code but the mistake is that in que query I putted the name of the Table and not the name of the class.
Not the answer you're looking for?Browse other questions tagged java ejb persistence or ask your own question. Hibernate is used as the implementation of JPA and HikariCP as the connection pool provider. In older versions of J2EE, the EJB 2.x specification was responsible for defining this layer.
It can also be described as an object-to-relational mapping engine (ORM), which means that the Java Persistence API can automatically map your Java objects to and from a relational database.
Instead, they are simple objects whose state can be synchronized with an underlying persistent storage provider. Because entities are plain Java objects, they do not become persistent until your code explicitly interacts with the EntityManager to make them persistent.
An entity bean instance is either managed (aka attached) by an entity manager or unmanaged (aka detached). For the purposes of this discussion, we may think of a transaction as a set of beginning and end boundaries.
An Employee reference is pulled from the EntityManager, and the setName() method is used to change the name of the employee. After the JTA transaction completes, the transaction-scoped persistence context is destroyed, and this Employee instance is no longer managed. Unlike a transaction-scoped persistence context, the Employee instance pointed to by this local variable remains managed. An interesting side effect is that detached entities can be serialized and sent across the network to a remote client. Application code, armed with the right tools, should be free of this kind of error-prone and excessive noise.
In Java SE environments, vendor-specific configuration must be used to define and configure these data sources. This name is used by injection annotations and XML deployment descriptor elements to reference this unit. This attribute defaults to JTA in Java EE environments and to RESOURCE_LOCAL in SE environments.
In Java EE and SE environments, the persistence implementation is pluggable: your vendor provides an appropriate implementation. One case where you might need one of these elements is when the same class is being used and mapped within two or more persistence units. If you are using EJB container-managed persistence contexts, you do not need to perform this extra step. By default, a transaction-scoped persistence context is injected when using this annotation. These features were defined to make it easier for developers to interact with entity beans.
The EntityManager API has methods to insert and remove entities from a database as well as merge updates from detached entity instances. TransactionRequiredException is thrown if this method is invoked on a transaction-scoped persistence context. During this period, you can change the state of the entity bean instance as you would any other object, and the updates will be synchronized automatically (depending on the flush mode) or when you call the flush() method directly. This copy is managed by the entity manager, and any additional setter methods called on this copy will be synchronized with the database when the EntityManager decides to flush. The TransactionRequiredException is thrown if this method is invoked on a transaction-scoped persistence context. We can still change his name just as we did before, but now these changes will not be synchronized with the database. This means that further calls to find() (or whatever) will return the same entity object instance. Also, if the entity has any relationships to other entity objects, those can be removed as well.The remove() operation can be undone only by recreating the entity instance using the persist() method. The refresh() method refreshes the state of the entity from the database, overwriting any changes made to that entity. If this particular object instance is currently being managed by the persistence context, it returns true.
Be aware that when you call clear(), any changes you have made to managed entities are lost. COMMIT means that changes are flushed only when the transaction commits, not before any query. The best way to tune a database application is to remove unnecessary calls to the database. Because locking behavior is closely related to the concept of transactions, using the lock() method is discussed.
Most vendors will have API extensions to the EntityManager interface that can be executed by obtaining and typecasting this delegate object to a provider’s proprietary interface. When you request that a domain entity be created, the EntityManager translates the entity into a new database record.
The EntityManager is easily the most important interface in JPA and is responsible for most of the ORM black magic in the API.
However, the JPA entity loses some that were available in the EJB 2 model because of its separation from the container. So if you catch someone calling JPA entities "beans," feel free to gently correct them! Making sense of the lifecycle is easy once you grasp a straightforward concept: the EtityManager knows nothing about a POJO regardless of how it is annotated, until you tell the manager to start treating the POJO like a JPA entity.
On the other hand, when an EntityManager stops managing an entity, the entity is said to be detached.
Also an entity becomes attached when you retrieve using the find method or a query within a transaction. It will become managed if the EntityManager’s persist method creates a new record in the database corresponding to the entity.
For example, the instance of the Bid entity will become detached when the underlying transaction commits. A retrieved instance of the entity becomes detached immediately if there is no associated transaction.
Detachment and merge operations become handy when you want to pass an entity across application tiers. Essentially, an attached entity becomes detached as soon as it goes out of the EntityManager context’s scope. This is because the EntityManager quite literally keeps track of entities through Java object references.
It is critical to understand how the persistence context works to use managed entities effectively. Although we perform persistence operations by invoking methods on the EntityManager, the EntityManager itself does not directly keep track of the lifecycle of an individual entity. We’ll explain how the persistence context and persistence scope relates to the EntityManager by first exploring what the persistence context is. If a persistence context is under transaction scope, entities attached during a transaction are automatically detached when the transaction ends. An extended EntityManager can only be used with stateful session beans and lasts as long as the bean instance is alive.
We’ll implement the itemManagerBean stateless session bean used to provide the operations to manipulate items. This method could be invoked from an administrative interface that allows a seller to update a listing after an item is posted.
Below you can find the figure we used in that chapter.Updating an entity requires that you get your object in persistent state. Then it copies the values of your old object to the new object and returns the loaded object.
Sometimes you want to limit the size of the session for example to keep memory consumption low.
Now I am pretty convinced that my mistake is a stupid one, but I have been searching over the web to see examples and everything looks fine to me. In addition to object mappings, this service also provides a query language that is very SQL-like but is tailored to work with Java objects rather than a relational schema. They are created just like any normal instance, typically using the new operator, and have no special APIs that must be implemented by the Entity class. Their state is not synchronized with persistent storage unless associated with an EntityManager.
When an entity is attached to an EntityManager, the manager tracks state changes to the entity and synchronizes those changes to the database whenever the entity manager decides to flush its state. The entity manager tracks all entity objects within a persistence context for changes and updates made, and flushes these changes to the database using the flush mode rules discussed later in this topic. Everything executed in between must either fully succeed or fully fail, and state changes made within a transaction are visible elsewhere in the system only when the end boundary completes successfully.
When the transaction completes, the transaction-scoped persistence context will be destroyed and all managed entity object instances will become detached. The Employee instance that the EntityManager returns will remain managed for the duration of the JTA transaction. This means that if setName() is called after it becomes detached, no changes will be made to any database.
Entity object instances that are attached to an extended context remain managed even after a transaction is complete. This is because extended persistence context stays alive past the completion of transaction 1. The client can make changes remotely to these serialized object instances and send them back to the server to be merged back and synchronized with the database.
For EJB 2.1 developers, this behavior will seem strange at first, since you are used to the container managing every aspect of the entity. Before you can even think about creating or querying entities with an entity manager, you must learn how to package a persistence unit for use within a Java SE (regular Java application) or Java EE (application server) environment. Although you can use the factory interface in Java EE, the platform provides some additional features that make it easier and less verbose to manage entity manager instances. Entity managers created using EntityManagerFactory are more error-prone because the application developer has to worry about more things.
To create an entity, we first allocate an instance of it, set its properties, and wire up any relationships it might have with other objects.
If persist() is called within a transaction, the insert may happen immediately, or it may be queued until the end of the transaction, depending on the flush mode (described later in this topic).


So, in the previous example, if we had auto key generation enabled, we could view the generated key after the persist() method completed. However, if the entity manager is an extended persistence context, it is legal to call persist() outside of a transaction scope; the insert is queued until the persistence context interacts with a transaction.
However, if the entity manager is an extended persistence context, it is legal to invoke this method outside of a transaction scope, and the update will be queued until the persistence context interacts with a transaction.
Changing his name via a traditional call to the setter method for his name will change the state of this object, and the EntityManager will propagate these changes to the database when the transaction completes. In this case, any object returned is detached if the EntityManager is transaction-scoped but remains managed if it is an extended persistence context.
However, if the EntityManager is an extended persistence context, it is legal to invoke this method outside of a transaction scope, and the remove will be queued until the persistence context interacts with a transaction. However, if the entity manager is an extended persistence context, it is legal to invoke this method outside of a transaction scope. By default, flushing automatically happens before a correlated query is executed (inefficient implementations may even flush before any query) and at transaction commit time. If you are doing a query on your database, you want to make sure that any updates you’ve made within your transaction are flushed so that your query will pick up these changes.
In theory, you should be able to write vendor-independent code, but in practice, most vendors provide a lot of extensions to Java Persistence that you may want to take advantage of in your applications. When you request that an entity be updated, it tracks down the relational data that corresponds to the entity and updates it.
The table lists some of the most commonly used methods defined in the Entity-Manager interface.
The default flush mode is AUTO, meaning that the EntityManager tries to automatically synch the entities with the database.
Because the EntityManager and not the container manages entities, they cannot directly use container services such as dependency injection, the ability to define method-level transaction and declarative security, remoteability, and so on.
This is the exact opposite of POJOs annotated to be session beans or MDBs, which are loaded and managed by the container as soon as the application starts. First, as soon as we ask an Entity-Manager to start managing an entity, it synchronizes the entity’s state with the database.
For example, you can detach an entity and pass it to the web tier, then update it and send it back to the EJB tier, where you can merge the detached entity to the persistence context.
Think of this as the expiration of the invisible link between an entity and the EntityManager at the end of a logical unit of work or a session. Since cloned or serialized instances don’t have the same object references as the original managed entity, the EntityManager has no way of knowing they exist. This makes perfect sense since this method removes the data associated with the entity from the database.
While the aircraft is outside the range of the airport (detached or new), it is not guided by the air traffic controller. We’ll examine the relationship between the persistence context, its scope, and the EntityManager in the next section.
In reality, the EntityManager delegates the task of managing entity state to the currently available persistence context.
Therefore, in persistence contexts with extended scope, how long entities remain managed has nothing to do with transaction boundaries.
Transaction-scoped persistence contexts can be compared to method local variables, in the sense that they are only in effect within the boundaries of a transaction.
As listing 9.1 demonstrates, the session bean provides methods for adding, updating, and removing item entities using the JPA EntityManager. As you might imagine, the additem method is used by the presentation layer to add an item posted by the seller to the database. The EntityManager’s refresh method is used in the undoitemChanges method to discard any changes made to an Item entity and to reload it with the data stored in the database Q.
If a version column is present, the version is incremented and verified during the update SQL statement. If a version column is defined and the object was changed, then the version is incremented and verified during the update.
Now, in EE6, we have a new revision called the Java Persistence API, Version 2.0, or more simply, JPA. It provides APIs for creating queries, finding objects, synchronizing, and inserting objects into the database. Once a persistence context is closed, all managed entity object instances become detached and are no longer managed. This means that the change made by calling the set Name() method will be synchronized with the database when the JTA transaction completes and commits. This feature is extremely useful in situations where you want to have a conversation with your database but not keep a long-running transaction, as transactions hold valuable resources such as JDBC connections and database locks. EJB 3.x eliminates the need for this pattern because persistent objects become value objects automatically when they are detached from a persistent context. If you do not enlist the EntityManager within the JTA transaction, the changes you make to your entities are not synchronized with the database. When you access this transaction-scoped EntityManager, a persistence context becomes associated with the transaction until it finishes.
This means that any entity object instances remain attached and managed as long as the stateful session bean is active. For instance, the developer could forget to close() an entity manager and subsequently leak resources. You can always force the insertion manually within a transaction by calling the EntityManager.flush() method.
If the entity manager didn’t flush, then these changes might not be reflected in the query.
Likewise, the EntityManager removes the relational data when you request that an entity be deleted. Once you’ve read and understood the material in this topic, though, we encourage you to explore them on your own. Moreover, the default behavior of the EntityManager is to manage an entity for as short a time as possible. Second, until the entity is no longer managed, the EntityManager ensures that changes to the entity’s data (caused by entity method invocations, for example) are reflected in the database. An Entity-Manager session could be limited to a single method call or span an arbitrary length of time.
This scenario occurs most often in situations where entities are sent across the network for session bean remote method calls. As far as the Entity-Manager is concerned, the entity no longer exists, so there is no need to continue managing it. However, when it does come into range (managed), the traffic controller manages the aircraft’s movement (state synchronized with database).
Once the persistence context detects that a transaction has either been rolled back or committed, it will detach all managed entities after making sure that all data changes until that point are synchronized with the database. In fact, once attached, entities pretty much stay managed as long as the EntityManager instance is around.
On the other hand, persistence contexts with extended scope are more like instance variables that are active for the lifetime of an object— they hang around as long as the EntityManager is around. The old object is not attached to the session.Pit fall using mergeImagine you have a reference to a detached object. If the object was changed, the update statement is most likely not yet send to the database. It also can provide caching and manage the interaction between an entity and transactional services in a Java EE environment such as the Java Transaction API (JTA). Once an object is detached from a persistence context, it can no longer be managed by an entity manager, and any state changes to this object instance will not be synchronized with the database. One side effect we encounter in dealing with detached entities revolves around entity relationships, which we’ll discuss later.
This means that if you interact with any entity managers within the context of a transaction, even if they are different instances that are injected into different beans, the same persistence context will be used.
You may call persist() outside of a transaction only if the entity manager is an EXTENDED persistence context. A flush does not need to happen when find() or getReference() is called, because finding by a primary key is not something that would be affected by any updates. Again, this is the opposite of container-managed beans, which remain managed until the application is shut down. The EntityManager accomplishes this feat by holding an object reference to the managed entity and periodically checking for data freshness.
Eventually, a grounded aircraft is guided into takeoff and goes out of airport range again (detached), at which point the pilot is free to follow her own flight plan (modifying a detached entity without state being managed).
Figure 9.3 depicts this relationship between entities, the transaction persistence scope, and persistence contexts.
As an example, for a stateful session bean, an EntityManager with extended scope will keep managing all attached entities until the EntityManager is closed as the bean itself is destroyed. If you do not want to loose your changes, then you should call flush to send all open changes to your database.
The EntityManager is well integrated with Java EE and EJB but is not limited to this environment; it also can be used in plain Java programs. When you call persist() outside of a transaction with an EXTENDED persistence context, the insert is queued until the persistence context is associated with a transaction. Using COMMIT limits the amount of time the transaction holds on to this database lock by holding it only for the duration of the JTA commit. If the EntityManager finds that any of the entity’s data has changed, it automatically synchronizes the changes with the database. As figure 9.4 shows, this means that unless explicitly detached through a remove method to end the life of the stateful bean instance, entities attached to an extended persistence context will remain managed across multiple transactions. The retrieved Seller entity is set as an association field of the newly instantiated Item entity along with all other item data.
An injected extended persistence context is automatically associated with a JTA transaction by the EJB container. The EntityManager stops managing the entity when the entity is either deleted or moves out of persistence provider’s reach. If this is not absolutely crystal clear right now, it will be once we talk about the EntityManager persistence context in the next section.



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