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The foo object still knows which persistence context it was loaded in. With getTransaction. The method getTransaction.

This pattern is problematic if the persistence context is too big to be stored during user think time, and if you don't know where to store it. This is indeed recommended, as the persistence context will soon also have stale data. It is up to you where you store the extended entity manager during requests, inside an EJB3 container you simply use a stateful session bean as described above. Don't transfer it to the web layer or even serialize it to a separate tier to store it in the HttpSession.

In a non-managed, two-tiered environment the HttpSession might indeed be the right place to store it. With this paradigm, each interaction with the data store occurs in a new persistence context.

However, the same persistent instances are reused for each interaction with the database. The application manipulates the state of detached instances originally loaded in another persistence context and then merges the changes using EntityManager.

Again, the entity manager will check instance versions during flush, throwing an exception if conflicting updates occurred. It is often useful for the application to react to certain events that occur inside the persistence mechanism. This allows the implementation of certain kinds of generic functionality, and extension of built-in functionality.

The JPA specification provides two related mechanisms for this purpose. A method of the entity may be designated as a callback method to receive notification of a particular entity life cycle event. Callbacks methods are annotated by a callback annotation. You can also define an entity listener class to be used instead of the callback methods defined directly inside the entity class.

An entity listener is a stateless class with a no-arg constructor. An entity listener is defined by annotating the entity class with the EntityListeners annotation:. The same callback method or entity listener method can be annotated with more than one callback annotation.

For a given entity, you cannot have two methods being annotated by the same callback annotation whether it is a callback method or an entity listener method. A callback method is a no-arg method with no return type and any arbitrary name. Object type allowing sharing of listeners across several entities. A callback method can raise a RuntimeException. The current transaction, if any, must be rolled back. The following callbacks are defined:. A callback method must not invoke EntityManager or Query methods!

You can define several entity listeners per entity at different level of the hierarchy. You can also define several callbacks at different level of the hierarchy. But you cannot define two listeners for the same event in the same entity or the same entity listener. EntityListeners for a given entity or superclass in the array order. You can stop the entity listeners inheritance by using the ExcludeSuperclassListeners , all superclasses EntityListeners will then be ignored.

There is also an additional feature that can be useful: default event listeners. You can override entity listeners on a given entity. An entity listener correspond to a given class and one or several event fire a given method call. You can also define event on the entity itself to describe the callbacks. Last but not least, you can define some default entity listeners that will apply first on the entity listener stack of all the mapped entities of a given persistence unit.

ORM is all about object state management, which implies that object state is available in memory. However, Hibernate has some features to optimize batch processing which are discussed in the Hibernate reference guide, however, EJB3 persistence differs slightly. Note that:. There can only be a single class named in the from-clause, and it cannot have an alias this is a current Hibernate limitation and will be removed soon.

No joins either implicit or explicit can be specified in a bulk JP-QL query. Sub-queries may be used in the where-clause. The int value returned by the Query.

This may or may not correlate with the number of rows effected in the database. The returned number indicates the number of actual entities affected by the statement. Going back to the example of joined-subclass, a delete against one of the subclasses may actually result in deletes against not just the table to which that subclass is mapped, but also the "root" table and potentially joined-subclass tables further down the inheritance hierarchy.

Both are therefore very close to SQL, but portable and independent of the database schema. For a type-safe approach to query, we highly recommend you to use the Criteria query, see Chapter 9, Criteria Queries. Queries are case-insensitive, except for names of Java classes and properties. FOO is not org. Foo and foo. This manual uses lowercase JP-QL keywords. Some users find queries with uppercase keywords more readable, but we find this convention ugly when embedded in Java code.

We don't usually need to qualify the class name, since the entity name defaults to the unqualified class name Entity. So we almost always just write:. As you may have noticed you can assign aliases to classes, the as keywork is optional.

An alias allows you to refer to Cat in other parts of the query. It is considered good practice to name query aliases using an initial lowercase, consistent with Java naming standards for local variables eg. You may also assign aliases to associated entities, or even to elements of a collection of values, using a join. The inner join , left outer join constructs may be abbreviated. In addition, a "fetch" join allows associations or collections of values to be initialized along with their parent objects, using a single select.

This is particularly useful in the case of a collection. It effectively overrides the fetching options in the associations and collection mapping metadata. See the Performance chapter of the Hibernate reference guide for more information. A fetch join does not usually need to assign an alias, because the associated objects should not be used in the where clause or any other clause. Also, the associated objects are not returned directly in the query results.

Instead, they may be accessed via the parent object. The only reason we might need an alias is if we are recursively join fetching a further collection:.

Note that the fetch construct may not be used in queries called using scroll or iterate. Nor should fetch be used together with setMaxResults or setFirstResult. It is possible to create a cartesian product by join fetching more than one collection in a query as in the example above , be careful the result of this product isn't bigger than you expect.

Join fetching multiple collection roles gives unexpected results for bag mappings as it is impossible for Hibernate to differentiate legit duplicates of a given bag from artificial duplicates created by the multi-table cartesian product.

If you are using property-level lazy fetching with bytecode instrumentation , it is possible to force Hibernate to fetch the lazy properties immediately in the first query using fetch all properties. This is Hibernate specific option:.

The select clause picks which objects and properties to return in the query result set. The query will select mate s of other Cat s. Actually, you may express this query more compactly as:. This is most useful when used together with select new map HQL specific feature :. You may use arithmetic operators, concatenation, and recognized SQL functions in the select clause dpending on configured dialect, HQL specific feature :.

The distinct and all keywords may be used and have the same semantics as in SQL. Hibernate queries may name any Java class or interface in the from clause portable JP-QL queries should only name mapped entities. The query will return instances of all persistent classes that extend that class or implement the interface. The following query would return all persistent objects:. The interface Named might be implemented by various persistent classes:.

This means that the order by clause does not correctly order the whole result set. It also means you can't call these queries using Query. The where clause allows you to narrow the list of instances returned. If no alias exists, you may refer to properties by name:. Compound path expressions make the where clause extremely powerful. This query translates to an SQL query with a table inner join. If you were to write something like.

The special property lowercase id may be used to reference the unique identifier of an object. You may also use its mapped identifer property name. Note that this keyword is specific to HQL. Properties of composite identifiers may also be used. Suppose Person has a composite identifier consisting of country and medicareNumber.

Likewise, the special property class accesses the discriminator value of an instance in the case of polymorphic persistence. A Java class name embedded in the where clause will be translated to its discriminator value. Once again, this is specific to HQL. You may also specify properties of components or composite user types and of components of components, etc. Never try to use a path-expression that ends in a property of component type as opposed to a property of a component.

For example, if store. An "any" type has the special properties id and class , allowing us to express a join in the following way where AuditLog. Any is specific to Hibernate. Notice that log. Expressions allowed in the where clause include most of the kind of things you could write in SQL:.

Any database-supported SQL scalar function like sign , trunc , rtrim , sin. SQL literals 'foo' , 69 , ' Java public static final constants eg. Likewise, is null and is not null may be used to test for null values.

Booleans may be easily used in expressions by declaring HQL query substitutions in Hibernate configuration:. You may test the size of a collection with the special property size , or the special size function HQL specific feature. For indexed collections, you may refer to the minimum and maximum indices using minindex and maxindex functions. Similarly, you may refer to the minimum and maximum elements of a collection of basic type using the minelement and maxelement functions.

These are HQL specific features. The SQL functions any, some, all, exists, in are supported when passed the element or index set of a collection elements and indices functions or the result of a subquery see below. Note that these constructs - size , elements , indices , minindex , maxindex , minelement , maxelement - may only be used in the where clause in Hibernate.

In HQL, elements of indexed collections arrays, lists, maps may be referred to by index in a where clause only :. HQL also provides the built-in index function, for elements of a one-to-many association or collection of values. If you are not yet convinced by all this, think how much longer and less readable the following query would be in SQL:.

The list returned by a query may be ordered by any property of a returned class or components:. The optional asc or desc indicate ascending or descending order respectively. A query that returns aggregate values may be grouped by any property of a returned class or components:. SQL functions and aggregate functions are allowed in the having and order by clauses, if supported by the underlying database eg.

Note that neither the group by clause nor the order by clause may contain arithmetic expressions. For databases that support subselects, JP-QL supports subqueries within queries. A subquery must be surrounded by parentheses often by an SQL aggregate function call.

Even correlated subqueries subqueries that refer to an alias in the outer query are allowed. For subqueries with more than one expression in the select list, you can use a tuple constructor:.

Note that on some databases but not Oracle or HSQLDB , you can use tuple constructors in other contexts, for example when querying components or composite user types:. There are two good reasons you might not want to do this kind of thing: first, it is not completely portable between database platforms; second, the query is now dependent upon the ordering of properties in the mapping document.

Hibernate queries can be quite powerful and complex. Here are some example queries very similar to queries that I used on a recent project. Note that most queries you will write are much simpler than these! The following query returns the order id, number of items and total value of the order for all unpaid orders for a particular customer and given minimum total value, ordering the results by total value.

In determining the prices, it uses the current catalog. What a monster! Actually, in real life, I'm not very keen on subqueries, so my query was really more like this:.

If I would have mapped the statusChanges collection as a list, instead of a set, the query would have been much simpler to write. The next query uses the MS SQL Server isNull function to return all the accounts and unpaid payments for the organization to which the current user belongs.

See Section 7. If your database supports subselects, you can place a condition upon selection size in the where clause of your query:. As this solution can't return a User with zero messages because of the inner join, the following form is also useful:.

Criteria queries are a programmatic, type-safe way to express a query. They are type-safe in terms of using interfaces and classes to represent various structural parts of a query such as the query itself, or the select clause, or an order-by, etc. They can also be type-safe in terms of referencing attributes as we will see in a bit.

Users of the older Hibernate org. Criteria queries are essentially an object graph, where each part of the graph represents an increasing as we navigate down this graph more atomic part of query. The first step in performing a criteria query is building this graph. The javax. CriteriaBuilder interface is the first thing with which you need to become acquainted to begin using criteria queries.

Its role is that of a factory for all the individual pieces of the criteria. You obtain a javax. CriteriaBuilder instance by calling the getCriteriaBuilder method of the javax. The next step is to obtain a javax. You do this by one of the 3 methods on javax. CriteriaBuilder for this purpose. Chapter 6 Criteria API of the [ JPA 2 Specification ] already contains a decent amount of reference material pertaining to the various parts of a criteria query.

So rather than duplicate all that content here, lets instead look at some of the more widely anticipated usages of the API. This might be an entity, an Integer, or any other object.

You have an entity and you want to select one or more of that entity based on some condition. We use the form createQuery Person. It was done here only for completeness of an example. We will use that form exclusively in this chapter. See Section 4. The simplest form of selecting a value is selecting a particular attribute from an entity. But this might also be an aggregation, a mathematical operation, etc.

Notice again the typing of the query based on the anticipated result type s. Here we are specifying java. Integer as the type of the Person age attribute is java. We need to bind the fact that we are interested in the age associated with the personRoot. We might have multiple references to the Person entity in the query so we need to identify aka qualify which Person age we mean.

Here we see javax. CriteriaBuilder used to obtain a MAX expression. These expression building methods return javax. Expression instances typed according to various rules. The rule for a MAX expression is that the expression type is the same as that of the underlying attribute.

There are actually a few different ways to select multiple values using criteria queries. We will explore 2 options here, but an alternative recommended approach is to use tuples as described in Section 9.

Technically this is classified as a typed query, but as you can see in handling the results that is sort of misleading. Anyway, the expected result type here is an array. Here we see the use of the array method of the javax. CriteriaBuilder which explicitly combines individual selections into a javax. Just as we saw in Example 9. This actually functions exactly the same as what we saw in Example 9. The multiselect method behaves slightly differently based on the type given when the criteria query was first built, but in this case it says to select and return an Object[].

Another alternative to Section 9. Going back to the example query there, rather than returning an array of [Person id, Person age] instead declare a class that holds these values and instead return that. First we see the simple definition of the wrapper object we will be using to wrap our result values.

Specifically notice the constructor and its argument types. Since we will be returning PersonWrapper objects, we use PersonWrapper as the type of our criteria query. Here we see another new javax. CriteriaBuilder method, construct , which is used to builder a wrapper expression. Basically for every row in the result we are saying we would like a PersonWrapper instantiated by the matching constructor.

This wrapper expression is then passed as the select. A better approach to Section 9. Tuple contract. Here we see the use of a new javax. CriteriaBuilder javax. CriteriaQuery building method, createTupleQuery. This is exactly equivalent to calling builder.

It signifies that we want to access the results through the javax. Again we see the use of the multiselect method, just like in Example 9. The difference here is that the type of the javax.

CriteriaQuery was defined as javax. Tuple so the compound selections in this case are interpreted to be the tuple elements.

Tuple allowing different types of access to the results, which we will expand on next. Tuple contract provides 3 basic forms of access to the underlying elements:. This allows typed access to the underlying tuple elements. We see this in Example 9.

Just about everything is a javax. Very similar to what we saw in Example 9. Only the second form here provides typing, because the user explicitly provides the typing on access. Again, only the second form here provides typing, because the user explicitly provides the typing on access. We have not seen an example of using this, but its trivial. We would simply, for example, have applies an alias to either of the paths like idPath.

A CriteriaQuery object defines a query over one or more entity, embeddable, or basic abstract schema types. The root objects of the query are entities, from which the other types are reached by navigation.

All the individual parts of the FROM clause roots, joins, paths implement the javax. From interface. Roots define the basis from which all joins, paths and attributes are available in the query. In a criteria query, a root is always an entity. Roots are defined and added to the criteria by the overloaded from methods on javax. CriteriaQuery :. Criteria queries may define multiple roots, the effect of which is to create a cartesian product between the newly added root and the others.

Here is an example matching all single men and all single women:. Joins allow navigation from other javax. From to either association or embedded attributes. Joins are created by the numerous overloaded join methods of the javax.

From interface:. Fetches are created by the numerous overloaded fetch methods of the javax. Technically speaking, embedded attributes are always fetched with their owner. However in order to define the fetching of Address country we needed a javax. Fetch for its parent path. Use the parameter method of javax. CriteriaBuilder to obtain a parameter reference. Use the parameter reference to bind the parameter value to the javax.

You may also express queries in the native SQL dialect of your database. Note that Hibernate allows you to specify handwritten SQL including stored procedures for all create, update, delete, and load operations please refer to the reference guide for more information. This is done using the SqlResultSetMapping annotation. Now that the result set is described, we are capable of executing the native SQL query.

EntityManager provides all the needed APIs. The first method is to use a SQL resultset name to do the binding, the second one uses the entity default mapping the column returned has to have the same names as the one used in the mapping.

A third one not yet supported by Hibernate entity manager , returns pure scalar results. This native query returns nights and area based on the GetNightAndArea result set.

The second version is useful when your SQL query returns one entity reusing the same columns as the ones mapped in metadata.

Your code doesn't need to know the difference between the two. Java Persistence 2. Introducing JPA Persistence 1. Architecture 1. Definitions 1. In container environment eg. EJB 3 1. Container-managed entity manager 1.

Application-managed entity manager 1. Persistence context scope 1. Persistence context propagation 1. Java SE environments 2. Setup and configuration 2.

Setup 2. Configuration and bootstrapping 2. Packaging 2. Bootstrapping 2. Event listeners 2. Various 3. Working with objects 3. Entity states 3. Making objects persistent 3.

Loading an object 3. Querying objects 3. Executing queries 3. Modifying persistent objects 3. Detaching a object 3. Modifying detached objects 3. Automatic state detection 3. Deleting managed objects 3. Flush the persistence context 3. In a transaction 3. Outside a transaction 3. Transitive persistence 3. Locking 3. Caching 3. Checking the state of an object 3. Native Hibernate API 4.

Metamodel 4. Static metamodel 5. Transactions and Concurrency 5. Entity manager and transaction scopes 5. Unit of work 5. Long units of work 5. Considering object identity 5.

Common concurrency control issues 5. Database transaction demarcation 5. Non-managed environment 5. Using JTA 5. Exception handling 5. Container Managed Entity Manager 5. Application Managed Entity Manager 5. Optimistic concurrency control 5.

Application version checking 5. Extended entity manager and automatic versioning 5. Detached objects and automatic versioning 6.

Entity listeners and Callback methods 6. Definition 6. Callbacks and listeners inheritance 6. XML definition 7. Batch processing 7. Case Sensitivity 8. The from clause 8. Associations and joins 8. The select clause 8. Aggregate functions 8. Polymorphic queries 8. The where clause 8. Expressions 8. The order by clause 8. The group by clause 8.

Subqueries 8. JP-QL examples 8. Criteria Queries 9. Typed criteria queries 9. Selecting an entity 9. Selecting a value 9. Selecting multiple values 9. Selecting a wrapper 9. Tuple criteria queries 9. Accessing tuple elements 9. FROM clause 9.

Roots 9. Joins 9. Fetches 9. Also used in managing persistence instances of entities, find entities by identity primary key, and also query the overall entities. We should pass entity to save data. EntityManager; import com. In this article, we learned about hibernate entityManager. The types of methods that can be used are studied which are available in the API. They work with persistence context.

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