Data filtering and interception If the association between Category and Item was one-to-many, you d created the following mapping:
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CHAPTER 12 Modifying objects efficiently List
Data filtering and interception directly to the database system, so you can use any SQL operator or function. It must evaluate to true if a record should pass the filter. In this example, you use a subquery to obtain the rank of the seller of the item. Unqualified columns, such as SELLER_ID, refer to the table to which the entity class is mapped. If the currently logged-in user s rank isn t greater than or equal than the rank returned by the subquery, the Item instance is filtered out. Here is the same in annotations on the Item entity: @Entity @Table(name = “ITEM”) @org.hibernate.annotations.Filter( name = “limitItemsByUserRank”, condition=”:currentUserRank >= ” + “(select u.RANK from USER u” + ” where u.USER_ID = SELLER_ID)” ) public class Item implements { … } You can apply several filters by grouping them within a @org.hibernate.annotations.Filters annotation. A defined and applied filter, if enabled for a particular unit of work, filters out any Item instance that doesn t pass the condition. Let s enable it. Enabling the filter You ve defined a data filter and applied it to a persistent class. It s still not filtering anything; it must be enabled and parameterized in the application for a particular Session (the EntityManager doesn t support this API you have to fall back to Hibernate interfaces for this functionality): Filter filter = session.enableFilter(”limitItemsByUserRank”); filter.setParameter(”currentUserRank”, loggedInUser.getRanking()); You enable the filter by name; this method returns a Filter instance. This object accepts the runtime arguments. You must set the parameters you have defined. Other useful methods of the Filter are getFilterDefinition() (which allows you to iterate through the parameter names and types) and validate() (which throws a HibernateException if you forgot to set a parameter). You can also set a list of arguments with setParameterList(), this is mostly useful if your SQL condition contains an expression with a quantifier operator (the IN operator, for example). Now every HQL or Criteria query that is executed on the filtered Session restricts the returned Item instances:
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CHAPTER 12 Modifying objects efficiently use aCategory.getItems() and navigate to these objects, all Item instances would be visible. You solve this problem with a dynamic filter. Defining a data filter A dynamic data filter is defined with a global unique name, in mapping metadata. You can add this global filter definition in any XML mapping file you like, as long as it s inside a
Data filtering and interception a restriction on collections; if you iterate through the Item objects in a Category, you ll see all of them. One possible solution for this problem uses database views. SQL doesn t standardize dynamic views views that can be restricted and moved at runtime with some parameter (the currently logged-in user, a time period, and so on). Few databases offer more flexible view options, and if they re available, they re pricey and/or complex (Oracle offers a Virtual Private Database addition, for example). Hibernate provides an alternative to dynamic database views: data filters with dynamic parameterization at runtime. We ll look at the use cases and application of data filters in the following sections. Another common issue in database applications is crosscutting concerns that require knowledge of the data that is stored or loaded. For example, imagine that you have to write an audit log of every data modification in your application. Hibernate offers an org.hibernate.Interceptor interface that allows you to hook into the internal processing of Hibernate and execute side effects such as audit logging. You can do much more with interception, and we ll show you a few tricks after we ve completed our discussion of data filters. The Hibernate core is based on an event/listener model, a result of the last refactoring of the internals. If an object must be loaded, for example, a Load- Event is fired. The Hibernate core is implemented as default listeners for such events, and this system has public interfaces that let you plug in your own listeners if you like. The event system offers complete customization of any imaginable operation that happens inside Hibernate, and should be considered a more powerful alternative to interception we ll show you how to write a custom listener and handle events yourself. Let s first apply dynamic data filtering in a unit of work. 12.3.1 Dynamic data filters The first use case for dynamic data filtering is related to data security. A User in CaveatEmptor has a ranking property. Now assume that users can only bid on items that are offered by other users with an equal or lower rank. In business terms, you have several groups of users that are defined by an arbitrary rank (a number), and users can trade only within their group. You can implement this with complex queries. For example, let s say you want to show all the Item objects in a Category, but only those items that are sold by users in the same group (with an equal or lower rank than the logged-in user). You d write an HQL or Criteria query to retrieve these items. However, if you
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CHAPTER 12 Modifying objects efficiently Modifications to objects aren t automatically detected (no dirty checking), and SQL operations aren t executed as late as possible (no write-behind). No modification of an object and no operation you call are cascaded to any associated instance. You re working with instances of a single entity class. Any modifications to a collection that is mapped as an entity association (one-to-many, many-to-many) are ignored. Only collections of value types are considered. You therefore shouldn t map entity associations with collections, but only the noninverse side with foreign keys to many-to-one; handle the relationship through one side only. Write a query to obtain data you d otherwise retrieve by iterating through a mapped collection. The StatelessSession bypasses any enabled org.hibernate.Interceptor and can t be intercepted through the event system (both features are discussed later in this chapter). You have no guaranteed scope of object identity. The same query produces two different in-memory detached instances. This can lead to data-aliasing effects if you don t carefully implement the equals() method of your persistent classes. Good use cases for a StatelessSession are rare; you may prefer it if manual batching with a regular Session becomes cumbersome. Remember that the insert(), update(), and delete() operations have naturally different semantics than the equivalent save(), update(), and delete()operations on a regular Session. (They probably should have different names, too; the StatelessSession API was added to Hibernate ad hoc, without much planning. The Hibernate developer team discussed renaming this interface in a future version of Hibernate; you may find it under a different name in the Hibernate version you re using.) So far in this chapter, we ve shown how you can store and manipulate many objects with the most efficient strategy through cascading, bulk, and batch operations. We ll now consider interception and data filtering, and how you can hook into Hibernate s processing in a transparent fashion. 12.3 Data filtering and interception Imagine that you don t want to see all the data in your database. For example, the currently logged-in application user may not have the rights to see everything. Usually, you add a condition to your queries and restrict the result dynamically. This becomes difficult if you have to handle a concern such as security or temporal data ( Show me only data from last week, for example). Even more difficult is
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Bulk and batch operations Another option that completely avoids memory consumption of the persistence context (by effectively disabling it) is the StatelessSession interface. 12.2.3 Using a stateless Session The persistence context is an essential feature of the Hibernate and Java Persistence engine. Without a persistence context, you wouldn t be able to manipulate object state and have Hibernate detect your changes automatically. Many other things also wouldn t be possible. However, Hibernate offers you an alternative interface, if you prefer to work with your database by executing statements. This statement-oriented interface, org.hibernate.StatelessSession, feels and works like plain JDBC, except that you get the benefit from mapped persistent classes and Hibernate s database portability. Imagine that you want to execute the same update all item objects procedure you wrote in an earlier example with this interface: Session session = sessionFactory.openStatelessSession(); Transaction tx = session.beginTransaction(); ScrollableResults itemCursor = session.createQuery(”from Item”).scroll(); while ( itemCursor.next() ) { Item item = (Item) itemCursor.get(0); modifyItem(item); session.update(item); } tx.commit(); session.close(); The batching is gone in this example you open a StatelessSession. You no longer work with objects in persistent state; everything that is returned from the database is in detached state. Hence, after modifying an Item object, you need to call update() to make your changes permanent. Note that this call no longer reattaches the detached and modified item. It executes an immediate SQL UPDATE; the item is again in detached state after the command. Disabling the persistence context and working with the StatelessSession interface has some other serious consequences and conceptual limitations (at least, if you compare it to a regular Session): A StatelessSession doesn t have a persistence context cache and doesn t interact with any other second-level or query cache. Everything you do results in immediate SQL operations.
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CHAPTER 12 Modifying objects efficiently A flush of the persistence context writes the changes you made to the last 100 Item objects to the database. For best performance, you should set the size of the Hibernate (and JDBC) configuration property hibernate.jdbc.batch_size to the same size as your procedure batch: 100. All UDPATE statements that are executed during flushing are then also batched at the JDBC level. (Note that you should disable the second-level cache for any batch operations; otherwise, each modification of an object during the batch procedure must be propagated to the second-level cache for that persistent class. This is an unnecessary overhead. You ll learn how to control the second-level cache in the next chapter.) The Java Persistence API unfortunately doesn t support cursor-based query results. You have to call org.hibernate.Session and org.hibernate.Query to access this feature. The same technique can be used to create and persist a large number of objects. Inserting many objects in batches If you have to create a few hundred or thousand objects in a unit of work, you may run into memory exhaustion. Every object that is passed to insert() or persist() is added to the persistence context cache. A straightforward solution is to flush and clear the persistence context after a certain number of objects. You effectively batch the inserts: Session session = sessionFactory.openSession(); Transaction tx = session.beginTransaction(); for ( int i=0; i<100000; i++ ) { Item item = new Item(...); session.save(item); if ( i % 100 == 0 ) { session.flush(); session.clear(); } } tx.commit(); session.close(); Here you create and persist 100,000 objects, 100 at a time. Again, remember to set the hibernate.jdbc.batch_size configuration property to an equivalent value and disable the second-level cache for the persistent class. Caveat: Hibernate silently disables JDBC batch inserts if your entity is mapped with an identity identifier generator; many JDBC drivers don t support batching in that case.
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Bulk and batch operations 12.2.2 Processing with batches Imagine that you have to manipulate all Item objects, and that the changes you have to make aren t as trivial as setting a flag (which you ve done with a single statement previously). Let s also assume that you can t create an SQL stored procedure, for whatever reason (maybe because your application has to work on database- management systems that don t support stored procedures). Your only choice is to write the procedure in Java and to retrieve a massive amount of data into memory to run it through the procedure. You should execute this procedure by batching the work. That means you cre ate many smaller datasets instead of a single dataset that wouldn t even fit into memory. Writing a procedure with batch updates The following code loads 100 Item objects at a time for processing: Session session = sessionFactory.openSession(); Transaction tx = session.beginTransaction(); ScrollableResults itemCursor = session.createQuery(”from Item”).scroll(); int count=0; while ( itemCursor.next() ) { Item item = (Item) itemCursor.get(0); modifyItem(item); if ( ++count % 100 == 0 ) { session.flush(); session.clear(); } } tx.commit(); session.close(); You use an HQL query (a simple one) to load all Item objects from the database. But instead of retrieving the result of the query completely into memory, you open an online cursor. A cursor is a pointer to a result set that stays in the database. You can control the cursor with the ScrollableResults object and move it along the result. The get(int i) call retrieves a single object into memory, the object the cursor is currently pointing to. Each call to next() forwards the cursor to the next object. To avoid memory exhaustion, you flush() and clear() the persistence context before loading the next 100 objects into it.
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CHAPTER 12 Modifying objects efficiently ownerFirstname, onwerLastname, ownerLogin, ownerEmailAddress, ownerHomeAddress) select c.type, c.number, c.expMonth, c.expYear, u.firstname, u.lastname, u.username, u.email, u.homeAddress from CreditCard c join c.user u where c.stolenOn is not null” ); int createdObjects = q.executeUpdate(); This operation does two things: First, the details of CreditCard records and the respective owner (a User) are selected. The result is then directly inserted into the table to which the StolenCreditCard class is mapped. Note the following: The properties that are the target of an INSERT … SELECT (in this case, the StolenCreditCard properties you list) have to be for a particular subclass, not an (abstract) superclass. Because StolenCreditCard isn t part of an inheritance hierarchy, this isn t an issue. The types returned by the SELECT must match the types required for the INSERT in this case, lots of string types and a component (the same type of component for selection and insertion). The database identifier for each StolenCreditCard object will be generated automatically by the identifier generator you map it with. Alternatively, you can add the identifier property to the list of inserted properties and supply a value through selection. Note that automatic generation of identifier values works only for identifier generators that operate directly inside the database, such as sequences or identity fields. If the generated objects are of a versioned class (with a version or timestamp property), a fresh version (zero, or timestamp of today) will also be generated. Alternatively, you can select a version (or timestamp) value and add the version (or timestamp) property to the list of inserted properties. Finally, note that INSERT … SELECT is available only with HQL; JPA QL doesn t standardize this kind of statement hence, your statement may not be portable. HQL and JPA QL bulk operations cover many situations in which you d usually resort to plain SQL. On the other hand, sometimes you can t exclude the application tier in a mass data operation.
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