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Defining the global fetch plan have to define this fetch plan, the part of your object network that you want to always load into memory. Let s assume that you always require the seller of an Item. In Hibernate XML mapping metadata you d map the association from Item to User as lazy=”false”: … … The same always load guarantee can be applied to collections for example, all bids of an Item: … … If you now get() an Item (or force the initialization of a proxied Item), both the seller object and all the bids are loaded as persistent instances into your persistence context: Item item = (Item) session.get(Item.class, new Long(123)); The persistence context after this call is shown graphically in figure 13.3. Other lazy mapped associations and collections (the bidder of each Bid instance, for example) are again uninitialized and are loaded as soon as you access them. Imagine that you close the persistence context after loading an Item. You can now navigate, in detached state, to the seller of the Item and iterate through all the bids for that Item. If you navigate to the categories this Item is assigned to, you get a LazyInitializationException! Obviously, this collection
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CHAPTER 13 Optimizing fetching and caching The JPA standard doesn t require an implementation with proxies; the word proxy doesn t even appear in the specification. Hibernate is a JPA provider that relies on proxies by default, so the switch that disables Hibernate proxies is available as a vendor extension: @Entity @Table(name = “USERS”) @org.hibernate.annotations.Proxy(lazy = false) public class User { … } Disabling proxy generation for an entity has serious consequences. All of these operations require a database hit: User user = (User) session.load(User.class, new Long(123)); User user = em.getReference(User.class, new Long(123)); A load() of a User object can t return a proxy. The JPA operation getReference() can no longer return a proxy reference. This may be what you desired to achieve. However, disabling proxies also has consequences for all associations that reference the entity. For example, the Item entity has a seller association to a User. Consider the following operations that retrieve an Item: Item item = (Item) session.get(Item.class, new Long(123)); Item item = em.find(Item.class, new Long(123)); In addition to retrieving the Item instance, the get() operation now also loads the linked seller of the Item; no User proxy is returned for this association. The same is true for JPA: The Item that has been loaded with find() doesn t reference a seller proxy. The User who is selling the Item must be loaded right away. (We answer the question how this is fetched later.) Disabling proxy generation on a global level is often too coarse-grained. Usually, you only want to disable the lazy loading behavior of a particular entity association or collection to define a fine-grained fetch plan. You want the opposite: eager loading of a particular association or collection. 13.1.5 Eager loading of associations and collections You ve seen that Hibernate is lazy by default. All associated entities and collections aren t initialized if you load an entity object. Naturally, you often want the opposite: to specify that a particular entity association or collection should always be loaded. You want the guarantee that this data is available in memory without an additional database hit. More important, you want a guarantee that, for example, you can access the seller of an Item if the Item instance is in detached state. You
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Defining the global fetch plan collections; they can be mapped as extra lazy. For example, consider the collection of bids of an Item: … The collection wrapper is now smarter than before. The collection is no longer initialized if you call size(), contains(), or isEmpty() the database is queried to retrieve the necessary information. If it s a Map or a List, the operations containsKey() and get() also query the database directly. A Hibernate extension annotation enables the same optimization: @OneToMany @org.hibernate.annotations.LazyCollection( org.hibernate.annotations.LazyCollectionOption.EXTRA ) private Set bids = new HashSet(); Let s define a fetch plan that isn t completely lazy. First, you can disable proxy generation for entity classes. 13.1.4 Disabling proxy generation Proxies are a good thing: They allow you to load only the data that is really needed. They even let you create associations between objects without hitting the database unnecessarily. Sometimes you need a different plan for example, you want to express that a User object should always be loaded into memory and no placeholder should be returned instead. You can disable proxy generation for a particular entity class with the lazy=”false” attribute in XML mapping metadata: …
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CHAPTER 13 Optimizing fetching and caching Figure 13.2 Proxies and collection wrappers represent the boundary of the loaded graph. perspective: the identifier values are all foreign key columns in the item s row. Collections also aren t loaded right away, but we use the term collection wrapper to describe this kind of placeholder. Internally, Hibernate has a set of smart collections that can initialize themselves on demand. Hibernate replaces your collections with these; that is why you should use collection interfaces only in your domain model. By default, Hibernate creates placeholders for all associations and collections, and only retrieves value-typed properties and components right away. (This is unfortunately not the default fetch plan standardized by Java Persistence; we ll get back to the differences later.) FAQ Does lazy loading of one-to-one associations work? Lazy loading for one-to-one associations is sometimes confusing for new Hibernate users. If you consider one-to-one associations based on shared primary keys (chapter 7, section 7.1.1, Shared primary key associations ), an association can be proxied only if it s constrained=”true”. For example, an Address always has a reference to a User. If this association is nullable and optional, Hibernate first would have to hit the database to find out whether a proxy or a null should be applied the purpose of lazy loading is to not hit the database at all. You can enable lazy loading through bytecode instrumentation and interception, which we ll discuss later. A proxy is initialized if you call any method that is not the identifier getter method, a collection is initialized if you start iterating through its elements or if you call any of the collection-management operations, such as size() and contains(). Hibernate provides an additional setting that is mostly useful for large
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Defining the global fetch plan A proxy is useful if you need the Item only to create a reference, for example: Item item = (Item) session.load(Item.class, new Long(123)); User user = (User) session.load(User.class, new Long(1234)); Bid newBid = new Bid(”99.99″); newBid.setItem(item); newBid.setBidder(user); session.save(newBid); You first load two objects, an Item and a User. Hibernate doesn t hit the database to do this: It returns two proxies. This is all you need, because you only require the Item and User to create a new Bid. The save(newBid)call executes an INSERT statement to save the row in the BID table with the foreign key value of an Item and a User this is all the proxies can and have to provide. The previous code snippet doesn t execute any SELECT! If you call get() instead of load() you trigger a database hit and no proxy is returned. The get() operation always hits the database (if the instance isn t already in the persistence context and if no transparent second-level cache is active) and returns null if the object can t be found. A JPA provider can implement lazy loading with proxies. The method names of the operations that are equivalent to load() and get() on the EntityManagerAPI are find() and getReference(): Item item = em.find(Item.class, new Long(123)); Item itemRef = em.getReference(Item.class, new Long(1234)); The first call, find(), has to hit the database to initialize an Item instance. No proxies are allowed it s the equivalent of the Hibernate get() operation. The second call, getReference(), may return a proxy, but it doesn t have to which translates to load() in Hibernate. Because Hibernate proxies are instances of runtime generated subclasses of your entity classes, you can t get the class of an object with the usual operators. This is where the helper method HibernateProxyHelper.getClassWithoutInitializingProxy(o) is useful. Let s assume you have an Item instance into memory, either by getting it explicitly or by calling one of its properties and forcing initialization of a proxy. Your persistence context now contains a fully loaded object, as shown in figure 13.2. Again, you can see proxies in the picture. This time, these are proxies that have been generated for all *-to-one associations. Associated entity objects are not loaded right away; the proxies carry the identifier values only. From a different
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CHAPTER 13 Optimizing fetching and caching 13.1.2 The lazy default fetch plan Hibernate defaults to a lazy fetching strategy for all entities and collections. This means that Hibernate by default loads only the objects you re querying for. Let s explore this with a few examples. If you query for an Item object (let s say you load it by its identifier), exactly this Item and nothing else is loaded into memory: Item item = (Item) session.load(Item.class, new Long(123)); This retrieval by identifier results in a single (or possibly several, if inheritance or secondary tables are mapped) SQL statement that retrieves an Item instance. In the persistence context, in memory, you now have this item object available in persistent state, as shown in figure 13.1. Figure 13.1 An uninitialized placeholder for an Item instance We ve lied to you. What is available in memory after the load() operation isn t a persistent item object. Even the SQL that loads an Item isn t executed. Hibernate created a proxy that looks like the real thing. 13.1.3 Understanding proxies Proxies are placeholders that are generated at runtime. Whenever Hibernate returns an instance of an entity class, it checks whether it can return a proxy instead and avoid a database hit. A proxy is a placeholder that triggers the loading of the real object when it s accessed for the first time: Item item = (Item) session.load(Item.class, new Long(123)); item.getId(); item.getDescription(); // Initialize the proxy The third line in this example triggers the execution of the SQL that retrieves an Item into memory. As long as you access only the database identifier property, no initialization of the proxy is necessary. (Note that this isn t true if you map the identifier property with direct field access; Hibernate then doesn t even know that the getId() method exists. If you call it, the proxy has to be initialized.)
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Defining the global fetch plan parsed and validated at compile time, whereas HQL expressions aren t parsed until runtime (or startup, if externalized named queries are used). The nice thing about the Hibernate CriteriaAPI is the Criterion framework. This framework allows extension by the user, which is more difficult in the case of a query language like HQL. Note that the Criteria API is native to Hibernate; it isn t part of the Java Persistence standard. In practice, Criteria will be the most common Hibernate extension you utilize in your JPA application. We expect that a future version of the JPA or EJB standard will include a similar programmatic query interface. Querying by example As part of the Criteria facility, Hibernate supports query by example (QBE). The idea behind query by example is that the application supplies an instance of the queried class, with certain property values set (to nondefault values). The query returns all persistent instances with matching property values. Query by example isn t a particularly powerful approach. However, it can be convenient for some applications, especially if it s used in combination with Criteria: Criteria criteria = session.createCriteria(User.class); User exampleUser = new User(); exampleUser.setFirstname(”John”); criteria.add( Example.create(exampleUser) ); criteria.add( Restrictions.isNotNull(”homeAddress.city”) ); List result = criteria.list(); This example first creates a new Criteria that queries for User objects. Then you add an Example object, a User instance with only the firstname property set. Finally, a Restriction criterion is added before executing the query. A typical use case for query by example is a search screen that allows users to specify a range of different property values to be matched by the returned result set. This kind of functionality can be difficult to express cleanly in a query language; string manipulations are required to specify a dynamic set of constraints. The Criteria API and the example query mechanism are discussed in more detail in chapter 15. You now know the basic retrieval options in Hibernate. We focus on the object-fetching plans and strategies for the rest of this section. Let s start with the definition of what should be loaded into memory.
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CHAPTER 13 Optimizing fetching and caching HQL is powerful, and even though you may not use the more advanced features all the time, they re needed for more difficult problems. For example, HQL supports The ability to apply restrictions to properties of associated objects related by reference or held in collections (to navigate the object graph using query language). The ability to retrieve only properties of an entity or entities, without the overhead of loading the entity itself into the persistence context. This is sometimes called a report query; it is more correctly called projection. The ability to order the results of the query. The ability to paginate the results. Aggregation with group by, having, and aggregate functions like sum, min, and max/min. Outer joins when retrieving multiple objects per row. The ability to call standard and user-defined SQL functions. Subqueries (nested queries). We discuss all these features in chapters 14 and 15, together with the optional native SQL query mechanism. Querying with a criteria The Hibernate query by criteria (QBC) API allows a query to be built by manipulation of criteria objects at runtime. This lets you specify constraints dynamically without direct string manipulations, but you don t lose much of the flexibility or power of HQL. On the other hand, queries expressed as criteria are often much less readable than queries expressed in HQL. Retrieving a user by first name is easy with a Criteria object: Criteria criteria = session.createCriteria(User.class); criteria.add( Restrictions.like(”firstname”, “John”) ); List result = criteria.list(); A Criteria is a tree of Criterion instances. The Restrictions class provides static factory methods that return Criterion instances. Once the desired criteria tree is build, it s executed against the database. Many developers prefer query by criteria, considering it a more object-oriented approach. They also like the fact that the query syntax may be
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Defining the global fetch plan persistent object network should be retrieved and how it should be retrieved. Your goal is to find the best retrieval method and fetching strategy for every use case in your application; at the same time, you also want to minimize the number of SQL queries for best performance. Before we look at the fetch plan options and fetching strategies, we ll give you an overview of the retrieval methods. (We also mention the Hibernate caching system sometimes, but we fully explore it later in this chapter.) You saw how objects are retrieved by identifier earlier in the previous chapter, so we won t repeat it here. Let s go straight to the more flexible query options, HQL (equivalent to JPA QL) and Criteria. Both allow you to create arbitrary queries. The Hibernate Query Language and JPA QL The Hibernate Query Language is an object-oriented dialect of the familiar database query language SQL. HQL bears some close resemblance to ODMG OQL, but unlike OQL, it s adapted for use with SQL databases and is easier to learn (thanks to its close resemblance to SQL) and fully implemented (we don t know of any OQL implementation that is complete). The EJB 3.0 standard defines the Java Persistence query language. This new JPA QL and the HQL have been aligned so that JPA QL is a subset of HQL. A valid JPA QL query is always also a valid HQL query; HQL has more options that should be considered vendor extensions of the standardized subset. HQL is commonly used for object retrieval, not for updating, inserting, or deleting data. Object state synchronization is the job of the persistence manager, not the developer. But, as we ve shown in the previous chapter, HQL and JPA QL support direct bulk operations for updating, deleting, and inserting, if required by the use case (mass data operations). Most of the time, you only need to retrieve objects of a particular class and restrict by the properties of that class. For example, the following query retrieves a user by first name. Query q = session.createQuery( “from User as u where u.firstname = :fname” ); q.setString(”fname”, “John”); List result = q.list(); After preparing query q, you bind a value to the named parameter :fname. The result is returned as a List of User objects.
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CHAPTER 13 Optimizing fetching and caching In this chapter, we ll show you how to retrieve objects from the database and how you can optimize the loading of object networks when you navigate from object to object in your application. We then enable caching; you ll learn how to speed up data retrieval in local and distributed applications. 13.1 Defining the global fetch plan Retrieving persistent objects from the database is one of the most interesting parts of working with Hibernate. 13.1.1 The object-retrieval options Hibernate provides the following ways to get objects out of the database: Navigating the object graph, starting from an already loaded object, by accessing the associated objects through property accessor methods such as aUser.getAddress().getCity(), and so on. Hibernate automatically loads (and preloads) nodes of the graph while you call accessor methods, if the persistence context is still open. Retrieval by identifier, the most convenient method when the unique identifier value of an object is known. The Hibernate Query Language (HQL), which is a full object-oriented query language. The Java Persistence query language (JPA QL) is a standardized subset of the Hibernate query language. The Hibernate Criteria interface, which provides a type-safe and object-oriented way to perform queries without the need for string manipulation. This facility includes queries based on example objects. Native SQL queries, including stored procedure calls, where Hibernate still takes care of mapping the JDBC result sets to graphs of persistent objects. In your Hibernate or JPA application, you use a combination of these techniques. We won t discuss each retrieval method in much detail in this chapter. We re more interested in the so-called default fetch plan and fetching strategies. The default fetch plan and fetching strategy is the plan and strategy that applies to a particular entity association or collection. In other words, it defines if and how an associated object or a collection should be loaded, when the owning entity object is loaded, and when you access an associated object or collection. Each retrieval method may use a different plan and strategy that is, a plan that defines what part of the
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