Optimizing fetching and caching in Java

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Optimizing fetching and caching
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13.1.2 The lazy default fetch plan
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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:
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Item item = (Item) session.load(Item.class, new Long(123));
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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.
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Figure 13.1 An uninitialized placeholder for an Item instance
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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.
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13.1.3 Understanding proxies
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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:
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Item item = (Item) session.load(Item.class, new Long(123)); item.getId(); item.getDescription(); // Initialize the proxy
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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.)
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 EntityManager API 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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