Hibernate Basics, Page 4
Applications usually require more than a simple database connection. Scalability, stability, and performance are core aspects of any enterprise application. Popular solutions to achieve these goals include database connection pooling, transaction strategies, and object caching. Hibernate supports each of these solutions.
Connection pools are a common way to improve application performance. Rather than opening a separate connection to the database for each request, the connection pool maintains a collection of open database connections that are reused. Application servers often provide their own connection pools using a JNDI DataSource, which Hibernate can take advantage of when configured to use a DataSource.
If you're running a standalone application or your application server doesn't support connection pools, Hibernate supports three connection pooling services: C3P0, Apache's DBCP library, and Proxool. C3P0 is distributed with Hibernate; the other two are available as separate distributions.
When you choose a connection pooling service, you must configure it for your environment. Hibernate supports configuring connection pools from the hibernate.cfg.xml file. The connection.provider_class property sets the pooling implementation:
Once the provider class is set, the specific properties for the pooling service can also be configured from the hibernate.cfg.xml file:
As you can see, the prefix for the C3P0 configuration parameters is c3p0. Similarly, the prefixes for DBCP and Proxool are dbcp and proxool, respectively. Specific configuration parameters for each pooling service are available in the documentation with each service. Table 1 lists information for the supported connection pools.
Hibernate ships with a basic connection pool suitable for development and testing purposes. However, it should not be used in production. You should always use one of the available connection pooling services, like C3P0, when deploying your application to production.
If your preferred connection pool API isn't currently supported by Hibernate, you can add support for it by implementing the org.hibernate.connection.ConnectionProvider interface. Implementing the interface is straightforward.
|Table 1: Connection pooling services|
|Pooling Service||Provider Class||Configuration Prefix|
There isn't much to using a connection pool, since Hibernate does most of the work behind the scenes. The next configuration topic we’ll look at deals with transaction management with the Hibernate Transaction API.
Transactions group many operations into a single unit of work. If any operation in the batch fails, all of the previous operations are rolled back, and the unit of work stops. Hibernate can run in many different environments supporting various notions of transactions. Standalone applications and some application servers only support simple JDBC transactions, whereas others support the Java Transaction API (JTA).
Hibernate needs a way to abstract the various transaction strategies from the environment. Hibernate has its own Transaction class that is accessible from the Session interface, demonstrated here:
Session session =factory.openSession();
Transaction tx =session.beginTransaction();
Event event =new Event();
//...populate the Event instance
In this example, factory is an initialized SessionFactory instance. This code creates an instance of the org.hibernate.Transaction class and then commits the Transaction instance.
Notice that you don't need to call session.flush(). Committing a transaction automatically flushes the Session object. The Event instance is persisted to the database when the transaction is committed. The transaction strategy you use (JDBC or JTA) doesn't matter to the application code-it's set in the Hibernate configuration file.
The transaction.factory_class property defines the transaction strategy that Hibernate uses. The default setting is to use JDBC transactions since they're the most common. To use JTA transactions, you need to set the following properties in hibernate.cfg.xml:
The transaction.factory_class property tells Hibernate that you'll be using JTA transactions. Currently, the only other option to JTA is JBDC transactions, which is the default. JTA transactions are retrieved from a JNDI URI, which is specified using the jta.User-Transaction property. If you don't know the URI for your specific application server, the default value is java:comp/UserTransaction.
There is some confusion about another property related to JTA transactions: transaction.manager_lookup_class. You only need to specify the manager lookup class when you're using a transactional cache. (We discuss caches in the next section–don't worry.) However, if you don't define the jta.UserTransaction property and transaction.manager_lookup_class is defined, the user transaction name in the lookup factory class is used. If neither of the properties are used, Hibernate falls back to java:comp/UserTransaction.
What's the benefit of using JTA transactions? JTA transactions are useful if you have multiple transactional resources, such as a database and a message queue. JTA allows you to treat the disparate transactions as a single transaction. Combining multiple transactions also applies within Hibernate. If you attempt to create multiple transactions from the same Session instance, all of the operations are batched into the first transaction. Let's look at an example that includes two transactions:
Transaction tx0 =session.beginTransaction();
Event event =new Event();
//...populate the event instance
Transaction tx1 =session.beginTransaction();
Location location =new Location();
//...populate the Location instance
This example begins by creating a new transaction. The second use of session.beginTransaction()just returns the first transaction instance. session.saveOrUpdate(location)commits the first transaction, and tx0.commit()recommits the first transaction.
Although you explicitly create two Transaction objects, only one is used. Of course, this creates a problem. Let's assume you have a Session object being used by two application threads. The first application thread begins the JTA transaction and starts adding objects. Meanwhile, the second thread, using the same transaction, deletes an object and commits the transaction. Where does this leave the first thread?
The first thread won't be committed, which is what you'd expect. The problem is that this issue can be hard to debug, bringing up an important point: Sessions should be used by only one application thread at a time. This is a common concern in web applications, which are multithreaded by their very nature.
In the next section, we discuss Hibernate's support for various caching providers.
As we mentioned earlier, caching is a common method used to improve application performance. Caching can be as simple as having a class store frequently used data, or a cache can be distributed among multiple computers. The logic used by caches can also vary widely, but most use a simple least recently used (LRU) algorithm to determine which objects should be removed from the cache after a configurable amount of time.
Before you get confused, let's clarify the difference between the Session–level cache, also called the first–level cache, and what this section covers. The Session–level cache stores object instances for the lifetime of a given Session instance. The caching services described in this section cache data outside of the lifetime of a given Session. Another way to think about the difference is that the Session cache is like a transactional cache that only caches the data needed for a given operation or set of operations, whereas a second–level cache is an application-wide cache.
Caching services are typically referred to as second-level caches elsewhere in this book and in other Hibernate documentation. When you see it mentioned in the text, we're referring to external caching services.
By default, Hibernate supports four different caching services, listed in table 2. EHCache (Easy Hibernate Cache) is the default service. If you prefer to use an alternative cache, you need to set the cache.provider_class property in the hibernate.cfg.xml file:
This snippet sets the cache provider to the OSCache caching service.
|Table 2: Caching Services Supported by Hibernate|
|Caching Service||Provider Class||Type|
The caching services support the caching of classes as well as collections belonging to persistent classes. For instance, suppose you have a large number of Attendee instances associated with a particular Event instance. Instead of repeatedly fetching the collection of Attendee s, you can cache it. Caching for classes and collections is configured in the mapping files, with the cache element:
Collections can also be cached:
Once you've chosen a caching service, what do you, the developer, need to do differently to take advantage of cached objects? Thankfully, you don't have to do anything. Hibernate works with the cache behind the scenes, so concerns about retrieving an outdated object from the cache can be avoided. You only need to select the correct value for the usage attribute.
The usage attribute specifies the caching concurrency strategy used by the underlying caching service. The previous configuration sets the usage to read–write , which is desirable if your application needs to update data. Alternatively, you may use the nonstrict–read–write strategy if it's unlikely two separate transaction threads could update the same object. If a persistent object is never updated, only read from the database, you may specify set usage to read-only.
Some caching services, such as the JBoss TreeCache, use transactions to batch multiple operations and perform the batch as a single unit of work. If you choose to use a transactional cache, you may set the usage attribute to transactional to take advantage of this feature. If you happen to be using a transactional cache, you'll also need to set the transaction.manager_lookup_class mentioned in the previous section.
The supported caching strategies differ based on the service used. Table 3 shows the supported strategies.
|Table 3: Supported Caching Service Strategies|
Clearly, the caching service you choose will depend on your application requirements and environment. Next, let's look at configuring EHCache.
By now you're probably tired of reading about configuring Hibernate, but EHCache is pretty simple. It's a single XML file, placed in a directory listed in your classpath. You'll probably want to put the ehcache.xml file in the same directory as the hibernate.cfg.xml file.
Listing 5 shows a simple configuration file for EHCache.
Listing 5. ehcache.xml file
In this example, the diskStore property sets the location of the disk cache store. Then, the listing declares two caches. The defaultCache element contains the settings for all cached objects that don't have a specific cache element: the number of cached objects held in memory, whether objects in the cache expire (if eternal is true , then objects don't expire), the number of seconds an object should remain the cache after it was last accessed, the number of seconds an object should remain in the cache after it was created, and whether objects exceeding maxElementsInMemory should be spooled to the diskStore. Next, for custom settings based on the class, the code defines a cache element with the fully qualified class name listed in the name attribute. (This listing only demonstrates a subset of the available configuration for EHCache. Please refer to the documentation found at http:// ehcache.sf.net for more information.)
With pooling, transactions, and caching behind us, we can look at a difference topic: how Hibernate handles inheritance.