The GlassFish Server provides a number of value additions that relate to EJB development. References to more in-depth material are included.
The following topics are addressed here:
Another feature that the GlassFish Server provides is the read-only bean, an EJB 2.1 entity bean that is never modified by an EJB client. Read-only beans avoid database updates completely.
Note - Read-only beans are specific to the GlassFish Server and are not part of the Enterprise JavaBeans Specification, v2.1. Use of this feature for an EJB 2.1 bean results in a non-portable application.
To make an EJB 3.0 entity read-only, use @Column annotations to mark its columns insertable=false and updatable=false.
A read-only bean can be used to cache a database entry that is frequently accessed but rarely updated (externally by other beans). When the data that is cached by a read-only bean is updated by another bean, the read-only bean can be notified to refresh its cached data.
The GlassFish Server provides a number of ways by which a read-only bean’s state can be refreshed. By setting the refresh-period-in-seconds element in the glassfish-ejb-jar.xml file and the trans-attribute element (or @TransactionAttribute annotation) in the ejb-jar.xml file, it is easy to configure a read-only bean that is one of the following:
Read-only beans are best suited for situations where the underlying data never changes, or changes infrequently. For further information and usage guidelines, see Using Read-Only Beans.
The pass-by-reference element in the glassfish-ejb-jar.xml file allows you to specify the parameter passing semantics for colocated remote EJB invocations. This is an opportunity to improve performance. However, use of this feature results in non-portable applications. See pass-by-reference in Oracle GlassFish Server 3.1 Application Deployment Guide.
The EJB container of the GlassFish Server pools anonymous instances (message-driven beans, stateless session beans, and entity beans) to reduce the overhead of creating and destroying objects. The EJB container maintains the free pool for each bean that is deployed. Bean instances in the free pool have no identity (that is, no primary key associated) and are used to serve method calls. The free beans are also used to serve all methods for stateless session beans.
Bean instances in the free pool transition from a Pooled state to a Cached state after ejbCreate and the business methods run. The size and behavior of each pool is controlled using pool-related properties in the EJB container or the glassfish-ejb-jar.xml file.
In addition, the GlassFish Server supports a number of tunable parameters that can control the number of “stateful” instances (stateful session beans and entity beans) cached as well as the duration they are cached. Multiple bean instances that refer to the same database row in a table can be cached. The EJB container maintains a cache for each bean that is deployed.
To achieve scalability, the container selectively evicts some bean instances from the cache, usually when cache overflows. These evicted bean instances return to the free bean pool. The size and behavior of each cache can be controlled using the cache-related properties in the EJB container or the glassfish-ejb-jar.xml file.
Pooling and caching parameters for the glassfish-ejb-jar.xml file are described in bean-cache in Oracle GlassFish Server 3.1 Application Deployment Guide.
One of the most important parameters for GlassFish Server pooling is steady-pool-size. When steady-pool-size is set to a value greater than 0, the container not only pre-populates the bean pool with the specified number of beans, but also attempts to ensure that this number of beans is always available in the free pool. This ensures that there are enough beans in the ready-to-serve state to process user requests.
Note that the steady-pool-size and max-pool-size parameters only govern the number of instances that are pooled over a long period of time. They do not necessarily guarantee that the number of instances that may exist in the JVM at a given time will not exceed the value specified by max-pool-size. For example, suppose an idle stateless session container has a fully-populated pool with a steady-pool-size of 10. If 20 concurrent requests arrive for the EJB component, the container creates 10 additional instances to satisfy the burst of requests. The advantage of this is that it prevents the container from blocking any of the incoming requests. However, if the activity dies down to 10 or fewer concurrent requests, the additional 10 instances are discarded.
Another parameter, pool-idle-timeout-in-seconds, allows the administrator to specify the amount of time a bean instance can be idle in the pool. When pool-idle-timeout-in-seconds is set to greater than 0, the container removes or destroys any bean instance that is idle for this specified duration.
GlassFish Server provides a way that completely avoids caching of entity beans, using commit option C. Commit option C is particularly useful if beans are accessed in large number but very rarely reused. For additional information, refer to Commit Options.
The GlassFish Server caches can be either bounded or unbounded. Bounded caches have limits on the number of beans that they can hold beyond which beans are passivated. For stateful session beans, there are three ways (LRU, NRU and FIFO) of picking victim beans when cache overflow occurs. Caches can also passivate beans that are idle (not accessed for a specified duration).
You can create multiple thread pools, each having its own work queues. An optional element in the glassfish-ejb-jar.xml file, use-thread-pool-id, specifies the thread pool that processes the requests for the bean. The bean must have a remote interface, or use-thread-pool-id is ignored. You can create different thread pools and specify the appropriate thread pool ID for a bean that requires a quick response time. If there is no such thread pool configured or if the element is absent, the default thread pool is used.
Normally, all entity bean updates within a transaction are batched and executed at the end of the transaction. The only exception is the database flush that precedes execution of a finder or select query.
Since a transaction often spans many method calls, you might want to find out if the updates made by a method succeeded or failed immediately after method execution. To force a flush at the end of a method’s execution, use the flush-at-end-of-method element in the glassfish-ejb-jar.xml file. Only non-finder methods in an entity bean can be flush-enabled. (For an EJB 2.1 bean, these methods must be in the Local, Local Home, Remote, or Remote Home interface.) See flush-at-end-of-method in Oracle GlassFish Server 3.1 Application Deployment Guide.
Upon completion of the method, the EJB container updates the database. Any exception thrown by the underlying data store is wrapped as follows:
If the method that triggered the flush is a create method, the exception is wrapped with CreateException.
If the method that triggered the flush is a remove method, the exception is wrapped with RemoveException.
For all other methods, the exception is wrapped with EJBException.
All normal end-of-transaction database synchronization steps occur regardless of whether the database has been flushed during the transaction.