Programming WebLogic Enterprise JavaBeans
This section describes how session beans work within the EJB container, and provides design and development guidelines that are specific to session beans. For a description of the overall bean development process, see Implementing Enterprise Java Beans.
It is assumed that the reader is familiar with Java programming and session bean features and capabilities. For an introduction to session bean features and how they are typically used in applications, see Session EJBs Implement Business Logic and Session Bean Features.
See Choosing Between Stateless and Stateful Beans for a discussion of when to use which type of session bean.
However, in a production environment, WebLogic Server can provide improved performance and throughput for stateless session EJBs by maintaining a free pool of unbound stateless session EJBs—instances that are not currently processing a method call. If an unbound instance is available to serve a request, response time improves, because the request does not have to wait for an instance to be created. The free pool improves performance by reusing objects and skipping container callbacks when it can.
Upon startup, WebLogic Server automatically creates and populates the free pool with the quantity of instances you specify in the bean's initial-beans-in-free-pool
weblogic-ejb-jar.xml file. By default,
initial-beans-in-free-pool is set to 0.
The following figure illustrates the WebLogic Server free pool, and the processes by which stateless EJBs enter and leave the pool. Dotted lines indicate the "state" of the EJB from the perspective of WebLogic Server.
If you configure a pool, WebLogic Server will service method calls with an EJB instance from the free pool, if one is available. The EJB remains active for the duration of the client's method call. After the method completes, the EJB instance is returned to the free pool. Because WebLogic Server unbinds stateless session beans from clients after each method call, the actual bean class instance that a client uses may be different from invocation to invocation.
If all instances of an EJB class are active and max-beans-in-free-pool has been reached, new clients requesting the EJB class will be blocked until an active EJB completes a method call. If the transaction times out (or, for non-transactional calls, if five minutes elapse), WebLogic Server throws a
RemoteException for a remote client or an
EJBException for a local client.
max-beans-in-free-pool. WebLogic Server allocates a new bean instance and gives it to you.
max-beans-in-free-pool.You application must wait until either your transaction times out or a bean instance that already exists in the pool becomes available.
WebLogic Server uses a cache of bean instances to improve the performance of stateful session EJBs. The cache stores active EJB instances in memory so that they are immediately available for client requests. The cache contains EJBs that are currently in use by a client and instances that were recently in use. Stateful session beans in cache are bound to a particular client.
No stateful session EJB instances exist in WebLogic Server at startup. Before a client begins accessing a stateful session bean, it creates a new bean instance to use during its session with the bean. When the session is over the instance is destroyed. While the session is in progress, the instance is cached in memory.
Passivation is the process by which WebLogic Server removes an EJB instance from cache while preserving its state on disk. While passivated, EJBs are not in memory and are not immediately available for client requests, as they are when in the cache.
The EJB developer must ensure that a call to the
ejbPassivate() method leaves a stateful session bean in a condition such that WebLogic Server can serialize its data and passivate the instance. During passivation, WebLogic Server attempts to serialize any fields that are not declared
transient. This means that you must ensure that all non-
transient fields represent serializable objects, such as the bean's remote or home interface. EJB 2.1 specifies the field types that are allowed.
max-beans-in-cache elements also affect passivation and removal behaviors, based on the value of cache-type.
When you configure eager passivation for a stateful session bean by setting cache-type to
LRU, the container passivates instances to disk:
idle-timeout-seconds, regardless of the value of
max-beans-in-cacheis reached, even though
idle-timeout-secondshas not expired.
The container removes a passivated instance from disk after it has been inactive for
idle-timeout-seconds after passivation. This is referred to as a lazy remove.
When lazy passivation is configured by setting cache-type to
NRU, the container avoids passivating beans, because of the associated systems overhead—pressure on the cache is the only event that causes passivation or eager removal of beans.
idle-timeout-secondsexpires, and does not passivate it to disk. This is referred to as a eager remove. An eager remove ensures that an inactive instance does not consume memory or disk resources.
max-beans-in-cacheis reached, even though
idle-timeout-secondshas not expired.
For a discussion of managing cache size to optimize performance in a production environment see Setting EJB Cache Size in WebLogic Server Performance and Tuning.
When a stateful session bean is passivated, its state is stored in a file system directory. Each server instance has its own directory for storing the state of passivated stateful session beans, known as the persistent store directory. The persistent store directory contains one subdirectory for each passivated bean.
weblogic-ejb-jar.xml. If no value is specified for
<persistent-store-dir>, the directory is named
In accordance with the EJB 2.0 specification, simultaneous access to a stateful session EJB results in a
RemoteException. This access restriction on stateful session EJBs applies whether the EJB client is remote or internal to WebLogic Server. To override this restriction and configure a stateful session bean to allow concurrent calls, set the allow-concurrent-calls deployment element.
If multiple servlet classes access a stateful session EJB, each servlet thread (rather than each instance of the servlet class) must have its own session EJB instance. To prevent concurrent access, a JSP/servlet can use a stateful session bean in request scope.
Stateless session beans are a good choice if your application does not need to maintain state for a particular client between business method calls. WebLogic Server is multi-threaded, servicing multiple clients simultaneously. With stateless session beans, the EJB container is free to use any available, pooled bean instance to service a client request, rather than reserving an instance for each client for the duration of a session. This results in greater resource utilization, scalability and throughput.
When you choose values for initial-beans-in-free-pool and max-beans-in-free-pool you must weigh memory consumption against slowing down your application. If the number of stateless session bean instances is too high, the free pool contains inactive instances that consume memory. If the number is too low, a client may not obtain an instance when it needs it. This leads to client threads blocking until an instance frees up, slowing down the application.
max-beans-in-free-pool should be equal to the number of worker threads in the server instance, so that when a thread tries to do work an instance is available.
Implementing Enterprise Java Beans, takes you through session bean implementation step-by-step. This section explains the details of configuring WebLogic-Server specific session bean behavior by setting bean-specific deployment descriptor elements.
Table 5-2 summarizes the deployment descriptor elements you set to configure the behavior of a stateless session bean and how the bean behaves if you do not configure the element. All of the elements listed are sub-elements of the stateless-session-descriptor element in
Table 5-3 summarizes the deployment descriptor elements you set to configure the behavior of a stateful session bean and how the bean behaves if you do not configure the element. All of the elements listed are sub-elements of the stateful-session-descriptor element in
The period of inactivity before a stateful session bean instance remains in cache (given that max-beans-in-cache has not been reached), and after passivation, remains on disk.
NRU (not recently used)—For a description of this behavior, see Lazy Passivation (NRU).