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Container-Managed Persistence in Borland Enterprise Server
Although the PM does not manage transactions (this is the Container's responsibility),
it is aware of transaction start and completion and can therefore manage entity state.
The PM uses the TxContext class to represent the root of managed entities during
transaction lifecycles. When the container manages a transaction it asks the PM for the
associated TxContext instance. If none exists, as is the case when a new transaction
has started, one is created by the PM. When a transaction is completing, the container
calls the method TxContext.beforeCompletion() to alert the PM to verify entity state.
The PM has complete responsibility for entity data storage and the maintenance of the
state of relationships between entities. Relationship editing is also managed by the PM.
This simplifies interactions with the container and allows the PM to optimize its read
and write operations. This approach also suppresses duplicate find requests by
tracking returned primary keys for requested entities. Data from duplicate find
operations can then be returned from the first load of the entity's data.
Borland CMP engine's CMP 2.0 implementation
In CMP 2.0, the details of constructing finder and select methods have been pushed
into the EJB 2.0 specification. Users should thoroughly inspect the specification for
details on implementing their database SQL. The Borland EJB Container is fullycompliant
with the EJB 2.0 specification and supports all of its features.
The implementation class for an entity bean using 2.0 container-managed persistence
is different from that of a bean using 1.1 container-managed persistence. The major
differences are as follows:
■
The class is declared as an abstract class.
■
There are no public declarations for the fields that are container-managed fields.
Instead, there are abstract get and set methods for container-managed fields.
These methods are abstract because the container provides their implementation.
For example, rather than declaring the fields balance and name, the CheckingAccount
class might include these get and set methods:
public abstract float getBalance();
public abstract void setBalance(float bal);
public abstract String getName();
public abstract void setName(String n);
■
Container-managed relationship fields are likewise not declared as instance
variables. The class instead provides abstract get and set methods for these fields,
and the container provides the implementation for these methods.
Table Mapping for CMP 2.0 is accomplished using the vendor-specific ejb-borland.xml
deployment descriptor. The descriptor is a companion to the ejb-jar.xml descriptor
described in the EJB 2.0 specification. Borland uses the XML tag as an
enclosure for table mapping data as needed. Then you use the and
its associated elements to specify particular information about the
entity bean's implementation. Use the DTD for syntax of the XML grammar.
Optimistic Concurrency Behavior
The container uses optimistic or pessimistic concurrency to control the behavior of
multiple transactions accessing the same data. BES has four optimistic concurrency
behaviors which are specified as Table Properties. These behaviors are:
■
SelectForUpdate
■
SelectForUpdateNoWAIT
■
UpdateAllFields
■
UpdateModifiedFields
130 BES Developer’s Guide