In Part 1, I provided an overview of available persistence mechanisms and their implementations, and introduced the Java Data Objects (JDO) approach to persistence. In Part 2, I conclude this series by looking more closely at the two competing JDO standards: the Sun Microsystems JDO and the Castor JDO. Both specifications provide unified, simple, and transparent persistence interfaces between Java application objects and data stores, and create an interesting alternative to entity beans.
Read the whole series on Java Data Objects:
The Sun JDO architecture provides application programmers a Java-centric view of persistent information. Sun's specification defines a standard API for data contained in various enterprise information systems, such as enterprise resource planning, mainframe transaction processing, and database systems. The architecture also follows the Java Connector Architecture (JCA), which defines a mechanism set for integrating an executive information system with an application server.
Figure 1 illustrates the Sun JDO architecture. The specification allows multiple JDO implementations -- possibly attached to different data stores -- to be plugged into an application server or used directly in a two-tier architecture. This approach lets application components access the underlying data stores using one consistent Java-centric data view. The JDO implementation provides the necessary mapping from Java objects into the underlying data store's special data types and relationships.
Before delving into the Sun JDO interfaces, I discuss the specification's three fundamental concepts:
- JDO instance
- First-class objects
- Second-class objects
A JDO instance is a Java class instance that implements the application functions and represents data in an enterprise data store. A JDO instance has an important limitation: its class must always implement the
PersistenceCapable interface (defined below), either explicitly by the class writer or implicitly by the enhancer's results.
Instances of the
PersistenceCapable classes that have JDO identity represent first-class objects; these objects are stored in a data store with their associated second-class objects (if any) and primitive values. First-class objects are unique: when a
PersistentManager (defined below) instantiates one into memory, that same
PersistentManager manages an instance representing that first-class object, though other
PersistentManagers might manage other instances of that same class.
PersistenceCapable instances, second-class objects differ from first-class objects in that they have no JDO identity of their own. Second-class objects notify their first-class objects of their modification; that modification reflects as a change to that first-class object. Second-class objects are stored in the data store as part of a first-class object only.
A good example of a first-class object: an object of class
Order usually has one or more instances of
OrderLine is a second-class object example.
Now, I'll detail the specification's major public interfaces:
PersistenceManager instances for application use. It also lets application developers configure the persistence layer behavior -- set transaction options, perform connection pool administration, and so on.
In a managed environment, the application uses JNDI (Java Naming and Directory Interface) lookup to retrieve an environment-named object, which is then cast to
PersistenceManager provides the primary interface for JDO-aware application components.
PersistenceManager administers persistent instances' lifecycles, and provides transaction and cache management. It also acts as the
Query interface's factory.
As mentioned before, any user domain class must implement the
PersistenceCapable interface. Note: There are no special methods for persistence; the
PersistenceCapable interface is really empty. You can implement this interface in one of three ways: through source code, an enhancer, or generation (tool-based).
Persistence operations usually occur within a transactional context. A one-to-one relationship exists between the
PersistenceManager and the
Transaction. In managed environments, the container provides actual transaction services, but the
Transaction interface provides methods for managing transaction options. In a standalone environment, the
Transaction implementation, provided by the JDO software vendor, must ensure a successful transaction -- commit or rollback.
Cache management and JDO instance lifecycle
Every JDO object (instance) goes through a series of state changes in its lifetime. The Sun JDO specification defines 10 JDO instance states. It requires seven transactional instance states (the remaining three are optional):
- Transient: When an instance is transient, the object lacks persistent identity. A transient instance changes its state to persistent-new (see below) in one of two ways: when passed as an argument to the
makePersistent()method or when referenced by a persistent instance's persistent field after that same instance commits.
- Persistent-new: An instance that is newly persistent in the current transaction. When an application component requests an instance to become persistent, that instance assumes the persistent-new state and receives a persistent identity.
- Persistent-dirty: An instance's state when one or more of its attributes have changed (within the current transaction), but not yet persisted.
- Hollow: A JDO instance that represents specific persistent data in the data store, but whose values are not in the JDO instance.
- Persistent-clean: A JDO instance that represents specific transactional persistent data in the data store, and whose values have not changed (within the current transaction).
- Persistent-deleted: JDO instances that represent specific persistent data in the data store and have been deleted in the current transaction.
- Persistent-new-deleted: JDO instances that represent new persistent instances deleted from the current transaction.
For more information regarding state transitions and JDO instances' persistence states, consult the Sun JDO spec.
An important part of every data manipulation subsystem is its query language. In the Sun JDO, the
PersistentManager instance is a factory for query instances, and queries execute in the context of the
Queries must conform to the Object Query Language (OQL) grammar, defined in the Object Management Group (OMG) 3.0 OQL Specification. The JDO OQL resembles SQL, except that it operates on Java classes and objects, not tables. A JDO OQL query has at least three elements:
- Class of result.
- JDO instances' candidate collection (usually extent).
- Query filter.
Optional query elements include the following:
- Parameter declaration(s) (follows formal Java syntax parameters).
- Value(s) to be bound.
- Ordering specification.
Query filters can be the following:
- Names of instance fields in the Java objects.
- Operators (a subset of Java operators, for example:
||, and so on). Of course not all Java operators make sense for data selection.
Please consult the JDO spec for more details regarding queries.
Below is a JDO query example:
// Example of a query Class target = Employee.class; Extent extent = pm.getExtent (target, false); String filter = "getEmpId () >= 1 && getEmpId () <= 10"; Query query = pm.newQuery (extent, filter); query.setClass (target); query.compile (); Collection result = (Collection) query.execute ();
Sun JDO: The good, the bad, and the ugly
Let's see how the Sun JDO compares to the ideal persistence layer presented in Part 1. As you might recall, the desirable traits I outlined include the following (Note: I reference only the most important features):
- Minimal intrusion
- Consistent, concise persistence APIs
The Sun JDO's major advantage: it provides a unified, standard persistence interface supported by multiple vendors delivering competing implementations. Another advantage is its transparency, or its data-store type independence.
The Sun JDO almost fulfils the simplicity trait, the only offending part being the OQL Query specification, which could be simplified. Sun also fails to offer minimal intrusion. In addition, Sun's JDO could do without the class enhancer or the obligatory
Some designed-by-committee traits are also evident: The API is not always consistent. Also, specification development progresses slowly; Sun JDO has been in draft form for several months now.
Overall, however, its standard persistence interface and transparency really set the tone for the Sun JDO. Its future looks quite bright.
Castor is an open source data-binding framework for Java. Castor's multiple projects target mapping between Java objects, XML documents, SQL tables, and LDAP (lightweight directory access protocol) directories. The Castor JDO project focuses on the Java object persistence-to-relational data stores. Despite its name and resemblance, Castor JDO is not compatible with Sun's spec. Note: Because Castor JDO concentrates on relational data stores exclusively, it does not support data-storage type transparency. Among the nonproprietary API (open source or multivendor) solutions available, the Castor JDO feature set is more than sufficient for most projects, and the price can't be beat.
Figure 2 illustrates the Castor JDO architecture. Unlike the Sun JDO, which can have multiple
PersistentManager instances in the JVM, Castor JDO maintains only one instance of its
PersistenceEngine in Castor. Castor does not require application objects to implement a special interface, but it provides a callback
Persistent interface, which can be implemented if the object wants to receive notification of Castor events -- such as object creation or deletion. This interface allows the creation of user-defined actions that an object will perform at various times during its lifecycle.
The Castor JDO's principal workhorse -- the one you will use most -- is the
Database module. It represents an active connection to the database, which you can use to perform transactional operations on the database.
Database's major methods:
public Object load( Class type, Object identity )
:Loads an object of a specified type and given identity into the cache. Once loaded, the object is marked as persistent.
public void create( Object object )
:Creates a new object in persistent storage. The object will persist only if the transaction commits.
public void remove( Object object )
:Removes the object from persistent storage -- only if the transaction commits.
public void update( Object object )
:Updates a data object queried/loaded/created in another transaction.
Database module also has the usual methods used for transaction demarcation:
public void begin()
public void commit()
public void rollback()
Castor JDO is designed to work in managed environments -- J2EE (Java 2 Platform, Enterprise Edition) application servers, for example -- and nonmanaged environments. The specification does not need or use a preprocessor (also known as a precompiler) or class enhancer (bytecodes modification) for data-binding and object persistence.
Castor JDO's central concept is the cache. Castor implements a data cache to reduce database access. The cache properly locks and isolates data objects from other transactions. Castor provides several alternative LRU (least recently used)-based caching strategies, such as instance eviction based on a cache's number of instances or an instance's age. Castor JDO also offers dirty-checking and deadlock detection. Caching modes are self-explanatory: none, count-limited, time-limited, and unlimited. The cache is write-through, as all changes to a transaction's objects persist at commit time without delay. Castor supports several locking modes, including shared, exclusive, database-locked, and read-only. In addition, Castor also supports long transactions, which allow objects to be read in one transaction, and modified and then committed in a second transaction, with built-in dirty-checking to prevent data that has changed since the initial transaction from being overwritten.
Castor distinguishes the relationships between two objects as either dependent or related, and maintains the lifecycle differently for the two relationship types. The mapping file explicitly defines the relationships. Castor JDO supports different relationship cardinalities, including one-to-one, one-to-many, and many-to-many. The framework distinguishes between related (i.e., association) and dependent (i.e., aggregation) relationships during an object's lifecycle, automatically creating and deleting dependent objects at appropriate times. It also supports multiple-column primary keys and a variety of unique key generators.