EJB Overview: Constraint Types and Enforcement in JBoss

1. EJB Overview:Constraint Types and Enforcement in JBoss Elissa Newman Fluid Meeting 6/3/04

2. EJB 2.0 Overview • Bean Types • Entity (EB) – represent persistent state that is stored in a database or other storage. Usually represent “nouns”. Can be accessed concurrently. Long-lived. • Session (SB) – models a workflow, or set of actions that a user may want to perform (actions) • Stateless – • Return values from methods do not depend on instance state • Instance state may be recorded, but it will not be persisted • A single instance can service multiple clients (long-lived) • Stateful – • Maintains conversational state between method calls • Dedicated to a single client for life of an instance • Cannot be accessed concurrently (vendor-specific) • Message-Driven (MDB) – process asynchronous (remote) messages for multiple clients, one message at a time

3. Ways to Access Beans • Home Interfaces – Lifecycle methods for use by client (remote or local) • Bean Interfaces – Business methods available for use by client (remote or local) • Handle – a serializable reference to a remote EJB object • Can get back same type of SB or unique instance of EB

4. The Big Picture: Remote Invocation • The container provides varying levels of services for the bean, including persistence, transaction management, naming directory service, and more

5. Remote EJB Class Diagram

6. Local EJB Class Diagram

7. Implementation Options • Container (CMP) vs. Bean managed persistence (BMP) • Responsibility for writing data to database, maintaining relationships between entity beans, implementing finder methods • Container (CMT) vs. Bean managed transactions (BMT) • Responsibility for transaction support and defining transaction boundaries

8. Local vs. Remote Interfaces • Local – intra-JVM • Pros: saves overhead of remote calls, pass by reference • Cons: • restricts deployment configurations • pass by reference semantics can lead to data corruption • Remote – inter-JVM, uses RMI • Pros: Location transparency, pass by value, MetaData access • Cons: • huge overhead (even intra-JVM) • casting requires use of narrow() method • parameter types must be Serializable • need to handle RemoteExceptions

9. Lifecycle Methods • All: • ejbCreate(…) • ejbRemove() • For SBs and EBs: • ejbFindByPrimaryKey() • EB only: • ejbPostCreate() -- 1 per create() method • ejbFind*() methods • ejbSelect*() methods • ejbLoad(), ejbStore() – for BMP only • ejbSet/UnsetEntityContext() • ejbHome() • Stateful SB only: • setSessionContext() • ejbActivate() • ejbPassivate() • MDB only: • onMessage()

10. Lifecycle of an Entity Bean Instance

11. Lifecycle of a Stateless Session Bean Instance

12. Lifecycle ofa Stateful Session BeanInstance

13. Lifecycle of a MDB instance

14. Major EJB Constraint Types • A _____ must inherit from _____ interface/class or one of its children • Certain lifecycle methods (per bean type) must exist and have (or not have) certain modifiers, parameters, return types • You shouldn’t use this <set> of methods for this <purpose>, instead use <other set> • Never call ____ from _____ • When ___ is called, do something

15. Some “Interesting” Constraints and Guidance • Remote Interfaces (and related) • Having both a remote and local interface is allowed, but not recommended • Question: When/why would this happen? • Should use EJBObject.isIdentical() instead of Object.equals() • Because remote objects can have additional networking state, but may still represent the same object • Remote interfaces may access persistence fields in a Bean class, but not relationship fields (references to other Beans) • BMP should use Collection type, although they are allowed to use Enumeration type • Make more compatible with CMP, which only uses Collection

16. “Interesting” (cont.) • Primary Keys (and related) • PKs only set once • In CMP, by container during creation • In BMP, in ejbCreate() method • ejbCreate() has a return type same as the Primary Key type (specified in XML file) • Always returns null for CMP • Must return the primary key for BMP • For BMP, should use EntityContext object to get the primary key, even if it is just a field in the EB • Data in bean could be stale • Rarely used outside of ejbLoad() and ejbStore() methods • PK classes must implement equals() and hashCode()

17. “Interesting” (cont.) • Inter-EB Relationships (and related) • If an entity bean is the target of a container-managed relationship, then it must have local interfaces and be collocated with the related bean(s) • Inter-EB relationships are not initialized until the ejbPostCreate() method • Primary key is required for use as a foreign key in other EBs, and may not be initialized until ejbCreate() has been called • Relationship fields can only access local interface of a bean • When creating related entities, need to preserve referential integrity by assigning relationship fields after new object creations • Otherwise, unconnected entries in database • Relationship fields with >1 multiplicity may never return null • Instead, they return an empty Collection

18. “Interesting” (cont.) • Stateful Session Beans • Must be activated and passivated to preserve conversational state of instance • On calls to ejbPassivate(), Bean should • close any open resources • Set all non-transient, non-serializable fields to null

19. “Interesting” (cont.) • Stateless Session Beans • Stateless SBs can only retain information between method calls via SessionContext and JNDI ENC • Also can have instance variables that won’t be persisted if instance is removed • Stateless SBs don’t ever have their create() method called, so no local object created, only one for the client

20. “Interesting” (cont.) • Etc. • In BMP, use only ejbLoad() and ejbStore() for synchronizing state with database • Do not use ejbActivate(),ejbPassivate(),setEntityContext(), or unsetEntityContext() for this purpose • If a system exception is thrown by a bean method, the ejbRemove() method is not invoked • Constructors should never be defined in an EB class • You should never throw RemoteExceptions from callback methods

21. Ensuring Constraints in JBoss • Package org.jboss.verifier provides implementation of an EJB checker • Verifier is (optionally) run during the Deployment process • Deployer may decide whether or not deployment should be terminated if an error is found by the verifier in any of the EJBs in a Jar file

22. Ensuring Constraints in JBoss (2) • strategy.EJBVerifier20 is most relevant class • Uses class loader to load bean class • Checks for appropriate restrictions based on if it’s a Session, Entity, or MD Bean • Checks subclassing of correct interfaces/classes by loading “to” class and calling Class.isAssignableFrom() • Checks that method names/arguments match expected • E.g., between local/remote interfaces and bean • Checks for correct return types • Checks for correct Exceptions thrown (or not) • Checks for no constructor

23. Future Work • Identify and describe all “interesting” EJB constraints, explicit and implicit, positive and negative • Find in JBoss implementation where these are checked, or what will happen if one of these constraints is violated • Using queries to identify possible constraint violations