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> ## Resource Adapter Contracts

> The resource adapter mediates communication between the J2EE server and the EIS via contracts. The application contract defines the API through which a J2EE component such as an enterprise bean accesses the EIS. This API is the only view that the component has of the EIS. The system contracts link the resource adapter to important services that are managed by the J2EE server. The resource adapter itself and its system contracts are transparent to the J2EE component.

> ### Management Contracts

> The J2EE Connector architecture defines system contracts that enable resource adapter life cycle and thread management.

> #### Life-Cycle Management

> The Connector architecture specifies a life-cycle management contract that allows an application server to manage the life cycle of a resource adapter. This contract provides a mechanism for the application server to bootstrap a resource adapter instance during the instance's deployment or application server startup. It also provides a means for the application server to notify the resource adapter instance when it is undeployed or when an orderly shutdown of the application server takes place.

> #### Work Management Contract

> The Connector architecture work management contract ensures that resource adapters use threads in the proper, recommended manner. It also enables an application server to manage threads for resource adapters.

> Resource adapters that improperly use threads can create problems for the entire application server environment. For example, a resource adapter might create too many threads or it might not properly release threads it has created. Poor thread handling inhibits application server shutdown. It also impacts the application server's performance because creating and destroying threads are expensive operations.

> The work management contract establishes a means for the application server to pool and reuse threads, similar to pooling and reusing connections. By adhering to this contract, the resource adapter does not have to manage threads itself. Instead, the resource adapter has the application server create and provide needed threads. When the resource adapter is finished with a given thread, it returns the thread to the application server. The application server manages the thread: It can return the thread to a pool and reuse it later, or it can destroy the thread. Handling threads in this manner results in increased application server performance and more efficient use of resources.

> In addition to moving thread management to the application server, the Connector architecture provides a flexible model for a resource adapter that uses threads:

> - The requesting thread can choose to block--stop its own execution--until the work thread completes.
> - Or the requesting thread can block while it waits to get the thread. When the application server provides a work thread, the requesting thread and the work thread execute in parallel.
> - The resource adapter can opt to submit the work for the thread to a queue. The thread executes the work from the queue at some later point. The resource adapter continues its own execution from the point it submitted the work to the queue, no matter of when the thread executes it.

> With the latter two approaches, the resource adapter and the thread may execute simultaneously or independently from each other. For these approaches, the contract specifies a listener mechanism to notify the resource adapter that the thread has completed its operation. The resource adapter can also specify the execution context for the thread, and the work management contract controls the context in which the thread executes.

> ### Outbound Contracts

> The J2EE Connector architecture defines system-level contracts between an application server and an EIS that enable outbound connectivity to an EIS: connection management, transaction management, and security.

> The connection management contract supports connection pooling, a technique that enhances application performance and scalability. Connection pooling is transparent to the application, which simply obtains a connection to the EIS.

> The transaction management contract between the transaction manager and an EIS supports transactional access to EIS resource managers. This contract lets an application server use a transaction manager to manage transactions across multiple resource managers. This contract also supports transactions that are managed inside an EIS resource manager without the necessity of involving an external transaction manager. Because of the transaction management contract, a call to the EIS may be enclosed in an XA transaction (a transaction type defined by the distributed transaction processing specification created by The Open Group). XA transactions are global: they can contain calls to multiple EISs, databases, and enterprise bean business methods. Although often appropriate, XA transactions are not mandatory. Instead, an application can use local transactions, which are managed by the individual EIS, or it can use no transactions at all.

> The security management contract provides mechanisms for authentication, authorization, and secure communication between a J2EE server and an EIS to protect the information in the EIS.

> ### Inbound Contracts

> The J2EE Connector architecture defines system contracts between a J2EE server and an EIS that enable inbound connectivity from the EIS: pluggability contracts for message providers and contracts for importing transactions.

> #### Messaging Contracts

> To enable external systems to connect to a J2EE application server, the Connector architecture extends the capabilities of message-driven beans to handle messages from any message provider. That is, message-driven beans are no longer limited to handling JMS messages. Instead, EISs and message providers can plug any message provider, including their own custom or proprietary message providers, into a J2EE server.

> To provide this feature, a message provider or an EIS resource adapter implements the messaging contract, which details APIs for message handling and message delivery. A conforming resource adapter is assured of the ability to send messages from any provider to a message-driven bean, and it also can be plugged into a J2EE server in a standard manner.

> #### Transaction Inflow

> The Connector architecture supports importing transactions from an EIS to a J2EE server. The architecture specifies how to propagate the transaction context from the EIS. For example, a transaction can be started by the EIS, such as the Customer Information Control System (CICS). Within the same CICS transaction, a connection can be made through a resource adapter to an enterprise bean on the application server. The enterprise bean does its work under the CICS transaction context and commits within that transaction context.

> The Connector architecture also specifies how the container participates in transaction completion and how it handles crash recovery to ensure that data integrity is not lost.

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