JMS is an API for using networked messaging services. A messaging system accepts messages from “producer” clients and delivers them to “consumer” clients. Data sent in a message is often intended as a sort of event notification (for example, an e-mail-handling process may need to be notified when a request is queued). Another common use for messaging (thus, JMS) is for interfacing with remote legacy applications. It can be complex and sometimes risky to use Remote Procedure Call (RPC) or a Java variant such as Remote Method Invocation (RMI) to directly invoke remote applications while a messaging solution can provide an easier and more reliable interconnection. In short, why write remote procedure calls when you have access to an API specifically designed for sending messages across a network from one object to another? JMS calls frequently rely on the Java Naming and Directory Interface (JNDI) to locate message recipients. JNDI is discussed later in this chapter. Java Transaction API Java Transaction API (JTA) provides developers with a mechanism for handling the commit and the rollback of transactions as well as ensuring the ACID (Atomicity, Consistency, Isolation, and Durability) properties of a transaction. JTA is used for managing distributed transactions (e.g., updates to multiple databases that must be handled in a single transaction). JTA is a low-level API and associated coding is complex and error-prone not in the spirit of J2EE! Fortunately, EJB containers or application servers generally provide support for distributed transactions using JTA. For this reason, the EJB developer is able to gain the benefit of distributed transactions, leaving the complex implementation details to the provider of the EJB container. Now, that s more in the J2EE spirit! Java Transaction Services The Java Transaction Service (JTS) provides developers with a means of communicating with transaction monitors and other transaction-oriented resources. Of course, JTS provides high-level support for JTA as well as other transaction services. The Java Transaction Service plays the role of an intermediary for all the constituent components of the EJB architecture. In JTS terminology, the director is called the transaction manager. The participants in the transaction that implement transaction-protected resources such as relational databases are called resource managers. When an application begins a transaction, it creates a transaction object that represents the transaction. You would use JNDI (Java Naming and Directory Interface) to access this transaction object. The application invokes the resource managers to perform the work of the transaction. As the transaction progresses, the transaction manager keeps track of each of the resource managers enlisted in the transaction. Often, JTS assists in managing the activities involved in a two-phase commit. JavaMail The JavaMail API offers a standard Java extension API to talk to all your favorite standard Internet mail protocols. The API provides a platform-independent and protocol-independent framework to build Java technology based mail and messaging applications. Put differently, JavaMail represents a standardized, extensible platform for communicating, presenting, and manipulating all current and future Multipurpose Internet Mail Extension (MIME) types. The JavaMail API is implemented as a Java platform standard extension. Say goodbye to writing your own classes for talking to mail protocols! Say goodbye to learning yet another unique third-party or in-house class library for dealing with e-mail or newsgroups! JavaMail was designed to communicate with popular protocols and MIME types.
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