The Java Enterprise APIs provide support for a number of the most commonly used distributed computing technologies and network services. These APIs are described in the sections that follow. The APIs are building blocks for distributed applications. At the end of the chapter, I'll present some enterprise computing scenarios that illustrate how these separate APIs can be used together to produce an enterprise application.
JDBC ( Java Database Connectivity) is the Java Enterprise API for working with relational database systems. JDBC allows a Java program to send SQL query and update statements to a database server and to retrieve and iterate through query results returned by the server. JDBC also allows you to get meta-information about the database and its tables from the database server.
The JDBC API is independent of vendor-specific APIs defined by particular database systems. The JDBC architecture relies upon a Driver class that hides the details of communicating with a database server. Each database server product requires a custom Driver implementation to allow Java programs to communicate with it. Major database vendors have made JDBC drivers available for their products. In addition, a "bridge" driver exists to enable Java programs to communicate with databases through existing ODBC drivers.
The JDBC API is found in the java.sql package, which was introduced in Java 1.1. Version 1.2 of the Java 2 platform adds a number of new classes to this package to support advanced database features. Java 1.2 also provides additional features in the javax.sql standard extension package. javax.sql includes classes for treating database query results as JavaBeans, for pooling database connections, and for obtaining database connection information from a name service. The extension package also supports scrollable result sets, batch updates, and the storage of Java objects in databases.
The JDBC API is simple and well-designed. Programmers who are familiar with SQL and database programming in general should find it very easy to work with databases in Java. See Chapter 2, "JDBC", for details on JDBC, and Chapter 8, "SQL Reference", for a quick reference to SQL.
Remote method invocation is a programming model that provides a high-level, generic approach to distributed computing. This model extends the object-oriented programming paradigm to distributed client-server programming; it allows a client to communicate with a server by invoking methods on remote objects that reside on the server. You invoke remote methods using the same syntax you would use to invoke methods of a normal local object. This model for distributed computing can be implemented in a number of ways. One of those ways is the Java Remote Method Invocation (RMI) API. RMI is implemented in the java.rmi package and its subpackages, which were introduced in Java 1.1 and have been enhanced for Version 1.2 of the Java 2 platform.
The Java RMI implementation is full-featured, but still simple and easy to use. It gains much of its simplicity by being built on top of a network-centric and dynamically extensible platform, of course. But it also gains simplicity by requiring both client and server to be implemented in Java. This requirement ensures that both client and server share a common set of data types and have access to the object serialization and deserialization features of the java.io package, for example. On the other hand, this means that RMI cannot be used with distributed objects written in languages other than Java, such as objects that exist on legacy servers. It also means that servers written using RMI can be used only by clients written in Java. In practice, RMI is an excellent distributed object solution for situations where it is clear that clients and servers will always be written in Java. Fortunately, there are many such situations.
One way to work around this restriction is to use native methods to create Java wrappers that interface directly with the legacy objects that are written in other languages.
The java.rmi package makes it easy to create networked, object-oriented programs. Programmers who have spent time writing networked applications using lower-level technologies are usually amazed by the power of RMI. By making RMI so easy, java.rmi points the way to future applications and systems that consist of loose groups of objects interacting with each other over a network. These objects may act both as clients, by calling methods of other objects, and as servers, by exposing their own methods to other objects. See Chapter 3, "Remote Method Invocation", for a tutorial on using RMI.
As we've just seen, RMI is a distributed object solution that works well when both client and server are written in Java. It does not work, however, in heterogenous environments where clients and servers may be written in arbitrary languages. For environments like these, the Java 2 platform includes a CORBA-based solution for remote method invocation on distributed objects.
CORBA (Common Object Request Broker Architecture) is a widely used standard defined by the Object Management Group (OMG). This standard is implemented as a core part of the Java 2 platform in the org.omg.CORBA package and its subpackages. The implementation includes an Object Request Broker (ORB) that a Java application can use to communicate, as both a client and a server, with other ORBs, and thus with other CORBA objects.
The interfaces to remote CORBA objects are described in a platform- and language-independent way with the Interface Description Language (IDL). Sun provides an IDL compiler (in "early access" release at the time of this writing) that translates an IDL description of a remote interface into the Java stub classes needed for implementing the IDL interface in Java or for connecting to a remote implementation of the interface from your Java code.
A number of Java implementations of the CORBA standard are available from various vendors. This book documents Sun's implementation, known as Java IDL. It is covered in detail in Chapter 4, "Java IDL". The syntax of the IDL language itself is summarized in Chapter 10, "IDL Reference".
JNDI ( Java Naming and Directory Interface) is the Java Enterprise API for working with networked naming and directory services. It allows Java programs to use name servers and directory servers to look up objects or data by name and search for objects or data according to a set of specified attribute values. JNDI is implemented in the javax.naming package and its subpackages as a standard extension to the Java 2 platform.
The JNDI API is not specific to any particular name or directory server protocol. Instead, it is a generic API that is general enough to work with any name or directory server. To support a particular protocol, you plug a service provider for that protocol into a JNDI installation. Service providers have been implemented for the most common protocols, such as NIS, LDAP, and Novell's NDS. Service providers have also been written to interact with the RMI and CORBA object registries. JNDI is covered in detail in Chapter 6, "JNDI".
Enterprise JavaBeans do for server-side enterprise programs what JavaBeans do for client-side GUIs. Enterprise JavaBeans (EJB) is a component model for units of business logic and business data. Thin client programming models that take business logic out of the client and put it on a server or in a middle tier have many advantages in enterprise applications. However, the task of writing this middleware has always been complicated by the fact that business logic must be mixed in with code for handling transactions, security, networking, and so on.
The EJB model separates high-level business logic from low-level housekeeping chores. A bean in the EJB model is an RMI remote object that implements business logic or represents business data. The difference between an enterprise bean and a run-of-the-mill RMI remote object is that EJB components run within an EJB container, which in turn runs within an EJB server. The container and server may provide features such as transaction management, resource pooling, lifecycle management, security, name services, distribution services, and so on. With all these services provided by the container and server, enterprise beans (and enterprise bean programmers) are free to focus purely on business logic. The particular set of services provided by an EJB server is implementation-dependent. The EJB specification is strongest in the areas of transaction management and resource pooling, so these are features that are expected in all EJB server implementations.
The EJB specification is a document that specifies the contracts to be maintained and conventions to be followed by EJB servers, containers, and beans. Writing EJB components is easy: you simply write code to implement your business logic, taking care to follow the rules and conventions imposed by the EJB model.
Unlike the other Java Enterprise APIs, EJB is not really an API; it is a framework for component-based enterprise computing. The key to understanding Enterprise JavaBeans lies in the interactions among beans, containers, and the EJB server. These interactions are described in detail in Chapter 7, "Enterprise JavaBeans". There is, of course, an API associated with the EJB application framework, in the form of the javax.ejb and javax.ejb.deployment packages. You'll find complete API quick-reference information for these packages in Part 3, "API Quick Reference".
A servlet is a piece of Java code that runs within a server to provide a service to a client. The name servlet is a takeoff on applet--a servlet is a server-side applet. The Java Servlet API provides a generic mechanism for extending the functionality of any kind of server that uses a protocol based on requests and responses.
The Servlet API differs from many other Java Enterprise APIs in that it is not a Java layer on top of an existing network service or protocol. Instead, servlets are a Java-specific enhancement to the world of enterprise computing. With the advent of the Internet and the World Wide Web, many enterprises are interested in taking advantage of web browsers--a universally available thin-client that can run on any desktop. Under this model, the web server becomes enterprise middleware and is responsible for running applications for clients. Servlets are a perfect fit here. The user makes a request to the web server, the web server invokes the appropriate servlet, and the servlet uses JNDI, JDBC, and other Java Enterprise APIs to fulfill the request, returning the result to the user, usually in the form of HTML-formatted text.
The Servlet API is a standard extension to the Java 2 platform, implemented in the javax.servlet and javax.servlet.http packages. The javax.servlet package defines classes that represent generic client requests and server responses, while the javax.servlet.http package provides specific support for the HTTP protocol, including classes for tracking multiple client requests that are all part of a single client session. See Chapter 5, "Java Servlets", for details on servlet programming.
JMS ( Java Message Service) is the Java Enterprise API for working with networked messaging services and for writing message-oriented middleware (fondly referred to as MOM).
The word "message" means different things in different contexts. In the context of JMS, a message is chunk of data that is sent from one system to another. The data serves as a kind of event notification and is almost always intended to be read by a computer program, not by a human. In a nondistributed system, an Event object notifies the program that some important event (such as the user clicking a mouse button) has occurred. In a distributed system, a message serves a similar purpose: it notifies some part of the system that an interesting event has occurred. So you can think of a networked message service as a distributed event notification system.
Like JNDI and JDBC, JMS is an API layered on top of existing, vendor-specific messaging services. In order to use JMS in your applications, you need to obtain a JMS provider implementation that supports your particular message server.
Although JMS is an important part of the Java Enterprise APIs, its use is not nearly as universal as APIs such as JDBC and JNDI, so this book does not contain a tutorial chapter on JMS. Chapter 21, "The javax.jms Package", does contain a complete API quick reference for the javax.jms package, however.
The JTA, or Java Transaction API, is a Java Enterprise API for managing distributed transactions. Distributed transactions are one of the things that make distributed systems more complicated than nondistributed programs. To understand distributed transactions, you must first understand simple, nondistributed transactions.
A transaction is a group of several operations that must behave atomically--as if they constituted a single, indivisible operation. Consider a banking application that allows a user to transfer money from a checking account to a savings account. If the two account balances are stored in a database, the application must perform two database updates to handle a transfer: it must subtract money from the checking account and add money to the savings account. These two operations must behave atomically. To see why, imagine what would happen if the database server crashed after money had been subtracted from the checking account but before it had been added to the savings account. The customer would lose money!
To make multiple operations atomic, we use transactions. In our banking example, we first begin a transaction, then perform the two database updates. While these updates are in progress, no other threads can see the updated account balances. If both updates complete successfully, we end the transaction by committing it. This makes the updated account balances available to any other clients of the database. On the other hand, if either of the database updates fails, we roll back the transaction, reverting the accounts to their original balances. Other clients are again given access to the database, and they see no changes in the account balances. The JDBC API supports transactions on databases. The database server is required to do some complex work to support transactions, but for the application programmer, the API is easy: simply begin a transaction, perform the desired operations, and then either commit or rollback the transaction.
Distributed transactions are, unfortunately, quite a bit more complex than the simple transactions just described. Imagine, for example, a program that transfers money from an account stored in one database to another account stored in a different database running on a different server. In this case, there are two different servers involved in the transaction, so the process of committing or rolling back the transaction must be externally coordinated. Distributed transactions are performed using a complex procedure known as the two-phase commit protocol; the details of the protocol are not important here. What is important is that we could write our account transfer code so that it implements the two-phase commit protocol itself, coordinating the entire distributed transaction with the two database servers. This would be tedious and error-prone, however. In practice, distributed transactions are coordinated by a specialized distributed transaction service.
This brings us, finally, to the JTA. The JTA is a Java API for working with transaction services. It defines a Java binding for the standard XA API for distributed transactions (XA is a standard defined by the Open Group). Using the JTA, we can write a program that communicates with a distributed transaction service and uses that service to coordinate a distributed transaction that involves a transfer of money between database records in two different databases.
Unfortunately, however, using the JTA in this way is still complex and error-prone. Modern enterprise applications are typically designed to run within some kind of application server, such as an Enterprise JavaBeans server. The server uses JTA to handle distributed transactions transparently for the application. Under this model, JTA becomes a low-level API used by server implementors, not by typical enterprise programmers. Therefore, this book doesn't include a tutorial chapter on JTA. It does, however, contain a complete API quick reference for the javax.transaction and javax.transactions.xa packages (see Chapter 28, "The javax.transaction Package" and Chapter 29, "The javax.transaction.xa Package").
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