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Introduction to Java in Oracle8i

Java applications are supported within the Oracle8i database. Java applications can range from the simple standalone application to large, enterprise solutions using EJB or CORBA. All supported Java APIs cannot be covered within a single document; thus, several books describe the full support for Java within Oracle8i. This book provides a general overview for how you should program your Java applications when loading and running these applications in the database. Secondly, this book helps you choose which type of Java application you might develop, and direct you to the corresponding book for detailed information on that subject.

This chapter contains the following information:

• Introduces the Java language for Oracle database programmers. Oracle PL/SQL developers are accustomed to developing server-side applications that have tight integration with SQL data. You can develop Java server-side applications that take advantage of the scalability and performance of the Oracle database. If you are not familiar with Java, see "Overview of Java".

• Examines why you should consider using Java within an Oracle8i database. See "Why Use Java in Oracle8i?". In addition, a brief description is given of the Java application development interfaces supported within Oracle8i. These include SQLJ, JDBC, Java stored procedures, EJB, and CORBA. See "Oracle's Java Application Strategy".

• Provides a roadmap to the Oracle8i Java documentation. Several Java application types are supported within Oracle8i. Each of these types are described generally in this book, and more intimately in their own books. "Overview of Oracle8i Java Documentation" shows you which books cover each Java application type in detail.

Contents

• Overview of Java

• Why Use Java in Oracle8i?

• Oracle's Java Application Strategy

• Overview of Oracle8i Java Documentation

Overview of Java

Java, which was developed at Sun Microsystems, has emerged over the last several years as the object-oriented programming language of choice. It includes the following concepts:

• A Java virtual machine (JVM), which provides the fundamental basis for platform independence

• Automated storage management techniques, the most visible of which is garbage collection

• Language syntax that borrows from C and enforces strong typing

The result is a language easily learned by existing C programmers, but which remains truly object-oriented and efficient for application-level programs.

Java and Object-Oriented Programming Terminology

This section covers some basic terminology for discussing details of Java application development in the Oracle8i environment. The terms should be familiar to experienced Java programmers. A detailed discussion of object-oriented programming or of the Java language is beyond the scope of this book. Many texts, in addition to the complete language specification, are available at your bookstore and on the Internet. See "Java Information Resources" in the Preface, and "Suggested Reading" in the Oracle8i Java Stored Procedures Developer's Guide. for pointers to reference materials and for places to find Java-related information on the Internet.

Classes

All object-oriented programming languages support the concept of a class. As with a table definition, a class provides a template for objects that share common characteristics. Each class can contain the following:

• Attributes--static or instance variables that each object of a particular class possesses.

• Methods--you can invoke methods defined by the class or inherited by any classes extended from the class.

When you create an object from a class, you are creating an instance of that class. The instance contains the fields of an object, which are known as its data, or state. Figure 1-1 shows an example of an Employee class defined with two attributes: last name (lastName) and employee identifier (ID).

Figure 1-1 Classes and Instances

When you create an instance, the attributes store individual and private information relevant only to the employee. That is, the information contained within an employee instance is known only for that single employee. The example in Figure 1-1 shows two instances of employee--Smith and Jones. Each instance contains information relevant to the individual employee.

Attributes

Attributes within an instance are known as fields. Instance fields are analogous to the fields of a relational table row. The class defines the fields, as well as the type of each field. You can declare fields in Java to be static, public, private, protected, or default access.

• Public, private, protected, or default access fields are created within each instance.

• Static fields are like global variables in that the information is available to all instances of the employee class.

The language specification defines the rules of visibility of data for all fields. Rules of visibility define under what circumstances you can access the data in these fields.

Methods

The class also defines the methods you can invoke on an instance of that class. Methods are written in Java and define the behavior of an object. This bundling of state and behavior is the essence of encapsulation, which is a feature of all object-oriented programming languages. If you define an Employee class, declaring that each employee's id is a private field, other objects can access that private field only if a method returns the field. In this example, an object could retrieve the employee's identifier by invoking the Employee.getId() method.

In addition, with encapsulation, you can declare that the Employee.getId() method is private, or you can decide not to write an Employee.getId() method. Encapsulation helps you write programs that are reusable and not misused. Encapsulation makes public only those features of an object that are declared public; all other fields and methods are private. Private fields and methods can be used for internal object processing.

Class Hierarchy

Java defines classes within a large hierarchy of classes. At the top of the hierarchy is the Object class. All classes in Java inherit from the Object class at some level, as you walk up through the inheritance chain of superclasses. When we say Class B inherits from Class A, each instance of Class B contains all the fields defined in class B, as well as all the fields defined in Class A. For example, in Figure 1-2, the FullTimeEmployee class contains the id and lastName fields defined in the Employee class because it inherits from the Employee class. In addition, the FullTimeEmployee class adds another field, bonus, which is contained only within FullTimeEmployee.

You can invoke any method on an instance of Class B that was defined in either Class A or B. In our employee example, the FullTimeEmployee instance can invoke methods defined only within its own class, or methods defined within the Employee class.

Figure 1-2 Inheritance Hierarchy

Instances of Class B are substitutable for instances of Class A, which makes inheritance another powerful construct of object-oriented languages for improving code reuse. You can create new classes that define behavior and state where it makes sense in the hierarchy, yet make use of pre-existing functionality in class libraries.

Interfaces

Java supports only single inheritance; that is, each class has one and only one class from which it inherits. If you must inherit from more than one source, Java provides the equivalent of multiple inheritance, without the complications and confusion that usually accompany it, through interfaces. Interfaces are similar to classes; however, interfaces define method signatures, not implementations. The methods are implemented in classes declared to implement an interface. Multiple inheritance occurs when a single class simultaneously supports many interfaces.

Polymorphism

Assume in our Employee example that the different types of employees must be able to respond with their compensation to date. Compensation is computed differently for different kinds of employees.

• FullTimeEmployees are eligible for a bonus

• NonExemptEmployees get overtime pay

In traditional procedural languages, you would write a long switch statement, with the different possible cases defined.

switch: (employee.type) {
          case: Employee

            return employee.salaryToDate;

          case: FullTimeEmployee

            return employee.salaryToDate + employee.bonusToDate

         ...

If you add a new kind of Employee, you must update your switch statement. If you modify your data structure, you must modify all switch statements that use it. In an object-oriented language such as Java, you implement a method, compensationToDate(), for each subclass of Employee class that requires any special treatment beyond what is already defined in Employee class. For example, you could implement the compensationToDate() method of NonExemptEmployee, as follows:

private float compensationToDate() {

         return super.compensationToDate() + this.overtimeToDate();

}

You implement FullTimeEmployee's method, as follows:

private float compensationToDate() {

         return super.compensationToDate() + this.bonusToDate();

}

The common usage of the method name compensationToDate() allows you to invoke the identical method on different classes and receive different results, without knowing the type of employee you are using. You do not have to write a special method to handle FullTimeEmployees and PartTimeEmployees. This ability for the different objects to respond to the identical message in different ways is known as polymorphism.

In addition, you could create an entirely new class that does not inherit from Employee at all--Contractor--and implement a compensationToDate() method in it. A program that calculates total payroll to date would iterate over all people on payroll, regardless of whether they were full-time, part-time, or contractors, and add up the values returned from invoking the compensationToDate() method on each. You can safely make changes to the individual compensationToDate() methods with the knowledge that callers of the methods will work correctly. For example, you can safely add new fields to existing classes.

The Java Virtual Machine (JVM)

As with other high-level computer languages, your Java source compiles to low-level machine instructions. In Java, these instructions are known as bytecodes (because their size is uniformly one byte of storage). Most other languages, such as C, compile to machine-specific instructions; for example, instructions specific to an Intel or HP processor. Your Java source compiles to a standard, platform-independent set of bytecodes, which interacts with a Java virtual machine (JVM). The JVM is a separate program optimized for the specific platform on which you execute your Java code. Figure 1-3 shows how Java can maintain platform independence. Your Java source is compiled into bytecodes, which are platform independent. Each platform has installed a JVM that is specific to its operating system. The Java bytecodes from your source get interpreted through the JVM into appropriate platform dependent actions.

Figure 1-3 Java Component Structure

When you develop a Java program, you use predefined core class libraries written in the Java language. The Java core class libraries are logically divided into packages that provide commonly-used functionality, such as basic language support (java.lang), input/output (java.io), and network access (java.net). Together, the JVM and core class libraries provide a platform on which Java programmers can develop with the confidence that any hardware and operating system that supports Java will execute their program. This concept is what drives the "write once, run anywhere" idea of Java.

Figure 1-4 illustrates how Oracle's Java applications sit on top of the Java core class libraries, which in turn sit on top of the JVM. Because Oracle's Java support system is located within the database, the JVM interacts with the Oracle database libraries, instead of directly with the operating system.

Figure 1-4 JServer Component Structure

Sun Microsystems furnishes publicly available specifications for both the Java language and the JVM. The Java language specification (JLS) defines things such as syntax and semantics; the JVM specification defines the necessary low-level behavior for the "machine" that executes the bytecodes. In addition, Sun Microsystems provides a compatibility test suite for JVM implementors to determine if they have complied with the specifications. This test suite is known as the Java Compatibility Kit (JCK). Oracle's JVM implementation complies fully with JCK. Part of the overall Java strategy is that an openly specified standard, together with a simple way to verify compliance with that standard, allows vendors to offer uniform support for Java across all platforms.

Key Features of the Java Language

The Java language has key features that make it ideal for developing server applications. These features include:

• Simplicity--Java is a simpler language to master than most others you use in server applications because of its consistent enforcement of the object model. The large, standard set of class libraries brings powerful tools to Java developers on all platforms.

• Portability--Java is portable across platforms. It is possible to write platform-dependent code in Java, but it is also simple to write programs that move seamlessly across machines. Oracle server applications, which do not support graphical user interfaces directly on the platform that hosts them, also tend to avoid the few platform portability issues that Java has.

• Automatic Storage Management--The Java virtual machine automatically performs all memory allocation and deallocation during program execution. Java programmers can neither allocate nor free memory explicitly. Instead, they depend on the JVM to perform these bookkeeping operations, allocating memory as they create new objects and deallocating memory when the objects are no longer referenced. The latter operation is known as garbage collection.

• Strong Typing--Before you use a Java variable, you must declare the class of the object it will hold. Java's strong typing makes it possible to provide a reasonable and safe solution to inter-language calls in the case of Java and PL/SQL and to integrate Java and SQL.

• No Pointers--Although Java retains much of the flavor of C in its syntax, it does not support direct pointers or pointer manipulation. You pass all parameters, except primitive types, by reference (that is, object identity is preserved), not by value. Java does not provide C's low level, direct access to pointers, which eliminates memory corruption and leaks.

• Exception Handling--Java exceptions are objects. Java requires developers to declare which exceptions can be thrown by methods in any particular class.

• Flexible Namespace--Java defines classes and holds them within a hierarchical structure that mirrors the Internet's domain namespace. You can distribute Java applications and avoid name collisions. Java extensions such as the Java Naming and Directory Interface (JNDI) provide a framework for multiple name services to be federated. Java's namespace approach is flexible enough for Oracle to incorporate the concept of a schema for resolving class names, while fully complying with the language specification.

• Security--The design of Java bytecodes and the JVM allow for built-in mechanisms to verify Java binary code has not been tampered with. Oracle8i is installed with an instance of SecurityManager, which, combined with Oracle database security, secures who can invoke any Java methods.

• Standards for Connectivity to Relational Databases--JDBC and SQLJ enable Java code to access and manipulate data resident in relational databases. Oracle provides drivers that allow vendor-independent, portable Java code to access the relational database.

Why Use Java in Oracle8i?

The only reason that you are allowed to write and load Java applications within the database is because it is a safe language. Java has been developed to prevent anyone tampering with the operating system that the Java code resides in. Some languages, such as C, can introduce problems within the database. Java, because of its design, is a safe language to allow within the database.

Although the Java language presents many advantages to developers, providing an implementation of a JVM that supports Java server applications in a scalable manner is a challenge. This section discusses some of these challenges.

• Multithreading

• Automated Storage Management

• Footprint

• Performance

• Dynamic Class Loading

Multithreading

Multithreading support is often cited as one of the key scalability features of the Java language. Certainly, the Java language and class libraries make it simpler to write multithreaded applications in Java than many other languages, but it is still a daunting task in any language to write reliable, scalable multithreaded code.

As a database server, Oracle8i efficiently schedules work for thousands of users. The Oracle8i Aurora JVM uses the facilities of the RDBMS server to concurrently schedule Java execution for thousands of users. Although Oracle8i supports Java language level threads required by the Java language specification (JLS) and Java Compatibility Kit (JCK), using threads within the scope of the database will not increase your scalability. Using the embedded scalability of the database eliminates the need for writing multithreaded Java servers. You should use the database's facilities for scheduling users by writing single-threaded Java applications. The database will take care of the scheduling between each application; thus, you achieve scalability without having to manage threads. You can still write multithreaded Java applications, but multiple Java threads will not increase your server's performance.

One difficulty multithreading imposes on Java is the interaction of threads and automated storage management, or garbage collection. The garbage collector executing in a generic JVM has no knowledge of which Java language threads are executing or how the underlying operating system schedules them.

• Non-Oracle8i model--A single user maps to a single Java language level thread; the same single garbage collector manages all garbage from all users. Different techniques typically deal with allocation and collection of objects of varying lifetimes and sizes. The result in a heavily multithreaded application is, at best, dependent upon operating system support for native threads, which can be unreliable and limited in scalability. High levels of scalability for such implementations have not been convincingly demonstrated.

• Oracle8i JServer model--Even when thousands of users connect to the server and execute the same Java code, each user experiences it as if he is executing his own Java code on his own Java virtual machine. The responsibility of the Oracle8i JServer is to make use of operating system processes and threads, using the scalable approach of the Oracle RDBMS. As a result of this approach, the JVM's garbage collector is more reliable and efficient because it never collects garbage from more than one user at any time. Refer to "Threading in JServer" for more information on the thread model implementation in JServer.

Automated Storage Management

Garbage collection is a major feature of Java's automated storage management, eliminating the need for Java developers to allocate and free memory explicitly. Consequently, this eliminates a large source of memory leaks that commonly plague C and C++ programs. There is a price for such a benefit: garbage collection contributes to the overhead of program execution speed and footprint. Although many papers have been written qualifying and quantifying the trade-off, the overall cost is reasonable, considering the alternatives.

Garbage collection imposes a challenge to the JVM developer seeking to supply a highly scalable and fast Java platform. Aurora's JVM meets these challenges in the following ways:

• Aurora JVM uses the Oracle8i scheduling facilities, which can manage multiple users efficiently.

• Garbage collection is consistently performant for multiple users because garbage collection is focused on a single user within a single session. The Oracle8i Aurora JVM enjoys a huge advantage because the burden and complexity of the memory manager's job does not increase as the number of users increases. The memory manager performs the allocation and collection of objects within a single session--which typically translates to the activity of a single user.

• Aurora JVM uses different garbage collection techniques depending on the type of memory used. These techniques provide high efficiency and low overhead.

Footprint

The footprint of an executing Java program is affected by many factors:

• Size of the program itself--how many classes and methods and how much code they contain.

• Complexity of the program--the amount of core class libraries Aurora uses as the program executes, as opposed to the program itself.

• Amount of state Aurora uses--how many objects Aurora allocates, how large they are, and how many must be retained across calls.

• Ability of the garbage collector and memory manager to deal with the demands of the executing program, which is often non-deterministic. The speed with which objects are allocated and the way they are held on to by other objects influences the importance of this factor.

From a scalability perspective, the key to supporting many concurrent clients is a minimum per-user session footprint. Aurora keeps the per-user session footprint to a minimum by placing all read-only data for users, such as Java bytecodes, in shared memory. Appropriate garbage collection algorithms are applied against call and session memories to maintain a small footprint for the user's session. Aurora uses three types of garbage collection algorithms to maintain the user's session memory:

• Generational scavenging for short-lived objects

• Mark and lazy sweep collection for objects that exist for the life of a single call

• Copying collector for long-lived objects--objects that live across calls within a session

Performance

JServer performance is enhanced by implementing a native compiler.

How Native Compilers Improve Performance

Java executes platform-independent bytecodes on top of a JVM, which in turn deals with the specific hardware platform. Anytime you add levels within software, your performance is degraded. Because Java requires going through an intermediary to interpret platform-independent bytecodes, a degree of inefficiency exists for Java applications that does not exists within a platform-dependent language, such as C. To address this issue, several JVM suppliers create native compilers. Native compilers translate Java bytecodes into platform-dependent native code. This eliminates the interpreter step and improves performance. The following describes two methods for native compilation:

Compiler	Description
Just In Time (JIT) Compilation	JIT compilers quickly compile Java bytecodes to native (platform-specific) machine code during runtime. This does not produce an executable to be executed on the platform; instead, it provides platform-dependent code from Java bytecodes that is executed directly after it is translated. This should be used for Java code that is run frequently, which will be executed at speeds closer to languages such as C.
Static Compilation	Static compilation translates Java bytecodes to platform-independent C code before runtime. Then, a standard C compiler compiles the C code into an executable for the target platform. This approach is more suitable for Java applications that are modified infrequently. This approach takes advantage of the mature and efficient platform-specific compilation technology found in modern C compilers.

Oracle8i uses static compilation to deliver its core Java class libraries, the Aurora/ORB, and JDBC code in natively compiled form. It is applicable across all the platforms Oracle supports, whereas a JIT approach requires low-level, processor-dependent code to be written and maintained for each platform. In a future release, this native compilation technology will be available for use with your own Java code. Refer to "Natively Compiled Code" for more information.

Dynamic Class Loading

Another strong feature of Java is dynamic class loading. The class loader loads classes from the disk (and places them in the JVM-specific memory structures necessary for interpretation) only as they are used during program execution. The class loader locates the classes in the CLASSPATH and loads them during program execution. This approach, which works well for applets, poses the following problems in a server environment:

Problem	Description	Solution
Predictability	The class loading operation places a severe penalty on first-time execution. A simple program can cause Aurora to load many core classes to support its needs. A programmer cannot easily predict or determine the number of classes Aurora loads.	Aurora loads classes dynamically, just as with any other Java virtual machine. The same one-time class loading speed hit is encountered. However, because Aurora loads the classes into shared memory, no other users of those classes will cause the classes to load again--they will simply use the same pre-loaded classes.
Reliability	A benefit of dynamic class loading is that it supports program updating. For example, you would update classes on a server, and clients who download the program and load it dynamically see the update whenever they next use the program. Server programs tend to emphasize reliability. As a developer, you must know that every client executes a specific program configuration. You do not want clients to inadvertently load some classes that you did not intend them to load.	Oracle8i separates the upload and resolve operation from the class loading operation at runtime. You upload Java code you developed to the server using the loadjava utility. Instead of using CLASSPATH, you specify a resolver at installation time. The resolver is analogous to CLASSPATH, but allows you to specify the schemas in which the classes reside. This separation of resolution from class loading means you always know what program users execute. Refer to Appendix A, "Tools", for details on loadjava and resolvers.

Oracle's Java Application Strategy

One appeal of Java is its ubiquity and the growing number of programmers capable of developing applications using it. Oracle furnishes enterprise application developers with an end-to-end Java solution for creating, deploying, and managing Java applications. The total solution consists of client- and server-side programmatic interfaces, tools to support Java development, and a Java virtual machine integrated with the Oracle8i database server. All of these products are 100 percent compatible with Java standards.

In addition to the Aurora JVM, the Oracle8i's Java programming environment consists of:

• Java stored procedures as the Java equivalent and companion for PL/SQL. Java stored procedures are tightly integrated with PL/SQL. You can call a Java stored procedure from a PL/SQL package; you can call PL/SQL procedures from a Java stored procedure.

• SQL data can be accessed through JDBC and SQLJ programming interfaces.

• Distributed enterprise application development through an Object Request Broker (the Aurora/ORB) and Enterprise JavaBeans support.

• Tools and scripts used in assisting in development, class loading, and class management.

To enable your decision making for which Java APIs to use, examine the following table:

Type of functionality you need	Java API to use
To have a Java procedure invoked from SQL, such as a trigger.	Java Stored Procedures
To invoke a static, simple SQL statement from a known table with known column names from a Java object.	SQLJ
To invoke dynamic, complex SQL statements from a Java object.	JDBC
To create a multi-tier Java application.	CORBA or EJB

Java Stored Procedures

If you are a PL/SQL programmer exploring Java, you will be interested in Java stored procedures. A Java stored procedure is a program you write in Java to execute in the server, exactly as a PL/SQL stored procedure. You invoke it directly with products like SQL*Plus or indirectly with a trigger and can access it from any Net8 client--OCI, PRO*, JDBC or SQLJ. The Oracle8i Java Stored Procedures Developer's Guide. explains how to write stored procedures in Java, how to access them from PL/SQL, and how to access PL/SQL functionality from Java.

In addition, you can use Java to develop powerful programs independently of PL/SQL. Oracle8i provides a fully compliant implementation of the Java programming language and JVM.

PL/SQL Integration and Oracle RDBMS Functionality

You can invoke existing PL/SQL programs from Java and invoke Java programs from PL/SQL. This solution protects and leverages your existing investment while opening up the advantages and opportunities of Java-based Internet computing.

Oracle offers two different application programming interfaces (APIs) for Java developers to access SQL data--JDBC and SQLJ. Both APIs are available on client and server, so you can deploy the same code in either place.

• JDBC Drivers--Used to build client/server 2-tier applications.

• SQLJ - Embedded SQL in Java--Used to access static SQL. You must know the name of the columns.

JDBC Drivers

JDBC is a database access protocol that enables you to connect to a database and then prepare and execute SQL statements against the database. Core Java class libraries provide only one JDBC API. JDBC is designed, however, to allow vendors to supply drivers that offer the necessary specialization for a particular database. Oracle delivers JServer with the following three distinct JDBC drivers.

Driver	Description
JDBC Thin Driver	You can use the JDBC thin driver to write 100% pure Java applications and applets that access Oracle SQL data. The JDBC thin driver is especially well-suited to Web browser-based applications and applets because you can dynamically download it from a Web page just like any other Java applet.
JDBC Oracle Call Interface Driver	The JDBC Oracle Call Interface (OCI) driver accesses Oracle-specific native code (that is, non-Java) libraries on the client or middle tier, providing a richer set of functionality and some performance boost compared to the JDBC thin driver, at the cost of significantly larger size.
JDBC Server-side Internal Driver	Oracle8i uses the JServer server-side internal driver when Java code executes on the server. It allows Java applications executing in the server's Java virtual machine to access locally defined data (that is, on the same machine and in the same process) with JDBC. It provides a further performance boost because of its ability to use underlying Oracle RDBMS libraries directly, without the overhead of an intervening network connection between your Java code and SQL data. By supporting the same Java-SQL interface on the server, Oracle 8i does not require you to rework code when deploying it.

For more information on JDBC, see "Utilizing SQLJ and JDBC for Querying Database" or a complete detailed description within the Oracle8i JDBC Developer's Guide and Reference.

SQLJ - Embedded SQL in Java

JDBC provides a low-level API for accessing SQL data from Java. It introduces Java classes that mirror their SQL equivalents. Oracle has worked with other vendors, including IBM, Tandem, Sybase, and Sun Microsystems, to develop a standard way to embed SQL statements in Java programs--SQLJ. This work has resulted in a new standard (ANSI x.3.135.10-1998) for a simpler and more highly productive programming API than JDBC. A user writes applications to this higher-level API and then employs a preprocessor to translate the program to standard Java source with JDBC calls. At runtime, the program can communicate with multi-vendor databases using standard JDBC drivers. SQLJ provides a simple, but powerful, way to develop both client-side and middle-tier applications that access databases from Java. You can use it in stored procedures, triggers, methods within the JServer environment, and with EJB and CORBA. In addition, you can combine SQLJ programs with JDBC.

The SQLJ translator is a Java program that translates embedded SQL in Java source code to pure JDBC-based Java code. Because JServer provides a complete Java environment, you can not only compile SQLJ programs on a client for execution on the JServer, but you can compile them directly on the server. Oracle8i's adherence to Internet standards allows you to choose the development style that fits your needs.

For more information on SQLJ, see "Utilizing SQLJ and JDBC for Querying Database" or for a complete detailed description, see the SQLJ Developer's Guide and Reference.

Distributed Application Development

In addition to support for traditional RDBMS-stored procedures, JServer comes with a built-in CORBA 2.0 ORB and support for Enterprise JavaBeans (EJB). CORBA and EJB allow you to distribute Java components and application logic between client, middle-tier, and database server.

OMG CORBA ORB	The ORB allows programs you develop in any language to communicate directly with the Oracle8i database through Internet Inter-ORB Protocol (IIOP), the standard wire-protocol defined by the Object Management Group (OMG).
Enterprise JavaBeans (EJB)	For 100% pure Java applications, EJB is the standard framework for deploying component-based, secure, transactional applications on JServer.

Using EJB Components

"Java and Object-Oriented Programming Terminology" discusses encapsulation as a key element of object-oriented programming. Each object maintains its own private state and supports a set of behaviors, which you implement as methods. Java provides a formal way to define components, using JavaBeans. A JavaBean component is a reusable object or group of objects (more precisely, an object graph) that you can manipulate in a builder tool of some type. IDEs, such as JDeveloper, provide tools to build user interfaces that use JavaBeans and create JavaBean components. Each bean specifies its public interface and properties that can be manipulated. JavaBeans do not have to be visually-oriented components. Virtually any Java programming abstraction can potentially be represented and manipulated as a bean.

A large component library provides the basis for assembling an application from pre-built, pre-tested building blocks. However, beans are limited in their ability to build complex business applications involving transactional logic. To address this limitation, a group of companies, including Oracle, Sun Microsystems, and IBM, developed the Enterprise JavaBean (EJB) specification. EJB introduces a declarative mechanism for specifying how components deal with transactions and security. Refer to the Oracle8i Enterprise JavaBeans and CORBA Developer's Guide for detailed information about using EJB components in Oracle8i.

There are alternative component models to JavaBeans and Enterprise JavaBeans-- notably, Microsoft's COM and COM+ models. If you have existing Microsoft COM-oriented applications, they can interact with open Internet standards, such as JavaBeans and EJB, with bridge products available from different vendors.

Development Tools

The introduction of Java to the Oracle8i server allows you to use several Java Integrated Development Environments. JServer's adherence to Java compatibility and open Internet standards and protocols ensures that your 100% pure Java programs work when you deploy them on JServer. Oracle delivers JServer with many tools or utilities, all written in Java, that make development and deployment of Java server applications easier. Oracle's JDeveloper has many features designed specifically to make deployment of Java stored procedures and Enterprise JavaBeans easier.

Overview of Oracle8i Java Documentation

This guide is the starting point for Oracle8i Java developers. It outlines some of the unique features of Java programming with Oracle8i, including aspects of the Aurora JVM, explaining how to take advantage of these features in your Java programs.

Once you have mastered the basics of Java development within the Oracle8i database, you might need more information for the specific protocol you will use in implementing your Java application. The following list includes other books within the documentation set that will help you in your application development:

Protocol	Description	Book Title
JDBC	Oracle8i Java developers should become familiar with Oracle's Java Database Connectivity (JDBC) product because it provides the basis for accessing SQL data from Java programs, as well as Oracle-specific extensions to this Java standard. JDBC is an industry standard.	Oracle8i JDBC Developer's Guide and Reference
SQLJ	You may find it easier to develop Java programs that access SQL data using embedded SQL in Java (SQLJ). SQLJ uses a preprocessor, written in Java, to translate embedded SQL statements to standard JDBC-style programs. SQLJ is an industry standard.	Oracle8i SQLJ Developer's Guide and Reference
JPublisher	JPublisher provides a simple and convenient tool to create Java programs that access existing Oracle relational database tables.	Oracle8i JPublisher User's Guide
Java Stored Procedures	If you are a PL/SQL programmer exploring Java, you will be interested in Java stored procedures. A Java stored procedure is a program you write in Java to execute in the server, exactly as a PL/SQL stored procedure. You invoke it directly with products like SQL*Plus or indirectly with a trigger and can access it from any Net8 client--OCI, PRO*, JDBC or SQLJ. The Oracle8i Java Stored Procedures Developer's Guide explains how to write stored procedures in Java, how to access them from PL/SQL, and how to access PL/SQL functionality from Java. In addition, you can use Java to develop powerful programs independently of PL/SQL. Oracle8i provides a fully compliant implementation of the Java programming language and JVM.	Oracle8i Java Stored Procedures Developer's Guide.
EJB and CORBA	For distributed applications, you will utilize either the ORB or EJB technology. Oracle's open distributed object technology is included in its Object Request Broker (the Aurora/ORB) and Enterprise JavaBeans (EJB) functionality. The Aurora/ORB and EJB furnish powerful standards-based frameworks and tools to help you build scalable Java applications that provide seamless transactional access to Oracle data across your intranet or the Internet.	Oracle8i Enterprise JavaBeans and CORBA Developer's Guide

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