Release 2 (9.2)
Part Number A96656-01
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| Oracle9i Java Developer's Guide Release 2 (9.2) Part Number A96656-01 |
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This book provides a general overview on how to develop, load, and execute your Java applications in the database.
This chapter contains the following information:
This chapter covers the following three missions:
The following sections describe the additions to this release:
In this release, the system classes are upgraded from JDK 1.2 to JDK 1.3. The JDK 1.3 is compatible with JDK 1.2. Sun Microsystems publishes the list of incompatibilities between the two versions at the following Web site:
http://java.sun.com/products/jdk/1.2/compatibility.html
With the introduction of Oracle9i Application Server Containers for J2EE (OC4J)--a new, lighter-weight, easier-to-use, faster, and certified J2EE container--Oracle will desupport the Java 2 Enterprise Edition (J2EE) and CORBA stacks from the database, starting with Oracle9i database release 2. However, the database-embedded Java VM (Oracle JVM) will still be present and will continue to be enhanced to offer Java 2 Standard Edition (J2SE) features, Java stored procedures, JDBC, and SQLJ in the database.
As of Oracle9i database release 2 (9.2.0), Oracle will no longer support the following technologies in the database:
Customers will no longer be able to deploy servlets, JSP pages, EJBs, and CORBA objects in Oracle databases. Oracle9i database release 1 (9.0.1) will be the last database release to support the J2EE and CORBA stack. Oracle is encouraging customers to migrate existing J2EE applications running in the database to OC4J now.
New options have been added to the loadjava tool. Specifically, the -genmissing option enables you to do the following:
Determines what classes and methods are referred to by the classes that loadjava is asked to process. Any classes not found in the database or file arguments are called "missing" classes. This option generates dummy definitions for missing classes containing all referred to methods. It then loads the generated classes into the database. This processing happens before the class resolution.
Because detecting references from source is more difficult than detecting references from classfiles, and because source is not generally used for distributing libraries, loadjava will not attempt to do this processing for source files.
The schema in which the missing classes are loaded will be the one specified by the -user command-line option, even when referring classes are created in some other schema. The created classes will be flagged as such so that tools can recognize them. In particular, this is needed, so that the verifier can recognize generated classes.
See "loadjava" for more information.
With the removal of the EJB, JSP, and servlet APIs, there is no longer a need for most of the functionality of the sess_sh tool. Instead, a scaled-down version of this tool is made available in the ovjmjava tool. See "ojvmjava" for more information.
Oracle9i furnishes a debugging capability that is useful for developers who use the JDK's jdb debugger. Two interfaces are supported.
DebugProxy makes remote Java programs appear to be local. It lets any debugger that supports the sun.tools.debug.Agent protocol connect to a program as if the program were local. The proxy forwards requests to the server and returns results to the debugger.
For detailed instructions, see the Oracle9i Java Developer's Guide.
http://java.sun.com/j2se/1.3/docs/guide/jpda/, http://java.sun.com/j2se/1.4/docs/guide/jpda/.) The use of this interface is documented on OTN. The JDWP protocol supports many new features, including the ability to listen for connections (no more DebugProxy), change the values of variables while debugging, and evaluate arbitrary Java expressions, including method evaluation.
Oracle's JDeveloper provides a user-friendly integration with these debugging features. See the JDeveloper documentation for more information on how to debug your Java application through JDeveloper. Other independent IDE vendors will be able to integrate their own debuggers with Oracle9i.
Java has emerged as the object-oriented programming language of choice. It includes the following concepts:
The result is a language that is object-oriented and efficient for application-level programs.
This section covers some basic terminology of Java application development in the Oracle9i 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 Oracle9i Java Stored Procedures Developer's Guide for pointers to reference materials and for places to find Java-related information on the Internet.
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:
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).
Text description of the illustration classins.gif
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 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.
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.
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.
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.
Text description of the illustration inherita.gif
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.
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.
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.
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.
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--such as 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 that is optimized for the specific platform on which you execute your Java code. Figure 1-3 illustrates 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.
Text description of the illustration jvmlayer.gif
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.
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.
The Java language has key features that make it ideal for developing server applications. These features include:
SecurityManager, which, when combined with Oracle database security, determines who can invoke any Java methods.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 from tampering with the operating system that the Java code resides in. Some languages, such as C, can introduce security 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 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, Oracle9i efficiently schedules work for thousands of users. The Oracle9i JVM uses the facilities of the RDBMS server to concurrently schedule Java execution for thousands of users. Although Oracle9i supports Java language level threads required by the JLS and 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.
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. The Oracle9i JVM meets these challenges in the following ways:
The footprint of an executing Java program is affected by many factors:
From a scalability perspective, the key to supporting many concurrent clients is a minimum per-user session footprint. The Oracle9i JVM 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. The Oracle9i JVM uses three types of garbage collection algorithms to maintain the user's session memory:
Oracle9i JVM performance is enhanced by implementing a native compiler.
Java executes platform-independent bytecodes on top of a JVM, which in turn interacts with the specific hardware platform. Any time 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, which eliminates the interpreter step and improves performance.
The following describes two methods for native compilation:
Oracle9i uses Ahead-of-Time compilation to deliver its core Java class libraries: 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. You can use this native compilation technology with your own Java code. Refer to "Natively Compiled Code" for more information.
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 |
|---|---|---|
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Predictability |
The class loading operation places a severe penalty on first-time execution. A simple program can cause the Oracle9i JVM to load many core classes to support its needs. A programmer cannot easily predict or determine the number of classes loaded. |
The Oracle9i JVM loads classes dynamically, just as with any other Java virtual machine. The same one-time class loading speed hit is encountered. However, because the classes are loaded 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. |
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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. |
Oracle9i separates the upload and resolve operation from the class loading operation at runtime. You upload Java code you developed to the server using the |
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-side and server-side programmatic interfaces, tools to support Java development, and a Java virtual machine integrated with the Oracle9i database server. All these products are 100 percent compatible with Java standards.
In addition to the Oracle9i JVM, the Java programming environment consists of:
To help you decide which Java APIs to use, examine the following table:
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. You can access it from any Oracle Net client--OCI, PRO*, JDBC or SQLJ. The Oracle9i 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. Oracle9i provides a fully-compliant implementation of the Java programming language and JVM.
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 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 the following three distinct JDBC drivers.
For more information on JDBC, see "Utilizing SQLJ and JDBC for Querying the Database". Or for a complete detailed description, see the Oracle9i JDBC Developer's Guide and Reference.
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, and methods within the Oracle9i environment. 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 Oracle9i provides a complete Java environment, you cannot only compile SQLJ programs on a client for execution on the server, but you can compile them directly on the server. The adherence of Oracle9i 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 the Database" or for a complete description, see the Oracle9i SQLJ Developer's Guide and Reference.
The introduction of Java to the Oracle9i server allows you to use several Java Integrated Development Environments. The adherence of Oracle9i to Java compatibility and open Internet standards and protocols ensures that your 100% pure Java programs work when you deploy them on Oracle9i. Oracle delivers 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.
This guide is the starting point for Oracle9i Java developers. It outlines some of the unique features of Java programming with Oracle9i, including aspects of the Oracle9i JVM, explaining how to take advantage of these features in your Java programs.
Once you have mastered the basics of Java development within the Oracle9i database, you may 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 |
|---|---|---|
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JDBC |
Oracle9i 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. |
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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. |
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JPublisher |
JPublisher provides a simple and convenient tool to create Java programs that access existing Oracle relational database tables. |
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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 Oracle Net client--OCI, PRO*, JDBC or SQLJ. The Oracle9i 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. Oracle9i provides a fully compliant implementation of the Java programming language and JVM. |
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 Copyright © 2000, 2002 Oracle Corporation. All Rights Reserved. |
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