1 Introduction to Java in Oracle Database

Java and Object-Oriented Programming Terminology

The following terms are common in Java application development in Oracle Database environment:

• Classes

• Objects

• Interfaces

• Encapsulation

• Inheritance

• Polymorphism

Objects

A Java object is an instance of a class and is analogous to a relational table row. Objects are collections of data values, the individual elements of which are described by the non-static field definitions of the class.

Figure 1-1 shows an example of an Employee class defined with two fields, id, which is the employee identifier, and lastName, which is the last name of the employee, and the getId() and setId(String anId) methods. The id field and the getId() method are private, and the lastName field and the setId(String anId) method are public.

Figure 1-1 Classes and Instances

Description of "Figure 1-1 Classes and Instances"

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

switch: (employee.type)
{
  case: Employee
        return employee.salaryToDate; 
  case: FullTimeEmployee
        return employee.salaryToDate + employee.bonusToDate
  ...
}

private float compensationToDate()
{
  return (super.compensationToDate() + this.overtimeToDate()); 
}

private float compensationToDate()
{
  return (super.compensationToDate() + this.bonusToDate());
}

Key Features of the Java Language

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

• Simplicity

  Java is simpler than most other languages that are used to create 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, and it is also simple to write programs that move seamlessly across systems.

  See Also:

  "Java Virtual Machine"

• Automatic storage management

  A JVM automatically performs all memory allocation and deallocation while the program is running. Java programmers cannot explicitly allocate memory for new objects or free memory for objects that are no longer referenced. Instead, they depend on a JVM to perform these operations. The process of freeing memory is known as garbage collection.

• Strong typing

  Before you use a field, you must declare the type of the field. Strong typing in Java makes it possible to provide a reasonable and safe solution to interlanguage calls between Java and PL/SQL applications, and to integrate Java and SQL calls within the same application.

• No pointers

  Although Java is quite similar to C in its syntax, it does not support direct pointers or pointer manipulation. You pass all parameters, except primitive types, by reference and not by value. As a result, the object identity is preserved. Java does not provide low level, direct access to pointers, thereby eliminating any possibility of 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 places them within a hierarchical structure that mirrors the domain namespace of the Internet. 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. The namespace approach of Java is flexible enough for Oracle to incorporate the concept of a schema for resolving class names in full compliance with the JLS.

• Security

  The design of Java bytecodes and JVM specification allow for built-in mechanisms to verify the security of Java binary code. Oracle Database is installed with an instance of Security Manager that, when combined with Oracle Database security, determines who can call any Java methods.

• Standards for connectivity to relational databases

  Java Database Connectivity (JDBC) and SQLJ enable Java code to access and manipulate data in relational databases. Oracle provides drivers that allow vendor-independent, portable Java code to access the relational database.

Java Virtual Machine

As with other high-level computer languages, the Java source compiles to low-level computer instructions. In Java, these instructions are known as bytecodes, because each instruction has a uniform size of one byte. Most other languages, such as C, compile to computer-specific instructions, such as instructions specific to an Intel or HP processor.

When compiled, the Java code gets converted to a standard, platform-independent set of bytecodes, which are executed by a Java Virtual Machine (JVM). A JVM is a separate program that is optimized for the specific platform on which you run your Java code.

Figure 1-2 illustrates how Java can maintain platform independence. Each platform has a JVM installed that is specific to the operating system. The Java bytecodes get interpreted through the JVM into the appropriate platform dependent actions.

Figure 1-2 Java Component Structure

Description of "Figure 1-2 Java Component Structure"

When you develop a Java application, 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. Basic language support is provided by the java.lang package, I/O support is provided by the java.io package, and network access is provided by the java.net package. Together, a JVM and the core class libraries provide a platform on which Java programmers can develop applications, which will run successfully on any operating system that supports Java. This concept is what drives the "write once, run anywhere" idea of Java.

Figure 1-3 illustrates how Oracle Java applications reside on top of the Java core class libraries, which reside on top of the JVM. Because the Oracle Java support system is located within the database, the JVM interacts with Oracle Database libraries, instead of directly interacting with the operating system.

Figure 1-3 Oracle Database Java Component Structure

Description of "Figure 1-3 Oracle Database Java Component Structure"

Sun Microsystems provides publicly available specifications for both the Java language and the JVM. The JLS defines the syntax and semantics, and JVM specification defines the necessary low-level actions for the system that runs the application. 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 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.

Java 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.

Figure 1-4 illustrates a generic Java class hierarchy. 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 call any method on an instance of Class B that was defined in either Class A or Class B. In the example, the FullTimeEmployee instance can call methods defined only in the FullTimeEmployee class and methods defined in the Employee class.

Figure 1-4 Class Hierarchy

Description of "Figure 1-4 Class Hierarchy"

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

Java and RDBMS: A Robust Combination

Oracle Database provides Java applications with a dynamic data-processing engine that supports complex queries and different views of the same data. All client requests are assembled as data queries for immediate processing, and query results are generated dynamically.

The combination of Java and Oracle Database helps you to create component-based, network-centric applications that can be easily updated as business needs change. In addition, you can move applications and data stores off the desktop and onto intelligent networks and network-centric servers. More important, you can access those applications and data stores from any client device.

Figure 1-5 shows a traditional two-tier, client/server configuration in which clients call Java stored procedures the same way they call PL/SQL stored procedures. The figure also shows how Oracle Net Services Connection Manager can combine many network connections into a single database connection. This enables Oracle Database to support a large number of concurrent users.

Figure 1-5 Two-Tier Client/Server Configuration

Description of "Figure 1-5 Two-Tier Client/Server Configuration"

Multithreading

Multithreading is one of the key scalability features of the Java language. 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.

Oracle Database server efficiently schedules work for thousands of users. The Oracle JVM takes advantage of the session architecture of Oracle database to concurrently run Java applications for hundreds to thousands of users. Although Oracle Database supports Java language-level threads required by the JLS and JCK, scalability will not increase by using threads within the scope of the database. By using the embedded scalability of the database, the need for writing multithreaded Java servers is eliminated.

You should use the facilities of Oracle Database for scheduling users by writing single-threaded Java applications. The database can schedule processes between each application, and thus, you achieve scalability without having to manage threads. You can still write multithreaded Java applications, but multiple Java threads will not increase the performance of the server.

One complication multithreading creates is the interaction of threads and automated storage management or garbage collection. The garbage collector running in a generic JVM has no knowledge of which Java language threads are running or how the underlying operating system schedules them. The difference between a non-Oracle Database model and an Oracle JVM model is as follows:

• Non-Oracle Database model

  A single user maps to a single Java thread and a 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.

• Oracle JVM model

  Even when thousands of users connect to the server and run the same Java code, each user experiences it as if he or she is running his or her own Java code on his or her own JVM. The responsibility of an Oracle JVM is to make use of operating system processes and threads and the scalable approach of Oracle Database. As a result of this approach, the garbage collector of the Oracle JVM is more reliable and efficient because it never collects garbage from more than one user at any time.

  See Also:

  "Threading in Oracle Database"

Memory Spaces Management

Garbage collection is a major function of the automated storage management feature of Java, eliminating the need for Java developers to allocate and free memory explicitly. Consequently, this eliminates a large source of memory leaks that are commonly found in C and C++ programs. However, garbage collection contributes to the overhead of program execution speed and footprint.

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

• The Oracle JVM uses Oracle Database scheduling facilities, which can manage multiple users efficiently.

• Garbage collection is performed consistently for multiple users, because garbage collection is focused on a single user within a single session. The Oracle JVM has an advantage, because the burden and complexity of the job of the memory manager 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.

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

The two types of memory space are call space and session space.

Figure 1-6 illustrates the different actions performed by the garbage collector.

Figure 1-6 Garbage Collection

Description of "Figure 1-6 Garbage Collection"

Garbage collection algorithms within an Oracle JVM adhere to the following rules:

1. New objects are created within a new call space.

2. Scavenging occurs at a set interval. Some programmers create objects frequently for only a short duration. These types of objects are created and garbage-collected quickly within the new call space. This is known as scavenging.

3. Any objects that have survived several iterations of scavenging are considered to be objects that can exist for a while. These objects are moved out of new call space into old call space. During the move, they are also compacted. Old call space is scavenged or garbage collected less often and, therefore, provides better performance.

4. At the end of the call, any objects that are to exist beyond the call are moved into session space.

Figure 1-6 illustrates the steps listed in the preceding text. This approach applies sophisticated allocation and collection schemes tuned to the types and lifetimes of objects. For example, new objects are allocated in fast and inexpensive call memory, designed for quick allocation and access. Objects held in Java static fields are migrated to the more precious and expensive session space.

Footprint

The footprint of a running Java program is affected by many factors:

• Size of the program

  The size of the program depends on the number of classes and methods and how much code they contain.

• Complexity of the program

  The complexity of the program depends on the number of core class libraries that the Oracle JVM uses as the program runs, as opposed to the program itself.

• Amount of space the JVM uses

  The amount of space the JVM uses depends on the number of objects the JVM allocates, how large these objects are, and how many objects must be retained across calls.

• Ability of the garbage collector and memory manager to deal with the demands of the program running

  This can not be determined often. 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 multiple clients concurrently is a minimum per-user session footprint. The Oracle 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 Oracle JVM uses the following 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, that is, objects that live across calls within a session

Performance of an Oracle JVM

The performance of an Oracle JVM is enhanced by the embedding of an innovative Just-In-Time compiler similar to HotSpot on Sun's JVM. The platform-independent Java bytecodes run on top of a JVM, and the JVM interacts with the specific hardware platform. Any time you add levels within software, the performance is degraded. Because Java requires going through an intermediary to interpret the bytecodes, a degree of inefficiency exists for Java applications as compared to applications developed using 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 table describes two methods for native compilation:

Oracle Database uses Just-In-Time (JIT) compilation to deliver its core Java class libraries, such as JDBC code, in natively compiled form. The JIT compiler is enabled without the support of any plug-ins and it is applicable across all the platforms that Oracle supports.

Figure 1-7 illustrates how natively compiled code runs up to 10 times faster than interpreted code. As a result, the more native code your program uses, the faster it runs.

Figure 1-7 Interpreter versus Accelerator

Description of "Figure 1-7 Interpreter versus Accelerator"

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. The class loader locates the classes in CLASSPATH and loads them only when they are used while the program is running. This approach, which works well for applets, poses the following problems in a server environment:

Process Area

In a standard Java environment, you run a Java application through the interpreter by issuing the following command on the command line, where classname is the name of the class that you want the JVM to interpret first:

java classname

When using the Oracle JVM, you must load the application into the database, publish the interface, and then run the application within a database session. The database session is the environment in which the Oracle JVM runs and as such is the analog of the operating system process in which a standard client JVM runs.

Java session initialization, duration and entrypoints

Standard client-based Java applications declare a single, top-level method, public static void main(String args[]). This method is executed once and the instantiation of the Java Virtual Machine lasts for the duration of that call. But, Oracle Java applications are not restricted to a single top-level main entrypoint, and the duration of the Oracle JVM instantiation is not determined by a single call and the exit of the call from this entrypoint. Each client begins a session, calls its server-side logic modules through top-level entry points, and eventually ends the session. The same JVM instance remains in place for the entire duration of the session, so data state such as static variable values can be used across multiple calls to multiple top-level entry points.

Class definitions that have been loaded and published in the database are generally available to all sessions in that database. The JVM instance in a given session and the Java data objects and global field values in that JVM instance are private to the session. This data is present for the duration of the session and may be used by multiple calls within the lifetime of that session. But, neither this data is visible to other sessions nor the data can be shared in any way with other sessions. This is analogous to how in a standard client Java application separate invocations of the main method share the same class definitions, but the data created and used during those invocations are separate.

The GUI

A server cannot provide GUIs, but it can provide the logic that drives them. The Oracle JVM supports only the headless mode of the Java Abstract Window Toolkit (AWT). All Java AWT classes are available within the server environment and your programs can use the Java AWT functionality, as long as they do not attempt to materialize a GUI on the server.

The IDE

The Oracle JVM is oriented to Java application deployment, and not development. You can write and test applications on any preferred integrated development environment (IDE), such as Oracle JDeveloper, and then deploy them within the database for the clients to access and run them.

The binary compatibility of Java enables you to work on any IDE and then upload the Java class files to the server. You need not move your Java source files to the database. Instead, you can use powerful client-side IDEs to maintain Java applications that are deployed on the server.

Internet Protocol Version 6 Support

This release of Oracle JVM supports Internet Protocol Version 6 (IPv6) addresses in the URL and computer names of the Java code in the database, which resolve to IPv6 addresses. IPv6 is a new Network layer protocol designed by the Internet Engineering Task Force (IETF) to replace the current version of Internet Protocol, Internet Protocol Version 4 (IPv4). The primary benefit of IPv6 is a large address space, derived from the use of 128-bit addresses. IPv6 also improves upon IPv4 in areas such as routing, network autoconfiguration, security, quality of service, and so on.

The following system properties enable you to configure IPv6 preferences:

java.net.preferIPv4Stack

If IPv6 is available on the operating system, then the underlying native socket will be an IPv6 socket. This enables Java applications to connect to, and accept connections from both IPv4 and IPv6 hosts. If you have an application that has a preference to use only IPv4 sockets, then you can set this property to true. If you set the property to true, then it implies that the application will not be able to communicate with IPv6 hosts.

java.net.preferIPv6Addresses

Even if IPv6 is available on the operating system, then for backward compatibility reasons, the addresses are mapped to IPv4. For example, applications that depend on access to only an IPv4 service, benefit from this type of mapping. If you want to change the preferences to use IPv6 addresses over IPv4 addresses, then you can set the java.net.preferIPv6Addresses property to true. This allows applications to be tested and deployed in environments, where the application is expected to connect to IPv6 services.

Library Manager

To store Java classes in Oracle Database, you use the loadjava command-line tool, which uses the SQL CREATE JAVA statements to do its work. When called by the CREATE JAVA {SOURCE | CLASS | RESOURCE} statement, the library manager loads Java source, class, or resource files into the database. These Java schema objects are not accessed directly, and only an Oracle JVM uses them.

Compiler

The Oracle JVM includes a standard Java compiler. When the CREATE JAVA SOURCE statement is run, it translates Java source files into architecture-neutral, one-byte instructions known as bytecodes. Each bytecode consists of an opcode followed by its operands. The resulting Java class files, which conform fully to the Java standard, are submitted to the interpreter at run time.

Interpreter

To run Java programs, the Oracle JVM includes a standard Java2 bytecode interpreter. The interpreter and the associated Java run-time system run standard Java class files. The run-time system supports native methods and call-in and call-out from the host environment.

Class Loader

In response to requests from the run-time system, the Java class loader locates, loads, and initializes Java classes stored in the database. The class loader reads the class and generates the data structures needed to run it. Immutable data and metadata are loaded into initialize-once shared memory. As a result, less memory is required for each session. The class loader attempts to resolve external references when necessary. In addition, if the source files are available, then the class loader calls the Java compiler automatically when Java class files must be recompiled.

Verifier

Java class files are fully portable and conform to a well-defined format. The verifier prevents the inadvertent use of spoofed Java class files, which might alter program flow or violate access restrictions. Oracle security and Java security work with the verifier to protect your applications and data.

Server-Side JDBC Internal Driver

JDBC is a standard and defines a set of Java classes providing vendor-independent access to relational data. Specified by Sun Microsystems and modeled after ODBC and the X/Open SQL Call Level Interface (CLI), the JDBC classes provide standard features, such as simultaneous connections to several databases, transaction management, simple queries, calls to stored procedures, and streaming access to LONG column data.

Using low-level entry points, a specially tuned JDBC driver runs directly inside Oracle Database, providing fast access to Oracle data from Java stored procedures. The server-side JDBC internal driver complies fully with the standard JDBC specification. Tightly integrated with the database, the JDBC driver supports Oracle-specific data types, globalization character sets, and stored procedures. In addition, the client-side and server-side JDBC APIs are the same, which makes it easy to partition applications.

Server-Side SQLJ Translator

SQLJ enables you to embed SQL statements in Java programs. It is more concise than JDBC and more responsive to static analysis and type checking. The SQLJ preprocessor, which itself is a Java program, takes as input a Java source file in which SQLJ clauses are embedded. Then, it translates the SQLJ clauses into Java class definitions that implement the specified SQL statements. The Java type system ensures that objects of those classes are called with the correct arguments.

A highly optimized SQLJ translator runs directly inside the database, where it provides run-time access to Oracle data using the server-side internal JDBC driver. SQLJ forms can include queries, data manipulation language (DML) statements, data definition language (DDL) statements, transaction control statements, and calls to stored procedures. The client-side and server-side SQLJ APIs are identical, making it easy to partition applications.

System Classes

A set of classes that constitute a significant portion of the implementation of Java in Oracle Database environment is known as the System classes. These classes are defined in the SYS schema and exported for all users by public synonym. A class with the same name as one of the System classes can be defined in a schema other than the SYS schema^Foot 1 . But, this is a bad practice because the alternate version of the class may behave in a manner that violates assumptions about the semantics of that class that are present in other System classes or in the underlying implementation of Java Virtual Machine. Oracle strongly discourages this practice.

Java in Database Application Development

The most important features of Java in database application development are:

• Providing flexible partitioning of Java2 Platform, Standard Edition (J2SE) applications for symmetric data access at the JDBC and SQLJ level.

• Bridging SQL and the Java2 Platform, Enterprise Edition (J2EE) world by:

  • Calling out Web components, such as JSP and servlet

  • Bridging SQL and Web Services

    • Calling out Web Services

  • Using an Oracle JVM as ERP Integration Hub

  • Invalidating cache

Java Programming Environment

In addition to the Oracle JVM, the Java programming environment provides:

• Java stored procedures as the Java equivalent and companion for PL/SQL. Java stored procedures are tightly integrated with PL/SQL. You can call Java stored procedures from PL/SQL packages and PL/SQL procedures from Java stored procedures.

• The JDBC and SQLJ programming interfaces for accessing SQL data.

• Tools and scripts that assist in developing, loading, and managing classes.

The following table helps you decide when to use which Java API:

Java Stored Procedures

Java stored procedures are Java programs written and deployed on a server and run from the server, exactly like 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, such as OCI and PRO*, or JDBC or SQLJ.

In addition, you can use Java to develop powerful, server-side programs, which can be independent of PL/SQL. Oracle Database provides a complete implementation of the standard Java programming language and a fully compliant JVM.

PL/SQL Integration and Oracle RDBMS Functionality

You can call existing PL/SQL programs from Java and Java programs from PL/SQL. This solution protects and leverages your PL/SQL and Java code and opens up the advantages and opportunities of Java-based Internet computing.

Oracle Database offers two different Java APIs for accessing SQL data, JDBC and SQLJ. Both these APIs are available on the client and the server. As a result, you can deploy your applications on the client and server, without modifying the code.

The following topics introduce the Java APIs and the JPublisher tool provided by Oracle Database:

• JDBC Drivers

• SQLJ

• JPublisher

Development Tools

The introduction of Java in Oracle Database enables you to use several Java IDEs. The adherence of Oracle Database to the Java standards and specifications and the open Internet standards and protocols ensures that your Java programs work successfully, when you deploy them on Oracle Database. Oracle provides many tools or utilities that are written in Java making development and deployment of Java server applications easier. Oracle JDeveloper, a Java IDE provided by Oracle, has many features designed specifically to make deployment of Java stored procedures and EJBs easier. You can download JDeveloper from: http://www.oracle.com/technology/software/products/jdev/index.html