The Oracle JVM is a standard, Java-compatible environment that runs any pure Java application. It is compatible with the standard JLS and the JVM specifications. It supports the standard Java binary format and the standard Java APIs. In addition, Oracle Database adheres to standard Java language semantics, including dynamic class loading at run time.
Java in Oracle Database introduces the following terms:
A session in Oracle Database Java environment is identical to the standard Oracle Database usage. A session is typically, although not necessarily, bounded by the time a single user connects to the server. As a user who calls a Java code, you must establish a session in the server.
A SQL client program runs a Java stored procedure.
A trigger runs a Java stored procedure.
A PL/SQL program calls a Java code.
In all the cases defined, a call begins, some combination of Java, SQL, or PL/SQL code is run to completion, and the call ends.
The concept of session and call apply across all uses of Oracle Database.
Unlike other Java environments, the Oracle JVM is embedded within Oracle Database and introduces a number of new concepts. This section discusses some important differences between an Oracle JVM and typical client JVMs based on:
1.4.1 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:
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.
1.4.2 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.
1.4.3 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.
1.4.4 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.