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# 1 Introduction to the Development Environment for GlassFish Server Add-On Components

Oracle GlassFish Server enables an external vendor such as an independent software vendor (ISV), original equipment manufacturer (OEM), or system integrator to incorporate GlassFish Server into a new product with the vendor's own brand name. External vendors can extend the functionality of GlassFish Server by developing add-on components for GlassFish Server. GlassFish Server provides interfaces to enable add-on components to be configured, managed, and monitored through existing GlassFish Server tools such as the Administration Console and the `asadmin` utility.

The following topics are addressed here:

- GlassFish Server Modular Architecture and Add-On Components

- OSGi Alliance Module Management Subsystem

- Hundred-Kilobyte Kernel

- Overview of the Development Process for an Add-On Component

## GlassFish Server Modular Architecture and Add-On Components

GlassFish Server has a modular architecture in which the features of GlassFish Server are provided by a consistent set of components that interact with each other. Each component provides a small set of functionally related features.

The modular architecture of GlassFish Server enables users to download and install only the components that are required for the applications that are being deployed. As a result, start-up times, memory consumption, and disk space requirements are all minimized.

The modular architecture of GlassFish Server enables you to extend the basic functionality of GlassFish Server by developing add-on components. An add-on component is an encapsulated definition of reusable code that has the following characteristics:

- The component provides a set of Java classes.

- The component offers services and public interfaces.

- The component implements the public interfaces with a set of private classes.

- The component depends on other components.

Add-on components that you develop interact with GlassFish Server in the same way as components that are supplied in GlassFish Server distributions.

You can create and offer new or updated add-on components at any time. GlassFish Server administrators can install add-on components and update or remove installed components after GlassFish Server is installed. For more information, see "Extending and Updating GlassFish Server" in Oracle GlassFish Server Administration Guide.

## OSGi Alliance Module Management Subsystem

To enable components to be added when required, GlassFish Server provides a lightweight and extensible kernel that uses the module management subsystem from the OSGi Alliance. Any GlassFish Server component that plugs in to this kernel must be implemented as an OSGi bundle. To enable an add-on component to plug in to the GlassFish Server kernel in the same way as other components, package the component as an OSGi bundle. For more information, see Packaging an Add-On Component.

The default OSGi module management subsystem in GlassFish Server is the Apache Felix OSGi framework. However, the GlassFish Server kernel uses only the OSGi Service Platform Release 4 API. Therefore, GlassFish Server supports other OSGi module management subsystems that are compatible with the OSGi Service Platform Release 4 API.

## Hundred-Kilobyte Kernel

The Hundred-Kilobyte Kernel (HK2) is the lightweight and extensible kernel of GlassFish Server. HK2 consists of the following technologies:

- Module subsystem. The HK2 module subsystem provides isolation between components of the GlassFish Server. The HK2 module subsystem is compatible with existing technologies such as the OSGi framework.

- Component model. The HK2 component model eases the development of components that are also services. GlassFish Server discovers these components automatically and dynamically. HK2 components use injection of dependencies to express dependencies on other components. GlassFish Server provides two-way mappings between the services of an HK2 component and OSGi services.

For more information, see Writing HK2 Components.

## Overview of the Development Process for an Add-On Component

To ensure that an add-on component behaves identically to components that are supplied in GlassFish Server distributions, the component must meet the following requirements:

- If the component generates management data or monitoring data, it must provide that data to other GlassFish Server components in the same way as other GlassFish Server components.

- If the component generates management data or monitoring data, it must provide that data to users through GlassFish Server administrative interfaces such as Administration Console and the `asadmin` utility.

- The component must be packaged and delivered as an OSGi bundle.

To develop add-on components that meet these requirements, follow the development process that is described in the following sections:

- Writing HK2 Components

- Extending the Administration Console

- Extending the `asadmin` Utility

- Adding Monitoring Capabilities

- Adding Configuration Data for a Component

- Adding Container Capabilities

- Creating a Session Persistence Module

- Packaging and Delivering an Add-On Component

### Writing HK2 Components

The Hundred-Kilobyte Kernel (HK2) is the lightweight and extensible kernel of GlassFish Server. To interact with GlassFish Server, add-on components plug in to this kernel. In the HK2 component model, the functions of an add-on component are declared through a contract-service implementation paradigm. An HK2 contract identifies and describes the building blocks or the extension points of an application. An HK2 service implements an HK2 contract.

For more information, see Writing HK2 Components.

### Extending the Administration Console

The Administration Console is a browser-based tool for administering GlassFish Server. It features an easy-to-navigate interface and online help. Extending the Administration Console enables you to provide a graphical user interface for administering your add-on component. You can use any of the user interface features of the Administration Console, such as tree nodes, links on the Common Tasks page, tabs and sub-tabs, property sheets, and JavaServer Faces pages. Your add-on component implements a marker interface and provides a configuration file that describes how your customizations integrate with the Administration Console.

For more information, see Extending the Administration Console.

### Extending the `asadmin` Utility

The `asadmin` utility is a command-line tool for configuring and administering GlassFish Server. Extending the `asadmin` utility enables you to provide administrative interfaces for an add-on component that are consistent with the interfaces of other GlassFish Server components. A user can run `asadmin` subcommands either from a command prompt or from a script. For more information about the `asadmin` utility, see the `asadmin`(1M) man page.

For more information, see Extending the `asadmin` Utility.

### Adding Monitoring Capabilities

Monitoring is the process of reviewing the statistics of a system to improve performance or solve problems. By monitoring the state of components and services that are deployed in the GlassFish Server, system administrators can identify performance bottlenecks, predict failures, perform root cause analysis, and ensure that everything is functioning as expected. Monitoring data can also be useful in performance tuning and capacity planning.

An add-on component typically generates statistics that the GlassFish Server can gather at run time. Adding monitoring capabilities enables an add-on component to provide statistics to GlassFish Server in the same way as components that are supplied in GlassFish Server distributions. As a result, system administrators can use the same administrative interfaces to monitor statistics from any installed GlassFish Server component, regardless of the origin of the component.

For more information, see Adding Monitoring Capabilities.

### Adding Configuration Data for a Component

The configuration data of a component determines the characteristics and runtime behavior of a component. GlassFish Server provides interfaces to enable an add-on component to store its configuration data in the same way as other GlassFish Server components. These interfaces are similar to interfaces that are defined in Java Specification Request (JSR) 222: Java Architecture for XML Binding (JAXB) 2.0. By using these interfaces to store configuration data, you ensure that the add-on component is fully integrated with GlassFish Server. As a result, administrators can configure an add-on component in the same way as they can configure other GlassFish Server components.

For more information, see Adding Configuration Data for a Component.

### Adding Container Capabilities

Applications run on GlassFish Server in containers. GlassFish Server enables you to create containers that extend or replace the existing containers of GlassFish Server. Adding container capabilities enables you to run new types of applications and to deploy new archive types in GlassFish Server.

For more information, see Adding Container Capabilities.

### Creating a Session Persistence Module

GlassFish Server enables you to create a session persistence module in the web container for high availability-related functionality by implementing the `PersistenceStrategyBuilder` interface . Using the `PersistenceStrategyBuilder` interface in an HK2 service makes the session manager extensible because you can implement a new persistence type without having to modify the web container code.

For information about other high-availability, session persistence solutions, see "Configuring High Availability Session Persistence and Failover" in Oracle GlassFish Server High Availability Administration Guide.

For more information, see Creating a Session Persistence Module.

### Packaging and Delivering an Add-On Component

Packaging an add-on component enables the component to interact with the GlassFish Server kernel in the same way as other components. Integrating a component with GlassFish Server enables GlassFish Server to discover the component at runtime. If an add-on component is an extension or update to existing installations of GlassFish Server, deliver the component through Update Tool.

For more information, see Packaging, Integrating, and Delivering an Add-On Component.