Section 3.1. Structural Overview

Eclipse isn't a single monolithic program, but rather a small kernel called a plug-in loader surrounded by hundreds (and potentially thousands) of plug-ins (see Figure 3-1) of which the Favorites example plug-in is one . Each plug-in may rely on services provided by another plug-in, and each may in turn provide services on which yet other plug-ins may rely.

This modular design lends itself to discrete chunks of functionality that can be more readily reused to build applications not envisioned by Eclipse's original developers.

3.1.1. Plug-in structure

The behavior of every plug-in is in code, yet the dependencies and services of a plug-in (see Section 2.3.1, The Plug-in manifests, on page 71) are declared in the MANIFEST.MF and plugin.xml files (see Figure 3-2). This structure facilitates lazy-loading of plug-in code on an as-needed basis, thus reducing both the startup time and the memory footprint of Eclipse.

On startup, the plug-in loader scans the MANIFEST.MF and plugin.xml files for each plug-in and then builds a structure containing this information. This structure takes up some memory, but it allows the loader to find a required plug-in much more quickly, and it takes up a lot less space than loading all the code from all the plug-ins all the time.

Plug-ins Are Loaded But Not Unloaded In Eclipse 3.1, plug-ins are loaded lazily during a session but not unloaded, causing the memory footprint to grow as the user requests more functionality. In future versions of Eclipse, this issue may be addressed by unloading plug-ins when they are no longer required (see eclipse.org/equinox; and for more specifics on deactivating plug-ins see dev.eclipse.org/viewcvs/indextech.cgi/~checkout~/ equinoxhome/dynamicPlugins/deactivatingPlugins.html).

3.1.2. Workspace

The Eclipse IDE displays and modifies files located in a workspace . The workspace is a directory hierarchy containing both user files such as projects, source code, and so on, and plug-in state information such as preferences (see Section 3.4.4, Plug-in preferences, on page 116). The plug-in state information located in the workspace directory hierarchy is associated only with that workspace, yet the Eclipse IDE, its plug-ins, the plug-in static resources (see Section 3.4.3, Static plug-in resources, on page 115) and plug-in configuration files (see Section 3.4.5, Plug-in configuration files, on page 116) are shared by multiple workspaces.

3.1. Structural Overview

Figure 3-1. Eclipse plug-in structure.

An example of how plug-ins depend on one another.

3.1.1. Plug-in structure

Figure 3-2. Declaring a new extension.

This is an example of how a new extension is declared in the plug-in manifest with lines highlighting how the plug-in manifest references various plug-in artifacts.

3.1.2. Workspace