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<title>Checking for internal references Requirements</title>
<p class="ueberschrift">Requirements for A Tool for Checking API Usage
in Eclipse-based Components</p>
<p>Jim des Rivieres<br>
Last updated Dec. 10, 2004</p>
<p>This document lists the general requirements for a tool for checking
API usage in Eclipse-based components and products.</p>
<p class="ueberschrift">1. The Problem</p>
<p>The context is the same as the current API Usage Scanner tool. That
is, we need a tool for checking Eclipse-based products that will help in
identifying inappropriate use of APIs and non-APIs. We want tools to be
able to check Eclipse-based plug-ins and products for obvious cases of
where plug-ins are not playing by the rules. The open-ended,
heterougeous, and heavily componentized nature of Eclipse are what make
it somewhat different from checking more monolithic applications.</p>
<p>Eclipse is based on the notion of a plug-in. Products end up being a
collection of plug-ins, collected from diverse development efforts
spanning open source projects, multiple IBM teams, and third-party tool
vendors. Set of these plug-ins are often bundled into various
&quot;platforms&quot; (e.g., Eclipse Platform, IBM Eclipse SDK), and
assembled into product stacks or families (IBM RAD, WBIT).
Component-based software is easier to develop, maintain, and evolve when
the inter-component coupling is well understood and well managed. This
is done by having each component offer its services through a
well-defined API that serves are the sole interface between it and other
components. To the extent that all inter-component coupling abides by
the contracts spelled out in the API specifications, replacing one
component by a different version supporting the same API contracts will
work the same way. Moreover, component APIs can be evolved in compatible
ways to meet changing requirements, without invalidating previously
working configurations. However, when inter-component coupling does not
follow the terms of the API contracts, replacing a component by a
different version supporting the same API contracts may break some
previously-working configurations. This is a destabilizing factor that
decreases robustness, increases maintenance costs, and inhibits
component growth.</p>
<p>In component-based software, the premium is on all inter-component
coupling being through the components API and in accord with all API
contracts. Although the Java language has some features for controlling
visibility that are enforced at compile-time (e.g., private fields and
methods cannot be referenced from outside the code of the class itself),
there are many constraints restrictions that the Java language cannot
express (e.g., that a particular public class is internal rather than
API, or that a particular interface must not be implemented outside its
own component). Detection of inappropriate inter-component coupling is
most useful when the component is under development; after a problematic
component has shipped in a product, the information can at least be
useful in managing the problem (although not solving it).</p>
<p>The problem we are addressing is helping development teams to manage
inter-component coupling in Eclipse-based components. In particular, we
aim to do this by providing an automatic tool that can detect obvious
cases where one component is more tightly coupled to another component
that it should be.</p>
<p class="ueberschrift">2. General Requirements</p>
<p>The tool should meet the following general requirements:</p>
<li><b>Tool will be used for checking Eclipse components.<br>
</b>The notion of a &quot;component&quot; in Eclipse is not formally
defined. Rather, a component is a loosely-defined grouping of a small
number of plug-ins and/or plug-in fragments that are designed and
maintained as a unit, generally by a small team of developers. For
instance, the Eclipse Platform itself is made of a number of
components each with between 1 and 5 plug-ins. The Java code for each
plug-in is housed in one or more JARs (this relationship is formalized
and captured in the plug-in's mainfest). Whereas coupling within a
component in intended to be tight/intimate, all coupling between
components is intended to be mediated through well-defined Java-based
APIs. The primary use of the tool will be used for checking
collections of Eclipse components to see whether the inter-component
couplings are in line with the component APIs. the Java code in the
various plug-ins.</li>
<li><b>Helpful to component developers.<br>
</b>Component developers (and development teams) are the primary
customers of this tool. The tool should be designed to meet their
needs. Their main need for such a tool is helping manage
inter-component dependencies to ensure that they do not accidentally
exceed the bounds of the component APIs. Early detection of problems
means they can be addressed while the component is still under
development. So the tool should detect and report cases of <i>illegitimate</i>
couplings from component A to component B are not covered by B's API,
the most common mistake being A referencing one of B's internal
classes. A secondary need is to detect and report <i>legitimate</i>
API couplings components, as an aid for planning and impact
<li><b>Modular, component-based input.<br>
</b>Eclipse components to be checked will be fed to the tool. Since
the notion of component and API are not formally specified in Eclipse,
the tool will also need to be fed some a description of each component
capturing things like the set of plug-ins that comprise the component,
and the rules for separating component API from component internals
and for distinguishing legitimate from illegitimate API usage.
Component owners should create and maintain the component descriptions
for their individual components (although it should also be easy to
furnish an after-market component description&nbsp; for a component
that does not come with its own). Each component description should be
in a file that the component owner can easily maintain. Component
description files should be in a well-specified XML-based format so
that they could be read or generated by other tools. A set of relevant
component description files would be fed to the tool whenever checking
<li><b>Performant and scalable.</b>&nbsp;<br>
The tool should be amenable to checking large numbers of Eclipse
components. We already have instances for Eclipse-based products with
1500 plug-ins. The tool should make it easy to check anything from a
single component, to a group of components that form a
&quot;platform&quot; of some ilk, to an entire product distribution.
The components may be a heterogeneous mix of product-specific, open
source, or third party. The tool should deal gracefully with
incompleteness, so that it is possible to check a component without
necessarily having complete information about other components. The
tool should be reasonably fast, with speed proportional to the number
of components being checked.</li>
<li><b>Modular, component-based output.<br>
</b>The tool should produce modular, component-based reports. The
outputs should be in a well-specified XML-based format so that they
could be post-processed for populating databases, generating web
pages, printing reports, etc.</li>
<li><b>Fully-automatic operation.</b><br>
The tool should be suitable for inclusion in automated build processes
that build the components, or repackage components into larger
aggregations. In other words, something without a GUI that can be run
from the command line, or from an Ant build file.</li>
<li><b>Open source.</b><br>
The tool itself should be open source, so that it can be used by
development teams working on open source Eclipse components, which
includes those at</li>
<p class="ueberschrift">References</p>
<li><a href="how_to_use_api.html">How to Use the Eclipse API<br>
</a>This is a good example of the ways some Eclipse-based components
separate API from internals at the Java package level, and of the
kinds of blanket API usage rules that are out there. Note that these
rules apply only to the components in the Eclipse Platform itself;
other Eclipse-based components will have their own rules and
Core &quot;Internal&quot; tool</a><br>
This simple tool is used for locating references to internal types in
Eclipse-based distributions. It has many of the right characteristics,
but is overly simplistic in assuming that 1 plug-in = 1 component and
that all components separate API from internal based on Eclipse's
package-level naming conventions.</li>