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<td align="left" valign="top" colspan="2" bgcolor="#0080c0"><b><font
face="Arial,Helvetica" color="#ffffff"> Eclipse Corner Article</font></b></td>
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<h1><img src="images/Idea.jpg" height="86" width="120" align="middle">
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<h1 align="center">Project Builders and Natures</h1>
<blockquote> <b>Summary</b> <br>
This article discusses two central mechanisms that are associated with projects
in an Eclipse workspace. The first of these is incremental project builders,
which create some built state based on the project contents, and then keep that
built state synchronized as the project contents change. The second is project
natures, which define and manage the association between a given project and
a particular plug-in or feature. The purpose and uses of builders and natures
will be described in detail, and working examples will be provided to highlight
the finer details of implementing them for your own plug-in. This article is
intended to supplement, rather than replace, the documentation on builders and
natures found in the Eclipse Platform Plug-in Developer Guide. <br>
<p><b> By John Arthorne, IBM OTI Labs</b><br>
<font size="-1">January 27, 2003</font><br>
<font size="-1">Updated November 23, 2004 for Eclipse 3.0</font></p>
</blockquote>
<hr width="100%"> <a name="1"> </a>
<h2><a name="1">Incremental project builders</a></h2>
<div align="right"><a name="1"><i>Castles in the air - they are so easy to
take refuge in. And so easy to build, too.</i> <br>
– Henrik Ibsen</a></div>
<p> <a name="1">All of the key points about incremental project builders
are captured in their carefully chosen name. The best way to introduce
them is to begin by describing each of the terms in this name: </a></p>
<ul>
<li><a name="1"><b>Builder - </b> Builders take raw materials and produce
some output based on those materials. In Eclipse, both the raw materials
and the output of builders are resources in the Eclipse workspace.</a></li>
<li><a name="1"><b>Incremental - </b> It would be inefficient if builders
rebuilt their entire output from scratch every time they were invoked. Instead,
builders in Eclipse are incremental. This means that after the first build,
subsequent builds should only rebuild based on what has changed since the
last build.</a></li>
<li><a name="1"><b>Project - </b> A builder operates on the resources in
a single project in the Eclipse workspace. If there are many projects in
the workspace, builders need to be installed on each one separately.</a></li>
</ul>
<p> <a name="1">All of the basic information about builders, including how to
define them, and how to install and run them, is found </a><a
href="http://help.eclipse.org/help30/topic/org.eclipse.platform.doc.isv/guide/resAdv_builders.htm" target="_blank">
here</a> in the Eclipse Platform Plug-in Developer Guide. This article will
dive into all the gory details about implementing builders that aren't found
in the basic documentation. If you haven't already done so, read through that
documentation first, then come back to this article. </p>
<a name="1a"> </a>
<h3><a name="1a">The kinds of builds</a></h3>
<p><a name="1a">From a user's point of view, builds in Eclipse come in several
different flavors. Under the covers, all of these different varieties are
implemented by the same builder instance, so your builder will need to be
able to understand all of them. Let's take a look at each of the different
varieties in turn. </a></p>
<h4><a name="1a">Incremental build</a></h4>
<p> <a name="1a">Incremental building is the variety of most interest in
Eclipse. This is what happens when <b>Build Project</b> or <b>Build All</b>
is selected from the <b>Project</b> menu in the Eclipse workbench. When an
incremental build is invoked, the builder is supplied with a <i>resource
delta</i>. A resource delta is a hierarchical description of what has changed
between two discrete points in the lifetime of the Eclipse workspace. In
particular, builders are given a delta that describes all changes in that
project since the last time that builder was called. The goal of incremental
building is to analyze this delta, and then rebuild only the builder output
that is affected by the change. </a></p>
<p> <a name="1a">For a detailed description of the structure of resource
deltas, and for tips on processing them, read the article on </a><a
href="http://www.eclipse.org/articles/Article-Resource-deltas/resource-deltas.html">
resource change listeners</a>. For this article, it is sufficient to point
out the differences between the resource deltas given to listeners, and
the resource deltas given to builders: </p>
<ul>
<li>Builder resource deltas are rooted at a single project. A builder can
request deltas for multiple projects, but each resource delta returned by
<code>IncrementalProjectBuilder.getDelta</code> has a single project
as its root.</li>
<li>Some resource change information that is available to resource change
listeners is omitted from build deltas. In particular, changes to resource
markers (<code>IResourceDelta.MARKERS</code>), and changes to synchronization
information (<code>IResourceDelta.SYNC</code>) are not included in builder
deltas. These change types are omitted for performance reasons, since this
information is not typically of interest to builders. </li>
<li>Builder deltas may be null. If a builder has never been invoked before,
any request for deltas will return null. Also, if a builder is not run for
a long time, the platform reserves the right to return a null delta. This
is done because keeping track of resource changes over an extended period
of time can be very costly, and it's generally more efficient to just rebuild
from scratch if the builder hasn't been run in a long time. When a builder
receives a null delta, it should just discard all existing built state, and
resort to doing a full build. </li>
</ul>
<h4>Auto-build</h4>
<p> From the point of view of the builder implementation, auto-build is exactly
the same as incremental build. The difference with auto-build is that
it is not triggered by an explicit build request, but as an automatic side
effect of resources changes in the workspace. When auto-build is turned
on, all installed builders will be invoked every time resources are added,
removed, or modified in the eclipse workspace. There is no guarantee about
precisely when auto-build will occur in relation to the operation that modified the
workspace. The build generally occurs in a background thread a short time after
all threads have stopped modifying the workspace. If a thread attempts to modify
the workspace while the build is running, the build is halted and the thread is allowed
to proceed with its changes. Auto-build will continue to attempt running until it
has sufficient time to complete the entire workspace build. As with resource change
events, you can avoid frequent auto-builds by wrapping long running operations in
an <tt>IWorkspaceRunnable</tt> passed to <tt>IWorkspace.run</tt>, or using
a <tt>WorkspaceJob</tt>.
<p>
There is a preference in the Eclipse workbench
for turning auto-build on or off (Preferences &gt; Workbench &gt; Build automatically).
Auto-build can also be turned on or off programatically by setting the auto-build flag
in the workspace description, although this is not generally recommended. The
following code snippet turns auto-build on: </p>
<font color="#4444cc">
<pre>
IWorkspace workspace = ResourcesPlugin.getWorkspace();
IWorkspaceDescription description = workspace.getDescription();
if (!description.isAutoBuilding()) {
description.setAutoBuilding(true);
workspace.setDescription(description);
}
</pre>
</font>
<h4>Full build</h4>
<p> When a full build is invoked, it is a request to discard all existing
built state and start over from scratch. There is currently no command
in the Eclipse workbench that allows a user to directly invoke a full build.
The first build after invoking clean will cause a full build in the cleaned projects.
Theoretically, full build should never be needed if your incremental build is
clever enough. However, if your builder is for some reason unable to correctly
build incrementally, it is useful to have an escape route available to rebuild
everything from scratch.
</p>
<h4>Clean build</h4>
<p>
The final variety of build, introduced in Eclipse 3.0, is <i>clean</i>. Clean
is inspired by the Unix <i>make</i> utility convention of using a target called <tt>clean</tt>
to discard all artifacts produced by a build. As with full build, clean should not
be required if all builders correctly perform their incremental builds. Clean is
intended largely as a safety blanket for the end user when things seem to go wrong
with their build process. Builders should implement clean by deleting all output
files produced by the build, and removing any problem markers associated with
that builder. Unlike other build types, the clean build is not implemented by the
<tt>build</tt> method on <tt>IncrementalProjectBuilder</tt>. For backwards
compatibility, there is a new <tt>clean</tt> method with an empty default implementation,
allowing older builders to continue running without modification.
The following is an example of a <tt>clean</tt>
implementation that just deletes all markers of a particular type from the project:
<font color="#4444cc">
<pre>
protected void clean(IProgressMonitor monitor) throws CoreException {
getProject().deleteMarkers("com.xyz.myproblems",
true, IResource.DEPTH_INFINITE);
}
</pre>
</font>
</p>
<a name="1b"> </a>
<h3><a name="1b">Gory details about the build process</a></h3>
<p> <a name="1b">The target of a build can either be a single project, or
the entire workspace. The entire workspace is built on every auto-build,
and also on the actions <b>Build All</b> and <b>Clean All</b>. Keep in
mind that on incremental builds, &quot;building&quot; the entire workspace may actually
only involve compiling a single file. So what happens when a workspace
build is invoked? Regardless of whether the build is full, incremental, or clean,
the process is more or less the same, and this section will outline exactly
what happens. </a></p>
<h4><a name="1b">Step 1: computing the workspace build order</a></h4>
<p> <a name="1b">First, the platform computes or retrieves the <i>workspace
build order</i>. This is simply an ordered list of projects to build. This
build order can be explicitly set using the API method <code>IWorkspaceDescription.setBuildOrder</code>.
The following example sets the workspace build order to be project "Foo"
followed by project "Bar":</a></p>
<a name="1b"><font color="#4444cc">
<pre>
IWorkspace workspace = ResourcesPlugin.getWorkspace();
IWorkspaceDescription description = workspace.getDescription();
String[] buildOrder = {"Foo", "Bar"};
description.setBuildOrder(buildOrder);
workspace.setDescription(description);
</pre>
</font>
</a>
<p> <a name="1b">In the Eclipse UI, a user can set the build order from the
"Build Order" preference page. Setting the build order manually, either
via API or on the preference page, is not recommended in most circumstances.
You may want to do this if you have a headless application that builds particular
projects, or if you have complex project inter-dependencies that the default
build order doesn't handle well. If no build order is set manually, the
workspace computes a default build order based on the <i>project references</i>.
To be precise, the default build order is a breadth-first traversal of
the project reference graph. If there are cycles in the project reference
graph, they are left in the build order, but the projects involved in the
cycle will be built in an indeterminate order. </a></p>
<p> <a name="1b">Once the workspace build order is figured out, the build
process proceeds as follows: </a></p>
<ol>
<li><a name="1b">Build each open project in the build order</a></li>
<li><a name="1b">Build each remaining open project in the workspace, in
no particular order</a></li>
</ol>
<p></p>
<p> <a name="1b">Note: Closed projects are ignored by the build process. </a></p>
<h4><a name="1b">Step 2: Build specifications and arguments</a></h4>
<p> <a name="1b">Now that we know the order in which projects are built,
we should take a closer look at what happens when a particular project is
built. There can be any number of incremental builders associated with a
given project. The set of builders to invoke for a project, and the ordering
of those builders, is specified by the project's <i>build spec</i>. The
build spec is found on the <code>IProjectDescription</code>, and is represented
as an array of <code>ICommand</code> objects. Each command is simply the
name of the builder extension to run, and an optional table of builder arguments.
Builder argument keys and values must be strings, to allow for easy serialization.
Thus, building a single project involves invoking each builder in the build
spec, one at a time, in the order specified in the build spec. </a></p>
<h4><a name="1b">Step 3: Decide if the builder needs to be invoked</a></h4>
<p> <a name="1b">There are some circumstances under which a builder might
be skipped during the build process. In particular, a builder will be skipped
if the build is an incremental or auto-build, and no resources have changed
in the project, or in any of the projects that the builder is interested
in. The return value of a builder's <code>build</code> method is an array
of projects. This list of projects represents the projects that the builder
requires resource deltas for (this implicitly includes the project that the
builder is installed on). If none of these projects have changed since the
last time the builder was invoked, the builder is skipped (for incremental and auto-build only).
Without this step, every builder in the workspace would be invoked after every single
resource change. This optimization ensures that only builders that are capable
of processing the changed resources will be invoked. On the downside, this
means builders cannot generally respond to external changes that occur outside
the workspace, because the builder might be skipped if no resources in the
workspace have changed. </a></p>
<p> <a name="1b"><img src="images/tip.gif">
If there are circumstances where your builder must be invoked regardless
of whether any resources have been changed, there is a way to circumvent
this behavior. If the builder invokes the method <code>IncrementalProjectBuilder.forgetLastBuiltState</code>,
the information required to compute the resource deltas will be discarded.
This means that the build mechanism cannot determine what has changed since
the last build, so the builder will always be invoked. The downside is that
deltas will not be available for such a builder, so the builder must always
perform a full build. </a></p>
<p> <a name="1b">Finally, if there are errors finding or initializing a builder,
it will be temporarily omitted from the build process. The builder is automatically
added back to the build once the problem is fixed. Error conditions that
can cause a builder to be skipped include: </a></p>
<ul>
<li><a name="1b">The plug-in that owns the builder is missing.</a></li>
<li><a name="1b">The plug-in that owns the builder has been marked as disabled.
This is caused either by an error during activation, or by a missing plug-in
prerequisite.</a></li>
<li><a name="1b">There was an error instantiating the builder. This typically
means a runtime exception was thrown either by the builder's constructor
or static initializer code.</a></li>
<li><a name="1b">The builder belongs to a particular project nature, and
that nature is missing or disabled. This will be discussed in more detail
in the section on project natures.</a></li>
</ul>
<p></p>
<a name="1c"> </a>
<h3><a name="1c">Progress, cancelation, and exception handling</a></h3>
<div align="right"><a name="1c"><i>Progress is the mother of all problems.
</i> <br>
– G. K. Chesterton</a></div>
<p> <a name="1c">There are a number of features that are available to builders
that are not available to simple resource change listeners. Most importantly,
builders are entities that the user is intended to be aware of and to interact
with, so there are a number of mechanisms available for builders to communicate
back with their caller. </a></p>
<h4><a name="1c">Reporting progress</a></h4>
<p>The <code>build</code> method on <code>IncrementalProjectBuilder</code>
takes as parameter an instance of <code>IProgressMonitor</code>. This object
should be used for reporting feedback and progress information to the user
as the build proceeds. The most important methods on <code>IProgressMonitor</code>
are <code>beginTask</code>, <code>subTask</code>, <code>worked</code>, and
<code>done</code>. The important concepts to remember about reporting progress
are:</p>
<ul>
<li><code>beginTask</a></code><a name="1c"> and <code>done</code>
must be called exactly once each.</a></li>
<li><a name="1c">The increments passed to the <code>worked</code> method
should eventually add up to the amount specified in <code>beginTask</code>.
If the code branches, you need to ensure every branch reports the same progress.</a></li>
<li><a name="1c">If you need to call API from another component during the
build, you should create a new <code>SubProgressMonitor</code> and pass that
to the API method. Never pass your own progress monitor across an API boundary,
because you have already called <code>beginTask</code>, which can only be
called once for a given progress monitor.</a></li>
<li><a name="1c">Some form of progress (either worked increments or a new
sub-task message), should be visible every few seconds. The purpose is to
reassure the user that something is actually happening.</a></li>
<li><a name="1c">The sub-task messages just need to give a general idea
of what is happening. You don't need to report exactly what is happening
at a low level in the code. Progress messages should be in a vocabulary
that the user will understand.</a></li>
<li>The user won't know or care if you are lying through your teeth.</li>
</ul>
<p></p>
<p> <a name="1c">Here is the recommended basic pattern for using progress
monitors: </a></p>
<a name="1c"><font color="#4444cc">
<pre>
final int TOTAL_WORK = 100;
try {
monitor.beginTask("Name of task", TOTAL_WORK);
for (int i = 0; i &lt; TOTAL_WORK; i++) {
monitor.subTask("Name of sub task");
//do some work
monitor.worked(1);
}
} finally {
monitor.done();
}
</pre>
</font> </a>
<p><a name="1c">In some circumstances, it may be too expensive or even impossible
to compute the exact amount of work units before the operation starts. For
example, when loading files from a server, you may not know exactly how many
files will be loaded or the size of the files until they have already arrived.
In these cases, it is necessary to "fake" the progress numbers a bit. For
example, you can start with a best estimate, and then revise the number of
work units as you approach the total work. If you don't even have an initial
estimate, you can use (warning: practical application of Calculus!) an infinite
series that converges at the total work. The goal is to keep the progress
bar moving, and only reach the end of the bar when the work is almost complete,
not always an easy task! </a></p>
<h4><a name="1c">Handling cancelation</a></h4>
<p> <a name="1c">Progress monitoring in Eclipse is always tied to the mechanism
for cancelation. Builder implementors are encouraged to check for cancelation
as often as possible. The important thing to keep in mind when adding cancelation
support, is that you are the one in control of cancelation. When the caller
attempts to cancel, this is really a cancelation request, not a demand.
If the request cannot be granted without leaving a corrupt or invalid state
behind, then it's ok to defer the cancelation until you are in a consistent
state. If the operation is almost finished and is not reversible, it is
sometimes better to ignore the cancelation request completely. When you decide
that safe cancelation is possible, the recommended approach is to throw a
new <code>OperationCanceledException</code> back from the builder's <code>build</code>
method. This special runtime exception will then be caught and handled
by the platform. The pattern for checking and responding to cancelation
is as follows: </a></p>
<a name="1c">
<font color="#4444cc">
<pre>
do some work
checkCancel(monitor);
//do some work
checkCancel(monitor);
//... etc ...
protected void checkCancel(IProgressMonitor monitor) {
if (monitor.isCanceled()) {
forgetLastBuiltState();//not always necessary
throw new OperationCanceledException();
}
}
</pre>
</font> </a>
<p> <a name="1c">It is sometimes necessary to call <code>IncrementalProjectBuilder.forgetLastBuiltState()</code>
in order to correctly recover from the cancelation the next time your builder
is called. If you don't call this method, the next build will be supplied
with the usual resource delta describing the changes since the last build.
Since the last build was canceled part way through, it might not be possible
to incrementally update your built state. Calling this method will
force the next incremental or auto-build to be a full build. An alternative is to extract the
information you need from the resource delta, and continue the incremental
build the next time one is requested.
</a></p>
<h4><a name="1c">Handling interruption</a></h4>
<p>
In Eclipse 3.0, auto-build was moved into a background thread. In a GUI environment,
this allows the user to continue working as the build progresses. However, if
the user attempts to make further resources changes while the build is running,
auto-build needs to interrupt itself. The workspace checks for this kind of interruption
immediately before calling each individual builder. If the contention occurs while
a builder is running, the user's activity will typically be blocked until the currently
executing builder completes. While this is fine for relatively quick builders, it can
become frustrating for users when a single builder takes a very long time to complete.
To alleviate this, long running builders can check for interruption periodically during
their execution. The method <tt>isInterrupted</tt> on <tt>IncrementalProjectBuilder</tt>
will return <tt>true</tt> if another thread is waiting to modify the workspace. When
this happens, a builder can chose to abort the build and allow the other operation to
proceed. Note that interruption should not always be treated the same as cancelation,
since it occurs implicitly, rather than based on an explicit cancelation request by the
user. It is acceptable to respond to cancelation by discarding build state and resorting
to a full build on the next builder invocation, but this is too drastic for interruption.
Interruption should not cause significant additional overhead for subsequent builds.
Builders that cannot gracefully handle interruption should not respond to it at all.
<h4><a name="1c">Handling exceptions and reporting problems</a></h4>
<p> <a name="1c">There are two categories of problems that a builder will
typically need to address. First, there may be internal errors in the code
of your builder, or errors thrown back by API that is called by the builder.
If it is not possible to recover from these errors, your <code>build</code>
method can throw a <code>CoreException</code> that includes a description,
severity, and optional nested exception. The text of this exception will
be reported directly to the user, and the remainder of the build for that
project will be aborted. Builds of other projects will proceed as normal.
</a></p>
<p> <a name="1c">The second category of problems are against the resources
being built. For example, this includes compilation errors in the user code,
or incorrect configuration information required by the user. These types
of problems are generally reported by creating <i>problem markers</i> that are
attached to the most specific resource that is relevant to that problem.
If no particular resource is relevant, you can attach the marker to the project
itself. You are encouraged to create your own custom marker type for your
builder problems, but the type you define should generally be a subtype of
the generic problem type, <code>IMarker.PROBLEM</code>. This ensures that
your problem type will appear in the task list of the Eclipse workbench.
See the article </a><a
href="http://www.eclipse.org/articles/Article-Mark%20My%20Words/Mark%20My%20Words.html">
Mark my Words</a> for more information on defining and using markers. </p>
<h3><a name="2b">Interacting with background auto-build</a></h3>
<p>
The fact that auto-build occurs in a background thread can sometimes introduce
challenges for code that relies on builder output. In a headless batch environment,
you cannot simply modify resources and assume auto-build will occur
immediately. Headless tools that previously made this assumption might exit from
their main thread of execution while the background build is still running. The best
solution in this situation is to simply turn off auto-build, and maintain complete control
over when builds occur by explicitly invoking <tt>IWorkspace.build</tt> if and when
required. There are other situations where a plug-in wants to allow auto-build to run,
but needs to wait until it completes before proceeding. For example, when the user
attempts to launch a Java program they have just created or modified, the launch
needs to wait until auto-build completes to ensure that the correct class files are
used. In this situation, the background build job can be joined using the following code:
<font color="#4444cc">
<pre>
Platform.getJobManager().join(ResourcesPlugin.FAMILY_AUTO_BUILD, null);
</pre>
</font>
The <tt>join</tt> invocation will block until the auto-build job completes, or until
the join is interrupted or canceled. You can report progress while waiting by passing
a progress monitor to the <tt>join</tt> method. Manual incremental build jobs can be joined
in a similar manner using the <tt>ResourcesPlugin.FAMILY_MANUAL_BUILD</tt> job family.
</p>
<table border="1" cellpadding="4" width="90%" bgcolor="FFFFCC"><tr>
<td bgcolor="0080c0"><font size="+1" color="FFFFCC"><b>Sidebar: the Java builder</b></font> </td>
</tr>
<tr><td>
<p>
The concept of automatic incremental compilation is not familiar to many developers.
A very frequent question from Eclipse beginners is, &quot;where is the compile button?&quot;
The answer is that an IDE with automatic compilation doesn't need a compile button. Every
time you make a change to a file, or a group of files, the incremental builder immediately rebuilds
every source file that was affected by the change. In this environment, the idea of compilation
as a task the user is involved in disappears -- the world is just always in a compiled state.
</p>
<p>
So what magic goes on behind the scenes to make this happen? How does the Java&trade; builder
know which files need to be recompiled when a given source file changes? This is no easy task,
but in broad brush strokes, this is what the Eclipse Java builder does:
<ul>
<li>The builder maintains a <b>built state</b> that includes a list of all types (classes or
interfaces) that are referenced by each type in the workspace. This information is returned
by the compiler each time a source file is compiled. This state is computed from scratch
on a full build, and updated incrementally on each subsequent build.</li>
<li>Whenever files are modified, the builder receives a resource delta that describes
which files were added, removed, or changed.</li>
<li>For deleted Java source files, the corresponding class files are deleted. Added and changed
source files are added to a queue of files that need to be compiled.</li>
<li>The builder then processes this queue as follows:
<ol>
<li>Remove a file from the queue, and compile it.</li>
<li>Compare the resulting type to the old class file, and see if the
type has <b>structural changes</b>. Structural changes are changes that can affect
the compilation of a referencing type: added or removed methods, fields or types, or changed method
signatures.</li>
<li>If the type has structural changes, find all types in the project that reference the changed type,
and add them to the queue.</li>
<li>If the type has changed at all, write it to disk in the builder's output folder.</li>
<li>Update the built state with the new reference information for the compiled type.</li>
<li>Repeat until the queue is empty.</li>
</ol>
</li>
<li>As a final step, the builder generates problem markers for each compiled type that had
compilation problems.</li>
</ul>
</p>
<p> The critical feature of the Java builder is that it is very fast 95% of the
time. Most times you edit a Java file, you don't make any structural changes.
This means the builder just has to compile that single file, and it's
done. When a type does have structural changes, all types that reference
it will be recompiled. However, those secondary types will almost never
have structural changes themselves, so the builder loop finishes with
at most a couple of iterations. On rare occasions, there will be significant
structure changes that cause many files to be recompiled. There is a lesson
here that you can apply when writing your own incremental builder. If
you can make your builder extremely fast for the most common cases, users
will generally tolerate a longer delay for those rare corner cases when
they come along. </p>
</td></tr></table>
<hr width="100%"> <a name="2"> </a>
<h2><a name="2">Project natures</a></h2>
<div align="right"><a name="2"><i>What nature delivers to us is never stale.
Because what nature creates has eternity in it.</i> <br>
– Isaac Bashevis Singer</a></div>
<p> <a name="2">The main purpose of project natures is to create an association
between a project and a given tool, plug-in, or feature set. By adding a
nature to a project, you indicate that your plug-in is configured to use
that project. For example, when the Java nature is added to a project, it
indicates that the Java Development Tool (JDT) plug-ins are aware of that project, and have
configured a classpath and a Java builder to work on that project. </a></p>
<p> <a name="2">In addition to defining the association between a tool and
a project, natures also provide a way of handling the lifecycle of a tool's
interaction with a project. When a nature is added to a project, the project
nature's <code>configure</code> method is called. This gives the tool an
opportunity to initialize its state for that project, and install any incremental
project builders that are needed for that project. Similarly, when a nature
is removed from a project, the nature's <code>deconfigure</code> method is
called. This gives the tool an opportunity to remove or clean up any meta-data
it has created for that project, and to remove any listeners and builders
associated with that tool. </a></p>
<p> <a name="2">Project natures also have significance in the eclipse UI. For
example, the icon for a project in the resource navigator is based on the installed
nature (using the org.eclipse.ui.projectNatureImages extension point). Actions
in the various menus can also be filtered to be visible only for resources that
belong to projects with a given nature. Refer to the </a>
<a
href="http://help.eclipse.org/help30/topic/org.eclipse.platform.doc.isv/reference/extension-points/index.html#workbench" target="_blank">
UI extension point documentation</a> for more details on action filtering. </p>
<a name="2a"> </a>
<h3><a name="2a">Nature constraints</a></h3>
<div align="right"><a name="2a"><i>The laws of nature are but the mathematical
thoughts of God.</i> <br>
– Euclid</a></div>
<p><a name="2a">Natures can be defined so that they are only enabled under
certain conditions. The two types of constraints that can currently be set
are <i>one-of</i> constraints and <i>requires</i> constraints. The one-of
constraint ensures that if two or more natures exist on a project that belong
to the same set, then all such natures will be disabled. The "set" in this
case is just an arbitrary string. As a fictitious example, say there are
several different plug-ins that can provide a project nature for managing
XML files in a project. However, since these plug-ins provide conflicting
services, it is not possible to have two of these XML natures installed on
any single project. These natures could specify membership in a common "xml-natures"
nature set. Then the platform would ensure that only one nature belonging
to that set could be installed on a project at any given time. </a></p>
<p> <a name="2a">The <i>requires</i> constraint ensures that a nature is
only enabled on a project if the nature(s) it requires are also enabled
on that project. For example, if your tools are only applicable for projects
managed by JDT, you can add a requires constraint on the Java nature.</a></p>
<p> <a name="2a">The platform will attempt to enforce these constraints whenever
possible. For example if an attempt to add or remove a nature from a project
would cause a constraint to be violated, the operation will fail. Also,
when several natures are added or removed at once, they will be serially
configured or deconfigured by the platform in an order that maintains the
validity of the constraints. For example, if your nature requires the JDT
nature, the platform will ensure that the JDT nature is configured before
your nature, and deconfigured after your nature. </a></p>
<p> <a name="2a">In some circumstances, the platform may not be able to ensure
the constraints are satisfied. For example, a nature may go missing or have
its definition changed between invocations of the platform. A more common
case is when two users are sharing a project in a repository, but have different
tools available, and hence different natures installed on the project. In
these circumstances, the natures whose constraints are not satisfied are
considered to be <i>disabled</i>. Disabled natures are tolerated by the
platform -- there will be no error messages and the nature will remain in
the project description. You can find out if a nature is enabled by calling
<code>IProject.isNatureEnabled()</code> on a project. </a></p>
<a name="2b"> </a>
<h3><a name="2b">Associating natures with builders</a></h3>
<p> <a name="2b">Incremental project builders and project natures often go
together. The nature life cycle methods provide a convenient place for adding
and removing builders from the project build spec, and the builder's association
with the project is often linked with other tools that benefit from the nature-based
presentation in the UI. Also, the ordering created by the nature "requires"
constraint makes it easy to correctly order the build spec. For example,
if you have a builder that must be run <b>after</b> the JDT builder, you
simply make your nature require the JDT nature, thus ensuring that your nature
will be configured after the JDT builder has already been installed. Then
if you add your nature to the end of the build spec, you are guaranteed that
your builder is run after the JDT builder (assuming nobody else reorders
the build spec). </a></p>
<p> <a name="2b">The relationship between a nature and a builder can be made
concrete by adding extra markup in the extension definitions. In the nature
definition, the builder can be specified by adding a builder attribute to
the nature definition. In the builder definition, you just need to add an
attribute specifying that the builder belongs to some nature. </a></p>
<p> <a name="2b"><img src="images/tryit.gif">
The following is an example of a builder and a nature that are explicitly
related. In this example, the id of the plug-in is "com.xyz.myplugin". </a></p>
<a name="2b"><font color="#4444cc">
<pre> &lt;extension <br> id="mynature" <br> name="My Nature" <br> point="org.eclipse.core.resources.natures"&gt; <br> &lt;runtime&gt; <br> &lt;run class="com.xyz.natures.MyNature"/&gt; <br> &lt;/runtime&gt; <br> &lt;builder id="com.xyz.myplugin.mybuilder"/&gt; <br> &lt;/extension&gt; <br> &lt;extension <br> id="mybuilder" <br> name="My Builder" <br> point="org.eclipse.core.resources.builders"&gt; <br> &lt;builder hasNature="true"&gt; <br> &lt;run class="com.xyz.builders.MyBuilder"/&gt;<br> &lt;/builder&gt; <br> &lt;/extension&gt; <br></pre>
</font> </a>
<p> <a name="2b">There are some benefits to be gained by explicitly associating
a nature with a builder. Primarily, this ensures that builders are automatically
omitted from the build spec when their corresponding nature is missing or
disabled. This gives builders the guarantee that, when they are run, they
can be sure that the builders of their prerequisite natures will also be run.
This also ensures smooth operation in the case where two users are sharing
a project, but only one has the plug-ins required for a particular nature.
All builders that belong to that nature, or that belong to natures that
require that nature, will be silently omitted from the build spec for the
user who does not have the necessary plug-ins. </a></p>
<a name="2c"> </a>
<h3><a name="2c">More on nature life cycle</a></h3>
<div align="right"><a name="2c"><i>Man masters nature not by force but by
understanding.</i> <br>
– Jacob Brownowski</a></div>
<p> <a name="2c">Since natures are intended to provide support for a project
life cycle, it is important to understand exactly when natures are configured
and deconfigured. Natures are only configured or deconfigured when a nature
is added or removed from the project description, and then committed using
<code>IProject.setDescription</code>. This is the only API method that will
cause natures to be configured or deconfigured. A nature is only ever configured
<b>once</b> for a given project, even if that project is shared using a repository
with multiple team members. The life cycle of a project nature on a shared
project goes like this: </a></p>
<ul>
<li><a name="2c">User 1 creates the project, and releases to the repository</a></li>
<li><a name="2c">Other users load this project from the repository into
their workspace</a></li>
<li><a name="2c">User 1 now adds nature X to the project. <code>configure()</code>
on nature X is called now.</a></li>
<li><a name="2c">User 1 releases this change to the repository</a></li>
<li><a name="2c">Other users load this change, and the nature is added to
their local project description. The <code>configure()</code> method for Nature X
is <b>not</b> called again for these other users.</a></li>
</ul>
<p></p>
<p> <a name="2c">The <code>configure()</code> method is also <b>not</b> called again when
a project is copied or moved, or when a project is created using <code>IProject.create(IProjectDescription,
IProgressMonitor)</code>. </a></p>
<a name="2c"><br>
</a><a name="3"> </a>
<h2><a name="3">Summary</a></h2>
<p> <a name="3">Incremental project builders provide a mechanism for processing
resources in a project and producing some build output. The builder framework
makes it easier to incrementally maintain that built state as the input resources
change. This article described implementation details about writing your
own builder, such as reporting problems, progress indication, and cancelation
support. Project natures create a concrete connection between a tool and
a project in the eclipse workspace, and they provide support for persisting,
sharing, and managing the relationship of the tool with the project. Natures
and builders are often used together, as this provides a way to install and
remove builders at appropriate times, and allows for builder ordering based
on nature dependencies. </a></p>
<p> <a name="3">For more information about builders and natures, consult the </a><a href="http://help.eclipse.org/help30/" target="_blank">
Eclipse Platform Plug-in Developer Guide</a>. The API Javadoc for the <a
href="http://help.eclipse.org/help30/topic/org.eclipse.platform.doc.isv/reference/api/org/eclipse/core/resources/package-summary.html" target="_blank">
<code>org.eclipse.core.resources</code></a> package is also an important source
of information on these concepts. </p>
<p><small>Java and all Java-based trademarks and logos are trademarks or registered
trademarks of Sun Microsystems, Inc. in the United States, other countries, or
both.</small></p>
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