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Copyright International Business Machines Corporation, 2004. All rights reserved</font>
<font face="Times New Roman, Times, serif" size="2">&copy; Copyright Bjorn Freeman-Benson,
2004. All rights reserved</font>
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<td align=LEFT valign=TOP colspan="2" bgcolor="#0080C0"><b><font face="Arial,Helvetica"><font color="#FFFFFF">&nbsp;Eclipse
Corner Article</font></font></b></td>
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<h1><img src="images/Idea.jpg" align=CENTER width="120" height="86"></h1>
<h1 ALIGN="CENTER"><br>
How to write an Eclipse debugger</h1>
One of the major tasks of adding a new language to an Eclipse-based IDE is debugging
support. A debugger needs to start and stop the program being debugged, suspend
and resume, single-step, manage breakpoints and watch points, and so on. This
article explains the Eclipse Platform debug framework and steps through a simple,
yet illustrative, example of adding debug support for a new language.
<p><b> By Darin Wright, IBM Rational Software Group<br>
Bjorn Freeman-Benson, Predictable Software</b><br>
<font size="-1">August 27, 2004</font> </p>
<hr width="100%">
<h2>Language support</h2>
<p>No matter how simple we want to make the topic, there is no getting around
the fact that adding a new language to an Eclipse IDE is not a trivial task.
This paper, together with its two companion papers, focuses on the
launching-running-debugging side of the process. Other aspects of adding new
language support include: editors, code assist, refactorings, new views,
builders, and other tools. These features are outside the scope of these papers.</p>
<p>The first paper in this series, <i><a href="../Article-Launch-Framework/launch.html">We
Have Lift-off: The Launching Framework in Eclipse</a></i>, describes the launching
framework through an applet launcher example. In this paper, we describe the
basic debugging framework using a small assembly language example. And in our
third paper (not yet published), <i>Enhancing the Eclipse Debugger</i>, we describe
how to enhance the UI for our assembly language debugger by adding all the little
bells and whistles that one expects in modern IDEs.</p>
<p>The Eclipse SDK provides a framework for building and integrating debuggers,
collectively known as the debug platform. The debug platform defines a set of
Java&#153; interfaces modeling a set of artifacts and actions common to many
debuggers, known as the debug model. For example, some common debug artifacts
are threads, stack frames, variables, and breakpoints; and some common actions
are suspending, stepping, resuming, and terminating. The platform does not provide
an implementation of a debugger – that is the job of language tool developers.
However, the platform does provide a basic debugger user interface (that is,
the debug perspective) that can be enhanced with features specific to a particular
debugger. The base user interface operates against the debug model interfaces,
providing views for a program’s call stack, variables, breakpoints, watch items,
and console I/O, and allows a user to step through source code. The debug platform
also provides a framework for launching applications from within the Eclipse
IDE, and a framework to perform source lookup.</p>
<p>In order to keep this paper to the stated goal of “how to write a debugger,”
and away from the larger problem of “how to add a support for a new language,”
we make the following assumptions:</p>
<li>The new language has an existing execution engine or interface or VM. This
paper does not describe how to write an interpreter, virtual machine, or compiler
run-time. For our example, the interpreter is implemented as a Perl program
(pdavm/ in the example code). The language and the debugger interface
are described in the next section. </li>
<li>An Eclipse launcher and launch configuration already exists for the new
language. At this point, the launcher needs to create a “run” configuration
– this paper will describe how to add a “debug” configuration. (For completeness,
here are the parts that comprise the example launcher support:
<li>org.eclipse.debug.examples.core contributes an org.eclipse.debug.core.launchConfigurationTypes
named org.eclipse.debug.examples.core.launchConfigurationType.pda, as
well as the two class implementation of the launch configuration (IPDAConstants
and PDALaunchDelegate)</li>
<li>org.eclipse.debug.examples.core contributes an org.eclipse.core.variables.valueVariables
that points to the Perl executable</li>
<li>org.eclipse.debug.examples.ui contributes an org.eclipse.debug.ui.launchConfigurationTabGroups
for that launch configuration type, as well as the two-class implementation
of the tab groups (PDATabGroup and PDAMainTab).</li>
<li>The source for the new language is text files that are displayed in a text
editor. The text editor might be a specialized editor with code coloring,
formatting, extra menu items, etc., or it might be just the standard Eclipse
text editor.</li>
<h2>Our language and its interpreter</h2>
<p>To demonstrate how to write a debugger for Eclipse, we need a language and
a run time to debug. For this example, we chose an enhanced push down automata
(PDA) assembly language and a simple interpreter implemented in Perl. Each line
contains a single operation and any number of arguments. Our language differs
from a standard PDA in two major ways:</p>
<li>Our language has a control stack and thus has call-return subroutines.</li>
<li>Our language allows data to be stored either on the data stack or in named
variables on the control stack.</li>
<blockquote>In order to actually run this example, you will need a Perl interpreter.
Linux&reg;&#153; comes with Perl. For Microsoft&reg; Windows&reg;, we use either
ActivePerl (<a href=""></a>)
or Indigo Perl (<a href=""></a>).
You also have to set the string substitution variable named “perlExecutable”
to the complete path to your Perl interpreter. (For example, ours was C:\perl\bin\perl.exe)
To set a string substitution variable, use the Windows &gt; Preferences &gt;
Run/Debug &gt; String Substitution preferences page.</blockquote>
<p>Here is an annotated example of the Fibonacci computation (note that the
annotations themselves are not valid syntax in this language – in this language,
all comments must start at column 1 and be the entire line):</p>
<pre>push 6
call Fibonacci <i>function call with one argument on the data stack</i>
output <i>print result to stdout</i>
# f(n) = f(n-1) + f(n-2)
# f(0) = 1
# f(1) = 1
var n <i>define variable n on control stack</i>
pop $n <i>get n from data stack</i>
push $n
branch_not_zero gt0
push 1 <i>f(0) = 1</i>
return <i>return with one value on data stack</i>
push $n
branch_not_zero gt1
push 1 <i>f(1) = 1</i>
return <i>return with one value on data stack</i>
push $n <i>stack: n</i>
dec <i>stack: n-1</i>
call fibonacci <i>stack: f(n-1)</i>
push $n <i>stack: f(n-1) n</i>
dec <i>stack: f(n-1) n-1</i>
dec <i>stack: f(n-1) n-2</i>
call Fibonacci <i>stack: f(n-1) f(n-2)</i>
add <i>stack: f(n-1)+f(n-2)</i>
return <i>return with one value on data stack</i></pre>
<h3><a name="Interpreter Debug Interface">Interpreter debug interface</a></h3>
<p>Our PDA assembly language interpreter can be started in either run mode or
debug mode. When started in debug mode, the interpreter listens for debug
commands on a specified local TCP socket and sends debug events to a separate
local TCP socket. The commands include:</p>
<li><samp>clear N</samp> – clear the breakpoint on line N</li>
<li><samp>data</samp> – return the contents of the data stack; the data is
returned from oldest to newest as a single string “value|value|value|…|value|”</li>
<li><samp>exit</samp> – end the interpreter</li>
<li><samp>resume</samp> – resume full speed execution of the program</li>
<li><samp>set N</samp> – set a breakpoint on line N</li>
<li><samp>stack</samp> – return the contents of the control stack (program
counters, function and variable names); the stack is returned from oldest to
newest as a single string “frame#frame#frame#…#frame”. Each frame is a string
“filename|pc|function name|variable name|variable name| …|variable name”</li>
<li><samp>step</samp> – single step forward</li>
<li><samp>suspend</samp> – end full speed execution and listen for debug
<li><samp>var N M</samp> – return the contents of a variable M from the
control stack frame N (stack frames are indexed from 0).</li>
<p>The debug events that are reported asynchronously to the second socket
<li><samp>started</samp> – the interpreter has started (guaranteed to be the
first event sent)</li>
<li><samp>terminated</samp> – the interpreter has terminated (guaranteed to
be the last event sent)</li>
<li><samp>suspended X</samp> – the interpreter has suspended and entered debug
mode; X is the cause of the suspension, either step or client or breakpoint
<li><samp>resumed X</samp> – the interpreter has resumed execution in run mode;
X is the cause of the resume, either step or client</li>
<li><samp>unimplemented instruction X</samp> – an unimplemented instruction
was encountered</li>
<li><samp>no such label X</samp> – a branch or call to an unknown label was
encountered </li>
<h2>Adding debugger support to the launch delegate</h2>
<blockquote><i>First Notation Note: </i>The class and object diagrams in this article use a
mostly UML syntax. It is only mostly UML because, unfortunately, UML does not
provide a way to model extension points and extensions. Therefore, we decided to
use a dashed box to indicate an extension in the plugin.xml file. We also use a
few non-standard, but labeled, lines with arrows to indicate certain semantics.<p>
<i>Second Notation Note:</i> The Eclipse design guidelines include separating the
code for the model from the code for the user interface. We abide by that design
principle in our example code resulting in two plug-ins. The extensions and code
discussed in this article come from both plug-ins.</p>
<p><i>Third Notation Note:</i> Each code and plugin.xml fragment in this article
includes a header describing the plug-in, package, class, and other details of
its location in the complete example code. In order to fit all this on one line,
the header abbreviates the org.eclipse.debug.examples.pda.core plug-in as
“core,” the org.eclipse.debug.examples.core.pda.model package as “pda.model,”
<p>Using the previous assumptions, we already have a launch configuration type
(org.eclipse.debug.examples.core.launchConfigurationType.pda) and a launch configuration
delegate (PDALaunchDelegate). To add debugger support, we modify the delegate
by adding code for DEBUG_MODE that performs the following actions:</p>
<li> <img src="images/tag_1.gif" align=CENTER width="24" height="13"> Modifies
the process command line to include the debug parameters. For details on what
the debug parameters for our interpreter are, see the previous section.</li>
<li> <img src="images/tag_2.gif" align=CENTER width="24" height="13"> Creates
and registers the PDADebugTarget object for this launch. For details on where
the debug target fits into the model, see the next section.</li>
<p><u> <font size="-1">Plug-in:</font> <font color="blue">core</font>, <font size="-1">Package:</font>
<font color="blue">pda.launching</font>, <font size="-1">Class:</font> <font color="blue">PDALaunchDelegate</font>,
<font size="-1">Method:</font> <font color="blue">launch</font></u>
<pre> commandList.add(file.getLocation().toOSString());
<img src="images/tag_1.gif" align=CENTER width="24" height="13"> int requestPort = -1;
<font color="#0000FF">.</font> int eventPort = -1;
<font color="#0000FF">.</font> if<b> </b>(mode.equals(ILaunchManager.DEBUG_MODE)) {
<font color="#0000FF">.</font> requestPort = findFreePort();
<font color="#0000FF">.</font> eventPort = findFreePort();
<font color="#0000FF">.</font> if (requestPort == -1 || eventPort == -1) {
<font color="#0000FF">.</font> abort(&quot;Unable to find free port&quot;, null);
<font color="#0000FF">.</font> }
<font color="#0000FF">.</font> commandList.add(&quot;-debug&quot;);
<font color="#0000FF">.</font> commandList.add(&quot;&quot; + requestPort );
<font color="#0000FF">.</font> commandList.add(&quot;&quot; + eventPort );
<font color="#0000FF">.</font> }
String[] commandLine = (String[])
commandList.toArray(new String[commandList.size()]);
Process process = DebugPlugin.exec(commandLine, null);
IProcess p = DebugPlugin.newProcess(launch, process, path);
<img src="images/tag_2.gif" align=CENTER width="24" height="13"> if (mode.equals(ILaunchManager.DEBUG_MODE)) {
<font color="#0000FF">.</font> IDebugTarget target = new PDADebugTarget(launch,p,requestPort,eventPort );
<font color="#0000FF">.</font> launch.addDebugTarget(target);
<font color="#0000FF">.</font> }</pre>
<h2>The debug model</h2>
<p>The Eclipse debug model is documented in the <i><cite>Platform Plug-in Developer
Guide</cite></i> under <b>Programmer’s Guide &gt; Program debug and launch support&gt;
Debugging a Program&gt; Platform debug model</b>. Being more visually oriented,
we prefer to look at pictures, so here is a mostly UML diagram of the Eclipse
platform debug model:</p>
<p align="center">
<img border="0" src="images/debug_model.gif" width="408" height="608"></p>
<p>To implement our debugger (the PDA debugger), we have to provide an
implementation of each of these debug model interfaces. Most of the
implementations are very straightforward except, perhaps, these small items:</p>
<li>The PDADebugElement class includes fireEvent, fireCreationEvent, fireSuspendEvent,
and other methods as a convenience to avoid code duplication in the subclasses.</li>
<li>The PDADebugTarget is the object that communicates with our interpreter
and is complex enough that we describe it in a following section of its own.</li>
<li>There is only one thread in a PDA program, so the PDAThread class delegates
all its operations (suspend, resume, isSuspended, getStackFrames, and so on)
to the debug target. This allows us to encapsulate all communication with
the interpreter in our single PDADebugTarget object.</li>
<li>The only thing the PDAStackFrame does that is mildly interesting is to parse
and cache the stack frame reply message from the interpreter (the string described
in the previous section). Other than that, the implementation of getLineNumber
returns the previously cached line number; the implementation of stepOver
delegates to the thread; and so on.</li>
<li>PDAVariable is just a wrapper for the variable name; it delegates the
fetching of the value to the debug target.</li>
<li>PDAValue is even simpler because the PDA language has only one type: a
value. Strings and numbers are stored identically in the interpreter and thus
the only thing the PDAValue object does is display the value.</li>
<li>The PDALineBreakpoint class is the second most complex class in our model
(behind the debug target class), but the Eclipse debug framework provides
an excellent abstract implementation of line-oriented breakpoints named LineBreakpoint.
Thus PDALineBreakpoint subclasses LineBreakpoint and adds its model identifier
and its own constructor. The constructor creates and associates a marker with
the breakpoint so that they show up in the annotation ruler. (See <a href="#Breakpoints">section
below</a> on breakpoints for more details about this code and the marker ID.)</li>
<pre> public PDALineBreakpoint(IResource resource, int lineNumber) throws CoreException {
IMarker marker = resource.createMarker(
ensureMarker().setAttribute(IMarker.LINE_NUMBER, lineNumber);
ensureMarker().setAttribute(IBreakpoint.ID, IPDAConstants.ID_PDA_DEBUG_MODEL);
<p>There are two main issues to keep in mind when implementing the debug model
<li>The Eclipse platform is inherently multi-threaded so it is important that
all the classes are thread safe, including those that must be single threaded.
For example, the debug socket channel to our PDA interpreter is single threaded:
the interpreter can only receive one command or request at a time. Thus the
methods in our model that communicate with the interpreter must be synchronized
to prevent overlapping requests.</li>
<li>The user interface actions, for example the step command, are issued from
Eclipse’s user interface thread. Thus these actions must be non-blocking.
We use step as the example here, because it might seem that step should block,
that it should send the step debug command and wait for the interpreter to
do the step, but that behavior would be wrong. As we will see below, the interpreter
communicates with the debug model by firing events rather than returning result
codes from commands.</li>
<p><a name="Blocking commands">The incorrect blocking command</a> would block the entire Eclipse
user interface
until the interpreter had finished stepping. In the case of infinite loops and
other client program defects, the worst case is that the interpreter might never
return thus leaving the Eclipse user interface completely frozen.</p>
<p align="center">
<img border="0" src="images/incorrect.gif" width="327" height="336"></p>
<p>By sending the step command asynchronously, the Eclipse user interface
remains responsive while the interpreter performs the command. (For those
readers who are not 100% familiar with UML, notice the difference in the arrows:
a synchronous call has a full arrow head and an asynchronous message has a half
arrow head.)</p>
<p align="center"> <img border="0" src="images/correct.gif" width="327" height="334"></p>
<h3>Code highlights of our debug target</h3>
<p>In our debug model, the PDADebugTarget is where most of the action happens
because the debug target is where we centralize the communication with the PDA
interpreter. For example, when the variable view asks a variable for its value,
the variable asks the debug target to ask the interpreter for the value.</p>
<p align="center">
<img border="0" src="images/get_variable_value.gif" width="451" height="180"></p>
<h4>Constructor and initialization</h4>
<p>As described <a href="#Interpreter Debug Interface">above</a>, the debug
interface to our interpreter consists of two sockets: one for debug commands and
one for debug events. Thus when we create our debug target,
<img src="images/tag_1.gif" align=CENTER width="24" height="13"> first we initialize some instance
variables (our launch, our process, our one thread, etc), then
<img src="images/tag_2.gif" align=CENTER width="24" height="13"> we open two debug sockets with readers
and writers, then <img src="images/tag_3.gif" align=CENTER width="24" height="13"> we start a thread to
listen on the event socket (we are using the Eclipse background process
mechanism), and lastly
<img src="images/tag_4.gif" align=CENTER width="24" height="13"> we register to listen for breakpoint
changes in order to send the “set” and “clear” commands to the interpreter.</p>
<p><u> <font size="-1">Plug-in:</font> <font color="blue">core</font>, <font size="-1">Package:</font>
<font color="blue"> pda.</font><font color="#0000FF">model</font>, <font size="-1">Class:</font>
<font color="#0000FF">PDADebugTarget</font></u>
<pre> public PDADebugTarget(ILaunch launch, IProcess process,
int requestPort, int eventPort) throws CoreException {
<img src="images/tag_1.gif" align=CENTER width="24" height="13"> fLaunch = launch;
<font color="#0000FF">.</font> fTarget = this;
<font color="#0000FF">.</font> fProcess = process;
<font color="#0000FF">.</font> fThread = new PDAThread(this);
<font color="#0000FF">.</font> fThreads = new IThread[] {fThread};
<img src="images/tag_2.gif" align=CENTER width="24" height="13"> try {
<font color="#0000FF">.</font> fRequestSocket = new Socket(&quot;localhost&quot;, requestPort);
<font color="#0000FF">.</font> fRequestWriter = new PrintWriter(fRequestSocket.getOutputStream());
<font color="#0000FF">.</font> fRequestReader = new BufferedReader(new InputStreamReader(
<font color="#0000FF">.</font> fRequestSocket.getInputStream()));
<font color="#0000FF">.</font> fEventSocket = new Socket(&quot;localhost&quot;, eventPort);
<font color="#0000FF">.</font> fEventReader = new BufferedReader(new InputStreamReader(
<font color="#0000FF">.</font> fEventSocket.getInputStream()));
<font color="#0000FF">.</font> } catch (UnknownHostException e) {
<font color="#0000FF">.</font> abort(&quot;Unable to connect to PDA VM&quot;, e);
<font color="#0000FF">.</font> } catch (IOException e) {
<font color="#0000FF">.</font> abort(&quot;Unable to connect to PDA VM&quot;, e);
<font color="#0000FF">.</font> }
<img src="images/tag_3.gif" align=CENTER width="24" height="13"> fEventDispatch = new EventDispatchJob();
<font color="#0000FF">.</font> fEventDispatch.schedule();
<img src="images/tag_4.gif" align=CENTER width="24" height="13"> DebugPlugin.getDefault().getBreakpointManager().addBreakpointListener(this);
<h4>Communicate with interpreter</h4>
<p>Now that communications with the debug sockets on the interpreter are
initialized, the debug target methods can send the appropriate commands. For
example, step and getVariableValue:</p>
<p><u> <font size="-1">Plug-in:</font> <font color="blue">core</font>, <font size="-1">Package:</font>
<font color="blue"> pda.</font><font color="#0000FF">model</font>, <font size="-1">Class:</font>
<font color="#0000FF">PDADebugTarget</font></u>
<pre> public void step() throws DebugException {
private void sendRequest(String request) throws DebugException {
synchronized (fRequestSocket) {
try {
// wait for &quot;ok&quot;
String response = fRequestReader.readLine();
} catch (IOException e) {
abort(&quot;Request failed: &quot; + request, e);
protected IValue getVariableValue(PDAVariable variable) throws DebugException {
synchronized (fRequestSocket) {
fRequestWriter.println(&quot;var &quot;
+ variable.getStackFrame().getIdentifier()
+ &quot; &quot; + variable.getName());
try {
String value = fRequestReader.readLine();
return new PDAValue(this, value);
} catch (IOException e) {
abort(MessageFormat.format(&quot;Unable to retrieve value for variable {0}&quot;,
new String[]{variable.getName()}), e);
return null;
<p>There are many more &quot;communicate with the interpreter&quot; methods. Notice
that all communication with the interpreter is serialized using synchronized
blocks – as mentioned previously, this is necessary because the Eclipse platform
is inherently multi-threaded but our PDA interpreter is not.</p>
<p>Note that our getVariableValue method is synchronous which goes against our
<a href="#Blocking commands">earlier caveat</a> to avoid using blocking communication
with the target due to the risk of freezing the Eclipse user interface. We chose
the synchronous &quot;send the request and wait for reply&quot; rather than
an asychronous &quot;send command and later receive the response in an event&quot;
for its simplicity in explaining the essential concepts of communication.</p>
<h4>Breakpoints are slightly more interesting</h4>
<p>The add-a-breakpoint method is a little more interesting because it has to
check if
<img src="images/tag_1.gif" align=CENTER width="24" height="13"> the breakpoint is valid and
<img src="images/tag_2.gif" align=CENTER width="24" height="13"> active, but otherwise it has the
same “communicate with the interpreter” style. The key to determining if the
breakpoint is valid is realizing that breakpoint listeners (like this one) get
notified of all breakpoint changes, thus we filter for breakpoints
<img src="images/tag_3.gif" align=CENTER width="24" height="13"> supported by our debug model and for
<img src="images/tag_4.gif" align=CENTER width="24" height="13"> our PDA program:</p>
<p><u> <font size="-1">Plug-in:</font> <font color="blue">core</font>, <font size="-1">Package:</font>
<font color="blue"> pda.</font><font color="#0000FF">model</font>, <font size="-1">Class:</font>
<font color="#0000FF">PDADebugTarget</font></u>
<pre> public void breakpointAdded(IBreakpoint breakpoint) {
<img src="images/tag_1.gif" align=CENTER width="24" height="13"> if (supportsBreakpoint(breakpoint)) {
try {
<img src="images/tag_2.gif" align=CENTER width="24" height="13"> if (breakpoint.isEnabled()) {
synchronized (fRequestSocket) {
try {
sendRequest(&quot;set &quot;
+ (((ILineBreakpoint)breakpoint).getLineNumber() - 1));
} catch (CoreException e) {
} catch (CoreException e) {
public boolean supportsBreakpoint(IBreakpoint breakpoint) {
<img src="images/tag_3.gif" align=CENTER width="24" height="13"> if (breakpoint.getModelIdentifier().equals(IPDAConstants.ID_PDA_DEBUG_MODEL)) {
try {
String program = getLaunch().getLaunchConfiguration()
if (program != null) {
IMarker marker = breakpoint.getMarker();
if (marker != null) {
IPath p = new Path(program);
<img src="images/tag_4.gif" align=CENTER width="24" height="13"> return marker.getResource().getFullPath().equals(p);
} catch (CoreException e) {
return false;
<h4>Synchronizing with the interpreter at startup</h4>
<p>The other interesting piece of our debug target is the startup code. When the
interpreter starts up, it has no breakpoints. Users, having set breakpoints in
the code using the Eclipse user interface, expect those breakpoints to work.
Thus, after the interpreter starts up, but before it processes any instructions,
the debug target has to reach in and set the initial breakpoints. These initial
breakpoints are known as <i>deferred breakpoints</i>.</p>
<p>The standard way this initial setup is accomplished is by having the
interpreter suspend on startup, <img src="images/tag_2.gif" align=CENTER width="24" height="13"> wait
for the debug target to set all the breakpoints, and then
<img src="images/tag_3.gif" align=CENTER width="24" height="13"> have the debug target resume the
interpreter. (Our apologies to the reader here, but we’ve gotten a little out of
order in describing things. The
<img src="images/tag_1.gif" align=CENTER width="24" height="13"> started method is called by our debug
event handler right after the interpreter starts. But at this point in the
paper, we have not described events, so you’ll just have to trust us on this
<p><u> <font size="-1">Plug-in:</font> <font color="blue">core</font>, <font size="-1">Package:</font>
<font color="blue">pda.launching</font>, <font size="-1">Class:</font> <font color="#0000FF">PDADebugTarget</font></u>
<pre><img src="images/tag_1.gif" align=CENTER width="24" height="13"> private void started() {
<img src="images/tag_2.gif" align=CENTER width="24" height="13"> installDeferredBreakpoints();
try {
<img src="images/tag_3.gif" align=CENTER width="24" height="13"> resume();
} catch( DebugException x ) {
private void installDeferredBreakpoints() {
<font color="#0000FF"> </font> IBreakpoint[] breakpoints = DebugPlugin.getDefault().getBreakpointManager()
<font color="#0000FF"> </font> .getBreakpoints(IPDAConstants.ID_PDA_DEBUG_MODEL);
<font color="#0000FF"> </font> for (int i = 0; i &lt; breakpoints.length; i++) {
<font color="#0000FF"> </font> breakpointAdded(breakpoints[i]);
<font color="#0000FF"> </font> }
<font color="#0000FF"> </font> }</pre>
<p>Here’s a sequence diagram showing how the debug target, the interpreter, and
the events interact:</p>
<p align="center"><img border="0" src="images/synchronize_w_interpreter.gif" width="432" height="390"></p>
<p>Note that because the interpreter events are delivered over a TCP socket to
the PDADebugTarget, even if the interpreter starts up and sends the &quot;started&quot;
event before the PDATarget is instantiated, the &quot;started&quot; event will
be queued in the socket. The potential race condition between the interpreter
and debug target is eliminated by having the interpreter suspend on startup.</p>
<h2>The debug events</h2>
<p>The Eclipse debug model uses debug events (DebugEvent) to describe events that
occur as a program is being debugged. Each element in the debug model has a
specific set of events that is supports – all of this is documented in the Javadoc
of DebugEvent and in the <i><cite>Platform Plug-in Developer Guide</cite></i>.</p>
<h3>Code highlights of our debug events</h3>
<p>As described <a href="#Interpreter Debug Interface">previously</a>, our example
PDA interpreter uses two sockets to communicate with its debugger: a command
channel and an event channel. Our debugger uses synchronous RPC over the command
channel (send command, block, receive reply), but the events can arrive asynchronously
so the debugger must use a separate thread to listen on the event channel.</p>
<p>We create the separate event listener thread when we create the debug target
(see previous). The main routine of the thread is a dispatch loop that reads
from the event channel and dispatches to the appropriate event behavior:</p>
<p><u> <font size="-1">Plug-in:</font> <font color="blue">core</font>, <font size="-1">Package:</font>
<font color="blue"> pda.model</font>, <font size="-1">Class:</font> <font color="#0000FF">PDADebugTarget.EventDispatchJob</font></u>
<pre> <font color="#0000FF"> </font> protected IStatus run(IProgressMonitor monitor) {
<font color="#0000FF"> </font> String event = &quot;&quot;;
<font color="#0000FF"> </font> while (!isTerminated() &amp;&amp; event != null) {
<font color="#0000FF"> </font> try {
<font color="#0000FF"> </font> event = fEventReader.readLine();
<font color="#0000FF"> </font> if (event != null) {
<font color="#0000FF"> </font> fThread.setBreakpoints(null);
<font color="#0000FF"> </font> fThread.setStepping(false);
<font color="#0000FF"> </font> …dispatch to the event behavior method…
<font color="#0000FF"> </font> }
<font color="#0000FF"> </font> } catch (IOException e) {
<font color="#0000FF"> </font> terminated();
<font color="#0000FF"> </font> }
<font color="#0000FF"> </font> }
<font color="#0000FF"> </font> return Status.OK_STATUS;
<font color="#0000FF"> </font> }</pre>
<p>Our interpreter sends six kinds of events (see <a href="#Interpreter Debug Interface">previous</a>),
but we only handle four kinds with five subtypes. We simply (and safely) ignore
the events that we do not yet handle.</p>
<pre> if (event.equals(&quot;started&quot;)) {
} else if (event.equals(&quot;terminated&quot;)) {
} else if (event.startsWith(&quot;resumed&quot;)) {
if (event.endsWith(&quot;step&quot;)) {
} else if (event.endsWith(&quot;client&quot;)) {
} else if (event.startsWith(&quot;suspended&quot;)) {
if (event.endsWith(&quot;client&quot;)) {
} else if (event.endsWith(&quot;step&quot;)) {
} else if (event.indexOf(&quot;breakpoint&quot;) &gt;= 0) {
<p>The event handlers are also simple: they turn around and fire the appropriate
DebugEvent to all the listeners. As implementers of the debug model, we don’t
have to know who those listeners are, but curiosity gets the better of us and we
want to know. The answer is “the debug views”. The debug views listen for events
and use those events to update the user interface to show the current state of
the debugged program.</p>
<p>In the debug target and the debug element:</p>
<p><u> <font size="-1">Plug-in:</font> <font color="blue">core</font>, <font size="-1">Package:</font>
<font color="blue"> pda.model</font>, <font size="-1">Class:</font> <font color="#0000FF">PDADebugTarget</font></u>
<pre> <font color="#0000FF"> </font> private void resumed(int detail) {
<font color="#0000FF"> </font> fSuspended = false;
<font color="#0000FF"> </font> fThread.fireResumeEvent(detail);
<font color="#0000FF"> </font> }</pre>
<p><u> <font size="-1">Plug-in:</font> <font color="blue">core</font>, <font size="-1">Package:</font>
<font color="blue"> pda.model</font>, <font size="-1">Class:</font> <font color="#0000FF">PDADebugElement</font></u>
<pre> public void fireResumeEvent(int detail) {
fireEvent(new DebugEvent(this, DebugEvent.RESUME, detail));
<p>Handling the suspended-due-to-breakpoint event in the debug target is a
little more involved because the IThread object keeps track of which breakpoint
was hit. The IThread object keeps track of the breakpoint that caused the
suspension in case the user interface wants to use that information in labels or
icons or some other view. To gather this information, we
<img src="images/tag_1.gif" align=CENTER width="24" height="13"> extract the breakpoint number from the
PDA interpreter event message,
<img src="images/tag_2.gif" align=CENTER width="24" height="13"> find the corresponding breakpoint
object, and then
<img src="images/tag_3.gif" align=CENTER width="24" height="13"> annotate the current thread object
with that breakpoint.</p>
<p><u> <font size="-1">Plug-in:</font> <font color="blue">core</font>, <font size="-1">Package:</font>
<font color="blue"> pda.model</font>, <font size="-1">Class:</font> <font color="#0000FF">PDADebugTarget</font></u>
<pre> private void breakpointHit(String event) {
int lastSpace = event.lastIndexOf(' ');
if (lastSpace &gt; 0) {
String line = event.substring(lastSpace + 1);
<img src="images/tag_1.gif" align=CENTER width="24" height="13"> int lineNumber = Integer.parseInt(line);
<img src="images/tag_2.gif" align=CENTER width="24" height="13"> IBreakpoint[] breakpoints = DebugPlugin.getDefault().getBreakpointManager()
<font color="#0000FF">.</font> .getBreakpoints(IPDAConstants.ID_PDA_DEBUG_MODEL);
<font color="#0000FF">.</font> for (int i = 0; i &lt; breakpoints.length; i++) {
<font color="#0000FF">.</font> IBreakpoint breakpoint = breakpoints[i];
<font color="#0000FF">.</font> if (supportsBreakpoint(breakpoint)) {
<font color="#0000FF">.</font> if (breakpoint instanceof ILineBreakpoint) {
<font color="#0000FF">.</font> ILineBreakpoint lineBreakpoint = (ILineBreakpoint) breakpoint;
<font color="#0000FF">.</font> try {
<font color="#0000FF">.</font> if (lineBreakpoint.getLineNumber() == lineNumber) {
<img src="images/tag_3.gif" align=CENTER width="24" height="13"> fThread.setBreakpoints(new IBreakpoint[]{breakpoint});
} catch (CoreException e) {
<h3>Pretty pictures of how our debug events work</h3>
<p>Here are two sequences showing how the events arrive asynchronously and are
processed by the Eclipse debug framework. First, here is how a step action from
the user interface gets sent to the interpreter as a “step” command on the command
channel. The interpreter operates asynchronously, sending events (“resume” and
then “suspend”) back to the debugger by way of the event channel.</p>
<p align="center"><img border="0" src="images/single_step.gif" width="444" height="435"></p>
<p>Very similarly, here is the sequence for a resume action, followed by the
interpreter running for a while and then hitting a breakpoint:</p>
<p align="center"><img border="0" src="images/run_until_breakpoint.gif" width="467" height="435"></p>
<p>The astute reader will notice that the sequence diagram is exactly the same
(except for the slightly different command and different subevents). This regularity
to the structure and behavior is a deliberate design decision in the Eclipse
debug framework.</p>
<h2>Source lookup</h2>
<p>Once our debug model and event handler are in place, the Eclipse debugger
will work with our interpreter, but its user interface will be quite generic and
uninteresting. For example, the generic debugger does not show or highlight
source code. </p>
<p>Highlighting the current source code line or statement is de rigueur in
modern debuggers, so our next step is to enable what is known as “source code
lookup” in our debugger. To do so, we add three new pieces:</p>
<li>Our launch object needs a source locator.</li>
<li>The source locator object translates stack frames into source elements. In
our case, the source elements are IFile objects.</li>
<li>Finally, our debug model presentation object maps the source elements (IFiles)
to editor ids and inputs. Our example uses the default Eclipse text editor.</li>
<p>To see how this works, let’s start by examining the Eclipse framework for
displaying source code of stack frames.</p>
<h3>The most general case</h3>
<p>In the most general case, the launch object has a source locator object. When
the source code for a stack frame needs to be displayed, the stack frame is
passed to the source locator method to retrieve the source element. The stack
frame’s model identifier is used to find the debug model presentation object
(by way of the corresponding extension). The debug model presentation maps the
source element to an editor id and editor input, which are then used by the
workbench to open an editor to display the source code.</p>
<p align="center"><img border="0" src="images/scl_most_general_case.gif" width="620" height="283"></p>
<h3>Extension defined source locator</h3>
<p>If the launch object does not have a source locator object (that is, a source
locator is not assigned by the launch delegate), the debug framework uses the
sourceLocatorId attribute of the launchConfigurationType extension to instantiate
an ISourceLocator object and store it in the launch object. Thereafter, the
rest of the code works as previously described.</p>
<p align="center"><img border="0" src="images/scl_extension_defined.gif" width="576" height="67"></p>
<h3>Standard kind of source locator</h3>
<p>If your source lookup mechanism is standard, that is, files in directories,
you can use the Eclipse debug framework’s standard source lookup. The ISourceLookupDirector
mechanism looks up source files in directories (and zips and jars) along a path
(known as the source lookup path). The framework has three pieces:</p>
<li>participants (ISourceLookupParticipant) – objects that map an IStackFrame
into a filename</li>
<li>containers (ISourceContainer) – objects that find files by filename in directories,
zips, jars, etc.</li>
<li>source lookup tab (SourceContainerLookupTab) – an ILaunchConfigurationTab
that provides a user interface for configuring and modifying a source lookup
<p>The ISourceLookupDirector is an ISourceLocator, so once you have implemented
an ISourceLookupDirector, you place the class name in the sourceLocator
<p align="center"><img border="0" src="images/scl_standard_kind.gif" width="599" height="259"></p>
<h3>Standard implementation of standard kind of source locator</h3>
<p>Although ISourceLookupDirector is available as an interface for
reimplementation, most debuggers use the default implementation provided by the
Eclipse debug framework. The AbstractSourceLookupDirector provides the algorithm
for looking source files up along a source path. The only unimplemented method
is initializeParticipants, which creates the collection of
ISourceLookupParticipants for mapping the stack frames to filenames.</p>
<p align="center"><img border="0" src="images/scl_standard_implementation.gif" width="352" height="352">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </p>
<h3>Default source path in the standard implementation</h3>
<p>The AbstractSourceLookupDirector has one more trick up its sleeve: if the user
has not specified the source path (that is, the source containers) either programmatically
or by way of the SourceContainerLookupTab, the AbstractSourceLookupDirector
will compute the default source path. Here’s how it works:</p>
<p>When initialized, the AbstractSourceLookupDirector has a default source path
of a single container: a DefaultSourceContainer. The DefaultSourceContainer uses
an ISourcePathComputer (which in turn uses an ISourcePathComputerDelegate) to
compute the source path. The source path computer is reused to recompute the
default source path until such time as the source containers in the source
lookup director are explicitly set to something other than the default.</p>
<p>The source path computer is defined for each launch configuration type using
the sourcePathComputer extension. The source path computer delegate has one
method: computeSourceContainers.</p>
<p align="center"><img border="0" src="images/scl_default_source_path.gif" width="477" height="277"></p>
<h3>Code highlights of our simple solution to source code lookup</h3>
<p>The simplest solution to source code lookup is to reuse that wonderful
framework the Eclipse debug framework provides. Being good developers, we seek
to reuse rather than reinvent, so our example uses an
AbstractSourceLookupDirector to lookup source files, and a source path computer
to compute the default source lookup path. </p>
<blockquote><i>Notation note: </i>
The six steps below are strongly interconnected and thus the code references
<img src="images/tag_1.gif" align=CENTER width="24" height="13">
<img src="images/tag_2.gif" align=CENTER width="24" height="13">
<img src="images/tag_3.gif" align=CENTER width="24" height="13">
<img src="images/tag_4.gif" align=CENTER width="24" height="13">
<img src="images/tag_5.gif" align=CENTER width="24" height="13"> ... are uniform across the six
steps. In other words, the
<img src="images/tag_1.gif" align=CENTER width="24" height="13"> in step 1 is the same as the
<img src="images/tag_1.gif" align=CENTER width="24" height="13"> in step 3.</blockquote>
<h4>Step 1. source locator</h4>
<p>For additional simplicity, we chose to use the “extension defined source
locator” described above rather than having our launch delegate create and
assign the source locator to the launch object. Thus we start by adding
<img src="images/tag_1.gif" align=CENTER width="24" height="13"> a source locator id to our launch
configuration type. Then we added an extension for our source locator id pointing to
<img src="images/tag_2.gif" align=CENTER width="24" height="13"> our source lookup director class.</p>
<p><u> <font size="-1">Plug-in:</font> <font color="blue">core</font>, <font size="-1">
Extension:</font> <font color="blue"> org.eclipse.debug.core.launchConfigurationTypes</font></u>
<pre> &lt;launchConfigurationType
…other attributes…
<img src="images/tag_1.gif" align=CENTER width="24" height="13"> sourceLocatorId=&quot;org.eclipse.debug.examples.core.sourceLookupDirector.pda&quot;
<p><u> <font size="-1">Plug-in:</font> <font color="blue">core</font>, <font size="-1">
Extension: </font></u><img src="images/tag_1.gif" align=center width="24" height="13"><u>
<font color="blue">org.eclipse.debug.core.sourceLocators</font></u>
<pre> &lt;sourceLocator
name=&quot;PDA Source Lookup Director&quot;
<img src="images/tag_2.gif" align=CENTER width="24" height="13"> class=&quot;org.eclipse.debug.examples.core.pda.launching.PDASourceLookupDirector&quot;
<img src="images/tag_1.gif" align=center width="24" height="13"> id=&quot;org.eclipse.debug.examples.core.sourceLookupDirector.pda&quot;&gt;
<h4>Step 2. source lookup director</h4>
<p>The abstract class does most of the work, but our PDASourceLocatorDirector
subclass of AbstractSourceLookupDirector still has to initialize
<img src="images/tag_3.gif" align=CENTER width="24" height="13"> the set of participants. In our case,
the set of participants is the singleton PDASourceLookupParticipant. Our
participant is trivial because in our debug model,
<img src="images/tag_4.gif" align=CENTER width="24" height="13"> the stack frames keep track of their
source filenames.</p>
<p><u> <font size="-1">Plug-in:</font> <font color="blue">core</font>, <font size="-1">Package:</font>
<font color="blue"> pda.launching</font>, <font size="-1">Class: </font></u>
<img src="images/tag_2.gif" align=center width="24" height="13"><u> <font color="#0000FF">PDASourceLocatorDirector</font></u>
<pre> public void initializeParticipants() {
<img src="images/tag_3.gif" align=CENTER width="24" height="13"> addParticipants(new ISourceLookupParticipant[]{new PDASourceLookupParticipant()});
<p><u> <font size="-1">Plug-in:</font> <font color="blue">core</font>, <font size="-1">Package:</font>
<font color="blue"> pda.launching</font>, <font size="-1">Class: </font></u>
<img src="images/tag_3.gif" align=center width="24" height="13"><u> <font color="#0000FF">PDASourceLocatorParticipant</font></u>
<pre> public String getSourceName(Object object) throws CoreException {
if (object instanceof PDAStackFrame) {
<img src="images/tag_4.gif" align=CENTER width="24" height="13"> return ((PDAStackFrame)object).getSourceName();
return null;
<p>And that’s all it takes to configure, instantiate, and use the framework’s
source path lookup director.</p>
<h4>Step 3. source path computer extension</h4>
<p>Next we have to deal with the default source lookup path issue. Again, being
efficient, we use the framework provided source path computer extension as
explained above. We again
<img src="images/tag_5.gif" align=CENTER width="24" height="13"> add to our launch configuration
followed by augmenting the corresponding extension to point to our source path
computer class.</p>
<p><u> <font size="-1">Plug-in:</font> <font color="blue">core</font>, <font size="-1">
Extension:</font> <font color="blue"> org.eclipse.debug.core.launchConfigurationTypes</font></u>
<pre> &lt;launchConfigurationType
…other attributes…
<img src="images/tag_1.gif" align=CENTER width="24" height="13"> sourceLocatorId=&quot;org.eclipse.debug.examples.core.sourceLookupDirector.pda&quot;
<img src="images/tag_5.gif" align=CENTER width="24" height="13"> sourcePathComputerId=&quot;org.eclipse.debug.examples.core.sourcePathComputer.pda&quot;
<p><u> <font size="-1">Plug-in:</font> <font color="blue">core</font>, <font size="-1">
Extension: </font></u> <img src="images/tag_5.gif" align=center width="24" height="13"><u>
<font color="blue">org.eclipse.debug.core.sourcePathComputers</font></u>
<pre> &lt;sourcePathComputer
<img src="images/tag_6.gif" align=CENTER width="24" height="13"> class=&quot;org.eclipse.debug.examples.core.pda.launching.PDASourcePathComputerDelegate&quot;
<img src="images/tag_5.gif" align=center width="24" height="13"> id=&quot;org.eclipse.debug.examples.core.sourcePathComputer.pda&quot;&gt;
<h4>Step 4. source path computer implementation</h4>
<p>Our source path computer returns a one-element source path where the one
element is the ISourceContainer that contains the source file. This is the
longest method we have to write to get source lookup to work.</p>
<p><u> <font size="-1">Plug-in:</font> <font color="blue">core</font>, <font size="-1">Package:</font>
<font color="blue"> pda.launching</font>, <font size="-1">Class: </font></u>
<img src="images/tag_6.gif" align=center width="24" height="13"><u> <font color="#0000FF">PDASourcePathComputerDelegate</font></u>
<pre> public ISourceContainer[] computeSourceContainers(
ILaunchConfiguration configuration,
IProgressMonitor monitor) throws CoreException {
String path = configuration.getAttribute(
IPDAConstants.ATTR_PDA_PROGRAM, (String)null);
ISourceContainer sourceContainer = null;
if (path != null) {
IResource resource = ResourcesPlugin.getWorkspace().getRoot()
.findMember(new Path(path));
if (resource != null) {
IContainer container = resource.getParent();
if (container.getType() == IResource.PROJECT) {
sourceContainer = new ProjectSourceContainer(
(IProject)container, false);
} else if (container.getType() == IResource.FOLDER) {
sourceContainer = new FolderSourceContainer(
container, false);
if (sourceContainer == null) {
sourceContainer = new WorkspaceSourceContainer();
return new ISourceContainer[]{sourceContainer};
<h4>Step 5. debug model presentation</h4>
<p>As a penultimate step, we add the debugModelPresentation extension that will
map our source element (an IFile) to an editor for the workbench to display.</p>
<p><u> <font size="-1">Plug-in:</font> <font color="#0000FF">ui</font>, <font size="-1">
Extension:</font> <font color="blue"> org.eclipse.debug.ui.debugModelPresentations</font></u>
<pre> &lt;debugModelPresentation
<img src="images/tag_7.gif" align=CENTER width="24" height="13"> class=&quot;org.eclipse.debug.examples.ui.pda.launching.PDAModelPresentation&quot;
<img src="images/tag_8.gif" align=center width="24" height="13"> id=&quot;org.eclipse.debug.examples.pda&quot;&gt;
<p>The debug model is linked to the debug model presentation in the usual way
that Eclipse user interfaces are linked to Eclipse models: through the extension
id. For the debugger, the IDebugElement knows which debug model it is a part of.</p>
<p><u> <font size="-1">Plug-in:</font> <font color="blue">core</font>, <font size="-1">Package:</font>
<font color="blue"> pda.model</font>, <font size="-1">Class:</font> <font color="#0000FF">
<pre> public String getModelIdentifier() {
<img src="images/tag_8.gif" align=center width="24" height="13"> return IPDAConstants.ID_PDA_DEBUG_MODEL;
<p>The model presentation class has many methods, but for now the only two
interesting methods are those that translate source elements into editor ids and
<p><u> <font size="-1">Plug-in:</font> <font color="#0000FF">ui</font>, <font size="-1">Package:</font>
<font color="blue"> pda.model</font>, <font size="-1">Class: </font></u> <img src="images/tag_7.gif" align=center width="24" height="13"><u>
<font color="#0000FF">PDAModelPresentation</font></u>
<pre> public IEditorInput getEditorInput(Object element) {
if (element instanceof IFile)
return new FileEditorInput((IFile)element);
if (element instanceof ILineBreakpoint)
return new FileEditorInput((IFile)((ILineBreakpoint)element).getMarker().getResource());
return null;
public String getEditorId(IEditorInput input, Object element) {
if (element instanceof IFile || element instanceof ILineBreakpoint)
return &quot;org.eclipse.ui.DefaultTextEditor&quot;;
return null;
<h4>Step 6. source path tab</h4>
<p>The final step is to add the source path tab to our launch configuration dialog
box. This is <img src="images/tag_9.gif" align=center width="24" height="13">
a simple one-line addition to our existing tab group class (you will recall
that we had created the tab group class as part of our original launching code
back when we read the first article of this series)</p>
<p><u> <font size="-1">Plug-in:</font> <font color="#0000FF">ui</font>, <font size="-1">Package:</font>
<font color="blue"> pda.launching</font>, <font size="-1">Class: </font><font color="#0000FF">
<pre> public void createTabs(ILaunchConfigurationDialog dialog, String mode) {
setTabs(new ILaunchConfigurationTab[] {
new PDAMainTab(),
<img src="images/tag_9.gif" align=center width="24" height="13"> new SourceContainerLookupTab(),
new CommonTab()
<h2><a name="Breakpoints">Breakpoints</a></h2>
<p>Breakpoints are the final major component of a modern debugger that our debugger
is still missing. The Eclipse debug framework support for breakpoints is documented
in the <cite><i>Platform Plug-in Developer Guide</i></cite> under <b>Programmer’s
Guide &gt; Program debug and launch support&gt; Debugging a program&gt; Breakpoints</b>.
Our implementation follows that design.</p>
<p>Because our assembly language is line-oriented, our breakpoints are line-oriented,
that is, breakpoints are associated with lines and there can be at most one
breakpoint per line. Other more complex languages have statement or expression-oriented
breakpoints and multiple statements or expressions on a single line. We have
left discussion of those to the third paper of this series <i> Enhancing the
Eclipse Debugger</i>.</p>
<blockquote><i>Notation note: </i> We use the same &quot;code references are uniform
across the three steps&quot; in this section that we used in the previous section.</blockquote>
<h4>Step 1. breakpoint objects</h4>
<p>The first step to implementing our line-oriented breakpoints is to define our
breakpoint data structure using the breakpoint and the resource marker. As
described in the Platform Plug-in Developer Guide, the resource marker is the
mechanism by which the breakpoint is persisted between Eclipse sessions. We
conveniently reuse code by
<img src="images/tag_1.gif" align=CENTER width="24" height="13"> creating the marker for our line-oriented breakpoints
as a subtype of the Eclipse framework line-oriented breakpoint marker. Our
marker inherits all the attributes of the Eclipse marker and we do not need,
thus we do not define, any additional attributes.</p>
<p><u> <font size="-1">Plug-in:</font> <font color="#0000FF">core</font>, <font size="-1">
Extension:</font> <font color="blue"> org.eclipse.debug.core.breakpoints</font></u>
<pre> &lt;breakpoint
<img src="images/tag_1.gif" align=center width="24" height="13"> markerType=&quot;org.eclipse.debug.examples.core.pda.lineBreakpoint.marker&quot;
<img src="images/tag_2.gif" align=CENTER width="24" height="13"> class=&quot;org.eclipse.debug.examples.core.pda.model.PDALineBreakpoint&quot;
<p><u> <font size="-1">Plug-in:</font> <font color="#0000FF">core</font></u>
<pre> &lt;extension
<img src="images/tag_1.gif" align=CENTER width="24" height="13"> id=&quot;pda.lineBreakpoint.marker&quot;
&lt;super type=&quot;org.eclipse.debug.core.lineBreakpointMarker&quot;/&gt;
&lt;persistent value=&quot;true&quot;/&gt;
<p>Note that the org.eclipse.core.resources.markers extension id is &quot;pda.lineBreakpoint.marker&quot;
rather than &quot;org.eclipse.debug.examples.core.pda.lineBreakpoint.marker&quot; because
the id is automatically prefixed with plug-in id, i.e., &quot;org.eclipse.debug.examples.core&quot;.</p>
<h4>Step 2. toggle breakpoint menu item</h4>
<p>The toggle breakpoint menu item in the Eclipse debug perspective uses a re-targetable
action provided by the debug platform to toggle the breakpoints using the appropriate
debug model. The retargetable action asks the active part (editor, view, and
so on), for its toggle breakpoint adapter. When available, the adapter is used
as a delegate to toggle breakpoints. We use plugin.xml to register the factory
for the particular editor we are using, then we define <img src="images/tag_3.gif" align=CENTER width="24" height="13">
the factory and <img src="images/tag_4.gif" align=center width="24" height="13">
the adapter. The adapter has to be an IToggleBreakpointsTarget, but other than
that, this is standard Eclipse platform code.</p>
<p><u> <font size="-1">Plug-in:</font> <font color="#0000FF">ui</font>, <font size="-1">
Extension:</font> <font color="blue"> org.eclipse.core.runtime.adapters</font></u>
<pre> &lt;factory
<img src="images/tag_3.gif" align=CENTER width="24" height="13"> class=&quot;org.eclipse.debug.examples.ui.pda.model.PDABreakpointAdapterFactory&quot;
&lt;adapter type=&quot;org.eclipse.debug.ui.actions.IToggleBreakpointsTarget&quot;/&gt;
<p><u> <font size="-1">Plug-in:</font> <font color="#0000FF">ui</font>, <font size="-1">Package:</font>
<font color="blue"> pda.model</font>, <font size="-1">Class: </font></u> <img src="images/tag_3.gif" align=center width="24" height="13"><u><font size="-1">
</font><font color="#0000FF"> PDABreakpointAdapterFactory</font></u>
<pre> public Object getAdapter(Object adaptableObject, Class adapterType) {
if (adaptableObject instanceof ITextEditor) {
ITextEditor editorPart = (ITextEditor) adaptableObject;
IResource resource = (IResource) editorPart.getEditorInput().getAdapter(IResource.class);
if (resource != null) {
String extension = resource.getFileExtension();
if (extension != null &amp;&amp; extension.equals(&quot;pda&quot;)) {
<img src="images/tag_4.gif" align=center width="24" height="13"> return new PDALineBreakpointAdapter();
return null;
<p>Note that we use the plugin.xml to register the adapter rather than
programmatically registering the adapter. Using plugin.xml allows our adapter
(and thus our toggle breakpoints) to be active before our lazily loaded plug-in
is active.</p>
<p>Also note that this isn’t a completely elegant solution because we are
registering an adapter factory for the text editor. Which means that if other
plug-ins register a toggle breakpoints adapter for the text editor, one of the
plug-ins will work and the others will not, but there is no way to know which
one will be &quot;the winner&quot;. What we,
as language IDE developers, should really do is supply our own source code
editor and then adapt that editor rather than the generic text editor. However,
source code editing is out of the scope of this article and thus we use the
simple text editor for demonstrative purposes.</p>
<h4>Step 3. toggle breakpoint action</h4>
<p>Toggling a breakpoint is as expected: look for an existing breakpoint on the
<img src="images/tag_5.gif" align=center width="24" height="13"> if there is a breakpoint, remove it;
<img src="images/tag_6.gif" align=center width="24" height="13"> if there is not a breakpoint, create
one (of the class we
<img src="images/tag_2.gif" align=CENTER width="24" height="13"> registered in plugin.xml).</p>
<p><u> <font size="-1">Plug-in:</font> <font color="#0000FF">ui</font>, <font size="-1">Package:</font>
<font color="blue"> pda.model</font>, <font size="-1">Class: </font></u> <img src="images/tag_4.gif" align=center width="24" height="13"><u><font size="-1">
</font><font color="#0000FF"> PDALineBreakpointAdapter</font></u>
<pre> public void toggleLineBreakpoints(IWorkbenchPart part, ISelection selection)
throws CoreException {
ITextEditor textEditor = getEditor(part);
if (textEditor != null) {
IResource resource = (IResource) textEditor.getEditorInput()
ITextSelection textSelection = (ITextSelection) selection;
int lineNumber = textSelection.getStartLine();
IBreakpoint[] breakpoints = DebugPlugin.getDefault().getBreakpointManager()
for (int i = 0; i &lt; breakpoints.length; i++) {
IBreakpoint breakpoint = breakpoints[i];
if (resource.equals(breakpoint.getMarker().getResource())) {
if (((ILineBreakpoint)breakpoint).getLineNumber() == (lineNumber + 1)) {
<img src="images/tag_5.gif" align=center width="24" height="13"> breakpoint.delete();
<img src="images/tag_2.gif" align=CENTER width="24" height="13"><img src="images/tag_6.gif" align=center width="24" height="13"> PDALineBreakpoint lineBreakpoint = new PDALineBreakpoint(resource, lineNumber + 1);
<p>The only mildly interesting twist in our code is that our breakpoints store
the user view of line numbers (the first line of a file is line 1) rather than
the implementation view of line numbers (the first line of the file is line 0).
But you, our dear reader, have vast experience dealing with off-by-one errors
and have already figured this out.</p>
<p>The generic Eclipse text editor does not support double clicking in the ruler
to create breakpoints the way the more specialized source code editors do, but
again that is a feature of the way we chose to implement (or not implement)
our source code editor rather than a feature of the Eclipse debug framework’s
support for breakpoints.</p>
<h2>Summary and further information</h2>
<p>The Eclipse platform provides a very powerful framework that enables developers
to add support for a new language quickly and easily. Having read this far,
you now realize that adding debug support for a new language is a relatively
simple task. There are a small number of classes to implement (eighteen in our
example), and an even smaller number of extensions to specify (ten in our example).
For more details on any aspect of the framework, we refer you to the API documentation
of the Debug Platform and, for a very complete example, of the Java Debugger.</p>
<p>Our journey, however, is not ended yet. The launch and debug facilities we
have described and implemented in these first two papers are useful, but like
a light read, will ultimately not be satisfying to most language implementers.
We have described launching, debug target control (step/resume/suspend), basic
debug element display, source lookup, and breakpoints. Most language implementers
will also want watch points, more descriptive labels, informative icons, custom
views and language specific actions. For details on these enhancements to the
debug support, we refer you to the third paper of this trilogy, <i>Enhancing
the Eclipse Debugger</i>, which is still under construction.</p>
<p>Thanks to Darin Swanson and Cameron Stevens for their careful reviews and
critiques that have improved this paper.</p>
<h2>Source code</h2>
<p><img src="images/tryit.gif" width="61" height="13"> To examine the entire source
code, or to run the example, download the <a href="">zip file</a>
and unzip it into your Eclipse directory. Both plug-ins come with complete source
code, although you will need your own installation of Perl to run the PDA interpreter.
This article describes, and the included code requires, features that are available
in Eclipse 3.0 or a later version.</p>
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