Custom markers and annotations – the bright side of Eclipse


I have discovered a truly remarkable proof of this theorem which this margin is too small to contain.
Pierre de Fermat

During the development of editors it is often needed to display additional information next to the document, such as error markers, search results or various coverage metrics. However, it is not a good idea to hardcode the list of supported markers during development, as the best ideas will be introduced by others – who do not want to have anything to do with our editor.

Luckily, Eclipse provides a simple way to attach markers to documents without explicit support needed from the editor developer: the markers (and annotations) created by their corresponding extensions are stored independently from the document, so these additions can be made without disturbing any existing user of the files.

As I described it in my previous post about error markers, such markers can be defined creating an extension for the extension point org.eclipse.core.resources.marker, and then using the IResource.createMarker() method to create the marker instances as needed. Then using some magic it can be displayed as the yellow or red wriggly lines during the text.

Other markers (and annotations) can be created in similarly: we create our own marker type, and register an annotation type for it, and the platform manages the rest. In my case I wanted to represent a calculated set of program statements similar to the code coverage display used by the EclEmma tool: the covered statements are highlighted with the green background.

To support such a display, I created a new marker type, that is a text marker; and to make the lifecycle-management easier, I did not set it persistent. This has been reached using the following code:

    name="GTASM Slice"

The most important difference between this marker and the one defined in the previous Markers and Annotations post is that the newly defined marker is not a problem marker, but a simple text marker. This means, the created marker will not show in the Problems view (that is nice – we don’t want to display problems now), and no default annotation is created (that will need fixing).

In theory it is possible to create annotations programmatically together with the markers, but in practice it is really a bad idea. First of all, annotations are created for the document model (thus assuming an open editor and a code dependency to that document model), are never persisted, and the resulting code fails indeterministically (I once debugged a code like that, every annotation creation code ran correctly, but some annotations were missing; re-running the code re-added the missing ones, but sometimes removed others), so this idea will not work.

Luckily, the platform provides two extension points that can be used to automatically assign annotations to marker instances. To define an automatically generated annotation type for the markers, the org.eclipse.ui.editors.annotationTypes extension point is used: basically a marker and an annotation ID is stored this way.




To define the appearance of the created annotation the org.eclipse.ui.editors.markerAnnotationSpecification extension point can be used. In the extensions we have to specify the annotation display methods used together with color and formatting settings.


            label="GTASM Slice Marker"

Even better, by assigning preference keys to the various settings the platform also offers the users the possibility to configure the annotations: the General/Editors/Text Editors/Annotations preference page will list our annotation, and allows setting its preference values. If the preference keys are unique (that is our responsibility 🙂 ), then creating this extension ends our work: annotations will be generated automatically, and they will be displayed in the opened text editors.

Annotation Preferences

The marker creation process is entirely the same as the creation of error markers: a new marker is initialized using the IResource.createMarker(String markerId) method, and the IMarker.CHAR_START and IMarker.CHAR_END markers are set for positioning the markers.

An editor opened with the annotated code

Alltogether, the markers and annotations provide an easy way to extend existing editors with meta-information (e.g. execution state, dependencies, coverage, etc.), that can be used with a very little amount of Java programming. Sadly, the content assists for extensions is not that discoverable as the Java APIs, so a lot of experimenting and/or documentation reading is needed. Luckily, documentation is available for these extension points, so knowing their name will be enough for most cases.

So, lets hope, Eclipse gives us a big enough margin to contain our proo metainformation.

Generating Java code from EMF models automatically

EMF provides a nice code generation facility for generating Java class hierarchy from ecore models. There is support for hand-made modifications to the generated code, or a large selection of generation parameters are available through a generator model.

However, we had a different need together with the VIATRA2 project: we explicitly forbid in our internal policy modifications to the generated Java code in order to avoid non-reusable models, and as this way the generated code does not contain any extra information to the generator model, we do not want to store the Java classes in our Subversion repository.

Another minor obstacle was that the code generation does not happen automatically (e.g. as in the LPG editor of the IMP project) – so simply not committing the generated code to the repository would mean a manual invocation of the generator that is inconvenient.

Luckily, the code generator is available using specific ANT tasks, allowing to write a build file that describes how to generate the required Java code, reusing the existing build model if necessary:

 reconcilegenmodel="reload" >

Some interesting points:

  • As the selected Ant task is provided by an OSGi plug-in, it is important to set the Runtime JRE to the same one as used by the workspace, otherwise the called task will not be found.
  • As the genmodel generator is not available as an Ant task, the ecore2Java task was used. In this case it is important to use the reconcilegenmodel attribute to avoid the unnecessary override of the genmodel
  • Regardless of the value of the generate*project attributes if the *project were not given as sub-tags, the corresponding code was not generated. This seems like a bug for me, as this duplicates information already present in the genmodel, but I am not sure.
  • The code generator does not set the generated files Derived attribute, thus it is not excluded from the svn repository. Unfortunately, it is not possible without writing custom tasks, as there is no such command (yet) in Eclipse (but hopefully the patch gets committed in Indigo).

This ant task mimics the GUI-based code generators functionality, thus fulfilling our needs, except two things: it does not exclude generated code from the repository, and does not run automatically.

The repository exclusion is resolved using a hack: the src folders are listed in all generated projects in the svn:ignore variable – thus manually disallowing their sharing. This approach works well, because if no code is generated, the src folder might be missing, that is marked with a very explicit error message (as it is still referenced in the project properties), but if the task is executed, the folder is created as well (and the source folder reference will be working).

For the automatic building Eclipse provides a neat way: External tools (e.g. Ant scripts, or shell scripts, etc.) can be added as project builders. A Run configuration has to be created for the selected task, that can be added to the project. This means, our script is executed each time the selected project changes

The Ant task has been set as a custom builder
Builders page in the Project properties

Finally, some other modifications were made to the task in order to provide a more standard build: first, as not the entire project is generated at every invocation of the script (the projects could not be deleted, because the generator does not support setting the version number of the generated edit and editor projects), if all project stubs are downloaded from the repository, the build order might not be adequate. To overcome this, a manual build is executed for all projects at the end.

A second modification is a clean task, that removes all source folders, in order to provide a “clean”, out-of-the-box experience for a regeneration. This task can also be registered at the builder properties.

The full, modified script looks as following (using Gist from Github):

Update: the tasks are not available in Eclipse, as mentioned by a comment. They are provided by the Buckminster tool, and are not required for the EMF code generation, so it is safe to remove them. However, in this case the refreshing and building of the projects have to be managed in another way (e.g. registering the Ant script as an External builder for the projects).

This simple script helps in a lot of cases – and because the ecore models does not change very often, its performance is not really an issue (we have quite a large ecore model, so the generator runs about 30-40 seconds). My main issues are the following:

  • I don’t like to reproduce information from the genmodel in the build script. I am not entirely sure which information is read from the genmodel and which is inferred from the parameters, making the solution a bit volatile.
  • The missing derived flag command. It would be quite a nice touch if the code would take care of it.

Alltogether, if you like it, feel free to use it (licensed under EPL). I also would be glad if somebody could give me a hint about how to enhance the script – this is only my second Ant build script, so I don’t know the language very well. 🙂

Markers and Annotations in Eclipse for Error Feedback


Ninety per cent of most magic merely consists of knowing one extra fact.
Terry Pratchett, Night Watch

Development in Eclipse keeps fascinating me, as there are a lot of very thoroughly designed services and features integrated to allow detailed customization – on the other hand, “with big power comes big responsibility”, but at least a steep learning curve. I had such an experience with the problem handling support not long ago.

We developed our own textual language, that provided error markers and editor annotations as error feedback. The problem was that the editor annotations were sometimes missing: our planned functionality worked like we like to add an annotation each time our marker was added, but the annotations were not always updated (so we got an inconsistent state between markers and annotations – but not every time).

Of course that was caused by the fact that we were rolling out our own implementation of marker/annotation synchronization – we were not aware that the JFace TextEditor provides such a service out of the box – and I did not found it documented on the various locations (that’s why this blog post is born 🙂 ).

Our requirements

First of all, lets assemble the various ways Eclipse editors can provide error feedback:

  • The most basic (and least usable) way is the use of pop-up windows – they are annoying, and not persistent (after pressing a button, they disappear, before the error could be solved).
  • Writing to the Error log view – a bit better, but the log could be overwhelmed, if the messages are too many, because they cannot be deleted when the problem is solved.
  • Displaying the error in the Problems view – that’s one of the standard places, where parse errors should be displayed. The problems view is generated by reading (and filtering) the error marker type (org.eclipse.core.resources.problemmarker).
  • Marking the files in the Project Navigator view with a small symbol, if they contain errors/warnings. This is also handled using the previously mentioned error marker.
  • Underlining the erroneous parts in the editor. For this reason annotations should be added to the JFace Document Model used by the JFace TextEditor component.

The basic idea is to create a marker and an annotation each time a problem is found. The problem with annotations, that creating them requires some connection with the editor object, or at least the document model, but our parser should not depend on the editor (core component should not depend on the GUI!).

Mark My Words!

The management of the markers is well described in the Mark My Words tutorial. Basically markers are attached to IResources, and provide a simple key-value interface with a freely definable key set (and the IMarker interface specifies some commonly used keys).

Following the tutorial we could create our own marker with the parent of the org.eclipse.core.resources.marker marker, after this adding the markers to the selected files managed the error display in the Problems and Project Navigator views.

So our marker definition looks as follows:

         name="VTCL Parsing Problems"

The problem marker understands severity, error message and line number parameters, and is capable of displaying them in the Problems view.

IMarker marker = file.createMarker(markerId);
 marker.setAttribute(IMarker.SEVERITY, severity);
 marker.setAttribute(IMarker.MESSAGE, message);
 marker.setAttribute(IMarker.LINE_NUMBER, line);

Annotating the document manually

As stated before, markers could be added easily without knowing anything about who has opened the file currently, so they are really useful for error feedback, but this information should be also available in the open editors.

Our first idea may be to create some glue code, that listens to the marker changes, and updates our editor accordingly. The drawbacks of this approach are first the fact that we have to code something that trivial, and the second (as we saw in our project), that the editor update could be quite tricky (missing or not deleted annotations).

My theory is, that we have encountered some kind of race condition with our naive implementation, as the annotation creation code was always executed, but sometimes the results were lost. So, in the end, this listener is not so trivial. 🙁

Automatic annotation display

This was the part that gave us the most headache – we did not understand, what caused the inconsistent display problem. On the other hand, knowing one extra fact (“magic”) the problem is trivially solvable.

So, here comes the Eclipse magic: we don’t have to create this service manually, we should reuse the already existing one. For that, we would need that our marker also become a child marker of the org.eclipse.core.resources.textmarker, and a minimal set of position data should be added to the marker.

This position information is described by the start and end offset value – instead of the human-readable line/column position information a machine-readable single position is used: the number of characters that has to be read from the start of the string stream. Its lucky, that most parser generators provide such output that this information is trivially available from the AST level

         name="VTCL Parsing Problems"

And the marker creator Java code looks as follows:

IMarker marker = file.createMarker(markerId);
marker.setAttribute(IMarker.SEVERITY, severity);
marker.setAttribute(IMarker.MESSAGE, message);
marker.setAttribute(IMarker.LINE_NUMBER, line);
if (pos.offset != 0) {

After that, the platform uses a default implementation, that provides the well-known wriggly underline to annotate our text files with the associated markers.


Altogether, the following steps are needed to provide a simple error reporting for our existing editor:

  • Create a custom marker type
    • With a supertype of org.eclipse.core.resources.problemmarker to display the report in the problems view
    • With a supertype of org.eclipse.core.resources.textmarker to underline the errors in the textual editor
  • Create a marker instance
    • Setting the IMarker.SEVERITY, IMarker.MESSAGE and IMarker.LINE_NUMBER attributes for the Problems view.
    • Setting the IMarker.CHAR_START and IMarker.CHAR_END for the editor annotations.


The Eclipse text editors provide a well-defined, easy to use error reporting mechanism nearly for free. The main catch is, we have to be careful to set everything – if something is missing, we got the silent failure issue – no exception is thrown, but something is not shown.

The biggest issue in our implementation was that the IMarker.CHAR_START and IMarker.CHAR_END marker attributes are ignored, if the used marker is not a text attribute, making this problem hard to identify.

A fine thing with this error reporting mechanism is, that it is independent of the concrete text editor used: when we changed our implementation in the VIATRA framework, the resulting reports were visible in every text editor (e.g. the dedicated VIATRA text editor, or the default text editor in Eclipse) – thus helping error recovery.

Presentation theme for Eclipse

In the last year I often had to held live presentations of Eclipse development. At that point a typical problem is, that the font sizes used during development are too small for the projector.

The possible solutions are either ignoring this fact, using a special environment, where all the appropriate settings have been made, or tweaking the development environment. The tweaking will not work, as several settings should be made, and they are easy to forget, while the special presentation environment seems too much of a hassle for me. This left as the only choice to ignore the fact.

But finally I had some hacking time, so I created a Presentation theme for Eclipse, that solves tweaking by a single selection of the user interface: Themes can be selected in the General/Appearance tab on the Preferences page.

The Presentation theme can be selected=

Currently, the theme description is a bit hacky: for every platform the default font is copied by hand to the plugin.xml (using a Windows 7, a OpenSuse 11.3 Linux and a Snow Leopard machine). More research is needed to solve this in a more generic way. Luckily only four fonts have been redefined by the theme.

Some things missing: support for resizing elements in JFace viewers (such as EMF tree editors or CNF views), and the theme was not tested with graphical editors – although they can be resized using the Zoom control.

Finally, an early alpha version can be downloaded from our update site:

I’m looking for some feedback, or either alternative implementation ideas; I think, others have already met these problems.

The source is available from GitHub.

Update: before/after screenshots below.

The default theme on OSX uses small fonts
Before: The default theme
The Presentation theme uses increased font sizes
After: Presentation theme

Ecore model analysis for the win

Today I tried to edit an Ecore/Generator model used in the VIATRA2 framework, but the system greated me with an error screen stating, there are 4 errors in the selected Genmodel.

Four validation errors at the opening of the genmodel

The error messages were a little bit cryptic (but all similar in nature): A containment reference of a type with a container feature platform:/resource/org.eclipse.viatra2.gtasm.model/model/gtasmmodel.ecore#//gtasm/metamodel/gt/GTPattern/namespace that requires instances to be contained elsewhere cannot be populated.

This was scary, as I don’t remember any new core features in EMF 2.6, that suggest such changes. My second guess was a misedited model, but after checking, that I have the same models as half year ago, this proved wrong. Even worse, these errors did not appear in either the Problems or Error log view. Checking the items mentioned in the error messages did not help, as the model elements looked “innocent enough” not to question their content.

It was only know when I saw that these errors are shown in a new, problem page in the editor (I often miss the editor pages option – except on form-based editors), and on the other page the tree is displayed, and code generation seemed also working.

At this point I looked on the internet for similar problems, and found a thread about the issue in the EMF newsgroup. The main idea was Ed Merks’s comment:

The point is supposed to be that you can’t be contained by more than one container, so you can’t have a container references that’s required as well as any other containment reference that’s not the opposite of that required container. I.e., a required container prevents other containment references from ever being populated.

Using this information I finally understood, what the error message meant, and begin looking for the cause in the Ecore model. After some searching (the error message only mentioned one model element from the three actors, and it is hard to get an overview of the inter modelelement relations in an EMF tree editor), I finally found the mentioned elements, that looked quite innocent at first – just take a look at the included graphical representation.

An Ecore diagram to illustrate the found model validation error
Illustration of the found problem

We have a machine, that contains both patterns and rules, while rules may contain addition pattern definitions (named local pattern definitions). On the other hand, patterns and rules have a backreference to the  machine (namespace). As we wanted to have the Machine reference available for local patterns, the pattern references exactly one machine.

The drawback of the solution is, that the machine reference is the inverse of the containment relation, so every pattern has to be contained in a machine. On the other hand, EMF requires that every EObject (not EClass) must have exactly one incoming containment reference for serialization, so it is not possible for a rule to be the container of a pattern – hence the error message.

This analysis is extremely useful: it shows, that our metamodel prevents having serializable models in some cases, which comes in handy (I wouldn’t like to be the one, who has to debug such an error after a runtime exception is caught…). The fix is not really straightforward, as this model is already used (interesting fact, that the issue has not been produced in runtime – this means, we don’t have 100% test coverage :)), but by having a map to the error it is easier to fix.

I’d like to thank for the EMF team for this fine analysis solution, but some minor additions: don’t add warnings to a new page, use the existing reporting options; and try to rephrase the message for it to become more understandable.