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MPS provides an API for creating custom debuggers as well as integrating with debugger for java. See Debugger Usage page for a description of MPS debugger features.

Integration with java debugger

To integrate your java-generated language with java debugger, provided by MPS, you should specify:

  • on which nodes breakpoints could be created;
  • nodes which should be traced;
  • how to start your application under debug;
  • custom viewers for your data.

Not all of those steps are absolutely necessary; which are – depends on the language. See next parts for details.

Nodes to trace and breakpoints

Suppose you have a language, let's call it highLevelLanguage, which generates code on some lowLevelLanguage, which in turn is generated directly into text (there can be several other languages between highLevelLanguage and lowLevelLanguage, it does not really metter). Suppose that the text generated from lowLevelLanguage is essentially java, and you want to have your highLevelLanguage integrated with java debugger. See the following explanatory table:


lowLevelLanguage is baseLanguage

lowLevelLanguage is not baseLanguage

highLevelLanguage extends baseLanguage
(uses concepts Statement,Expression, BaseMethodDeclaration etc)

Do not have to do anything.

Specify traceable concepts for lowLevelLanguage.

highLevelLanguage does not extend baseLanguage

Specify breakpointable concepts for highLevelLanguage.

Specify traceable concepts for lowLevelLanguage.
Specify breakpointable concepts in DebugInfoInitializer for highLevelLanguage.

Startup of a run configuration under java debugger

MPS provides a special language for creating run configurations for languages generated into java – jetbrains.mps.baseLanguage.runConfigurations. Those run configurations are able to start under debugger automatically. See Run configurations for languages generated into java for details.

Custom viewers

When one views variables and fields in a variable view, one may want to define one's own way to show certain values. For instance, collections could be shown as a collection of elements rather than as an ordinary object with all its internal structure.

For creating custom viewers MPS has jetbrains.mps.debug.customViewers language.

A language jetbrains.mps.debug.customViewers enables one to write one's own viewers for data of certain form.

A main concept of customViewers language is a custom data viewer. It receives a raw java value (an objects on stack) and returns a list of so-called watchables. A watchable is a pair of a value and its label (a string which cathegorizes a value, i.e. whether a value is a method, a field, an element, a size etc.) Labels for watchables are defined in custom watchables container. Each label could be assigned an icon.

The viewer for a specific type is defined in a custom viewer root. In the following table custom viewer parts are described:



for type

A type for which this viewer is intended.

can wrap

An additional filter for viewed objects.

get presentation

A string representation of an object.

get custom watchables

Subvalues of this object. Result of this funtion must be of type watchable list.

Custom Viewers language introduces two new types: watchable list and watchable.

This is the custom viewer specification for java.util.Map.Entry class:

And here we see how a map entry is displayed in debugger view:

Creating a non-java debugger

You can create a non-java debugger using the API provided by MPS. You can see how it is done in jetbrains.mps.samples.nanoc language – a toy language generated into C. This language is supplied with the "Debugger API for MPS" plugin. The project location is %MPS_INSTALLATION_PATH%/mps-debugger-api/ See also Debugger API description.

Traceable Nodes

This section describes how to spesify which nodes require to save some additional information in file (like information about positions text, generated from the node, visible variables, name of the file the node was generated into etc.). files contain information allowing to connect nodes in MPS with generated text. For example, if a breakpoint is hit, java debugger tells MPS the line number in source file and to get the actual node from this information MPS uses information from files.

Specifically, files contain the following information:

  • position information: name of text file and position in it where the node was generated;
  • scope information: for each "scope" node (such that has some variables, associated with it and visible in the scope of the node) – names and ids of variables visible in the scope;
  • unit information: for each "unit node" (such that represent some unit of a language, for example a class in java) – name of the unit the node is generated into.

Concepts TraceableConcept, ScopeConcept and UnitConcept of language jetbrains.mps.lang.traceable are used for that purpose. To save some information into file, user should derive from one of those concepts and implement the specific behavior method. The concepts are described in the table below.



Behavior method to implement



Concepts for which location in text is saved and for which breakpoints could be created.

getTraceableProperty – some property to be saved into file.


Concepts which have some local variables, visible in the scope.

getScopeVariables – variable declarations in the scope.


Concepts which are generated into separate units, like classes or inner classes in java.

getUnitName – name of the generated unit. files are created on the last stage of generation – while generating text. So the decsribed concepts are only to be used in languages generated into text.

Breakpoint Creators

To specify how breakpoints are created on various nodes, root breakpoint creators is used. This is a root of concept BreakpointCreator from jetbrains.mps.debug.apiLang language. The root should be located in the language plugin model. It contains a list of BreakpointableNodeItem, each of them specify a list of concept to create breakpoint for and a method actually creating a breakpoint. jetbrains.mps.debug.apiLanguage provides several concepts to operate with debuggers, and specifially to create breakpoints. They are described below.

  • DebuggerReference – a reference to a specific debugger, like java debugger;
  • CreateBreakpointOperation – an operation which creates a location breakpoint of specified kind on a given node for a given project;
  • DebuggerType – a special type for references to debuggers.

On the following example breakpoint creators node from baseLanguage is shown.

Since MPS 2.5 brakpoint creators can use isApplicable function instead of plain concept list. There is an intention to switch to using this function.

Changes in MPS 2.5

In order to make MPS more modular, the debugger API and the Java debugger itself were moved into separate MPS plugins. This allows the user to completely turn the functionality off when not needed. A migration script to upgrade code to the new debugger API has been included in the MPS 2.5 migration wizard. Additionally, a migration script named "Fix references to debugger classes" is available for manual execution.

New debugger features introduced in MPS 2.5:

  1. In the generic debugger:
    • Cell-based highlighting in tables.
    • Toolbar with stepping actions in the Debugger Tool.
  2. In the Java debugger:
    • "Copy value" action in the variables tree.
    • Special highligting for breakpoints on which the debugger is not able to stop.
    • Evaluate selection: Alt+F8 copies the selected code to the Evaluate window.
    • High-level types and variable names are calculated for variables in the evaluation and watches windows. Low-level types (i.e. types from the generated Java code) are shown in brackets.
  3. For developers of debugger extension:
    • isApplicable function is available in the breakpoints creators.
    • When automatical tracing is impossible, $TRACE$ macro can be used in order to set input node for generated code (since MPS 2.5.2).

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