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Franca IDL overview

Franca IDL files can be written in any text editor.

We recommend using the Franca Eclipse tooling to simplify the process (syntax highlighting, code completion, model validation etc.): Franca IDL (

Note: Since the necessary Franca dependencies are currently not available from Maven Central Repository, we ship Franca dependencies together with the joynr source code in the <JOYNR>/tools/generator/dependency-libs/ directory.

If you build joynr yourself using the provided docker and / or Maven infrastructure, the Franca dependencies are installed to your local Maven repository during the build.

The following sections provide a brief overview of Franca.

Franca Data types

Basic data types

The following Franca basic data types are supported by joynr:

Name Meaning
Int8, Int16, Int32, Int64 signed 8, 16, 32, 64-bit integer value
UInt8, UInt16, UInt32, UInt64 unsigned 8, 16, 32, 64-bit integer value
Boolean boolean value
Float, Double single / double precision floating point number
String String
ByteBuffer buffer of bytes

Complex data types

The following complex data types are supported by joynr:

Name Meaning
struct { elements } Data record
enumeration { values } Enumeration
map { KeyType to ValueType } associative array, hash
type[] inline array

Each element is a <Type> variableName pair.

Franca Packages

A Franca package definition creates a unique namespace.

package <Package>

Franca Type Collections

Type collections serve as namespace for one or more named custom complex types.

typeCollection <TypeCollection> {
    ... complex type declarations ...

It is recommended to use only one type collection in joynr, since at the moment the names of the types in all type collections must be unique.

Outside of a type collection, data types are referenced using hierachical notation like <TypeCollection>.<complexType>.

Franca Interfaces

A Franca interface defines a namespace for attributes, methods and broadcasts, which must be defined in exactly that order.

interface <Interface> {
    ... attributes (optional) ...
    ... methods (optional)  ...
    ... broadcasts (optional) ...

The following restrictions apply for joynr:

  • Franca Contracts are not supported.
  • For a given interface, all attribute, method and broadcast names must be unique, e.g. a name for an attribute may not be reused as name for a method or broadcast etc.
  • While technically data types can be defined inside an interface as well, it is strongly recommended to place them inside a type collection.


Attributes are values to which a consumer may subscribe, in order to get informed upon changes and/or periodically via a publication.

attribute <Type> <Attribute>


A Franca method defines an RPC call that can be invoked by a Consumer application. The call is then sent to and handled by a Provider application.

RPC supports optional input- and output parameters as well as an optional enumeration for error codes.

For Java and C++, the consumer may make synchronous or asynchronous calls to a proxy.

method <Method> {
    in {
        ... collection of "<Type> <inputVariable>" pairs ...
    out {
        ... collection of "<returnType> <outputVariable>" pairs ...
    error <enumerationType>


A Franca broadcast defines an asynchronous event that can be fired by the Provider and be received by the Consumer. A consumer can selectively subscribe to desired events, optionally with filter parameters to limit the number of broadcasts it gets. Typically broadcasts have optional output parameter(s) to provide details about the event.

broadcast <Broadcast> {
    out {
        <Type> <outputVariable>

Selective (filtered) broadcasts

If a broadcast supports filtering, the filter parameter must be defined using Franca comments as follows:

    @description: <description text>
    @param: <filterParam1> (<filterParamType1>) descriptionParam1
    @param: <filterParamN> (<filterParamTypeN>) descriptionParamN
broadcast <Broadcast> selective {
    out {

Important things to notice for the declaration of selective broadcasts:

  • Each description text can hold multiple lines.
  • The parenthesis in the parameters definition do not mean that the type information is optional, but rather mean that it has indeed to be provided inbetween parenthesis, e.g. (Boolean) or (String) etc.
  • The keyword selective


package myPackage
typeCollection myTypeCollection {
    struct myStruct {
        String myElement1
        String myElement2

interface myInterface {
    method myMethod {
        in {
            myTypeCollection.myStruct myInput
        out {
            String myOutput
        error {

    broadcast myBroadcast {
        out {
            myTypeCollection.myStruct myOutput

        @param: filterMinValue (Integer) filter parameter
                that defines the minimal value of interest
                for the subscriber
    broadcast mySelectiveBroadcast selective {
        out {
            Integer myIntValue

Further Reading

Franca IDL documentation on GitHub (external link:

For details about using joynr with Java see joynr Java Developer Guide. For details about using joynr with C++ see joynr C++ Developer Guide. For details about using joynr with JavaScript see joynr JavaScript Developer Guide.

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