Defining XPCOM Components

This document explains how to write a components.conf file. For documentation on the idl format see XPIDL. For a tutorial on writing a new XPCOM interface, see Tutorial for Writing a New XPCOM Interface.

Native XPCOM components are registered at build time, and compiled into static data structures which allow them to be accessed with little runtime overhead. Each module which wishes to register components must provide a manifest describing each component it implements, its type, and how it should be constructed.

Manifest files are Python data files registered in files in a XPCOM_MANIFESTS file list:


The files may define any of the following special variables:

# Optional: A function to be called once, the first time any component
# listed in this manifest is instantiated.
InitFunc = 'nsInitFooModule'
# Optional: A function to be called at shutdown if any component listed in
# this manifest has been instantiated.
UnloadFunc = 'nsUnloadFooModule'

# Optional: A processing priority, to determine how early or late the
# manifest is processed. Defaults to 50. In practice, this mainly affects
# the order in which unload functions are called at shutdown, with higher
# priority numbers being called later.
Priority = 10

# Optional: A list of header files to include before calling init or
# unload functions, or any legacy constructor functions.
# Any header path beginning with a `/` is loaded relative to the root of
# the source tree, and must not rely on any local includes.
# Any relative header path must be exported.
Headers = [

# A list of component classes provided by this module.
Classes = [
        # ...
    # ...

# A list of category registrations
Categories = {
    'category': {
        'name': 'value',
        'other-name': ('value', ProcessSelector.MAIN_PROCESS_ONLY),
        # ...
    # ...

Class definitions may have the following properties:

name (optional)

If present, this component will generate an entry with the given name in the mozilla::components namespace in mozilla/Components.h, which gives easy access to its CID, service, and instance constructors as (e.g.,) components::Foo::CID(), components::Foo::Service(), and components::Foo::Create(), respectively.


A UUID string containing this component’s CID, in the form '{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}'.

contract_ids (optional)

A list of contract IDs to register for this class.

categories (optional)

A dict of category entries to register for this component’s contract ID. Each key in the dict is the name of the category. Each value is either a string containing a single entry, or a list of entries. Each entry is either a string name, or a dictionary of the form {'name': 'value', 'backgroundtasks': BackgroundTasksSelector.ALL_TASKS}. By default, category entries are registered for no background tasks: they have 'backgroundtasks': BackgroundTasksSelector.NO_TASKS.

type (optional, default=``nsISupports``)

The fully-qualified type of the class implementing this component. Defaults to nsISupports, but must be provided if the init_method property is specified, or if neither the constructor nor legacy_constructor properties are provided.

headers (optional)

A list of headers to include in order to call this component’s constructor, in the same format as the global Headers property.

init_method (optional)

The name of a method to call on newly-created instances of this class before returning them. The method must take no arguments, and must return a nsresult. If it returns failure, that failure is propagated to the getService or createInstance caller.

constructor (optional)

The fully-qualified name of a constructor function to call in order to create instances of this class. This function must be declared in one of the headers listed in the headers property, must take no arguments, and must return already_AddRefed<iface> where iface is the interface provided in the type property.

This property is incompatible with legacy_constructor.

esModule (optional)

If provided, must be the URL of a JavaScript module which contains a JavaScript implementation of the component. The constructor property must contain the name of an exported function which can be constructed to create a new instance of the component.

jsm (deprecated, optional)

Do not use. Use esModule instead.

legacy_constructor (optional)

This property is deprecated, and should not be used in new code.

The fully-qualified name of a constructor function to call in order to create instances of this class. This function must be declared in one of the headers listed in the headers property, and must have the signature nsresult(const nsID& aIID, void** aResult), and behave equivalently to nsIFactory::CreateInstance.

This property is incompatible with constructor.

singleton (optional, default=``False``)

If true, this component’s constructor is expected to return the same singleton for every call, and no mozilla::components::<name>::Create() method will be generated for it.

overridable (optional, default=``False``)

If true, this component’s contract ID is expected to be overridden by some tests, and its mozilla::components::<name>::Service() getter will therefore look it up by contract ID for every call. This component must, therefore, provide at least one contract ID in its contract_ids array.

If false, the Service() getter will always retrieve the service based on its static data, and it cannot be overridden.

Note: Enabling this option is expensive, and should not be done when it can be avoided, or when the getter is used by any hot code.

external (optional, default=``False`` if any headers are provided, True otherwise)

If true, a constructor for this component’s type must be defined in another translation unit, using NS_IMPL_COMPONENT_FACTORY(type). The constructor must return an already_AddRefed<nsISupports>, and will be used to construct instances of this type.

This option should only be used in cases where the headers which define the component’s concrete type cannot be easily included without local includes.

Note: External constructors may not specify an init_method, since the generated code will not have the necessary type information required to call it. This option is also incompatible with constructor and legacy_constructor.

processes (optional, default=``ProcessSelector.ANY_PROCESS``)

An optional specifier restricting which types of process this component may be loaded in. This must be a property of ProcessSelector with the same name as one of the values in the Module::ProcessSelector enum.

Conditional Compilation

This manifest may run any appropriate Python code to customize the values of the Classes array based on build configuration. To simplify this process, the following globals are available:


A function which returns true if the given build config setting is defined and true.


The buildconfig python module, with a substs property containing a dict of all available build substitutions.

Component Constructors

There are several ways to define component constructors, which vary mostly depending on how old the code that uses them is:

Class Constructors

This simplest way to define a component is to include a header defining a concrete type, and let the component manager call that class’s constructor:

'type': 'mozilla::foo::Foo',
'headers': ['mozilla/Foo.h'],

This is generally the preferred method of defining non-singleton constructors, but may not be practicable for classes which rely on local includes for their definitions.

Singleton Constructors

Singleton classes are generally expected to provide their own constructor function which caches a singleton instance the first time it is called, and returns the same instance on subsequent calls. This requires declaring the constructor in an included header, and implementing it in a separate source file:

'type': 'mozilla::foo::Foo',
'headers': ['mozilla/Foo.h'],
'constructor': 'mozilla::Foo::GetSingleton',


class Foo final : public nsISupports {
  static already_AddRefed<Foo> GetSingleton();


already_AddRefed<Foo> Foo::GetSingleton() {
  // ...

External Constructors

For types whose headers can’t easily be included, constructors can be defined using a template specialization on an incomplete type:

'type': 'mozilla::foo::Foo',
'external: True,'


  return do_AddRef(new Foo()).downcast<nsISupports>();

Legacy Constructors

These should not be used in new code, and are left as an exercise for the reader.

Registering Categories

Classes which need define category entries with the same value as their contract ID may do so using the following:

'contract_ids': [';1'],
'categories': {
    'content-policy': 'm-foo',
    'Gecko-Content-Viewers': ['image/jpeg', 'image/png'],

This will define each of the following category entries:

  • "content-policy" "m-foo", ";1"

  • "Gecko-Content-Viewers" "image/jpeg" ";1"

  • "Gecko-Content-Viewers" "image/png" ";1"

Some category entries do not have a contract ID as a value. These entries can be specified by adding to a global Categories dictionary:

Categories = {
    'update-timer': {
        'nsUpdateService': ';1,getService,background-update-timer,app.update.interval,43200,86400',

It is possible to limit these on a per-process basis by using a tuple as the value:

Categories = {
    ';1': {
        '?from=gzip&to=uncompressed': ('', ProcessSelector.ALLOW_IN_SOCKET_PROCESS),