Getting started with libmapper and Java

Since libmapper uses GNU autoconf, getting started with the library is the same as any other library on Linux; use ./configure and then make to compile it. You'll need the Java Developer Kit (JDK) available if you want to compile the Java bindings.

Once you have libmapper installed, it can be imported into your program:

import mapper.*;

Overview of the API

The libmapper API is is divided into the following sections:

For this tutorial, the only sections to pay attention to are Devices and Signals. Networks are reserved for providing custom networking configurations, and in general you don't need to worry about it.

The Database module is used to keep track of what devices, signals and maps are on the network. It is used mainly for creating user interfaces for mapping design and will also not be covered here.


Creating a device

To create a libmapper device, it is necessary to provide a device name to the constructor. There is a brief initialization period after a device is created during which a unique ordinal is chosen to append to the device name. This allows multiple devices with the same name to exist on the network.

If no other arguments are given, libmapper will randomly choose a port to use for exchanging signal data. If desired, a second argument setting a specific "starting port" can be given, but the allocation algorithm will possibly choose another port number close to it if the port is in use.

A third optional parameter of the constructor is a network object. It is not necessary to provide this, but can be used to specify different networking parameters, such as specifying the name of the network interface to use.

An example of creating a device:

final Device dev = new Device("my_device");

Polling the device

The device lifecycle looks like this, in terrible ASCII diagram art:

creation --> poll --+--> destruction
              |     |

In other words, after a device is created, it must be continuously polled during its lifetime.

The polling is necessary for several reasons: to respond to administrative messages; to check for incoming signals. Therefore even a device that does not have signals must be polled. The user program must organize to have a timer or idle handler which can poll the device often enough. The polling interval is not extremely sensitive, but should be 100 ms or less. The more often it is polled, the faster it can handle incoming and outgoing signals.

The poll() function can be blocking or non-blocking, depending on how you want your application to behave. It takes a number of milliseconds during which it should do some work, or 0 if it should check for any immediate actions and then return without waiting:

dev.poll(int block_ms);

An example of calling it with non-blocking behaviour:


If your polling is in the middle of a processing function or in response to a GUI event for example, non-blocking behaviour is desired. On the other hand if you put it in the middle of a loop which reads incoming data at intervals or steps through a simulation for example, you can use poll() as your "sleep" function, so that it will react to network activity while waiting.

It returns the number of messages handled, so optionally you could continue to call it until there are no more messages waiting. Of course, you should be careful doing that without limiting the time it will loop for, since if the incoming stream is fast enough you might never get anything else done!

Note that an important difference between blocking and non-blocking polling is that during the blocking period, messages will be handled immediately as they are received. On the other hand, if you use your own sleep, messages will be queued up until you can call poll(); stated differently, it will "time-quantize" the message handling. This is not necessarily bad, but you should be aware of this effect.

Since there is a delay before the device is completely initialized, it is sometimes useful to be able to determine this using ready(). Only when ready() returns non-zero is it valid to use the device's name.


Now that we know how to create a device and poll it, we only need to know how to add signals in order to give our program some input/output functionality. While libmapper enables arbitrary connections between any declared signals, we still find it helpful to distinguish between two type of signals: inputs and outputs.

This can become a bit confusing, since the "reverb" parameter of a sound synthesizer might be updated locally through user interaction with a GUI, however the normal use of this signal is as a destination for control data streams so it should be defined as an input signal. Note that this distinction is to help with GUI organization and user-understanding – libmapper enables connections from input signals and to output signals if desired.

Creating a signal

We'll start with creating a "sender", so we will first talk about how to update output signals. A signal requires a bit more information than a device, much of which is optional:

for input signals there is an additional argument:


Signal in = dev.addInputSignal("my_input", 1, 'f', "m/s",
                               new Value(-10.f), null,
                               new UpdateListener() {
    public void onUpdate(Signal sig, float[] value, TimeTag tt) {
        System.out.println("got input for signal ";

Signal out = dev.addOutputSignal("my_output", 4, 'i', null, 0, 1000);

The only required parameters here are the signal "length", its name, and data type. Signals are assumed to be vectors of values, so for usual single-valued signals, a length of 1 should be specified. Finally, supported types are currently 'i', 'f' or 'd' for int, float or double values, respectively.

The other parameters are not strictly required, but the more information you provide, the more the mapper can do some things automatically. For example, if the minimum and maximum properties are provided, it will be possible to create linear-scaled connections very quickly. If unit is provided, the mapper will be able to similarly figure out a linear scaling based on unit conversion (centimeters to inches for example). Currently automatic unit-based scaling is not a supported feature, but will be added in the future. You can take advantage of this future development by simply providing unit information whenever it is available. It is also helpful documentation for users.

Lastly, it is usually necessary to be informed when input signal values change. This is done by providing a function to be called whenever its value is modified by an incoming message. It is passed in the UpdateListener parameter.

An example of creating a "barebones" integer scalar output signal with no unit, minimum, or maximum information:

Signal outA = dev.addOutputSignal("outA", 1, 'i', null, null, null);

An example of a float signal where some more information is provided:

Signal sensor1 = dev.addOutputSignal("sensor1", 1, 'f', "V", 0.0, 5.0)

So far we know how to create a device and to specify an output signal for it. To recap, let's review the code so far:

import mapper.*;
import mapper.signal.*;

class test {
    public static void main() {
        final Device dev = new Device("testDevice");
        Signal sensor1 = dev.addOutputSignal("sensor1", 1, 'f', "V",
        new Value('f', 0.0),
        new Value('f', 5.0));
        while (1) {
            ... do stuff ...
            ... update signals ...

It is possible to retrieve a device's inputs or outputs at a later time using the functions inputs() and outputs().

Updating signals

We can imagine the above program getting sensor information in a loop. It could be running on an network-enabled ARM device and reading the ADC register directly, or it could be running on a computer and reading data from an Arduino over a USB serial port, or it could just be a mouse-controlled GUI slider. However it's getting the data, it must provide it to libmapper so that it will be sent to other devices if that signal is mapped.

This is accomplished by the update() function:


So in the "sensor 1" example, assuming we have some code which reads sensor 1's value into a float variable called v1, the loop becomes:

while (1) {

    // call a hypothetical function that reads a sensor
    v1 = read_sensor_1();

This is about all that is needed to expose sensor 1's value to the network as a mappable parameter. The libmapper GUI can now be used to create a mapping between this value and a receiver, where it could control a synthesizer parameter or change the brightness of an LED, or whatever else you want to do.

Signal conditioning

Most synthesizers of course will not know what to do with the value of sensor1 -- it is an electrical property that has nothing to do with sound or music. This is where libmapper really becomes useful.

Scaling or other signal conditioning can be taken care of before exposing the signal, or it can be performed as part of the mapping. Since end users can demand any mathematical operation be performed on the signal, they can perform whatever mappings between signals they wish.

As a developer, it is therefore your job to provide information that will be useful to the end user.

For example, if sensor 1 is a position sensor, instead of publishing "voltage", you could convert it to centimeters or meters based on the known dimensions of the sensor, and publish a "/sensor1/position" signal instead, providing the unit information as well.

We call such signals "semantic", because they provide information with more meaning than a relatively uninformative value based on the electrical properties of the sensing technique. Some sensors can benefit from low-pass filtering or other measures to reduce noise. Some sensor data may need to be combined in order to derive physical meaning. What you choose to expose as outputs of your device is entirely application-dependent.

You can even publish both "/sensor1/position" and "/sensor1/voltage" if desired, in order to expose both processed and raw data. Keep in mind that these will not take up significant processing time, and zero network bandwidth, if they are not mapped.

Receiving signals

Now that we know how to create a sender, it would be useful to also know how to receive signals, so that we can create a sender-receiver pair to test out the provided mapping functionality.

As mentioned above, the addInputSignal() function takes an optional UpdateListener. This is a function that will be called whenever the value of that signal changes. To create a receiver for a synthesizer parameter "pulse width" (given as a ratio between 0 and 1), specify a handler when calling addInputSignal(). We'll imagine there is some Java synthesizer implemented as a class Synthesizer which has functions setPulseWidth() which sets the pulse width in a thread-safe manner, and startAudioInBackground() which sets up the audio thread.

We need to create a handler function for libmapper to update the synth:

UpdateListener freqHandler = new UpdateListener() {
    public void onUpdate(Signal sig, float[] value, TimeTag tt) {

Then our program will look like this:

import mapper.*;
import mapper.signal.*;

# Some synth stuff

UpdateListener freqHandler = new UpdateListener() {
    public void onUpdate(Signal sig, float[] value, TimeTag tt) {

final Device dev = new Device("mySynth");
Signal pw = dev.addInputSignal("pulseWidth", 1, 'f', "Hz",
new Value('f', 0.0), new Value('f', 1.0),

while (1) {


Alternately, we can declare the UpdateListener as part of the addInputSignal() function:

Signal pulseWidth = dev.addInputSignal("pulseWidth", 1, 'f', "Hz",
                                       new Value('f', 0.0),
                                       new Value('f', 1.0),
                                       new UpdateListener() {
    public void onUpdate(Signal sig, float[] value, TimeTag tt) {

Working with timetags

libmapper uses the TimeTag class to store NTP timestamps associated with signal updates. For example, the handler function called when a signal update is received contains a timetag argument. This argument indicates the time at which the source signal was sampled (in the case of sensor signals) or generated (in the case of sequenced or algorithimically-generated signals).

The signal update() function for output signals is overloaded; calling the function without a timetag argument will automatically label the outgoing signal update with the current time. In cases where the update should more properly be labeled with another time, this can be accomplished by simply adding the timetag as a second argument. This timestamp should only be overridden if your program has access to a more accurate measurement of the real time associated with the signal update, for example if you are writing a driver for an outboard sensor system that provides the sampling time.

libmapper also provides helper functions for getting the current time:

TimeTag tt = new TimeTag();;

Working with signal instances

libmapper also provides support for signals with multiple instances, for example:

The important qualities of signal instances in libmapper are:

All signals possess one instance by default. If you would like to reserve more instances you can use:

<sig>.reserveInstances(int num);

After reserving instances you can update a specific instance:

Signal.Instance inst = <sig>.instance();


inst.update(<value>, TimeTag tt);

Associating signal instances with Java objects

Signal instances can be associated with an arbitrary object, for example:

int[] my_obj = new int[]{1,2,3,4};
Signal.Instance inst = <sig>.instance(my_obj);

The object can be retrieved:

Object o = inst.userReference();

Receiving instances

To receive updates to multiple instances of an input signal you will need to declare an InstanceUpdateListener for the signal in question. Here is the listener prototype:

new InstanceUpdateListener(Signal.Instance inst, float[] value, TimeTag tt);

The listener can be added using the function setInstanceUpdateListener():

<sig>.setInstanceUpdateListener(new InstanceUpdateListener() {
    public void onUpdate(Signal.Instance inst, float[] v, TimeTag tt) {
        System.out.println("in onInstanceUpdate() for "
                           +inst.signal().name()+" instance "
                 ": "+inst.userReference()+", val= "

Remember that you will need to reserve instances for your input signal using <sig>.reserveInstances() if you want to receive instance updates.

Instance Stealing

For handling cases in which the sender signal has more instances than the receiver signal, the instance allocation mode can be set for an input signal to set an action to take in case all allocated instances are in use and a previously unseen instance id is received. Use the function:


The argument mode can have one of the following values:

If you want to use another method for determining which active instance to release (e.g. the sound with the lowest volume), you can create an InstanceEventListener for the signal and write the method yourself:

InstanceEventListener myHandler = new InstanceEventListener() {
    public void onEvent(Signal.Instance inst, InstanceEvent event,
                        TimeTag tt) {
        System.out.println("onInstanceEvent() for "
                           + inst.signal().name() + " instance "
                           + + ": " + event.value());
        // call user function that chooses an instance to release
        Signal.Instance release_me = choose_instance(inst.signal());

For this function to be called when instance stealing is necessary, we need to register it for mapper.signal.InstanceEvent.OVERFLOW events:


Publishing metadata

Things like device names, signal units, and ranges, are examples of metadata – information about the data you are exposing on the network.

libmapper also provides the ability to specify arbitrary extra metadata in the form of name-value pairs. These are not interpreted by libmapper in any way, but can be retrieved over the network. This can be used for instance to label a device with its location, or to perhaps give a signal some property like "reliability", or some category like "light", "motor", "shaker", etc.

Some GUI could then use this information to display information about the network in an intelligent manner.

Any time there may be extra knowledge about a signal or device, it is a good idea to represent it by adding such properties, which can be of any OSC-compatible type. (So, numbers and strings, etc.)

The property interface is through the functions,

<object>.setProperty(String key, Value value);

where the value can any OSC-compatible type. This function can be called for devices or signals.

For example, to store a float vector indicating the 2D position of a device dev, you can call it like this:

dev.setProperty("position", new Value(new float[] {12.5f, 40.f}));

To specify a string property of a signal sig:

sig.setProperty("sensingMethod", new Value("resistive"));

Reserved keys

You can use any property name not already reserved by libmapper.

Reserved keys for devices

description, host, id, is_local, libversion, name, num_incoming_maps, num_outgoing_maps, num_inputs, num_outputs, port, synced, value, version, user_data

Reserved keys for signals

description, direction, id, is_local, length, max, maximum, min, minimum, name, num_incoming_maps, num_instances, num_outgoing_maps, rate, type, unit, user_data

Reserved keys for links

description, id, is_local, num_maps

Reserved keys for maps

description, expression, id, is_local, mode, muted, num_destinations, num_sources, process_location, ready, status

Reserved keys for map slots

bound_max, bound_min, calibrating, causes_update, direction, length, maximum, minimum, num_instances, use_as_instance, type