Calling Matlab From Java

Here is a class I built as part of my matlab distributed computing tool. It uses Java 1.5 with generics and variable argument lists to create a reasonably simple interface to Matlab functions.

The function object is thread safe and blocks until matlab returns the result.

import com.mathworks.jmi.*;

import java.util.concurrent.*;

/**
 * Represents a blocking matlab function call. Adds the ability to call a matlab
 * function in a thread safe manner. The function call to execute the matlab
 * function takes a variabe length argument list.
 *
 * @author Brad
 *
 */
public class MatlabFunction<T> {

    private class Completion implements CompletionObserver {

        private T returnValue;

        private Semaphore semaphore = new Semaphore(0);

        /*
         *
         * @see com.mathworks.jmi.CompletionObserver#completed(int,
         *      java.lang.Object)
         */
        @SuppressWarnings("unchecked")
        public void completed(int arg0, Object arg1) {
            returnValue = (T) arg1;
            semaphore.release();
        }

        T getReturnValue() {
            try {
                semaphore.acquire();
                return returnValue;
            } catch (InterruptedException e) {
                return null;
            }
        }
    }

    /** The name of the matlab function */
    String function;

    /** Handle to the matlab object */
    Matlab matlab = new Matlab();

    /**
     * Create a thread safe blocking matlab function object
     *
     * @param function
     *            The name of the matlab function
     */
    public MatlabFunction(String function) {
        this.function = function;
    }

    /**
     * Execute the matlab function
     *
     * @param args
     *            Variable input arguments
     * @return the result of the matlab function.
     */
    public T execute(Object... args) {
        Completion completion = new Completion();
        matlab.feval(function, args, completion);
        return completion.getReturnValue();
    }

}

Use the class like

MatlabFunction<double []> f = new MatlabFunction<double []>("times");
double x[] = f.execute(10,20);

The function should return the value 20

Just remember that you can't call this from the Matlab command line. It will deadlock and hang your Matlab. This should only be run from a seperate thread not directly invoked by the Matlab thread.

If you get a ClassCastException when running this then Matlab is returning a datatype different to what you have specified in the generics specification. Use a debugger to figure out what kind of datatype is being returned by matlab.

Collect multiple return values into a cell array.

The min function can return two values, Y and I. Y being the min values and I being the index of the min value. Normally you would call it like this.

[y i] = min(X);

However sometimes this is not convienient. What if you don't know before hand how many left hand values you wish to collect. Imagine you wanted to wrap up a call to feval with your own feval.

function varargout = my_feval(varargin)
   ...
   % Do some stuff
   ...
   varargout= feval(varargin);
   ...
   % Do some stuff
   ...
end

will not work. You need to do some fancy indexing.

function varargout = my_feval(varargin)
   ...
   % Do some stuff
   ...
   [varargout{1:nargout}] = feval(varargin);
   ...
   % Do some stuff
   ...
end