Pass Java Objects to MATLAB

Purpose

This example shows how to:

Use the MATLAB^® Compiler SDK™ product to create a package that applies MATLAB optimization routines to objective functions implemented as Java^® objects.
Access the MATLAB functions in a Java application , including use of the MWJavaObjectRef class to create a reference to a Java object and pass it to the generated Java methods.
Build and run the application.

OptimDemo Package

The OptimDemo package finds a local minimum of an objective function and returns the minimal location and value. The package uses the MATLAB optimization function fminsearch, and this example optimizes the Rosenbrock banana function used in the fminsearch documentation. The class, Optimizer, performs an unconstrained nonlinear optimization on an objective function implemented as a Java object. A method of this class, doOptim, accepts an initial guess and Java object that implements the objective function, and returns the location and value of a local minimum. The second method, displayObj, is a debugging tool that lists the characteristics of a Java object. These two methods, doOptim and displayObj, encapsulate MATLAB functions. The MATLAB code for these two methods is in doOptim.m and displayObj.m, which can be found in matlabroot\toolbox\javabuilder\Examples\ObjectRefExample\ObjectRefDemoComp.

Procedure

If you have not already done so, copy the files for this example as follows:
1. Copy the following folder that ships with MATLAB to your work folder: matlabroot\toolbox\javabuilder\Examples\ObjectRefExample
2. At the MATLAB command prompt, cd to the new ObjectRefExample subfolder in your work folder.
If you have not already done so, set the environment variables that are required on a development machine. See Configure Your Java Environment.

Write the MATLAB code that you want to access. This example uses doOptim.m and displayObj.m, which are already in your work folder in ObjectRefExample\ObjectRefDemoComp.

For reference, the code of doOptim.m is displayed here:

function [x,fval] = doOptim(h, x0)

% FMINSEARCH can't operate directly on Java 
%    objective functions, 
% so you must create an anonymous function with the correct 
% signature to wrap the Java object.
% Here, we assume our object has a method evaluateFunction() 
% that takes an array of doubles and returns a double. 
% This could become an Interface, 
% and we could check that the object implements that Interface.
mWrapper = @(x) h.evaluateFunction(x);

% Compare two ways of evaluating the objective function
% These eventually call the same Java method, and return the
% same results.
directEval = h.evaluateFunction(x0)
wrapperEval = mWrapper(x0)

[x,fval] = fminsearch(mWrapper,x0)

For reference, the code of displayObj.m is displayed here:

function className = displayObj(h)

h
className = class(h)
whos('h')
methods(h)

Open the Library Compiler app.
Create a Java package using the Library Compiler app to build a Java class that wraps around your MATLAB code.
Use the following information as you work through this example in Compile Java Packages with Library Compiler App:
Project Name OptimDemo

Class Name Optimizer

File to compile doOptim.m and displayObj.m

Write source code for a class that implements an object function to optimize. The sample application for this example is in ObjectRefExample\ObjectRefDemoJavaApp\BananaFunction.java. The program listing is shown here:

public class BananaFunction {
	public BananaFunction() {}
	public double evaluateFunction(double[] x)
	{
		/* Implements the Rosenbrock banana function described in 
		 * the FMINSEARCH documentation
		 */
		double term1 = 100*java.lang.Math.pow((x[1]-Math.pow(x[0],2.0)),2.0); 
		double term2 =  Math.pow((1-x[0]),2.0);
		return term1 + term2;
	}

}

The class implements the Rosenbrock banana function described in the fminsearch documentation.

Write source code for an application that accesses the MATLAB functions. The sample application for this example is in ObjectRefExample\ObjectRefDemoJavaApp\PerformOptim.java. The program listing is shown here:

/* Necessary package imports */
import com.mathworks.toolbox.javabuilder.*;
import OptimDemo.*;

/*
 * Demonstrates the use of the MWJavaObjectRef class
 * Takes initial point for optimization as two arguments:
 *    PerformOptim -1.2 1.0
 */
class PerformOptim
{
	public static void main(String[] args)
	{
		Optimizer theOptimizer = null;		/* Stores component 
                                       instance */
		MWJavaObjectRef origRef = null;	/* Java object reference to 
                                     be passed to component */
		MWJavaObjectRef outputRef = null;	/* Output data extracted 
                                        from result */
		MWNumericArray x0 = null;	/* Initial point for optimization */
		MWNumericArray x = null;	/* Location of minimal value */
		MWNumericArray fval = null;	/* Minimal function value */
		Object[] result = null;	/* Stores the result */


		try
		{
			/* If no input, exit */
			if (args.length < 2)
			{
				System.out.println("Error: must input initial x0_1 
                                            and x0_2 position");
				return;
			}

			/* Instantiate a new Java object */
			/* This should only be done once per application instance */
			theOptimizer = new Optimizer();

			try {
				/* Initial point --- parse data from text fields */
				double[] x0Data = new double[2];
				x0Data[0] = Double.valueOf(args[0]).doubleValue();
				x0Data[1] = Double.valueOf(args[1]).doubleValue();
				x0 = new MWNumericArray(x0Data, MWClassID.DOUBLE);
				System.out.println("Using x0 =");
				System.out.println(x0);

				/* Create object reference to objective function object */
				BananaFunction objectiveFunction = new BananaFunction();
				origRef = new MWJavaObjectRef(objectiveFunction);

				/* Pass Java object to a MATLAB function that lists its 
                              methods, etc */            
				System.out.println("*********************************");
				System.out.println("** Properties of Java object   **");
				System.out.println("*********************************");
				result = theOptimizer.displayObj(1, origRef);     
				MWArray.disposeArray(result);
				System.out.println("** Finished DISPLAYOBJ **********");

				/* Call the Java component to optimize the function */
				/* using the MATLAB function FMINSEARCH */
				System.out.println("**********************************");
				System.out.println("** Unconstrained nonlinear optim**");
				System.out.println("**********************************");
				result = theOptimizer.doOptim(2, origRef, x0);
				try {
					System.out.println("** Finished DOOPTIM ****** *********");
					x = (MWNumericArray)result[0];
					fval = (MWNumericArray)result[1];

					/* Display the results of the optimization */
					System.out.println("Location of minimum: ");
					System.out.println(x);
					System.out.println("Function value at minimum: ");
					System.out.println(fval.toString());
				}
				finally
				{
					MWArray.disposeArray(result);
				}
			}
			finally
			{
				/* Free native resources */
				MWArray.disposeArray(origRef);
				MWArray.disposeArray(outputRef);
				MWArray.disposeArray(x0);
			}
		}
		catch (Exception e)
		{
			System.out.println("Exception: " + e.toString());
		}

		finally
		{
			/* Free native resources */
			if (theOptimizer != null)
				theOptimizer.dispose();
		}
	}
}

The program does the following:

Instantiates an object of the BananaFunction class above to be optimized.
Creates an MWJavaObjectRef that references the BananaFunction object, as shown: origRef = new MWJavaObjectRef(objectiveFunction);
Instantiates an Optimizer object
Calls the displayObj method to verify that the Java object is being passed correctly
Calls the doOptim method, which uses fminsearch to find a local minimum of the objective function
Uses a try/catch block to handle exceptions
Frees native resources using MWArray methods

Compile the PerformOptim.java application and BananaFunction.java helper class using javac. When entering this command, ensure there are no spaces between path names in the matlabroot argument. For example, there should be no space between javabuilder.jar; and .\distrib\OptimDemo.jar in the following example.
1. Open a Command Prompt window and cd to the matlabroot\ObjectRefExample folder.
2. Compile the application according to which operating system you are running on:
  - On Windows^®, execute this command:
```
javac -classpath
.;matlabroot\toolbox\javabuilder\jar\javabuilder.jar;
.\distrib\OptimDemo.jar BananaFunction.java
javac -classpath
.;matlabroot\toolbox\javabuilder\jar\javabuilder.jar;
.\distrib\OptimDemo.jar PerformOptim.java
```
  - On UNIX^®, execute this command:
```
javac -classpath
.:matlabroot/toolbox/javabuilder/jar/javabuilder.jar:
./distrib/OptimDemo.jar BananaFunction.java
javac -classpath
.:matlabroot/toolbox/javabuilder/jar/javabuilder.jar:
./distrib/OptimDemo.jar PerformOptim.java
```

Execute the PerformOptim class file as follows:

On Windows, type:

      java -classpath 
      .;matlabroot\toolbox\javabuilder\jar\javabuilder.jar 
      .\distrib\OptimDemo.jar 
      PerformOptim -1.2 1.0

On UNIX, type:

      java -classpath 
      .:matlabroot/toolbox/javabuilder/jar/javabuilder.jar: 
      ./distrib/OptimDemo.jar 
      PerformOptim -1.2 1.0

Note: You should be using the same version of Java that ships with MATLAB. To find out what version of Java MATLAB is running, enter the following MATLAB command:

version -java

Caution MathWorks only supports the Oracle^® JDK™ and JRE™. A certain measure of cross-version compatibility resides in the Oracle software and it may be possible to run applications with compiled MATLAB functions with non-Oracle JDKs under some circumstances—however, compatibility is not guaranteed.

Note: If you are running on the Mac 64-bit platform, you must add the -d64 flag in the Java command. See Limitations of the MATLAB Compiler SDK Java Target for more specific information.

When run successfully, the PerformOptim program should display the following output:

Using x0 =
-1.2000    1.0000
*****************************************************
** Properties of Java object                       **
*****************************************************
 
h =
 
BananaFunction@1766806
 
 
className =
 
BananaFunction
 
  Name      Size            Bytes  Class             Attributes
 
  h         1x1                    BananaFunction              
 
 
 
Methods for class BananaFunction:
 
 
BananaFunction    getClass          notifyAll         
equals            hashCode          toString          
evaluateFunction  notify            wait              
 
** Finished DISPLAYOBJ ******************************
*****************************************************
** Performing unconstrained nonlinear optimization **
*****************************************************
 
directEval =
 
   24.2000
 
 
wrapperEval =
 
   24.2000
 
 
x =
 
    1.0000    1.0000
 
 
fval =
 
   8.1777e-10
 
Optimization successful
** Finished DOOPTIM *********************************
Location of minimum: 
1.0000    1.0000
Function value at minimum: 
8.1777e-10

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