A Guide to JavaHMI

A Guide to JavaHMI

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2 Developing with JavaHMI

You can download the latest JavaHMI version here: http://sourceforge.net/projects/javahmi/

JavaHMI's CVS repository can be checked out through anonymous (pserver) CVS with the following instruction set. When prompted for a password for anonymous, simply press the Enter key.

cvs -d:pserver:anonymous@cvs.sourceforge.net:/cvsroot/javahmi login cvs -z3 -d:pserver:anonymous@cvs.sourceforge.net:/cvsroot/javahmi co javahmi

2.3 Integrated Development Environments (IDEs)

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2.3.1 Eclipse

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2.3.2 NetBeans

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2.4 Development Considerations

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2.5 Writing JNI (Java™ Native Interface) wrappers

2.5.1 What is JNI

The Java Native Interface enables Java code to be embedded in C code and vice versa. The advantage of JNI to JavaHMI is that most interface cards and PLC have C interfaces available to them. To make these resources available to JavaHMI, JNI can be used to "wrap" the C interfaces in Java classes. These classes are then available to be incorporated with JavaHMI.

2.5.2 A JNI Primer

Excerpts of the following example were taken from chapter 2 of Sheng Liang's The Java Native Interface.

Step 1: Declare the native method in a Java™ program.

Code Example 2.1. HelloWorld.java

class HelloWorld {

	private native void print();

	public static void main(String [] args) {
		new HelloWorld().print();
	}

	static {
		System.loadLibrary("HelloWorld");
	}

Compile this program:

%> javac HelloWorld.java

This creates a file HelloWorld.class in the same directory.

Step 2: Create the native method header.

%> javah -jni HelloWorld

This assumes the current directory is included in your class path. The command above creates the file HelloWorld.h.

Step 3: Write native method implementation.

Code Example 2.2. HelloWorld.c

#include <jni.h>
#include <stdio.h>
#include "HelloWorld.h"

JNIEXPORT void JNICALL
Java_HelloWorld_print(JNIEnv *env, jobject obj)
}
	printf("Hello World!\n");
	return;
}

%> gcc -shared \
> -I$JAVA_HOME/include \
> -I$JAVA_HOME/include/linux \
> HelloWorld.c -o libHelloWorld.so

In the command above the "\" are line continuations. The command above assumes you are running a Linux system. Please consult your c compiler documentation for variances on your system. This command also assumes you have an environment variable JAVA_HOME that points to your Java installation.

The command above produces the file libHelloWorld.so which is a shared library file.

Step 4: Run the program.

%> java -Djava.library.path=. \
> -Djava.class.path=.:$CLASSPATH HelloWorld
Hello World!

2.6 Extending ControlObserver and its subclasses

The following example shows how net.sourceforge.javahmi.mockup.MockDigitalOutput extends net.sourceforge.javahmi.control.ControlDigitalOutput which extends ControlObserver.

Code Example 2.2. MockDigitalOutput.java

package net.sourceforge.javahmi.mockup;

import net.sourceforge.javahmi.control.ControlDigitalOutput;

public class MockDigitalOutput extends ControlDigitalOutput {

	public final Integer ON = new Integer(1);
	public final Integer OFF = new Integer(0);

	private Integer _value = ON;

	public MockDigitalOutput() {
		start();
	}

	public Number read() {
		return _value;
	}

	public void write(Number value) {
		if (value instanceof Integer) {
			if (value.intValue() != ON.intValue()
				&& value.intValue() != OFF.intValue()) {
				StringBuffer msg = new StringBuffer();
				msg.append("Input parameter must be ");
				msg.append("either MockDigitalObservable.ON ");
				msg.append("or MockDigitalObservable.OFF.");
				throw new IllegalArgumentException(msg.toString());
			}
			_value = (Integer) value;
		} else {
			StringBuffer msg = new StringBuffer();
			msg.append("Input parameter is not of type Integer.");
			throw new IllegalArgumentException(msg.toString());
		}
	}
}

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