Java Native Interface is a standard programming interface for writing Java native methods. It is known to be a gray area not so many do and is perfectly fine not to develop .dll/.so libraries. After all, it's just one line away from the JVM crash if not done properly.

Most JNI developers use C/C++ environment, Sun releases header files only in C format. But Delphi (Object Pascal) has a very active community and JNI headers have been ported for us. Delphi JNI.pas home page hosts an excellent examples and JNI.pas file.

Use JavaToDPR tool to create MyDelphi.dpr project file from .java source code. It creates all necessary function declarations from native tagged java functions. It simplifies .dll process to automate tedious mandatory JNI function naming convention.

Enhanced JNI.pas
Updated JNI.pas file
Changes are very simple and does not modify anything from the earlier versions.

• JStringToWideString: convert jstring to widestring
• WideStringToJString: convert widestring to jstring
• BytesToJByteArray: convert byte array to JByteArray
• IntegersToJIntArray: convert integer array to JIntArray
• ThrowException: wrap native exception to java.lang.Exception
• SetIntMethod: call public void myClass.setInt(int id) method
• SetStringMethod: call public void myClass.setString(String s) method

Java has no problems using unicode strings because the internal format is unicode already. This is not so easy in native side where you have a choice of several string types. Old legacy string is ansistring and unicode-aware is widestring variable type. If you want to send strings back and fort you should keep values in widestring variable. Knowing characters are within the range of system-default encoding you may use ansistring. But doing things right at the year +2006, you should stick to widestring variables.

SimpleDemo example
SimpleDemo.zip example (uses enhanced JNI.pas)
My goal was a short and simple Delphi study.

• unicode string interchange between Java and Delphi program
• simple integer interchange between Java and Delphi program
• Java class instantiation and "setter" methods from Delphi program

Test program calls several native functions and print results to UTF-8 formatted html file. Web browsers have a good unicode support, so character results can be see easily. Verify SimpleDemo.png image what you should see in SimpleDemo.html page generated by test program.

You need a good unicode font, some Windows has "Arial Unicode MS" which usually works fine. Its installed by some MS software (MSOffice, ...).

See more info in SimpleDemo.zip/readme.txt file about how to compile project.

ScreenShot example
screenshot.zip example (uses enhanced JNI.pas)
Use native code to take a screenshot from the desktop. Code is able to "see" alphablended windows unlike Java Robot sceenshot method. Example introduces the following technique.

• return byte array from Delphi to Java
• return integer array from Delphi to Java

Turbo Delphi
Turbo Delphi IDE
Turbo Delphi is a new release from Borland Delphi. Now you can get one of the best win32 development tool reasonable price....or even free of charge. Turbo Delphi Professional version provides a cutting edge version.
The free Turbo Delphi Explorer edition lets beginners and hobbyists learn win32 programming. But you can install only one Explorer version to the same machine, so be sure to take your favourite version. This page goes for Delphi release of course.

SimpleDemo example: Java and Delphi code
Java class has one or more native method declarations, that are implemented in .dll library. Java program may use functions just like any other pure Java functions.

You need three files in this example:
SimpleDemo.java, SimpleDemo.dpr, uSimpleDemo.pas

file: SimpleDemo.java
public class SimpleDemo {
public static void main(String[] args) throws Exception {
// call native Delphi method from Java
SimpleDemo sd = new SimpleDemo();
System.out.println( sd.getStringDelphiJ("Hello from Java") );
}
// declare native functions
native public String getStringDelphiJ(String s);
// load .dll library at startup
static {
System.loadLibrary("SimpleDemo");
}
}

• Declare library SimpleDemo; to compile SimpleDemo.dll file
• import JNI.pas; file, that implements JNI interface
• declare and implement native public String getStringDelphiJ(String s); method
• function must use a strict naming convention
  Java_ prefix is mandatory for each jni entrypoint method
  package_classname_ from java class, this example does not have a package in a .java source file, so its just classname.
  functionname from java class
  PEnv: PJNIEnv; Obj:JObject args are always present
  optional params if java method declared any
  

file: SimpleDemo.dpr
library SimpleDemo;
// import JNI.pas file
uses
SysUtils,
uSimpleDemo in 'uSimpleDemo.pas',
JNI in '..\..\JNI\JNI.pas';
{$R *.res}
function Java_SimpleDemo_getStringDelphiJ
(PEnv: PJNIEnv; Obj: JObject; Arg1: JString): JString; stdcall;
begin
// - convert JNI-JString to Widestring (unicode)
// - call uSimpleDemo.getStringDelphiJ(s: Widestring) method
// - convert widestring to JNI-JString
// - return JNI-String back to java program
// try-catch all exceptions and wrap it to
// to java.lang.Exception
try
Result := JNI_WideStringToJString(PEnv,
uSimpleDemo.getStringDelphiJ(
JNI_JStringToWideString(PEnv, Arg1)
)
);
except
on E: Exception do begin
Result := nil;
JNI_ThrowException(PEnv, E);
end;
end;
end;
exports
// export public JNI .dll functions
Java_SimpleDemo_getStringDelphiJ;
end.

file: uSimpleDemo.pas

unit uSimpleDemo;
interface
// declare public functions
function getStringDelphiJ(ws: WideString): WideString; forward;
implementation
uses
SysUtils;
// implement function
function getStringDelphiJ(ws: WideString): WideString;
var
ws2: WideString;
begin
ws2 := WideUpperCase(ws);
Result := 'Uppercased in dll: ' + ws2
+ ' (len ' + IntToStr(Length(ws2)) + ')';
end;
end.