UnsatisfiedLinkError at JNI invocation

Question

I've already found a lot of things and questions about UnsatisfiedLinkError in combination with JNI but none of them could help me out though. I'm trying to hide a window using Java, so I need 3 WinAPI functions.

I have the following Java source:

package hide.window;
public class HideWindow {

    public native static int hideWindow(char[] windowTitle);

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

    public static void main(String[] args) {
        System.out.println("loadLibrary was successful");
        System.out.println(hideWindow("Session Window - Windows Internet Explorer".toCharArray()));
    }
}

Then I have my hide_window_HideWindow.h generated by javah. There's one thing I had to change, because it didn't find the jni.h file in the library path so I just put it into my project and included it the following way:

#include "jni.h"

May this cause my Error?? The whole file:

/* DO NOT EDIT THIS FILE - it is machine generated */
#include "jni.h"
/* Header for class hide_window_HideWindow */

#ifndef _Included_hide_window_HideWindow
#define _Included_hide_window_HideWindow
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     hide_window_HideWindow
 * Method:    hideWindow
 * Signature: ([C)I
 */
JNIEXPORT jint JNICALL Java_hide_window_HideWindow_hideWindow
  (JNIEnv *, jobject, jcharArray);

#ifdef __cplusplus
}
#endif
#endif

The code I compiled to my DLL looks as follows:

#include <windows.h>
#include "hide_window_HideWindow.h"
using namespace std;

JNIEXPORT jint JNICALL Java_hide_window_HideWindow_hideWindow(char windowTitle[20])
{
    HWND windowHandle = FindWindowA(NULL, windowTitle);
    if (windowHandle){
        if (IsWindowVisible(windowHandle)){
            ShowWindow(windowHandle, 0);
            return 0;
        }
        else{
            ShowWindow(windowHandle, 1);
            return 1;
        }
    }
    return -1;
}

There's no difference between a call from eclipse and the following call via cmd (Path is correct):

java -Djava.library.path=.. hide.window.HideWindow

The resulting output looks like that:

loadLibrary was successful
Exception in thread "main" java.lang.UnsatisfiedLinkError: hide.window.HideWindow.hideWindow([C)I
  at hide.window.HideWindow.hideWindow(Native Method)
  at hide.window.HideWindow.main(HideWindow.java:12)

That indicates to me that the Java program is able to load the DLL but it fails at the function call. I use Eclipse as Java and Codeblocks as C++ IDE, and I have already added the following to the mingw build options.

-Wl,--kill-at

I hope this question is not too stupid and thank you in advance, Tom

Answer 1

Look at the prototype:

JNIEXPORT jint JNICALL Java_hide_window_HideWindow_hideWindow
    (JNIEnv *, jobject, jcharArray);

and your implementation:

JNIEXPORT jint JNICALL Java_hide_window_HideWindow_hideWindow
    (char windowTitle[20])

Of course there's an UnsatisfiedLinkError because you didn't implement the correct function.

So you would need something like:

JNIEXPORT jint JNICALL Java_hide_window_HideWindow_hideWindow
    (JNIEnv* env, jobject thiz, jcharArray windowTitle)

And to get the chars out of the array have a look at GetPrimitiveArrayCritial or GetCharArrayRegion.

The generated header also look suspicious to me, because you defined the Java method to be static but the generated prototype wants to pass a jobject ... static methods need jclass (because there is no instance object for static method calls):

JNIEXPORT jint JNICALL Java_hide_window_HideWindow_hideWindow
    (JNIEnv* env, jclass clazz, jcharArray windowTitle)

Another thing: Java's char (jchar) is an unsigned int16, C/C++ char is signed or unsigned int8. So it might be better to pass a String (jstring) and get the contents as an UTF8 string using GetStringUTFChars.