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copied!<p>The Unsatisfied Link Error can mean many things went wrong. I would use</p> <pre><code>System.loadLibrary("HelloWorld"); </code></pre> <p>Instead of</p> <pre><code>System.load(); </code></pre> <p>As TwentyMiles suggested.</p> <p>Also, when invoking your program you need to (assuming your DLL is on the same directory as your class files:</p> <p>java -Djava.library.path=. HelloWorld</p> <p>Here's a simple demo I made that calls a Win32 API function (MessageBox)</p> <h2>Java class</h2> <pre><code>class CallApi{ private native String showMessageBox(String msg); private native double getRandomDouble(); static{ try{ System.loadLibrary("CallApi"); System.out.println("Loaded CallApi"); }catch(UnsatisfiedLinkError e){ //nothing to do System.out.println("Couldn't load CallApi"); System.out.println(e.getMessage()); } } public static void main(String args[]){ CallApi api = new CallApi(); double randomNumber = api.getRandomDouble(); String retval = api.showMessageBox("Hello from Java!\n"+ "The native random number: "+randomNumber); System.out.println("The native string: "+retval); } } </code></pre> <h2>Generated header file</h2> <pre><code>/* DO NOT EDIT THIS FILE - it is machine generated */ #include <jni.h> /* Header for class CallApi */ #ifndef _Included_CallApi #define _Included_CallApi #ifdef __cplusplus extern "C" { #endif /* * Class: CallApi * Method: showMessageBox * Signature: (Ljava/lang/String;)Ljava/lang/String; */ JNIEXPORT jstring JNICALL Java_CallApi_showMessageBox (JNIEnv *, jobject, jstring); /* * Class: CallApi * Method: getRandomDouble * Signature: ()D */ JNIEXPORT jdouble JNICALL Java_CallApi_getRandomDouble (JNIEnv *, jobject); #ifdef __cplusplus } #endif #endif </code></pre> <h2>The C DLL code</h2> <pre><code>#include "CallApi.h" #include <windows.h> #include <stdlib.h> #include <time.h> #pragma comment(lib,"user32.lib") JNIEXPORT jstring JNICALL Java_CallApi_showMessageBox (JNIEnv *env, jobject thisObject, jstring js) { //first convert jstring to const char for use in MessageBox const jbyte* argvv = (*env)->GetStringUTFChars(env, js, NULL); char* argv =(char *) argvv; //Call MessageBoxA MessageBox(NULL, argv, "Called from Java!", MB_ICONEXCLAMATION | MB_OK); return js; } JNIEXPORT jdouble JNICALL Java_CallApi_getRandomDouble (JNIEnv *env, jobject thisObject) { double num1; srand((unsigned)(time(0))); num1 = ((double)rand()/(double)RAND_MAX); return num1; } </code></pre> <h2>Compile instructions</h2> <p>I compile with the Visual C++ express 2008 cl, removing the -ML flag since it causes an exception when the Java code tries to call the native code:</p> <p>cl /I"c:\Program Files\Java\jdk1.6.0_10\include" /I"c:\Program Files\Java\jdk1.6.0_10\include\win32" -LD CallApi.c -FeCallApi.dll</p> <p>Then, to run the code:</p> <p>java -Djava.library.path=. CallApi</p>

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