I used to have the book Essential JNI. And while it is kinda dated (1998), much of it still works today. If I recall correctly, in C, Java constructs are simply pointers. Thus, in your code, (*env)-> is dereferencing pointers to give you access to the underlying methods. For C++, env is actually an object … Read more
You cannot cache the JNIEnv pointer. Read about it here: The JNI interface pointer (JNIEnv) is valid only in the current thread. Should another thread need to access the Java VM, it must first call AttachCurrentThread() to attach itself to the VM and obtain a JNI interface pointer. Once attached to the VM, a native … Read more
We just code utility methods for each of the types of exceptions we want to throw. Here are some examples: jint throwNoClassDefError( JNIEnv *env, char *message ) { jclass exClass; char *className = “java/lang/NoClassDefFoundError”; exClass = (*env)->FindClass( env, className); if (exClass == NULL) { return throwNoClassDefError( env, className ); } return (*env)->ThrowNew( env, exClass, message … Read more
Quick profiler test yields: Java class: public class Main { private static native int zero(); private static int testNative() { return Main.zero(); } private static int test() { return 0; } public static void main(String[] args) { testNative(); test(); } static { System.loadLibrary(“foo”); } } C library: #include <jni.h> #include “Main.h” JNIEXPORT int JNICALL Java_Main_zero(JNIEnv … Read more
You can use the Android logging facilities: #include <android/log.h> #define APPNAME “MyApp” __android_log_print(ANDROID_LOG_VERBOSE, APPNAME, “The value of 1 + 1 is %d”, 1+1); Make sure you also link against the logging library, in your Android.mk file: LOCAL_LDLIBS := -llog
The Unsatisfied Link Error can mean many things went wrong. I would use System.loadLibrary(“HelloWorld”); Instead of System.load(); As TwentyMiles suggested. Also, when invoking your program you need to (assuming your DLL is on the same directory as your class files: java -Djava.library.path=. HelloWorld Here’s a simple demo I made that calls a Win32 API function … Read more
I’m aware of two compelling projects: posix for Java (based on JNI) [derived from Jython] jna-posix (based on JNA) [derived from JRuby] Personally I like very much JNA. Take a look at this example of mine, mapping link(2): import com.sun.jna.Library; import com.sun.jna.Native; class Link { private static final C c = (C) Native.loadLibrary(“c”, C.class); private … Read more
You can always access the directory in which your app is placed. This directory is usually : data/data/<Your_package_name_usually com.android.appName>/files/<your_filename> but better use getFilesDir() to ensure valid filepath with future version changes of android. If you wanna use external storage instead, make sure you place this line of code in your app manifest to gain permission. … Read more
You can achieve what you’re looking for with SWIG+Java using “Directors”, however it’s not quite as straightforward mapping from the C++ abstract classes onto Java as you might hope. My answer therefore is split into three parts – firstly the simple example of implementing a C++ pure virtual function in Java, secondly an explanation of … Read more
[DISCLAIMER: Note that this solution always was hackish. And in most cases won’t work anymore nowadays. Check Benjamins answer for more info] There is a way to determine all currently loaded native libraries if you meant that. Already unloaded libraries can’t be determined. Based on the work of Svetlin Nakov (Extract classes loaded in JVM … Read more