The rules here are clear. Method ID and field ID values are forever. You can hang onto them. The lookups take some time. jclass, on the other hand, is generally a local reference. A local reference survives, at most, the duration of a single call to a JNI function. If you need to optimize, you … Read more
There are many valid ways to callback into Java from C/C++. I’m going to show you a technique using C (easy to adjust env for C++) that makes it fairly easy to pass data from native code to Java code. This example passes strings ( easy to modify for any data type ). In native … Read more
I suspect that jni.h is trying to #include <jni_md.h>, which is then failing because you haven’t added its location to your include path. Try adding both of these entries to your C compiler’s include path: C:\Program Files\Java\jdk1.7.0\include C:\Program Files\Java\jdk1.7.0\include\win32 The win32 path might not be necessary, depending on how jni.h is set up.
From the source: package jnahelloworldtest; import com.sun.jna.Library; import com.sun.jna.Native; import com.sun.jna.NativeLong; import com.sun.jna.Platform; import com.sun.jna.*; /** Simple example of native library declaration and usage. */ public class Main { public interface simpleDLL extends Library { simpleDLL INSTANCE = (simpleDLL) Native.loadLibrary( (Platform.isWindows() ? “simpleDLL” : “simpleDLLLinuxPort”), simpleDLL.class); // it’s possible to check the platform on which … Read more
Since Android Studio 3.1 its possible easy way: 1. Create cpp folder inside app\src\main. 2. Create <YOUR_FILE_NAME>.cpp file in app\src\main\cpp path (e.g. native-lib.cpp) 3. Add CMakeLists.txt file to app folder. In that file name of the library, .cpp file path and some other settings should be defined, e.g. (from new, empty Android Studio Project with … Read more
explanation the sample code shows how to store 2 different bitmaps (small ones, but it’s just a demo), recycle the original java ones, and later restore them to java instances and use them. as you might guess, the layout has 2 imageViews. i didn’t include it in the code since it’s quite obvious. do remember … 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
On Linux, use colon : as separator (as you will do with the classpath option) as in: -Djava.library.path=/opt/hdf-java/build/bin:/opt/opencv-2.4.10/build/lib
JNA is much slower than JNI, but much easier. If performance is not an issue use JNA. Using direct buffers have the advantage that the most critical operations don’t use JNI or JNA and are thus faster. They use intrinsic when means they get turned into single machine code instructions. If Java code is significantly … Read more
Since the objects in Java and C code actually use the same memory locations (the object passed to the native code is the same memory reference in both worlds), a simple if (someJObject == NULL) {} in the C code should be just fine I guess. I haven’t tested it though 🙂