JNI - Java & C in the same project

1. JNI Karol Wrótniak

2. • Java Native Interface • Native: • Platform-specific shared library (.dll, .so, .dylib) • Non-hybrid mobile app, React Native • C, C++ • Call native functions from Java and vice versa • Other JVM languages supported • JNI ≠ JNA Overview

3. • Implement operations not achievable in pure Java: • System clock - System.currentTimeMillis() • Filesystem access • Sockets (e.g. for HTTP requests) • Reuse existing native libraries • Hinder reverse-engineering • Increase performance (in certain cases only): • computation Why JNI?

4. Sample projects using JNI

5. static { System.loadLibrary("foo"); } public native int getTextLength(String text); Foo.java:

6. static { System.loadLibrary("foo"); } public native int getTextLength(String text); Foo.java:

7. static { System.loadLibrary("foo"); } public native int getTextLength(String text); #include <jni.h> JNIEXPORT jint JNICALL Java_pl_droidsonroids_ndkdemo_Foo_getTextLength( JNIEnv *env, jobject thiz, jstring text ) { return (*env)->GetStringUTFLength(env, text); } foo.c: Foo.java:

17. • System.loadLibrary("foo") • System.mapLibraryName("foo") ➔ "libfoo.so" • java.library.path property • System.load("/usr/local/lib/libfoo.so") Java API

18. • jbyte, jchar, jshort, jint, jlong, jfloat, jdouble • jboolean: JNI_TRUE, JNI_FALSE • jstring, jthrowable • jarray, jintarray... • jclass, jobject • jmethodID, jfieldID JNI types

19. Calling Java from C jclass mathClass = (*env)->FindClass(env, "java/lang/Math"); jmethodID minMethodID = (*env)->GetStaticMethodID(env, mathClass, "min", "(JJ)J"); jlong min = (*env)->CallStaticLongMethod(env, mathClass, minMethodID, x, y); public static long min(long a, long b)

22. Calling Java methods jclass mathClass = (*env)->FindClass(env, "java/lang/Math"); jmethodID minMethodID = (*env)->GetStaticMethodID(env, mathClass, "min", "(JJ)J"); jlong min = (*env)->CallStaticLongMethod(env, mathClass, minMethodID, x, y); public static long min(long a, long b)

23. Accessing Java ﬁelds jobject calendar = … jclass calendar_class = (*env)->GetObjectClass(env, calendar); jfieldID time_field_id = (*env)->GetFieldID(env, calendar_class, "time", "J"); jlong time = (*env)->GetLongField(env, calendar, time_field_id);

24. Creating Java objects jclass foo_class = (*env)->FindClass(env, "pl/droidsonroids/ndkdemo/Foo"); jmethodID constructor_id = (*env)->GetMethodID(env, foo_class, "<init>", "()V"); jobject foo = (*env)->NewObject(env, foo_class, constructor_id);

25. Type signatures Type signature Java type Example Z boolean B byte C char S short I int J long F float D double L<FQCN>; FQCN Lcom/foo/Foo; [<type> type[] [Z V void javap -s <class file> abstract ArrayList<String> foo(String[] a, boolean b); ([Ljava/lang/String;Z)Ljava/util/ArrayList;

26. • Local • Global • WeakGlobal (weaker than Java Weak and Soft) Reference types

27. Local references JNIEXPORT void JNICALL Java_pl_droidsonroids_ndkdemo_Foo_foo( JNIEnv *env, jobject thiz, jobject foo ) { //... } Local reference

28. Global References jobject persistent_foo; JNIEXPORT void JNICALL Java_pl_droidsonroids_ndkdemo_Foo_foo( JNIEnv *env, jobject thiz, jobject foo ) { persistent_foo = (*env)->NewGlobalRef(env, foo); } (*env)->DeleteGlobalRef(env, persistent_foo);

29. Finalizers public final class Foo { @Override protected void finalize() throws Throwable { releaseBar(bar); super.finalize(); } private final long bar; public Foo() { bar = createBar(); } private native long createBar(); private native void releaseBar(long bar); ...

30. JNI_OnLoad JNI_OnUnload void *foo; JavaVM *global_vm; JNIEXPORT jint JNI_OnLoad(JavaVM *vm, void *reserved) { global_vm = vm; foo = malloc(1024); if (foo == NULL) { return JNI_ERR; } return JNI_VERSION_1_8; } JNIEXPORT void JNI_OnUnload(JavaVM *vm, void *reserved) { free(foo); }

31. Registering native methods //Foo.java public native int plus(int a, int b); //Foo.c static jint add(JNIEnv *env, jobject thiz, jint a, jint b) { return a + b; }

32. JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *vm, void *reserved) { JNINativeMethod methods[] = { {"plus", "(II)I", (void *) add} }; return JNI_VERSION_1_6; } Registering native methods //Foo.java public native int plus(int a, int b); //Foo.c static jint add(JNIEnv *env, jobject thiz, jint a, jint b) { return a + b; }

33. JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *vm, void *reserved) { JNINativeMethod methods[] = { {"plus", "(II)I", (void *) add} }; JNIEnv *env; (*vm)->GetEnv(vm, (void **) &env, JNI_VERSION_1_6); jclass foo_class = (*env)->FindClass(env, "pl/droidsonroids/ndkdemo/Foo"); (*env)->RegisterNatives(env, foo_class, methods, 1); return JNI_VERSION_1_6; } Registering native methods //Foo.java public native int plus(int a, int b); //Foo.c static jint add(JNIEnv *env, jobject thiz, jint a, jint b) { return a + b; }

34. • JNI functions: • ExceptionCheck • ExceptionOccured • ExceptionClear • FatalError • ExceptionDescribe Handling exceptions

35. (*env)->CallVoidMethod(env, foo, fooMethodID); if ((*env)->ExceptionCheck(env) == JNI_TRUE) { (*env)->ExceptionClear(env); (*env)->CallVoidMethod(env, foo, barMethodID); } Catching exceptions catch • Need to clear exception or return before JNI calls

36. Arrays JNIEXPORT void JNICALL Java_pl_droidsonroids_ndkdemo_Foo_foo(JNIEnv *env, jobject thiz, jintArray points) { jsize length = (*env)->GetArrayLength(env, points); jint *native_points = (*env)->GetIntArrayElements(env, points, NULL); //... (*env)->ReleaseIntArrayElements(env, points, native_points, 0); }

37. Arrays JNIEXPORT void JNICALL Java_pl_droidsonroids_ndkdemo_Foo_foo(JNIEnv *env, jobject thiz, jintArray points) { jsize length = (*env)->GetArrayLength(env, points); jint *native_points = (*env)->GetIntArrayElements(env, points, NULL); //... (*env)->ReleaseIntArrayElements(env, points, native_points, 0); }

38. Arrays JNIEXPORT void JNICALL Java_pl_droidsonroids_ndkdemo_Foo_foo(JNIEnv *env, jobject thiz, jintArray points) { jsize length = (*env)->GetArrayLength(env, points); jint *native_points = (*env)->GetIntArrayElements(env, points, NULL); //... (*env)->ReleaseIntArrayElements(env, points, native_points, 0); } Release mode

39. Arrays JNIEXPORT void JNICALL Java_pl_droidsonroids_ndkdemo_Foo_foo(JNIEnv *env, jobject thiz, jintArray points) { jsize length = (*env)->GetArrayLength(env, points); jint *native_points = (*env)->GetIntArrayElements(env, points, NULL); //... (*env)->ReleaseIntArrayElements(env, points, native_points, 0); } *isCopy

40. JNIEXPORT void JNICALL Java_pl_droidsonroids_ndkdemo_Foo_foo(JNIEnv *env, jobject thiz, jintArray points) { jsize start = 0; jsize size = 2; jint native_points[2]; (*env)->GetIntArrayRegion(env, points, start, size, native_points); native_points[0] = native_points[1]; (*env)->SetIntArrayRegion(env, points, start, size, native_points); } Arrays JNIEXPORT void JNICALL Java_pl_droidsonroids_ndkdemo_Foo_foo(JNIEnv *env, jobject thiz, jintArray points) { jsize length = (*env)->GetArrayLength(env, points); jint *native_points = (*env)->GetIntArrayElements(env, points, NULL); //... (*env)->ReleaseIntArrayElements(env, points, native_points, 0); }

41. JNIEXPORT void JNICALL Java_pl_droidsonroids_ndkdemo_Foo_foo(JNIEnv *env, jobject thiz, jintArray points) { jsize start = 0; jsize size = 2; jint native_points[2]; (*env)->GetIntArrayRegion(env, points, start, size, native_points); native_points[0] = native_points[1]; (*env)->SetIntArrayRegion(env, points, start, size, native_points); } Arrays JNIEXPORT void JNICALL Java_pl_droidsonroids_ndkdemo_Foo_foo(JNIEnv *env, jobject thiz, jintArray points) { jsize length = (*env)->GetArrayLength(env, points); jint *native_points = (*env)->GetIntArrayElements(env, points, NULL); //... (*env)->ReleaseIntArrayElements(env, points, native_points, 0); }

42. GC compaction • Disable GC between Get and Release • Pin • Copy Java heap native heap

43. Mode Copy back Free buffer 0 ✓ ✓ JNI_COMMIT ✓ ❌ JNI_ABORT ❌ ✓ Release modes & isCopy Need to release with another mode *isCopy == JNI_FALSE: • skip useless commit • restore original contents before abort

44. Direct byte buffers ByteBuffer.allocateDirect(1024); void *foo = (*env)->GetDirectBufferAddress(env, buffer); jlong capacity = (*env)->GetDirectBufferCapacity(env, buffer);

45. 1. Java Native Interface Specification 2. JNI Tips 3. The Java™ Native Interface Programmer’s Guide and Specification References

46. Q&A Android dev @DroidsOnRoids Co-organizer @GDG Wrocław karol.wrotniak@droidsonroids.pl koral-- Karol Wrótniak karol-wrotniak @karol.wrotniak

JNI - Java & C in the same project

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