My environment:

• OS: Mac 10.10.1
• c compiler: Apple LLVM version 6.0

This is based on the JavaNativeInterface and Java Fundamentals Tutorial : Java Native Interface Jni.

Notice the header file, listed below, has many types -- Type Conversion .

• Primitive type matching:

  

• Reference types

  JNI passes objects to native methods as opaque references, C pointer types that refer to internal data structures in the JVM.

   JNIEXPORT void JNICALL Java_com_marakana_jniexamples_Hello_sayHi
     (JNIEnv *, jobject, jstring, jint);

• Native Method Arguments:

  • JNIEnv:

    pointer that points to another pointer pointing to a function table (array of pointer). Each entry in this function table points to a JNI function. These are the functions we are going to use for type conversion

  • jobject:

    It's the second argument for instance method, refering to the object on which the method is invoked.

  • jclass:

    It's the second argument for static method refering to the class in which the method is define.

• String Conversion

  • (*env)->GetStringUTFChars(env, who, &iscopy);

    Convert java String to C string.

  • Other mathod can be found in JNI Functions.

HelloJNI.o : HelloJNI.c HelloJNI.h
gcc -I"/System/Library/Frameworks/JavaVM.framework/Headers" -fpic -c $< -o $@

.jnilib extension is used. Refer to Mac Developer Library.

For example, if the files hello.c and hola.c contain the implementations of the native methods to be built into a dynamic shared JNI library that will be called with System.loadLibrary(“hello”), you would build the resultant library, libhello.jnilib, with this code:

cc -c -I/System/Library/Frameworks/JavaVM.framework/Headers hola.c
cc -c -I/System/Library/Frameworks/JavaVM.framework/Headers hello.c
cc -dynamiclib -o libhello.jnilib hola.o hello.o -framework JavaVM

Often JNI libraries have interdependencies. For example assume the following:

• libA.jnilib contains a function foo().
• libB.jnilib needs to link against libA.jnilib to make use of foo().

Refer to the example/hellojni :

System.loadLibrary("HelloImpl");

gcc -o libHelloImpl.jnilib -lc -shared 
	-I"/System/Library/Frameworks/JavaVM.framework/Headers" Hello.c # -> jnilib

export LD_LIBRARY_PATH=.

When we load libray AAA, LD_LIBRARY_PATH must be libAAA.

Common mistakes resulting in java.lang.UnsatisfiedLinkError usually come from incorrect naming of the shared library (O/S-dependent), the library not being in the search path, or wrong library being loaded by Java code.