JNI

이글은, 구글 안드로이드 상의 Java 애플리케이션에서 native c/c++ 모듈을 Call 할 수 있는 가에

대한 것이다. 결론적으로 말하면 쉽지 않을 것 같지만 불가능 하지는 않을 듯 하다.

직접 JNI를 통해 결과를 검증해 보지는 않았지만, 대략적인 방법들과 문제점들을 정리해 보고자

한다.

 

 

1. JNI란 무엇인가? - JNI example 중심으로
 
This section presents the ReadFile example program. This example shows how you can use
the Java Native Interface (JNI) to invoke a native method that makes C function calls to map a
file into memory.

About the Example

You can call code written in any programming language from a program written in the Java
language by declaring a native Java method, loading the library that contains the native code,
and then calling the native method. The ReadFile source code below does exactly this.
However, successfully running the program requires a few additional steps beyond compiling
the Java language source file. After you compile, but before you run the example, you have to
generate a header file. The native code implements the function defintions contained in the
generated header file and implements the business logic as well. The following sections walk
through all the steps.

import java.util.*;

class ReadFile {
 //Native method declaration
   native byte[] loadFile(String name);
 //Load the library
   static {
     System.loadLibrary("nativelib");
   }
 
   public static void main(String args[]) {
      byte buf[];
 //Create class instance
      ReadFile mappedFile=new ReadFile();
 //Call native method to load ReadFile.java
      buf=mappedFile.loadFile("ReadFile.java");
 //Print contents of ReadFile.java
      for(int i=0;i<buf.length;i++) {
          System.out.print((char)buf[i]);
      }
   }
}

Native Method Declaration 

The native declaration provides the bridge to run the native function in the Java virtual machine.
In this example, the loadFile function maps onto a C function called Java_ReadFile_loadFile.
The function implementation accepts a String that represents a file name and returns the
contents of that file in the byte array.

native byte[] loadFile(String name); Load the Library

The library containing the native code implementation is loaded by a call to System.loadLibrary().
Placing this call in a static initializer ensures this library is only loaded once per class.
The library can be loaded outside of the static block if your application requires it. You might
need to configure your environment so the loadLibrary method can find your native code library.

static {
     System.loadLibrary("nativelib");
}

Compile the Program

 

To compile the program, just run the javac compiler command as you normally would:
javac ReadFile.java
Next, you need to generate a header file with the native method declaration and implement the
native method to call the C functions for loading and reading a file.

Generate the Header File 

To generate a a header file, run the javah command on the ReadFile class. In this example,
the generated header file is named ReadFile.h. It provides a method signature that you have
to use when you implement the loadfile native function.

javah -jni ReadFile
Note: When running javah on your own classes, be sure to use the fully-qualified class name.

Method Signature 

The ReadFile.h header file defines the interface to map the Java language method to the native
C function. It uses a method signature to map the arguments and return value of the Java
language mappedfile.loadFile method to the loadFile native method in the nativelib library.
Here is the loadFile native method mapping (method signature):

/* * Class: ReadFile * Method: loadFile * Signature: (Ljava/lang/String;)[B */
JNIEXPORT jbyteArray JNICALL Java_ReadFile_loadFile (JNIEnv *, jobject, jstring);

The method signature parameters function as follows:

JNIEnv *: A pointer to the JNI environment. This pointer is a handle to the current thread
in the Java virtual machine, and contains mapping and other hosuekeeping information.
jobject: A reference to the method that called this native code. If the calling method is
static, this parameter would be type jclass instead of jobject.
jstring: The parameter supplied to the native method. In this example, it is the name of
the file to be read.
 

Implement the Native Method

 

In this native C source file, the loadFile definition is a copy and paste of the C declaration
contained in ReadFile.h. The definition is followed by the native method implementation.
JNI provides a mapping for both C and C++ by default.

#include <jni.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

 

JNIEXPORT jbyteArray JNICALL Java_ReadFile_loadFile
  (JNIEnv * env, jobject jobj, jstring name) {
    caddr_t m;
    jbyteArray jb;
    jboolean iscopy;
    struct stat finfo;
    const char *mfile = (*env)->GetStringUTFChars(
                env, name, &iscopy);
    int fd = open(mfile, O_RDONLY);

    if (fd == -1) {
      printf("Could not open %sn", mfile);
    }
    lstat(mfile, &finfo);
    m = mmap((caddr_t) 0, finfo.st_size,
                PROT_READ, MAP_PRIVATE, fd, 0);
    if (m == (caddr_t)-1) {
      printf("Could not mmap %sn", mfile);
      return(0);
    }
    jb=(*env)->NewByteArray(env, finfo.st_size);
    (*env)->SetByteArrayRegion(env, jb, 0,
 finfo.st_size, (jbyte *)m);
    close(fd);
    (*env)->ReleaseStringUTFChars(env, name, mfile);
    return (jb);
}
 

You can approach calling an existing C function instead of implementing one, in one of two ways:
Map the name generated by JNI to the existing C function name. The Language Issues
section shows how to map between Xbase database functions and Java language code
Use the shared stubs code available from the JNI page on the java.sun.com web site.

Compile the Dynamic or Shared Object Library 

The library needs to be compiled as a dynamic or shared object library so it can be loaded at
runtime. Static or archive libraries are compiled into an executable and cannot be loaded at
runtime. The shared object or dynamic library for the loadFile example is compiled as follows:

Gnu C/Linux:

gcc  -o libnativelib.so -shared -Wl,-soname,libnative.so  
     -I/export/home/jdk1.2/include
     -I/export/home/jdk1.2/include/linux nativelib.c  
     -static -lc

Gnu C++/Linux with Xbase

g++ -o libdbmaplib.so -shared -Wl,-soname,libdbmap.so  
    -I/export/home/jdk1.2/include
    -I/export/home/jdk1.2/include/linux
    dbmaplib.cc -static -lc -lxbase

Win32/WinNT/Win2000

cl -Ic:/jdk1.2/include
   -Ic:/jdk1.2/include/win32
   -LD nativelib.c -Felibnative.dll

Run the Example

 

To run the example, the Java virtual machine needs to be able to find the native library. To do this,
set the library path to the current directory as follows:

Unix or Linux: LD_LIBRARY_PATH=`pwd` export LD_LIBRARY_PATH
Windows NT/2000/95: set PATH=%path%;.

With the library path properly specified for your platform, invoke the program as you normally would
with the interpreter command:

 java ReadFile

 

 

2. 구글 안드로이드 상에서 JNI 지원 API

아래의 두가지 함수에 대한 안드로이드 상에서의 특별한 제약이 없다면 JNI 을 지원하기 위한 최소한의
조건은 만족된 듯하다

The core functional component is the use of the 'native' keyword and the bindings to the
java.lang.System.loadLibary() API, which, does seem to be documented within the android reference:

 

java.lang.System

java.lang
public final class

java.lang.System
java.lang.Object  
       java.lang.System

Class System provides a standard place for programs to find system related information.
All System API is static.

static  void  load(String pathName) : Loads the specified file as a dynamic library.
static  void  loadLibrary(String libName) : Loads and links the library specified by the argument.

3. Dynamic & Shard Object Library 만들기

 

the question here is how to build a native .so (shared library), but we can refer to the JNI example
above for instructions. we have also been informed, through this list that you can build native code
on the device, but without UI bindings:

 

이 부분 중, 가장 어렵게 생각되는 부분은 Cross Compiler를 통해 shard object library를 만들 수 있느냐 일듯.
현재 안드로이드의 /system/lib 아래에는 안드로이드 Library들이 .so 형태로 존재함.