The Java Native Interface (JNI)

The Java Native Interface (JNI) enables the integration of code written in the Java programming language with code written in other languages such as C, C++ and assembly. Therefore, you can natively run heavy tasks in pursuance of a better performance.

Is it platform-independent?

Your application can be platform-independent if you store the compiled JNI libraries (for all possible architectures) in the jar alongside the class files. However, since System.load() can't cope with loading libraries from within a jar, you'll therefore need a custom loader which extracts the library to a temporary file at runtime.

So, what do I need ?

• Microsoft Windows SDK (recomended if you are using Windows)
• Microsoft Visual Studio (same here)
• Java SDK

And, which are the magic steps?

1. Declare your native methods in a normal Java class.

public class JavaJNI {

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

public static native String write(String s);

public static void main(String[] argv) {
System.out.print(JavaJNI.write("Hello"));
}

}

2. Compile the Java file normally.

javac JavaJNI.java

3. Generate the .h file.

javah -jni JavaJNI

4. Write the native code using the file generated above.

In this case you will find the following method inside:

JNIEXPORT jstring JNICALL 
Java_JavaJNI_write(JNIEnv *, jclass, jstring);

Now you have the green light to implement it.

#include "JavaJNI.h"

JNIEXPORT jstring JNICALL Java_JavaJNI_write(JNIEnv *env, 
jobject thisobject, jstring js){
const char *temp;
jstring result = NULL;
temp = (*env)->GetStringUTFChars(env, js, 0);

/** Your normal C code */
printf("Input: %s", temp);

result = (*env)->NewStringUTF(env, 
(const char*) "\nResult: http://www.think-techie.com");
(*env)->ReleaseStringUTFChars(env, js, temp);
return result;
}

If you are trying to use C++ use the following methods instead:

env->GetStringUTFChars(js, 0);
env->NewStringUTF((const char*) temp);
env->ReleaseStringUTFChars(js, temp);

5. Compile the code

Windows with MS C++ Compiler

cl -c /Ic:\Programs\JavaSDK\include /I"C:\Programs\Microsoft Visual Studio 9.0\VC\include" /Ic:\Programs\JavaSDK\include\win32 NativeLib.c

link -LIBPATH:"C:\Programs\Microsoft SDKs\Windows\v6.0\VC\LIB" -LIBPATH:"C:\Programs\Microsoft SDKs\Windows\v6.0\Lib" NativeLib.obj /dll

Or this one:

cl -I C:\Programs\JavaSDK\include -I C:\Programs\JavaSDK\include\win32 -LD NativeLib.c -FeNativeLib.dll

Solaris with Sun Compiler

cc -G -I JavaSDK/include -I JavaSDK/include/solaris -o libNativeLib.so NativeLib.c

Linux with GCC

gcc -fPIC -I JavaSDK/include -I JavaSDK/include/linux -shared -o libNativeLib.so NativeLib.c

If your prefer to use Ant take a look at cc Task.

6. Run the example

java JavaJNI

Application output:

Input: Hello
Result: http://www.think-techie.com

7. Did you got errors ?

There are some common errors such as:

java.lang.UnsatisfiedLinkError: no NativeLib in 
java.library.path
at java.lang.ClassLoader.loadLibrary(Unknown Source)
at java.lang.Runtime.loadLibrary0(Unknown Source)
at java.lang.System.loadLibrary(Unknown Source)
at ...

The manner through which java searches for the needed files are different in each scenario:

1. Java searches for *.class files through the "-cp"/"-classpath" command line switch. If neither are specified on the command line, CLASSPATH is searched.
2. Java searches for JNI files through the "-Djava.library.path" command line switch. If "-Djava.library.path" is not specified, LD_LIBRARY_PATH is searched, but the /etc/ld.so.conf mechanism is not used.
3. The way a JNI glue library searches for the underlying C++ *.so libraries is not controlled by the Java process at all. The normal operating system rules are applied (LD_LIBRARY_PATH, /etc/ld.so.conf, etc.).

Consequently, one way of fixing this is to include the current location of your newly compiled shared library. If you are using linux you can do this:

LD_LIBRARY_PATH=`pwd`
export LD_LIBRARY_PATH

Another way is to define the location when running the program:

java -Djava.library.path=. JavaJNI

8. Did you got more errors ?

java.lang.UnsatisfiedLinkError: <library location>: 
<library location>: only ET_DYN and ET_EXEC can be loaded
at java.lang.ClassLoader$NativeLibrary.load(Native Method)
at java.lang.ClassLoader.loadLibrary0(ClassLoader.java:1778)
at java.lang.ClassLoader.loadLibrary(ClassLoader.java:1674)
at java.lang.Runtime.load0(Runtime.java:770)
at java.lang.System.load(System.java:1005)
...

If you are using g++, try to include the option "-shared" when compiling the cpp file.

g++ -shared NativeLib.cpp -o NativeLib.so -I JavaSDK/include
-I JavaSDK/include/linux

Do not include the "-c" option. It will skip the linking stage.

References

[1] JNI – Java Native Interface. White Magician. wmagician.wordpress.com