jni Getting started with jni Getting started with JNI

Example

JNI means Java Native Interface. It is a mechanism how to call a native code from Java application running under JVM control and vice versa. Native code means code compiled for the target platform. Native code code is typically written in C or C++ but it may be written in any language that has compiler for the target platform.

JNI is useful when

• A java application need to access a platform specific resources and there is no java library with the required functionality. The resource may be a specific hardware, sensors or whatever.
• A java application has to do a performance critical task and native code may be faster or with less footprint than java bytecode. Nevertheless do be too self-confident JVM is able to do a lot of optimization and a naive implementation in C/C++ will be probably slower.
• An application in C/C++ (or another language) wants to use a feature provided in a java library.

To start with JNI you need

• JDK or something that is able to compile java to bytecode.
• Compiler for compiling the native code.

The following hello world example is a simple java application that calls a C function. The example can be compiled by javac from JDK and gcc C compiler.

Java code:

public class JNIExample {

    public static void main(String[] args) {
       // The loadLibrary search for the native library (libnative.so in this case)
       System.loadLibrary("native");
       String s = "Hello JNI";
       JNIExample example = new JNIExample();
       example.doPrint(s);
   }

   // The method with native code (written in C) must be declared with native prefix
   public native void doPrint(String message);

}

C code:

#include <jni.h>
#include <stdio.h>

/* the function that is called from java must be declered with decorators
 * JNIEXPORT and JNICALL.
 * The function name is constructed as Java_ClassName_MethodName
 * Function parameters correspond parameters in java but there are 2 extra parameters
 * JNIEnv is a pointer to java envoronmet and jobject is a reference to caller object.
 * Caller object is the instance of the JNIExample in this case.
 */
JNIEXPORT void JNICALL Java_JNIExample_doPrint(JNIEnv *e, jobject obj, jstring message) {
    const char *c_message;
    /* It is necessary to convert java objects like string to something C native */
    c_message = (*e)->GetStringUTFChars(e, message, NULL);
    printf("%s\n", c_message);
    /* in the end it is necessary to free resources allocated by Get above */
    (*e)->ReleaseStringUTFChars(e, message, c_message);
}