Java Native Interface with C tutorial

# Makefile: jni
JNI_INCLUDE=/usr/lib/jvm/default-java/include
JNI_LIB=/usr/lib/jvm/default-java/lib

build: libHello.so

blog/rock/Hello.class: blog/rock/Hello.java
	javac blog/rock/Hello.java

hello-glue.h: blog/rock/Hello.class
	javah -jni -o hello-glue.h blog.rock.Hello
	touch hello-glue.h

libHello.so: blog/rock/Hello.class hello-glue.h hello-native.c
	gcc -I$(JNI_INCLUDE) -L$(JNI_LIB) \
	    -fPIC -shared -o libHello.so hello-native.c

sig: blog/rock/Hello.class
	javap -s -p blog.rock.Hello

clean:
	rm -f blog/rock/Hello.class libHello.so hello-glue.h

package blog.rock;

class Hello {
    //匯入此類別時，一併載入包含原生方法的共享函數庫。
    static {
        //此例的共享函數庫文件名稱是 libHello.so，故此處給的參數是 "Hello".
        System.loadLibrary("Hello");
    }

    private String message1; //Accessing Strings
    private char[] message2; //Accessing Arrays of Primitive Types
    private int x; //Accessing Primitive Type and Field.
    private static int y; //Class field.

    public native boolean say(String msg, int x);
    public native Hello setMessage1(String msg);
    public native String getMessage1();
    public native Hello setMessage2(char[] msg);
    public native char[] getMessage2();
    public native Hello setX(int v);
    public native int getX();
    public native static void setY(int v);
    public native static int getY();
    
    //Java 對照組
    public boolean jsay(String msg, int x) {
        System.out.println("[J] msg: " + msg);
        System.out.println("[J] x: " + x);
        return true;
    }

    //這只是用來測試的方法。
    public static void main(String[] args) {
        String s;
        char[] msg2 = {'c', 'h', 'a', 'r'};
        Hello h = new Hello();

        h.say("world", 1);
        h.jsay("world", 1);
        s = h.setMessage1("rock").getMessage1();
        System.out.println( "h.message1: " + s );
        h.setMessage2(msg2);
        System.out.println( h.getMessage2() );
        h.setX(11);
        System.out.println( "h.x: " + h.getX() );
        Hello.setY(21);
        System.out.println( "Hello.y: " + Hello.getY() );
    }
}

#include "hello-glue.h"
#include <stdio.h>
#include <stdlib.h>

/*
 * Class:     blog_rock_Hello
 * Method:    say
 * Signature: (Ljava/lang/String;I)Z
 */
JNIEXPORT jboolean JNICALL Java_blog_rock_Hello_say
  (JNIEnv *env, jobject this, jstring msg, jint x)
{
}

/*
 * Class:     blog_rock_Hello
 * Method:    setMessage1
 * Signature: (Ljava/lang/String;)Lblog/rock/Hello;
 */
JNIEXPORT jobject JNICALL Java_blog_rock_Hello_setMessage1
  (JNIEnv *env, jobject this, jstring msg)
{
}

/*
 * Class:     blog_rock_Hello
 * Method:    getMessage1
 * Signature: ()Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_blog_rock_Hello_getMessage1
  (JNIEnv *env, jobject this)
{
}

/*
 * Class:     blog_rock_Hello
 * Method:    setMessage2
 * Signature: ([C)Lblog/rock/Hello;
 */
JNIEXPORT jobject JNICALL Java_blog_rock_Hello_setMessage2
  (JNIEnv *env, jobject this, jcharArray msg)
{
}

/*
 * Class:     blog_rock_Hello
 * Method:    getMessage2
 * Signature: ()[C
 */
JNIEXPORT jcharArray JNICALL Java_blog_rock_Hello_getMessage2
  (JNIEnv *env, jobject this)
{
}

/*
 * Class:     blog_rock_Hello
 * Method:    setX
 * Signature: (I)Lblog/rock/Hello;
 */
JNIEXPORT jobject JNICALL Java_blog_rock_Hello_setX
  (JNIEnv *env , jobject this, jint v)
{
}

/*
 * Class:     blog_rock_Hello
 * Method:    getX
 * Signature: ()I
 */
JNIEXPORT jint JNICALL Java_blog_rock_Hello_getX
  (JNIEnv *env, jobject this)
{
}

/*
 * Class:     blog_rock_Hello
 * Method:    setY
 * Signature: (I)V
 */
JNIEXPORT void JNICALL Java_blog_rock_Hello_setY
  (JNIEnv *env, jclass Hello, jint v)
{
}

/*
 * Class:     blog_rock_Hello
 * Method:    getY
 * Signature: ()I
 */
JNIEXPORT jint JNICALL Java_blog_rock_Hello_getY
  (JNIEnv *env, jclass Hello)
{
}

// C 
    jclass cls = (*env)->GetObjectClass(env, this);
    jfieldID fid = (*env)->GetFieldID(env, cls, "x", "I");
    jint value = (*env)->GetIntField(env, this, fid);

// Java
    Class cls = this.getClass();
    Field fid = cls.getField("x");
    int value = fid.getInt(this);

/*
 * Class:     blog_rock_Hello
 * Method:    say
 * Signature: (Ljava/lang/String;I)Z
 */
JNIEXPORT jboolean JNICALL Java_blog_rock_Hello_say
  (JNIEnv *env, jobject this, jstring msg, jint x)
{
    const char *c_msg;

    //get String type data.
    //存取參考型別(Reference type)資料時，需要透過 JNI 提供的特定方法。
    c_msg = (*env)->GetStringUTFChars(env, msg, NULL); //need ReleaseStringUTFChars
    printf("[C] msg: %s\n", c_msg);

    (*env)->ReleaseStringUTFChars(env, msg, c_msg);

    //get Primitive type data.
    //存取基本型別資料時，可以直接存取。
    printf("[C] x: %d\n", x);

    return JNI_TRUE; 
}

/*
 * Class:     blog_rock_Hello
 * Method:    setMessage1
 * Signature: (Ljava/lang/String;)Lblog/rock/Hello;
 */
JNIEXPORT jobject JNICALL Java_blog_rock_Hello_setMessage1
  (JNIEnv *env, jobject this, jstring msg)
{
    jclass cls = (*env)->GetObjectClass(env, this);
    jfieldID fid; // store the field ID

    //use javap -s -p to see data type signature.
    fid = (*env)->GetFieldID(env, cls, "message1", "Ljava/lang/String;");
    (*env)->SetObjectField(env, this, fid, msg);
    return this;
}

/*
 * Class:     blog_rock_Hello
 * Method:    getMessage1
 * Signature: ()Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_blog_rock_Hello_getMessage1
  (JNIEnv *env, jobject this)
{
    jclass cls = (*env)->GetObjectClass(env, this);
    jfieldID fid; // store the field ID
    jstring msg;
    
    fid = (*env)->GetFieldID(env, cls, "message1", "Ljava/lang/String;");
    msg = (*env)->GetObjectField(env, this, fid);
    return msg;  //return NULL;
}

/*
 * Class:     blog_rock_Hello
 * Method:    setMessage2
 * Signature: ([C)Lblog/rock/Hello;
 */
JNIEXPORT jobject JNICALL Java_blog_rock_Hello_setMessage2
  (JNIEnv *env, jobject this, jcharArray msg)
{
    jclass cls = (*env)->GetObjectClass(env, this);
    jfieldID fid; // store the field ID

    jchar *c_msg;
    int len, i;
    len = (*env)->GetArrayLength(env, msg);
    printf("[C] length of msg: %d\n", len);
    
    c_msg = (*env)->GetCharArrayElements(env, msg, NULL);

    printf("[C] dump char array: ");
    for (i = 0; i < len; ++i) {
        printf("%c", c_msg[i]);
    }
    printf("\n");

    (*env)->ReleaseCharArrayElements(env, this, c_msg, 0);

    fid = (*env)->GetFieldID(env, cls, "message2", "[C");
    (*env)->SetObjectField(env, this, fid, msg);
    return this;
}

/*
 * Class:     blog_rock_Hello
 * Method:    getMessage2
 * Signature: ()[C
 */
JNIEXPORT jcharArray JNICALL Java_blog_rock_Hello_getMessage2
  (JNIEnv *env, jobject this)
{
    jclass cls = (*env)->GetObjectClass(env, this);
    jfieldID fid; // store the field ID
    jstring msg;
    
    fid = (*env)->GetFieldID(env, cls, "message2", "[C");
    msg = (*env)->GetObjectField(env, this, fid);
    return msg;  //return NULL;
}

/*
 * Class:     blog_rock_Hello
 * Method:    setX
 * Signature: (I)Lblog/rock/Hello;
 */
JNIEXPORT jobject JNICALL Java_blog_rock_Hello_setX
  (JNIEnv *env , jobject this, jint v)
{
    jclass cls = (*env)->GetObjectClass(env, this);
    jfieldID fid; // store the field ID

    fid = (*env)->GetFieldID(env, cls, "x", "I");
    (*env)->SetIntField(env, this, fid, v);
    return this;
}

/*
 * Class:     blog_rock_Hello
 * Method:    getX
 * Signature: ()I
 */
JNIEXPORT jint JNICALL Java_blog_rock_Hello_getX
  (JNIEnv *env, jobject this)
{
    jclass cls = (*env)->GetObjectClass(env, this);
    jfieldID fid; // store the field ID
    
    fid = (*env)->GetFieldID(env, cls, "x", "I");
    return (*env)->GetIntField(env, this, fid);
}

/*
 * Class:     blog_rock_Hello
 * Method:    setY
 * Signature: (I)V
 */
JNIEXPORT void JNICALL Java_blog_rock_Hello_setY
  (JNIEnv *env, jclass Hello, jint v)
{
    jfieldID fid; // store the field ID

    fid = (*env)->GetStaticFieldID(env, Hello, "y", "I");
    (*env)->SetStaticIntField(env, Hello, fid, v);
    return;
}

/*
 * Class:     blog_rock_Hello
 * Method:    getY
 * Signature: ()I
 */
JNIEXPORT jint JNICALL Java_blog_rock_Hello_getY
  (JNIEnv *env, jclass Hello)
{
    jfieldID fid; // store the field ID
    
    fid = (*env)->GetStaticFieldID(env, Hello, "y", "I");
    return (*env)->GetStaticIntField(env, Hello, fid);
}