Invoking the Java Virtual Machine

�� 忡�� native ��α׷��, JVM�� Ű�� ؼ� ��Ѵ�. �̰�� ϰ� �ִ�. �� , Java ��ø�� ϴ� �� Ѵٰ� �� , JVM�� Ѽ� �� ִ�.

Invoking the Java Virtual Machine

�� κп�� JVM�� native ��̺귯��κ�� native ��α׷�� ε��ϴ� �� ؼ� ��Ѵ�. JVM�� ʱ�ȭ�ؼ� Java �޼ҵ带 ȣ��ϴ� �� ؼ�� ȴ�. Invocation API�� ؼ�, native ��带 ��ϰ� �ִ� JVM�� ٿ�� ü�� Java �� ϰ� �� ִ�. �� Win32�� ǰ� �ִ�.

Invoking the Java Virtual Machine

JNI�� ̿��ؼ�, native�� ̺귯�� native ��α׷�� Ѽ� �ű⿡ JVM�� ų �� ִ�.

JNI�� Invocation API�� ϴµ�, �̰��, ��java�� ͸� �� ʰ�, C�� ٷ� Java ��α׷�� ֵ�� ִ�.

Invoking the Java Virtual Machine

�� ϱ� �� Ѵ�.

public class Prog {
    public static void main(String[] args) {
        System.out.println("Hello World" + args[0]);
    }
}

�� Java ��α׷�� C ��α׷�� ϰ� �� JVM�� ̴�.

#include <jni.h>

#ifdef _WIN32
#define PATH_SEPARATOR ';'
#else /* UNIX */
#define PATH_SEPARATOR ':'
#endif

#define USER_CLASSPATH "." /* where Prog.class is */

main() {
    JNIEnv *env;
    JavaVM *jvm;
    JDK1_1InitArgs vm_args;
    jint res;
    jclass cls;
    jmethodID mid;
    jstring jstr;
    jobjectArray args;
    char classpath[1024];

    /* IMPORTANT: specify vm_args version # if you use JDK1.1.2 and beyond */
    vm_args.version = 0x00010001;

    JNI_GetDefaultJavaVMInitArgs(&vm_args);

    /* Append USER_CLASSPATH to the end of default system class path */
    sprintf(classpath, "%s%c%s",
            vm_args.classpath, PATH_SEPARATOR, USER_CLASSPATH);
    vm_args.classpath = classpath;

    /* Create the Java VM */
    res = JNI_CreateJavaVM(&jvm,&env,&vm_args);
    if (res < 0) {
        fprintf(stderr, "Can't create Java VM\n");
        exit(1);
    }

    cls = (*env)->FindClass(env, "Prog");
    if (cls == 0) {
        fprintf(stderr, "Can't find Prog class\n");
        exit(1);
    }

    mid = (*env)->GetStaticMethodID(env, cls, "main", "([Ljava/lang/String;)V");
    if (mid == 0) {
        fprintf(stderr, "Can't find Prog.main\n");
        exit(1);
    }

    jstr = (*env)->NewStringUTF(env, " from C!");
    if (jstr == 0) {
        fprintf(stderr, "Out of memory\n");
        exit(1);
    }
    args = (*env)->NewObjectArray(env, 1,
                        (*env)->FindClass(env, "java/lang/String"), jstr);
    if (args == 0) {
        fprintf(stderr, "Out of memory\n");
        exit(1);
    }
    (*env)->CallStaticVoidMethod(env, cls, mid, args);

    (*jvm)->DestroyJavaVM(jvm);
}

JNI_GetDefaultJavaVMInitArgs() �Լ�� ̿��ؼ�, JVM�� ʱ�ȭ�ϱ� �� ڵ�� �� Ѵ�. �׷� ��, JNI_CreateJavaVM() �� ̿��ؼ� VM�� ε��ϰ� �ʱ�ȭ�Ѵ�. �� Լ�� ڸ� �ʿ�� Ѵ�.

jvm�� Ǵ� JVM�� ǹ��Ѵ�.
env�� JNI ��̽� ��̴�.

JVM�� Ű�� Ͱ� native �޼ҵ带 ��ϴ� �Ͱ�� ϴ�. �׷��, ��̰� �� ̶�� ʴ� ��̴�. �׷��Ƿ�, �� ۷�� DestroyJavaVM() �� Ǳ� �� ʴ´�. VM�� ε�� , �Ϲ�� JNI �Լ�� ȣ��ؼ� Java ��α׷�� ؼ� �� , ��, DestroyJavaVM() �� VM�� Ѵ�.
�� ִ�. �� Solaris�� ,

cc -I<where jni.h is> -L<where libjava.so is> -ljava invoke.c

Win32�� VC++ 4.0�� 쿡��

cl -I<where jni.h is> -MT invoke.c -link<where javai.lib is>\javai.lib

Attaching Native Threads

native ��带 JVM�� ִ�. �� Win32ȯ�濡�� ǰ�, ��δ� Solaris�� δ�.

�� Win32��̸�, �տ�� ߴ� �� ̱� ��ؼ� �� ̴�.

/* Note: This program only works on Win32.
*/
#include <windows.h>
#include <jni.h>

JavaVM *jvm;

void thread_fun(void *arg)
{
    jint res;
    jclass cls;
    jmethodID mid;
    jstring jstr;
    jobjectArray args;
    JNIEnv *env;
    char buf[100];
    int threadNum = (int)arg;

    /* Pass NULL as the third argument */
    res = (*jvm)->AttachCurrentThread(jvm, &env, NULL);
    if (res < 0) {
       fprintf(stderr, "Thread %d: attach failed\n", threadNum);
       return;
    }

    cls = (*env)->FindClass(env, "Prog");
    if (cls == 0) {
        fprintf(stderr, "Thread %d: Can't find Prog class\n", threadNum);
        goto detach;
    }

    mid = (*env)->GetStaticMethodID(env, cls, "main", "([Ljava/lang/String;)V");
    if (mid == 0) {
        fprintf(stderr, "Thread %d: Can't find Prog.main\n", threadNum);
          goto detach;
    }

    sprintf(buf, " from Thread %d", threadNum);
    jstr = (*env)->NewStringUTF(env, buf);
    if (jstr == 0) {
        fprintf(stderr, "Thread %d: Out of memory\n", threadNum);
        goto detach;
    }
    args = (*env)->NewObjectArray(env, 1,
                        (*env)->FindClass(env, "java/lang/String"), jstr);
    if (args == 0) {
        fprintf(stderr, "Thread %d: Out of memory\n", threadNum);
        goto detach;
    }
    (*env)->CallStaticVoidMethod(env, cls, mid, args);

detach:
    if ((*env)->ExceptionOccurred(env)) {
        (*env)->ExceptionDescribe(env);
    }
    (*jvm)->DetachCurrentThread(jvm);
}

#ifdef _WIN32
#define PATH_SEPARATOR ';'
#else /* UNIX */
#define PATH_SEPARATOR ':'
#endif

#define USER_CLASSPATH "." /* where Prog.class is */

main() {
    JNIEnv *env;
    JDK1_1InitArgs vm_args;
    int i;
    jint res;
    char classpath[1024];

    /* IMPORTANT: specify vm_args version # if you use JDK1.1.2 and beyond */
    vm_args.version = 0x00010001;

    JNI_GetDefaultJavaVMInitArgs(&vm_args);

    /* Append USER_CLASSPATH to the end of default system class path */
    sprintf(classpath, "%s%c%s",
            vm_args.classpath, PATH_SEPARATOR, USER_CLASSPATH);
    vm_args.classpath = classpath;

    /* Create the Java VM */
    res = JNI_CreateJavaVM(&jvm, &env, &vm_args);
    if (res < 0) {
        fprintf(stderr, "Can't create Java VM\n");
        exit(1);
    }

    for (i=0; i<5; i++)
        /* We pass the thread number as the argument to every thread */
        _beginthread(thread_fun,0,(void *)i);

    Sleep(5000); /* wait for threads to finish */

    (*jvm)->DestroyJavaVM(jvm);
}

ũ�� ޶�� . ��, ��带 ��ϱ� ��ؼ� Win32�� ϴ� �޼ҵ带 ��ؼ� ��带 ��, �� ȿ�, AttachCurrentThread() �� ȣ��ϴ� �Ϳ� ��ؼ� �� Ѵ�. �̰�� ν� �ٵ�� ִ� ��̸�, ��° ��ڴ� �� Ǿ� �־, NULL�� ָ� �ȴ�.

DetachCurrentThread() �� ȣ��ϰ� �Ǹ� �� 忡 �ҼӵǾ� �ִ� �� ۷�� ϰ� �ȴ�.

Limitation of the Invocation API in JDK1.1

�� JNI�� invocation API�� Ѵ�.

Solaris �󿡼� Java ��带 ��ϱ� ��ؼ�� JVM�� Solaris �ý�� ݷ� ��ϵ�� ؾ� �Ѵ�. �̷�� ý�� ݷδ� read, readv, write, writev, getmsg, putmsg, poll, open, close, pipe, fcntl, dup, create, accept, recv, send �� ִ�.
Solaris �󿡼�� native ��带 JVM�� 忡�� . ��, AttachCurrentThread�� Solaris�� ȴ�.
��μ�� ʰ�� JVM�� unload�� . DestroyJavaVM() �� ڵ常 ��Ѵ�.