Android JNI 编程常见小问题

extern "C" {
jint
Java_com_cmcc_media_MicphoneService_setMicVolume(JNIEnv * jEnv, jobject jobj, jint volume)
{
   ...
}

cpp文件中如果不添加 extern "C" {}的话可能会导致java link lib error；

jstring 转string：
static char* jstring2string(JNIEnv* env, jstring jstr)
{
char* rtn = NULL;

jclass clsstring = (*env)->FindClass(env, "java/lang/String");

jstring strencode = (*env)->NewStringUTF(env, "utf-8");
jmethodID mid = (*env)->GetMethodID(env, clsstring, "getBytes", "(Ljava/lang/String;)[B");
jbyteArray barr= (jbyteArray)(*env)->CallObjectMethod(env, jstr, mid, strencode);
jsize alen = (*env)->GetArrayLength(env, barr);
jbyte* ba = (*env)->GetByteArrayElements(env, barr, JNI_FALSE);

if(alen > 0)
{
rtn = (char*)malloc(alen + 1);
memcpy(rtn, ba, alen);
rtn[alen] = 0;
}

(*env)->ReleaseByteArrayElements(env, barr, ba, 0);

return rtn;
}

或者
char* bytes = const_cast< char* > (env->GetStringUTFChars(string, NULL));
用完后 env->ReleaseStringUTFChars(string, bytes);


string转jstring：
static jstring string2jstring(JNIEnv* env, const char* pat)
{
       jclass strClass = (*env)->FindClass(env, "java/lang/String");
       jmethodID ctorID = (*env)->GetMethodID(env, strClass, "<init>", "([BLjava/lang/String;)V");
       jbyteArray bytes = (*env)->NewByteArray(env, strlen(pat));
       (*env)->SetByteArrayRegion(env, bytes, 0, strlen(pat), (jbyte*)pat);
       jstring encoding = (*env)->NewStringUTF(env, "utf-8");
       return (jstring)(*env)->NewObject(env, strClass, ctorID, bytes, encoding);
}

String16转String8：
String16 value;
String8 propvalue(value);