Trouble calling on a Java method from a native thread using JNI (C++)

问题:

I have a JNI problem which I hope someone can help me out with.
I'm trying to call on a constructor of a Java class called LUSOutputJNI from a native thread.
It keeps failing on FindClass(...) of this specific class.

Here is the code:

 LOGE("1");     JNIEnv *env = NULL;      LOGE("2");     int res = -1;     res = g_vm->AttachCurrentThread(&env, NULL);      if(env == NULL)     {         LOGE("env is NULL, AttachCurrentThread failed");;     }       if(res >= 0)         LOGE("AttachCurrentThread was successful");     jclass clazz = NULL;     jmethodID cid;      jclass clazzESEngine;     jmethodID callbackid;      jobject jCoreOut;     static jfieldID fid_ActionState = NULL;     static jfieldID fid_nSpeed = NULL;     static jfieldID fid_nType = NULL;     static jfieldID fid_nInProcess = NULL;     static jfieldID fid_nX = NULL;     static jfieldID fid_nY = NULL;     LOGE("3");      static const char* const ECOClassName = "lus/android/sdk/LUSOutputJNI";     //static const char* const ECOClassName = "android/widget/TextView";     clazz = env->FindClass(ECOClassName);     if (clazz == NULL) {         LOGE("Can't find class LUSOutputJNI");      }     else         LOGE("lus/android/sdk/LUSOutputJNI was found, YEY!!");      LOGE("4");     cid = env->GetMethodID(clazz,"<init>", "()V");     LOGE("5");     jCoreOut = env->NewObject(clazz, cid);      LOGE("6");     

Here is the logcat output from when it fails:

E/lusCore_JNI( 3040): 1 E/lusCore_JNI( 3040): 2 E/lusCore_JNI( 3040): AttachCurrentThread was successful E/lusCore_JNI( 3040): 3 E/lusCore_JNI( 3040): Can't find class LUSOutputJNI E/lusCore_JNI( 3040): 4 W/dalvikvm( 3040): JNI WARNING: JNI method called with exception raised   

Observations:

• I get a result from AttachCurrentThread which is 0, which means that this attachment was successful + the env pointer is no longer NULL.
• I'm sure about the package name declaration of LUSOutputJNI (triple checked it...)
• When I try to run FindClass(..) with a more popular class name such as android/widget/TextView , I get a positive match. It is there. Meaning the thread attachment and the env variables are ok. (Can I assume that?)
• When I try to run the following code from a JNI method which has a JNI thread running it, it finds the LUSOutputJNI class without a problem.

What am I doing wrong?

Help will be much appreciated :)

Thanks for your time,

Ita

回答1:

Found the answer \ work around in here. (Look for FAQ: "FindClass didn't find my class" in JNI tips)

I basicaly saved a global ref to the needed jclass objects.
Did however had to overcome some evil JNI changes between C/JNI and C++/JNI in order for the code to compile.
This is how I got the NewGlobalRef to compile and work.

jclass localRefCls = env->FindClass(strLUSClassName); if (localRefCls == NULL) {     LOGE("Can't find class %s",strLUSCoreClassName);     return result; }  //cache the EyeSightCore ref as global  /* Create a global reference */  clazzLUSCore = (_jclass*)env->NewGlobalRef(localRefCls);   /* The local reference is no longer useful */  env->DeleteLocalRef(localRefCls);   /* Is the global reference created successfully? */  if (clazzLUSCore == NULL) {      LOGE("Error - clazzLUSCore is still null when it is suppose to be global");      return result; /* out of memory exception thrown */  }   

I hope this helps anyone.

回答2:

To get a bit more information about whats going wrong ( it will log to stderr, not LOGE, and I'm not sure how to change that) you can add some exception printing - you can change this:

//static const char* const ECOClassName = "android/widget/TextView"; clazz = env->FindClass(ECOClassName); if (clazz == NULL) {     LOGE("Can't find class LUSOutputJNI");  } else     LOGE("lus/android/sdk/LUSOutputJNI was found, YEY!!"); 

To this:

    //static const char* const ECOClassName = "android/widget/TextView";     clazz = env->FindClass(ECOClassName);     if (clazz == NULL) {         LOGE("Can't find class LUSOutputJNI");         jthrowable exc = env->ExceptionOccurred();         if(exc)         {           env->ExceptionDescribe();           env->ExceptionClear();         }         }     else         LOGE("lus/android/sdk/LUSOutputJNI was found, YEY!!"); 

文章来源: Trouble calling on a Java method from a native thread using JNI (C++)