NDK programming -- JNI tool class call

11. The function of the tool is to java To c String represented by character pointer in language

<span style="font-size:14px;">// take const char Type conversion to jstring type 
jstring CStr2Jstring( JNIEnv* env, const char* pat )
{
    //  Definition java String class  strClass
    jclass strClass = (env)->FindClass("Ljava/lang/String;");
    //  obtain java String Class method String(byte[],String) Constructor , For local byte[] Convert array to a new String
    jmethodID ctorID = (env)->GetMethodID(strClass, "<init>", "([BLjava/lang/String;)V");
    //  establish byte Array 
    jbyteArray bytes = (env)->NewByteArray((jsize)strlen(pat));
    //  take char*  Convert to byte Array 
    (env)->SetByteArrayRegion(bytes, 0, (jsize)strlen(pat), (jbyte*)pat);
    // Set up String,  Save the language type , be used for byte Array conversion to String Time parameters 
    jstring encoding = (env)->NewStringUTF("GB2312"); 
    // take byte Array to java String, And the output 
    return (jstring)(env)->NewObject(strClass, ctorID, bytes, encoding);

}
char * Jstring2CStr( JNIEnv * env, jstring jstr ) 
{ 
    char * rtn = NULL; 
    jclass clsstring = env->FindClass("java/lang/String"); 
    jstring strencode = env->NewStringUTF("GB2312"); 
    jmethodID mid = env->GetMethodID(clsstring, "getBytes", "(Ljava/lang/String;)[B"); 
    jbyteArray barr= (jbyteArray)env->CallObjectMethod(jstr,mid,strencode); 
    jsize alen = env->GetArrayLength(barr); 
    jbyte * ba = env->GetByteArrayElements(barr,JNI_FALSE); 
    if(alen > 0) 
    { 
        rtn = (char*)malloc(alen+1); //new char[alen+1]; 
        memcpy(rtn,ba,alen); 
        rtn[alen]=0; 
    } 
    env->ReleaseByteArrayElements(barr,ba,0); 

    return rtn;
}</span>

 error: conflicting types for 'Jstring2CStr'
 note: previous implicit declaration of 'Jstring2CStr' was here

stay jni Of C The above error will appear in the partial call of language , This is because in the C In a language, the function to be called must be defined in front of the calling function , Otherwise, the function cannot be found .

F:/android/Crack1/jni/com_example_crack1_MainActivity.c: In function 'Jstring2CStr':
F:/android/Crack1/jni/com_example_crack1_MainActivity.c:14:27: error: request for member 'FindClass' in something not a structure or union
F:/android/Crack1/jni/com_example_crack1_MainActivity.c:15:28: error: request for member 'NewStringUTF' in something not a structure or union
F:/android/Crack1/jni/com_example_crack1_MainActivity.c:16:24: error: request for member 'GetMethodID' in something not a structure or union
F:/android/Crack1/jni/com_example_crack1_MainActivity.c:17:37: error: request for member 'CallObjectMethod' in something not a structure or union
F:/android/Crack1/jni/com_example_crack1_MainActivity.c:18:21: error: request for member 'GetArrayLength' in something not a structure or union
F:/android/Crack1/jni/com_example_crack1_MainActivity.c:19:21: error: request for member 'GetByteArrayElements' in something not a structure or union
F:/android/Crack1/jni/com_example_crack1_MainActivity.c:26:8: error: request for member 'ReleaseByteArrayElements' in something not a structure or union

There will be the above problems of finding relevant methods , The solution is as follows ：

If it is c Program , Use (*env)->
If it is C++ Use env->

ps: stay linux Next if .c Used in documents “env->” The compiler will not find this structure , Must use “(*env)->”, Or change it into .cpp file , With c++ To compile .

Here's the difference between the two ：

namely C++ Use in
env->FindClass("java/lang/String")

C Use in
(*env)->FindClass(env, "java/lang/String")