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Can't GetMethodID of constructor from C++

Posted on 1997-08-17
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Last Modified: 2011-10-03
I am trying to instantiate a new Java object from C++ using the JNI.  I've read extensively on the JNI and have compiled and run all the example code in the tutorial.

Now, I'm trying to do something more complicated and am failing miserable.  My C++ program is able to initialize VM args and load the Java VM.  Next I am attempting to execute the following code segment:

 //This statement succeeds, although the pointer returned
 //is always 0x00000002 which seems wrong to me.
 jniENV->FindClass("myClass");

 //This next statement fails (returns NULL).  I've tried
 //many variations using "myClass" and "<init>" as the
 //method name as well as several different signatures,
 //specifically the signature I get from javap -s -p myClass
 jmethodID mid = jniENV->GetMethodID(cls, "<init>", "()V");

 //This next statement I've never made it to
 jobject obj = jniENV->NewObject(cls, mid);

Also, I've tested the Java code and it works to my satisfaction.  My problem is only that I am unable to load and run it starting from a C++ application.

Any advice would be greatly appreciated.
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Question by:jerrym
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Accepted Solution

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Arjan092097 earned 200 total points
ID: 1225222
I think I understand your problem. Probably the JNI cant find the  class 'myClass' because your classpath is not correct. There are two solutions for that:
I:  use the complete path name in the findClass function:
    cls = JNIEnv->FindClass("path/classfile")
    where path is the path where the classfile is to be found
II: set the Javaclasspath to the path where your class files are  
    to be found.

I think your GetMethodID is alright. For the default constructor the 'method name' is indeed <init> and the signature is "()V".

So I think that solves your problem and you can now create an instance of myClass from C.
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