Query about "env- GetByteArrayElements(jImageBlob, 0);" Returning null

Question

Hi All,
I am calling a C++ method using JNI with a byte array as argument.
The C++ code snippet is:
if( jImageBlob == NULL ) {
          env->ThrowNew( jnullPointerException, "Image buffer is null." );
          return 0;
imageBlob = env->GetByteArrayElements(jImageBlob, 0);
     if (imageBlob == NULL)
     env->ThrowNew( jexception, ImgErrorString("Error Message") );
          goto onError;
This code works fine with normal working conditions. But, with heavy load under multi threaded execution, this call fails with "Error message". As the byte array passes first check for null, it is sure that the contents are not null. Still "env->GetByteArrayElements(jImageBlob, 0);" returns null.
What can be the probable reason here?
Thanks in advance,
Daya

Answer 1

imageBlob = env->GetByteArrayElements(jImageBlob, 0);
if (imageBlob == NULL)
env->ThrowNew( jexception, ImgErrorString("Error Message") );That is certainly wrong. When that null is returned java, by definition, has already thrown an exception.
JNI exception MUST be dealt with via one of the following
1. Clear it
2. Return immediately to java.
You are not doing the above.
Other than that
1. you might want to make sure that the byte array is not being accessed by more than one thread.
2. your code example uses the same var name for completely different types. Hopefully that isn't how your code works.