What could cause SIGSEGV when calling NewObjectArray for JNI in Android?

Question

I just started working with the Android NDK but I keep getting SIGSEGV when I have this call in my C code:

jobjectArray someStringArray;
someStringArray = (*env)->NewObjectArray(env, 10, 
(*env)->FindClass(env,"java/lang/String"),(*env)->NewStringUTF(env, ""));

Base on all the example I can find, the above code is correct but I keep getting SIGSERGV and everything is ok if the NewObjectArray line is commented out. Any idea what could cause such a problem?

Answer 1

that looks right, so i'm guessing you've done something else wrong. i assume you're running with checkjni on? you might want to break that up into multiple lines: do the FindClass and check the return value, do the NewStringUTF and check the return value, and then call NewObjectArray.

btw, you might want to pass NULL as the final argument; this pattern of using the empty string as the default value for each element of the array is commonly used (i think it's copy & pasted from some Sun documentation and has spread from there) but it's rarely useful, and it's slightly wasteful. (and it doesn't match the behavior of "new String[10]" in Java.)

Answer 2

I guess one of the possible causes is that in a long-run JNI method, the VM aborts when running out of the per-method-invocation local reference slots (normally 512 slots in Android).

Since FindClass() and NewStringUTF() functions would allocate local references, if you stay in a JNI method for a long time, the VM do not know whether a specific local reference should be recycled or not. So you should explicitly call DeleteLocalRef() to release the acquired local references when not required anymore. If you don't do this, the "zombie" local references will occupy slots in VM, and the VM aborts while running out of all the local reference slots.

In short-run JNI method, this may not be a problem due to all the local references would be recycled when exiting from a JNI method.