Discovered by code inspection as part of JDK-8022584.

According to the JNI spec:

GetStringUTFChars

const char * GetStringUTFChars(JNIEnv *env, jstring string,
jboolean *isCopy);

Returns a pointer to an array of bytes representing the string in modified UTF-8 encoding. This array is valid until it is released by ReleaseStringUTFChars().

If isCopy is not NULL, then *isCopy is set to JNI_TRUE if a copy is made; or it is set to JNI_FALSE if no copy is made.

LINKAGE:
Index 169 in the JNIEnv interface function table.
PARAMETERS:

env: the JNI interface pointer.

string: a Java string object.

isCopy: a pointer to a boolean.
RETURNS:

Returns a pointer to a modified UTF-8 string, or NULL if the operation fails.

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The JNI book goes further and states that an OutOfMemoryException will be posted if the system runs out of memory.

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The actual code is somewhat different:

  oop java_string = JNIHandles::resolve_non_null(string);
  size_t length = java_lang_String::utf8_length(java_string);
  char* result = AllocateHeap(length + 1, mtInternal);
  java_lang_String::as_utf8_string(java_string, result, (int) length + 1);
  if (isCopy != NULL) *isCopy = JNI_TRUE;
  return result;

To return NULL, AllocateHeap must return NULL, but the form of AllocateHeap used will actually abort the VM on an out-of-memory condition. We should switch to using the "return NULL" variant of AllocateHeap thus allowing this function to return NULL. I also think we should post an OOME - though that may be difficult if we have already hit an OOM condition.