I understand that using GetDoubleArrayElements, it is the JVM who decides whether or not to copy the elements of Array. In this case, is there any way to avoid the copy? If not, is there an other way to transfer from Java to C without copying? I'm passing very big Arrays, and I wish I could avoid the copy. Thanks
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2You can use a DoubleBuffer to wrap a direct byte buffer. This updates an area of memory can be used in Java/C directly.Peter Lawrey– Peter Lawrey2011-03-08 11:13:17 +00:00Commented Mar 8, 2011 at 11:13
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1 Answer
The JNI guide says:
In JDK/JRE 1.1, programmers can use Get/ReleaseArrayElements functions to obtain a pointer to primitive array elements. If the VM supports pinning, the pointer to the original data is returned; otherwise, a copy is made.
New functions introduced in JDK/JRE 1.3 allow native code to obtain a direct pointer to array elements even if the VM does not support pinning.
These "new functions" are GetPrimitiveArrayCritical and ReleasePrimitiveArrayCritical which disable garbage collection completely and have thus to be used with care. So in summary it is a VM problem rather than an API problem. Don't forget that without pinning the garbage collector might decide to compact the heap and physically move your array, so the direct pointer would be of little use after all.
As Peter suggested you could work with a java.nio.DoubleBuffer instead of using arrays. The JNI function
void* GetDirectBufferAddress(JNIEnv* env, jobject buf);
allows you to access it.
2 Comments
NoDataDumpNoContribution
I wonder what is preferred, byte[] with
GetPrimitiveArrayCritical or ByteBuffer with GetDirectBufferAddress?
Tilo
That really depends on your use case. One consideration might be to use
GetPrimitiveArrayCritical or GetArrayElements when your data is an output of your Java code and an input of your C code. You would use GetDirectBufferAddress when your data flows the other way.