I have a Java program which is designed to display images in a custom file format, read through a C++ library using JNI. The data is loaded into an char array on the C++ side, and transferred to a BufferedImage on the Java side. Since sections of the image could be removed from memory and need to be reloaded fairly regularly, so I want these operations to be as fast as possible.
The way I'm currently doing this is data gets read from the file into a buffer in the C++ library's memory. In order to populate the BufferedImage, the Java code makes a JNI function call for each pixel to read from this buffer and, if necessary, load another chunk of data into the buffer. This works, but with a higher overhead than I'd like.
What I've been planning to do to improve this is pass the BufferedImage object to the C++ code using a JNI call, and make function calls on it from that side. I've been researching JNI as much as I can, but I haven't been able to find out if there's any cost in modifying Java objects from a C++ library which was loaded by the JVM. Is this a good way to implement this, or is there a faster way to transfer large amounts of byte data with JNI?
The most efficient way to move data across the JNI boundary is probably via direct ByteBuffers. You should move as much as you can at a time, as it is crossing the boundary itself that is inefficient, not the transfer. Certainly more than one pixel, preferably megabytes at a time.