how to get a direct byte buffer from an address location

问题

In this opencv example, the Mat object has a nativeObj field, returning a long that represents the address of the object (i.e 140398889556640). Because the size of the data within the object is known, I wish to access the contents of the Mat object directly, returning a byte buffer.

What is the best way to do so?

回答1:

You can wrap the address with a DirectByteBuffer or use Unsafe.

While you can do this, you probably shouldn't. I would explore all other options first.

// Warning: only do this if there is no better option

public static void main(String[] args) {
    ByteBuffer bb = ByteBuffer.allocateDirect(128);
    long addr = ((DirectBuffer) bb).address();

    ByteBuffer bb2 = wrapAddress(addr, bb.capacity());

    bb.putLong(0, 0x12345678);
    System.out.println(Long.toHexString(bb2.getLong(0)));
}

static final Field address, capacity;
static {
    try {
        address = Buffer.class.getDeclaredField("address");
        address.setAccessible(true);
        capacity = Buffer.class.getDeclaredField("capacity");
        capacity.setAccessible(true);

    } catch (NoSuchFieldException e) {
        throw new AssertionError(e);
    }
}

public static ByteBuffer wrapAddress(long addr, int length) {
    ByteBuffer bb = ByteBuffer.allocateDirect(0).order(ByteOrder.nativeOrder());
    try {
        address.setLong(bb, addr);
        capacity.setInt(bb, length);
        bb.clear();
    } catch (IllegalAccessException e) {
        throw new AssertionError(e);
    }
    return bb;
}

回答2:

If you don't want to use Unsafe and want something that works without warnings in Java 9 and is actually portable across JVMs you can use JNIs NewDirectByteBuffer. This is API and guaranteed to work.

You will need to write some C (or C++) code however and ship a native library with your code.

回答3:

There is a tiny framework called "nalloc" which is designed to help developer with memory/pointers manipulations, it could be useful for whatever purposes you are looking for direct memory address accessing.

Also it brings you ability to write your Java program in a C-style, doing memory stuff manually.

Check it out: https://github.com/alaisi/nalloc

来源：https://stackoverflow.com/questions/52624395/how-to-get-a-direct-byte-buffer-from-an-address-location

标签

java

OpenCV

bytebuffer

unsafe