如何从地址位置获取直接字节缓冲区 [英] how to get a direct byte buffer from an address location

问题描述

在这个opencv示例中，Mat 对象有一个 nativeObj 字段，返回一个表示对象地址的 long(即 140398889556640).因为对象内数据的大小是已知的，所以我想直接访问 Mat 对象的内容，返回一个字节缓冲区.

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.

最好的方法是什么?

推荐答案

您可以使用 DirectByteBuffer 包装地址或使用 Unsafe.

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

虽然您可以这样做，但您可能不应该这样做.我会先探索所有其他选项.

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;
}

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