How comes .array() doesn't work on ByteBuffers returned from map'ed FileChannels?

Question

I'm doing memory-mapped IO in Java. The FileChannel class allows you to map a ByteBuffer to a particular part of a file. I'm doing that with a file opened read only.

The problem I am having is that I'm getting an exception when I attempt to call the .array() method on the resulting ByteBuffer. Perhaps that's because the .array() returns a byte[] array, and I really want a finalized byte array?

Is there any way around this?

Answer 1

I'm going to assume this is about the FileChannel.map method which can map a file to memory which can be accessed by a MappedByteBuffer.

In the documentation for the FileChannel.map method, if the file is mapped as read-only, the any attempt to modify the buffer will result in a ReadOnlyBufferException:

A region of a file may be mapped into memory in one of three modes:

• Read-only: Any attempt to modify the resulting buffer will cause a ReadOnlyBufferException to be thrown. (MapMode.READ_ONLY)

In terms of the exceptions thrown by the ByteBuffer.array method, there are two types of exceptions which are thrown depending on the reason for the problem:

Throws:

• ReadOnlyBufferException - If this buffer is backed by an array but is read-only
• UnsupportedOperationException - If this buffer is not backed by an accessible array

Although the exception being thrown is not mentioned in the question, perhaps the file being read-only is causing the ReadOnlyBufferException to be thrown by the array method.

Also, it should also be mentioned that the ByteBuffer.array method is an optional operation:

Returns the byte array that backs this buffer (optional operation).

To be sure that the array method will return a byte[] that can be used, invoke the hasArray method as suggested in the documentation for the array method:

Invoke the hasArray method before invoking this method in order to ensure that this buffer has an accessible backing array.