@Test
  public void testClosedChannel() throws Throwable {
    RegularFile file = regularFile(15);
    ExecutorService executor = Executors.newSingleThreadExecutor();

    try {
      JimfsAsynchronousFileChannel channel = channel(file, executor, READ, WRITE);
      channel.close();

      assertClosed(channel.read(ByteBuffer.allocate(10), 0));
      assertClosed(channel.write(ByteBuffer.allocate(10), 15));
      assertClosed(channel.lock());
      assertClosed(channel.lock(0, 10, true));
    } finally {
      executor.shutdown();
    }
  }
Just tests the main read/write methods... the methods all delegate to the non-async channel
anyway.
jimfs/src/test/java/com/google/common/jimfs/JimfsAsynchronousFileChannelTest.java