public MappedByteBuffer map(MapMode mode, long position, long size)throws IOException
    {ensureOpen();if (position < 0L)throw new IllegalArgumentException("Negative position");if (size < 0L)throw new IllegalArgumentException("Negative size");if (position + size < 0)throw new IllegalArgumentException("Position + size overflow");//最大2Gif (size > Integer.MAX_VALUE)throw new IllegalArgumentException("Size exceeds Integer.MAX_VALUE");int imode = -1;if (mode == MapMode.READ_ONLY)
            imode = MAP_RO;else if (mode == MapMode.READ_WRITE)
            imode = MAP_RW;else if (mode == MapMode.PRIVATE)
            imode = MAP_PV;assert (imode >= 0);if ((mode != MapMode.READ_ONLY) && !writable)throw new NonWritableChannelException();if (!readable)throw new NonReadableChannelException();long addr = -1;int ti = -1;try {begin();
            ti = threads.add();if (!isOpen())return null;//size()返回实际的文件大小//如果实际文件大小不符合，则增大文件的大小，文件的大小被改变，文件增大的部分默认设置为0。if (size() < position + size) { // Extend file sizeif (!writable) {throw new IOException("Channel not open for writing " +"- cannot extend file to required size");
                }int rv;do {                   //增大文件的大小rv = nd.truncate(fd, position + size);
                } while ((rv == IOStatus.INTERRUPTED) && isOpen());
            }//如果要求映射的文件大小为0，则不调用操作系统的mmap调用，只是生成一个空间容量为0的DirectByteBuffer//并返回if (size == 0) {
                addr = 0;// a valid file descriptor is not requiredFileDescriptor dummy = new FileDescriptor();if ((!writable) || (imode == MAP_RO))return Util.newMappedByteBufferR(0, 0, dummy, null);elsereturn Util.newMappedByteBuffer(0, 0, dummy, null);
            }//allocationGranularity的大小在我的系统上是4K//页对齐，pagePosition为第多少页int pagePosition = (int)(position % allocationGranularity);//从页的最开始映射long mapPosition = position - pagePosition;//因为从页的最开始映射，增大映射空间long mapSize = size + pagePosition;try {// If no exception was thrown from map0, the address is valid//native方法，源代码在openjdk/jdk/src/solaris/native/sun/nio/ch/FileChannelImpl.c,//参见下面的说明addr = map0(imode, mapPosition, mapSize);
            } catch (OutOfMemoryError x) {// An OutOfMemoryError may indicate that we&#39;ve exhausted memory// so force gc and re-attempt mapSystem.gc();try {
                    Thread.sleep(100);
                } catch (InterruptedException y) {
                    Thread.currentThread().interrupt();
                }try {
                    addr = map0(imode, mapPosition, mapSize);
                } catch (OutOfMemoryError y) {// After a second OOME, failthrow new IOException("Map failed", y);
                }
            }// On Windows, and potentially other platforms, we need an open// file descriptor for some mapping operations.FileDescriptor mfd;try {
                mfd = nd.duplicateForMapping(fd);
            } catch (IOException ioe) {unmap0(addr, mapSize);throw ioe;
            }assert (IOStatus.checkAll(addr));assert (addr % allocationGranularity == 0);int isize = (int)size;
            Unmapper um = new Unmapper(addr, mapSize, isize, mfd);if ((!writable) || (imode == MAP_RO)) {return Util.newMappedByteBufferR(isize,
                                                 addr + pagePosition,
                                                 mfd,
                                                 um);
            } else {return Util.newMappedByteBuffer(isize,
                                                addr + pagePosition,
                                                mfd,
                                                um);
            }
        } finally {
            threads.remove(ti);end(IOStatus.checkAll(addr));
        }
    }

    DirectByteBuffer(int cap) {                   // package-private

        super(-1, 0, cap, cap);
        //直接内存是否要页对齐，我本机测试的不用
        boolean pa = VM.isDirectMemoryPageAligned();
        //页的大小，本机测试的是4K
        int ps = Bits.pageSize();
        //如果页对齐，则size的大小是ps+cap，ps是一页，cap也是从新的一页开始，也就是页对齐了
        long size = Math.max(1L, (long)cap + (pa ? ps : 0));
        //JVM维护所有直接内存的大小，如果已分配的直接内存加上本次要分配的大小超过允许分配的直接内存的最大值会
        //引起GC，否则允许分配并把已分配的直接内存总量加上本次分配的大小。如果GC之后，还是超过所允许的最大值，
        //则throw new OutOfMemoryError("Direct buffer memory");
        Bits.reserveMemory(size, cap);

        long base = 0;
        try {
           //是吧，unsafe可以直接操作底层内存
            base = unsafe.allocateMemory(size);
        } catch (OutOfMemoryError x) {、
            //没有分配成功，把刚刚加上的已分配的直接内存的大小减去。
            Bits.unreserveMemory(size, cap);
            throw x;
        }
        unsafe.setMemory(base, size, (byte) 0);
        if (pa && (base % ps != 0)) {
            // Round up to page boundary
            address = base + ps - (base & (ps - 1));
        } else {
            address = base;
        }
        cleaner = Cleaner.create(this, new Deallocator(base, size, cap));
        att = null;
    }