java 中Buffer源码的分析

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2G    if (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 size        if (!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 required        FileDescriptor dummy = new FileDescriptor();        if ((!writable) || (imode == MAP_RO))          return Util.newMappedByteBufferR(0, 0, dummy, null);        else          return 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/class="lazy" data-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've exhausted memory        // so force gc and re-attempt map        System.gc();        try {          Thread.sleep(100);        } catch (InterruptedException y) {          Thread.currentThread().interrupt();        }        try {          addr = map0(imode, mapPosition, mapSize);        } catch (OutOfMemoryError y) {          // After a second OOME, fail          throw 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));    }  }