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()返回實際的文件大小
      //如果實際文件大小不符合，則增大文件的大小，文件的大小被改變，文件增大的部分默認(rèn)設(shè)置為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為0，則不調(diào)用操作系統(tǒng)的mmap調(diào)用，只是生成一個空間容量為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的大小在我的系統(tǒng)上是4K
      //頁對齊，pagePosition為第多少頁
      int pagePosition = (int)(position % allocationGranularity);
      //從頁的最開始映射
      long mapPosition = position - pagePosition;
      //因為從頁的最開始映射，增大映射空間
      long mapSize = size + pagePosition;
      try {
        // If no exception was thrown from map0, the address is valid
        //native方法，源代碼在openjdk/jdk/src/solaris/native/sun/nio/ch/FileChannelImpl.c,
        //參見下面的說明
        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));
    }
  }

JNIEXPORT jlong JNICALL
Java_sun_nio_ch_FileChannelImpl_map0(JNIEnv *env, jobject this,
                   jint prot, jlong off, jlong len)
{
  void *mapAddress = 0;
  jobject fdo = (*env)->GetObjectField(env, this, chan_fd);
  //linux系統(tǒng)調(diào)用是通過整型的文件id引用文件的，這里得到文件id
  jint fd = fdval(env, fdo);
  int protections = 0;
  int flags = 0;

  if (prot == sun_nio_ch_FileChannelImpl_MAP_RO) {
    protections = PROT_READ;
    flags = MAP_SHARED;
  } else if (prot == sun_nio_ch_FileChannelImpl_MAP_RW) {
    protections = PROT_WRITE | PROT_READ;
    flags = MAP_SHARED;
  } else if (prot == sun_nio_ch_FileChannelImpl_MAP_PV) {
    protections = PROT_WRITE | PROT_READ;
    flags = MAP_PRIVATE;
  }
  //這里就是操作系統(tǒng)調(diào)用了，mmap64是宏定義，實際最后調(diào)用的是mmap
  mapAddress = mmap64(
    0,          /* Let OS decide location */
    len,         /* Number of bytes to map */
    protections,     /* File permissions */
    flags,        /* Changes are shared */
    fd,          /* File descriptor of mapped file */
    off);         /* Offset into file */

  if (mapAddress == MAP_FAILED) {
    if (errno == ENOMEM) {
      //如果沒有映射成功，直接拋出OutOfMemoryError
      JNU_ThrowOutOfMemoryError(env, "Map failed");
      return IOS_THROWN;
    }
    return handle(env, -1, "Map failed");
  }

  return ((jlong) (unsigned long) mapAddress);
}

 public static ByteBuffer allocateDirect(int capacity) {
    return new DirectByteBuffer(capacity);
  }

  DirectByteBuffer(int cap) {          // package-private

    super(-1, 0, cap, cap);
    //直接內(nèi)存是否要頁對齊，我本機(jī)測試的不用
    boolean pa = VM.isDirectMemoryPageAligned();
    //頁的大小，本機(jī)測試的是4K
    int ps = Bits.pageSize();
    //如果頁對齊，則size的大小是ps+cap，ps是一頁，cap也是從新的一頁開始，也就是頁對齊了
    long size = Math.max(1L, (long)cap + (pa ? ps : 0));
    //JVM維護(hù)所有直接內(nèi)存的大小，如果已分配的直接內(nèi)存加上本次要分配的大小超過允許分配的直接內(nèi)存的最大值會
    //引起GC，否則允許分配并把已分配的直接內(nèi)存總量加上本次分配的大小。如果GC之后，還是超過所允許的最大值，
    //則throw new OutOfMemoryError("Direct buffer memory");
    Bits.reserveMemory(size, cap);

    long base = 0;
    try {
      //是吧，unsafe可以直接操作底層內(nèi)存
      base = unsafe.allocateMemory(size);
    } catch (OutOfMemoryError x) {、
      //沒有分配成功，把剛剛加上的已分配的直接內(nèi)存的大小減去。
      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;
  }

UNSAFE_ENTRY(jlong, Unsafe_AllocateMemory(JNIEnv *env, jobject unsafe, jlong size))
 UnsafeWrapper("Unsafe_AllocateMemory");
 size_t sz = (size_t)size;
 if (sz != (julong)size || size < 0) {
  THROW_0(vmSymbols::java_lang_IllegalArgumentException());
 }
 if (sz == 0) {
  return 0;
 }
 sz = round_to(sz, HeapWordSize);
 //最后調(diào)用的是 u_char* ptr = (u_char*)::malloc(size + space_before + space_after)，也就是malloc。
 void* x = os::malloc(sz, mtInternal);
 if (x == NULL) {
  THROW_0(vmSymbols::java_lang_OutOfMemoryError());
 }
 //Copy::fill_to_words((HeapWord*)x, sz / HeapWordSize);
 return addr_to_java(x);
UNSAFE_END