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));
        }
    }

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系统调用是通过整型的文件id引用文件的，这里得到文件idjint 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;
    }//这里就是操作系统调用了，mmap64是宏定义，实际最后调用的是mmapmapAddress = 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) {//如果没有映射成功，直接抛出OutOfMemoryErrorJNU_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);
    }

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);  //最后调用的是 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