Android设备如何保证数据同步写入磁盘的实现

在一些特定的工作场景中，我们把数据及时写出磁盘，而不是暂时保存在系统的文件缓存区，防止掉电导致数据丢失

/**
 * Force all system buffers to synchronize with the underlying
 * device. This method returns after all modified data and
 * attributes of this FileDescriptor have been written to the
 * relevant device(s). In particular, if this FileDescriptor
 * refers to a physical storage medium, such as a file in a file
 * system, sync will not return until all in-memory modified copies
 * of buffers associated with this FileDescriptor have been
 * written to the physical medium.
 *
 * sync is meant to be used by code that requires physical
 * storage (such as a file) to be in a known state For
 * example, a class that provided a simple transaction facility
 * might use sync to ensure that all changes to a file caused
 * by a given transaction were recorded on a storage medium.
 *
 * sync only affects buffers downstream of this FileDescriptor. If
 * any in-memory buffering is being done by the application (for
 * example, by a BufferedOutputStream object), those buffers must
 * be flushed into the FileDescriptor (for example, by invoking
 * OutputStream.flush) before that data will be affected by sync.
 *
 * @exception SyncFailedException
 *    Thrown when the buffers cannot be flushed,
 *    or because the system cannot guarantee that all the
 *    buffers have been synchronized with physical media.
 * @since   JDK1.1
 */
public native void sync() throws SyncFailedException;

可能一看到这个场景，很多人会想到数据库的事务，查看Android数据库sqlite的源码可以看到，数据库事务只能保证n个操作，要么都执行，要么都不执行。数据库事务在所有操作完成后，会提醒文件系统与磁盘同步，但是不会等到所有系统缓冲区与磁盘同步完成才返回！

  FileDescriptor.getFd().sync();会强制所有系统缓冲区与磁盘同步
File file = new File("/sdcard/a.txt");
    try {
      FileOutputStream outputStream = new FileOutputStream(file);
      outputStream.write("kuangxf".getBytes());
      outputStream.flush();
      //强制文件系统刷新
      outputStream.getFD().sync();
    } catch (Exception e) {
      e.printStackTrace();
    }

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