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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