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
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/** * 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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