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
/** *Forceallsystembufferstosynchronizewiththeunderlying *device.Thismethodreturnsafterallmodifieddataand *attributesofthisFileDescriptorhavebeenwrittentothe *relevantdevice(s).Inparticular,ifthisFileDescriptor *referstoaphysicalstoragemedium,suchasafileinafile *system,syncwillnotreturnuntilallin-memorymodifiedcopies *ofbuffersassociatedwiththisFileDescriptorhavebeen *writtentothephysicalmedium. * *syncismeanttobeusedbycodethatrequiresphysical *storage(suchasafile)tobeinaknownstateFor *example,aclassthatprovidedasimpletransactionfacility *mightusesynctoensurethatallchangestoafilecaused *byagiventransactionwererecordedonastoragemedium. * *synconlyaffectsbuffersdownstreamofthisFileDescriptor.If *anyin-memorybufferingisbeingdonebytheapplication(for *example,byaBufferedOutputStreamobject),thosebuffersmust *beflushedintotheFileDescriptor(forexample,byinvoking *OutputStream.flush)beforethatdatawillbeaffectedbysync. * *@exceptionSyncFailedException *Thrownwhenthebufferscannotbeflushed, *orbecausethesystemcannotguaranteethatallthe *buffershavebeensynchronizedwithphysicalmedia. *@sinceJDK1.1 */ publicnativevoidsync()throwsSyncFailedException;
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
Filefile=newFile("/sdcard/a.txt");
try{
FileOutputStreamoutputStream=newFileOutputStream(file);
outputStream.write("kuangxf".getBytes());
outputStream.flush();
//强制文件系统刷新
outputStream.getFD().sync();
}catch(Exceptione){
e.printStackTrace();
}
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