FileOutputstream.close（）并不总是字节写入文件系统？ [英] FileOutputstream.close() is not always writing bytes to file system?

问题描述

看完<一个href=\"http://stackoverflow.com/questions/7849528/fileoutputstream-close-is-really-slow-when-writing-large-file#7849941\">this从@Peter Lawrey回答特别是这一句：

  

的close（）可以确保文件实际上被写到磁盘（或不依赖于操作系统）

<子>（重点是我的。）

我有3个问题：

• 这是真的，我们无法保证在调用后的所有字节将可在磁盘的close（）？ （我猜，因为它从@Peter Lawrey来这是真的）




• 在一般（即有效上的所有OS），什么是要确保所有字节有效地写入磁盘的最佳方式？ （我能想象计数写入流的字节数，和等到的 file.length（）== BYTECOUNT ...但有一个更好的办法？）




• 在特定的，在 Android版，是足以称之为 fileOutputStream.close（），以确保所有字节都有效写入文件系统？

下面是一些code（忽略例外，......保持简单）来说明我的文章

 最后的InputStream河道= getInputStreamFromSomewhere（）;
    最终的FileOutputStream的OutputStream =新的FileOutputStream（someExistingFile）;
    INT升;
    最后一个字节[] TMP =新的字节[1024];
    而（（1- = instream.read（TMP））！=  -  1）{
            outstream.write（TMP，0,1）;
    }
    instream.close（）;
    //outputStream.flush（）; //因为outputStream.flush（）无用的呼叫做什么的FileOutputStream
    outputStream.close（）;
    //在这点：都字节写入磁盘？
 

解决方案

  fileOutputstream.getFD（）。同步（）
 

应该做你想要什么。请参见 http://docs.oracle。 COM / JavaSE的/ 7 /文档/ API / JAVA / IO / FileDescriptor.html＃同步（）

After reading this answer from @Peter Lawrey and especially this sentence :

close() can ensure the file is actually written to disk (or not depending on the OS)

_{(emphasis is mine.)}

I have 3 questions :

• Is it true that there is no guaranty that all the bytes will be available on disk after calling close() ? (I guess it's true since it came from @Peter Lawrey)

• In general (i.e. valid on all OS), what is the best way to be sure that all bytes are effectively written to disk ? (I can imagine counting the bytes written to the stream, and waiting until file.length() == byteCount ... but is there a better approach ?)

• In particular, on Android, is it enough to call fileOutputStream.close() to be sure that all bytes are effectively written to the file system ?

Here is some code (ignoring exceptions, ... to keep it simple) to illustrate my post

    final InputStream instream = getInputStreamFromSomewhere();
    final FileOutputStream outputstream = new FileOutputStream(someExistingFile);
    int l;
    final byte[] tmp = new byte[1024];
    while ((l = instream.read(tmp)) != -1) {
            outstream.write(tmp, 0, l);
    }
    instream.close();
    //outputStream.flush(); //useless call since outputStream.flush() do nothing on FileOutputStream
    outputStream.close();
    //at this point : are all bytes written to disk ?

解决方案

fileOutputstream.getFD().sync()

should do what you want. See http://docs.oracle.com/javase/7/docs/api/java/io/FileDescriptor.html#sync()

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