FileInputStream 和 FileOutputStream 到同一个文件：read() 是否保证看到“之前发生过”的所有 write()？

Question

I am using a file as a cache for big data. One thread writes to it sequentially, another thread reads it sequentially.

Can I be sure that all data that has been written (by write() ) in one thread can be read() from another thread, assuming a proper "happens-before" relationship in terms of the Java memory model? Is this behavior documented?

In my JDK, FileOutputStream does not override flush() , and OutputStream.flush() is empty. That's why I'm wondering...

The streams in question are owned exclusively by a class that I have full control of. Each stream is guaranteed to be accesses by one thread only. My tests show that it works as expected, but I'm still wondering if this is guaranteed and documented.

See also this related discussion .

Answer 1

Assuming you are using a posix file system, then yes.

FileInputStream and FileOutputStream on *nix use the read and write system calls internally. The documentation for write says that reads will see the results of past writes ,

After a write() to a regular file has successfully returned:

Any successful read() from each byte position in the file that was modified by that write shall return the data specified by the write() for that position until such byte positions are again modified.

I'm pretty sure ntfs on windows will have the same read() write() guarantees.

Answer 2

You can't talk about "happens-before" relationship in terms of the Java memory model between your FileInputStream and FileOutputStream objects since they don't share any memory or thread. VM is free to reorder them just honoring your synchronization requirements. When you have proper synchronization between reads and writes without application level buffering, you are safe.

However FileInputStream and FileOutputStream share a file, which leaves things up to the OS which in main stream ones you can expect to read after write in order.

Answer 3

No, you need to flush() the Streams (at least for Buffered(Input|Output)Streams), otherwise you could have data in a buffer.

Maybe you need a concurrent data structure ?

Answer 4

如果 FileOutputStream 没有覆盖flush()，那么我认为您可以确保所有由 write() 写入的数据都可以由 read() 读取，除非您的操作系统对数据做了一些奇怪的事情（例如启动一个等待硬盘驱动器以正确的速度旋转而不是阻塞等），以便它不会立即写入。