Prepend lines to file in Java

Question

Is there a way to prepend a line to the File in Java, without creating a temporary file, and writing the needed content to it?

Answer 1

No, there is no way to do that SAFELY in Java. (Or AFAIK, any other programming language.)

No filesystem implementation in any mainstream operating system supports this kind of thing, and you won't find this feature supported in any mainstream programming languages.

Real world file systems are implemented on devices that store data as fixed sized "blocks". It is not possible to implement a file system model where you can insert bytes into the middle of a file without significantly slowing down file I/O, wasting disk space or both.

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The solutions that involve an in-place rewrite of the file are inherently unsafe. If your application is killed or the power dies in the middle of the prepend / rewrite process, you are likely to lose data. I would NOT recommend using that approach in practice.

Use a temporary file and renaming. It is safer.

Answer 2

There is a way, it involves rewriting the whole file though (but no temporary file). As others mentioned, no file system supports prepending content to a file. Here is some sample code that uses a RandomAccessFile to write and read content while keeping some content buffered in memory:

public static void main(final String args[]) throws Exception {     File f = File.createTempFile(Main.class.getName(), "tmp");     f.deleteOnExit();      System.out.println(f.getPath());      // put some dummy content into our file     BufferedWriter w = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(f)));     for (int i = 0; i < 1000; i++) {         w.write(UUID.randomUUID().toString());         w.write('\n');     }     w.flush();     w.close();              // append "some uuids" to our file     int bufLength = 4096;     byte[] appendBuf = "some uuids\n".getBytes();     byte[] writeBuf = appendBuf;     byte[] readBuf = new byte[bufLength];      int writeBytes = writeBuf.length;      RandomAccessFile rw = new RandomAccessFile(f, "rw");     int read = 0;     int write = 0;      while (true) {                     // seek to read position and read content into read buffer         rw.seek(read);         int bytesRead = rw.read(readBuf, 0, readBuf.length);                      // seek to write position and write content from write buffer         rw.seek(write);         rw.write(writeBuf, 0, writeBytes);                      // no bytes read - end of file reached         if (bytesRead < 0) {                             // end of             break;         }                      // update seek positions for write and read         read += bytesRead;         write += writeBytes;         writeBytes = bytesRead;                      // reuse buffer, create new one to replace (short) append buf         byte[] nextWrite = writeBuf == appendBuf ? new byte[bufLength] : writeBuf;         writeBuf = readBuf;         readBuf = nextWrite;     };      rw.close();              // now show the content of our file     BufferedReader reader = new BufferedReader(new InputStreamReader(new FileInputStream(f)));      String line;     while ((line = reader.readLine()) != null) {         System.out.println(line);     } }