Inserting text into an existing file via Java

Question

I would like to create a simple program (in Java) which edits text files - particularly one which performs inserting arbitrary pieces of text at random positions in a text file. This feature is part of a larger program I am currently writing.

Reading the description about java.util.RandomAccessFile, it appears that any write operations performed in the middle of a file would actually overwrite the exiting content. This is a side-effect which I would like to avoid (if possible).

Is there a simple way to achieve this?

Thanks in advance.

Answer 1

Okay, this question is pretty old, but FileChannels exist since Java 1.4 and I don't know why they aren't mentioned anywhere when dealing with the problem of replacing or inserting content in files. FileChannels are fast, use them.

Here's an example (ignoring exceptions and some other stuff):

public void insert(String filename, long offset, byte[] content) {   RandomAccessFile r = new RandomAccessFile(new File(filename), "rw");   RandomAccessFile rtemp = new RandomAccessFile(new File(filename + "~"), "rw");   long fileSize = r.length();   FileChannel sourceChannel = r.getChannel();   FileChannel targetChannel = rtemp.getChannel();   sourceChannel.transferTo(offset, (fileSize - offset), targetChannel);   sourceChannel.truncate(offset);   r.seek(offset);   r.write(content);   long newOffset = r.getFilePointer();   targetChannel.position(0L);   sourceChannel.transferFrom(targetChannel, newOffset, (fileSize - offset));   sourceChannel.close();   targetChannel.close(); } 

Answer 2

Well, no, I don't believe there is a way to avoid overwriting existing content with a single, standard Java IO API call.

If the files are not too large, just read the entire file into an ArrayList (an entry per line) and either rewrite entries or insert new entries for new lines.

Then overwrite the existing file with new content, or move the existing file to a backup and write a new file.

Depending on how sophisticated the edits need to be, your data structure may need to change.

Another method would be to read characters from the existing file while writing to the edited file and edit the stream as it is read.