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How to append text to an existing file in Java?


Question

I need to append text repeatedly to an existing file in Java. How do I do that?

2020/08/13
1
692
8/13/2020 8:37:33 PM

Accepted Answer

Are you doing this for logging purposes? If so there are several libraries for this. Two of the most popular are Log4j and Logback.

Java 7+

If you just need to do this one time, the Files class makes this easy:

try {
    Files.write(Paths.get("myfile.txt"), "the text".getBytes(), StandardOpenOption.APPEND);
}catch (IOException e) {
    //exception handling left as an exercise for the reader
}

Careful: The above approach will throw a NoSuchFileException if the file does not already exist. It also does not append a newline automatically (which you often want when appending to a text file). Steve Chambers's answer covers how you could do this with Files class.

However, if you will be writing to the same file many times, the above has to open and close the file on the disk many times, which is a slow operation. In this case, a buffered writer is better:

try(FileWriter fw = new FileWriter("myfile.txt", true);
    BufferedWriter bw = new BufferedWriter(fw);
    PrintWriter out = new PrintWriter(bw))
{
    out.println("the text");
    //more code
    out.println("more text");
    //more code
} catch (IOException e) {
    //exception handling left as an exercise for the reader
}

Notes:

  • The second parameter to the FileWriter constructor will tell it to append to the file, rather than writing a new file. (If the file does not exist, it will be created.)
  • Using a BufferedWriter is recommended for an expensive writer (such as FileWriter).
  • Using a PrintWriter gives you access to println syntax that you're probably used to from System.out.
  • But the BufferedWriter and PrintWriter wrappers are not strictly necessary.

Older Java

try {
    PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("myfile.txt", true)));
    out.println("the text");
    out.close();
} catch (IOException e) {
    //exception handling left as an exercise for the reader
}

Exception Handling

If you need robust exception handling for older Java, it gets very verbose:

FileWriter fw = null;
BufferedWriter bw = null;
PrintWriter out = null;
try {
    fw = new FileWriter("myfile.txt", true);
    bw = new BufferedWriter(fw);
    out = new PrintWriter(bw);
    out.println("the text");
    out.close();
} catch (IOException e) {
    //exception handling left as an exercise for the reader
}
finally {
    try {
        if(out != null)
            out.close();
    } catch (IOException e) {
        //exception handling left as an exercise for the reader
    }
    try {
        if(bw != null)
            bw.close();
    } catch (IOException e) {
        //exception handling left as an exercise for the reader
    }
    try {
        if(fw != null)
            fw.close();
    } catch (IOException e) {
        //exception handling left as an exercise for the reader
    }
}
2017/06/03
804
6/3/2017 8:20:48 PM

You can use fileWriter with a flag set to true , for appending.

try
{
    String filename= "MyFile.txt";
    FileWriter fw = new FileWriter(filename,true); //the true will append the new data
    fw.write("add a line\n");//appends the string to the file
    fw.close();
}
catch(IOException ioe)
{
    System.err.println("IOException: " + ioe.getMessage());
}
2017/04/24

Shouldn't all of the answers here with try/catch blocks have the .close() pieces contained in a finally block?

Example for marked answer:

PrintWriter out = null;
try {
    out = new PrintWriter(new BufferedWriter(new FileWriter("writePath", true)));
    out.println("the text");
} catch (IOException e) {
    System.err.println(e);
} finally {
    if (out != null) {
        out.close();
    }
} 

Also, as of Java 7, you can use a try-with-resources statement. No finally block is required for closing the declared resource(s) because it is handled automatically, and is also less verbose:

try(PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("writePath", true)))) {
    out.println("the text");
} catch (IOException e) {
    System.err.println(e);
}
2019/02/05

Edit - as of Apache Commons 2.1, the correct way to do it is:

FileUtils.writeStringToFile(file, "String to append", true);

I adapted @Kip's solution to include properly closing the file on finally:

public static void appendToFile(String targetFile, String s) throws IOException {
    appendToFile(new File(targetFile), s);
}

public static void appendToFile(File targetFile, String s) throws IOException {
    PrintWriter out = null;
    try {
        out = new PrintWriter(new BufferedWriter(new FileWriter(targetFile, true)));
        out.println(s);
    } finally {
        if (out != null) {
            out.close();
        }
    }
}

2011/12/07

Slightly expanding on Kip's answer, here is a simple Java 7+ method to append a new line to a file, creating it if it doesn't already exist:

try {
    final Path path = Paths.get("path/to/filename.txt");
    Files.write(path, Arrays.asList("New line to append"), StandardCharsets.UTF_8,
        Files.exists(path) ? StandardOpenOption.APPEND : StandardOpenOption.CREATE);
} catch (final IOException ioe) {
    // Add your own exception handling...
}

Further notes:

  1. The above uses the Files.write overload that writes lines of text to a file (i.e. similar to a println command). To just write text to the end (i.e. similar to a print command), an alternative Files.write overload can be used, passing in a byte array (e.g. "mytext".getBytes(StandardCharsets.UTF_8)).

  2. The CREATE option will only work if the specified directory already exists - if it doesn't, a NoSuchFileException is thrown. If required, the following code could be added after setting path to create the directory structure:

    Path pathParent = path.getParent();
    if (!Files.exists(pathParent)) {
        Files.createDirectories(pathParent);
    }
    
2020/08/13

Make sure the stream gets properly closed in all scenarios.

It's a bit alarming how many of these answers leave the file handle open in case of an error. The answer https://stackoverflow.com/a/15053443/2498188 is on the money but only because BufferedWriter() cannot throw. If it could then an exception would leave the FileWriter object open.

A more general way of doing this that doesn't care if BufferedWriter() can throw:

  PrintWriter out = null;
  BufferedWriter bw = null;
  FileWriter fw = null;
  try{
     fw = new FileWriter("outfilename", true);
     bw = new BufferedWriter(fw);
     out = new PrintWriter(bw);
     out.println("the text");
  }
  catch( IOException e ){
     // File writing/opening failed at some stage.
  }
  finally{
     try{
        if( out != null ){
           out.close(); // Will close bw and fw too
        }
        else if( bw != null ){
           bw.close(); // Will close fw too
        }
        else if( fw != null ){
           fw.close();
        }
        else{
           // Oh boy did it fail hard! :3
        }
     }
     catch( IOException e ){
        // Closing the file writers failed for some obscure reason
     }
  }

Edit:

As of Java 7, the recommended way is to use "try with resources" and let the JVM deal with it:

  try(    FileWriter fw = new FileWriter("outfilename", true);
          BufferedWriter bw = new BufferedWriter(fw);
          PrintWriter out = new PrintWriter(bw)){
     out.println("the text");
  }  
  catch( IOException e ){
      // File writing/opening failed at some stage.
  }
2017/05/23

Source: https://stackoverflow.com/questions/1625234
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