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?

Answer 1

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+

For a one-time task, 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). Another approach is to pass both CREATE and APPEND options, which will create the file first if it doesn't already exist:

private void write(final String s) throws IOException {     Files.writeString(         Path.of(System.getProperty("java.io.tmpdir"), "filename.txt"),         s + System.lineSeparator(),         CREATE, APPEND     ); } 

However, if you will be writing to the same file many times, the above snippets must open and close the file on the disk many times, which is a slow operation. In this case, a BufferedWriter is faster:

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.

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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 } 

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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     } } 

Answer 2

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()); }