I need to append text repeatedly to an existing file in Java. How do I do that?
Are you doing this for logging purposes? If so there are several libraries for this. Two of the most popular are Log4j and Logback.
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:
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.)BufferedWriter
is recommended for an expensive writer (such as FileWriter
).PrintWriter
gives you access to println
syntax that you're probably used to from System.out
.BufferedWriter
and PrintWriter
wrappers are not strictly necessary.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
}
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
}
}