Dumping a Java StringBuilder to File

Question

What is the most efficient/elegant way to dump a StringBuilder to a text file?

You can do:

outputStream.write(stringBuilder.toString().getBytes());

But is this efficient for a very long file?

Is there a better way?

Answer 1

As pointed out by others, use a Writer, and use a BufferedWriter, but then don't call writer.write(stringBuilder.toString()); instead just writer.append(stringBuilder);.

EDIT: But, I see that you accepted a different answer because it was a one-liner. But that solution has two problems:

1. it doesn't accept a java.nio.Charset. BAD. You should always specify a Charset explicitly.

2. it's still making you suffer a stringBuilder.toString(). If the simplicity is really what you're after, try the following from the Guava project:

Files.write(stringBuilder, file, Charsets.UTF_8)

Answer 2

You should use a BufferedWriter to optimize the writes (always write character data using a Writer instead of an OutputStream). If you weren't writing character data, you would use a BufferedOutputStream.

File file = new File("path/to/file.txt");
BufferedWriter writer = null;
try {
    writer = new BufferedWriter(new FileWriter(file));
    writer.write(stringBuilder.toString());
} finally {
    if (writer != null) writer.close();
}

or, using try-with-resources (Java 7 and up)

File file = new File("path/to/file.txt");
try (BufferedWriter writer = new BufferedWriter(new FileWriter(file))) {
    writer.write(stringBuilder.toString());
}

Since you're ultimately writing to a file, a better approach would be to write to the BufferedWriter more often instead of creating a huge StringBuilder in-memory and writing everything at the end (depending on your use-case, you might even be able to eliminate the StringBuilder entirely). Writing incrementally during processing will save memory and will make better use of your limited I/O bandwidth, unless another thread is trying to read a lot of data from the disk at the same time you're writing.

Answer 3

Well, if the string is huge, toString().getBytes() will create duplicate bytes (2 or 3 times). The size of the string.

To avoid this, you can extract chunk of the string and write it in separate parts.

Here is how it may looks:

final StringBuilder aSB = ...;
final int    aLength = aSB.length();
final int    aChunk  = 1024;
final char[] aChars  = new char[aChunk];

for(int aPosStart = 0; aPosStart < aLength; aPosStart += aChunk) {
    final int aPosEnd = Math.min(aPosStart + aChunk, aLength);
    aSB.getChars(aPosStart, aPosEnd, aChars, 0);                 // Create no new buffer
    final CharArrayReader aCARead = new CharArrayReader(aChars); // Create no new buffer

    // This may be slow but it will not create any more buffer (for bytes)
    int aByte;
    while((aByte = aCARead.read()) != -1)
        outputStream.write(aByte);
}

Hope this helps.

Answer 4

You could use the Apache Commons IO library, which gives you FileUtils:

FileUtils.writeStringToFile(file, stringBuilder.toString(), Charset.forName("UTF-8"))

Answer 5

For character data better use Reader/Writer. In your case, use a BufferedWriter. If possible, use BufferedWriter from the beginning on instead of StringBuilder to save memory.

Note that your way of calling the non-arg getBytes() method would use the platform default character encoding to decode the characters. This may fail if the platform default encoding is for example ISO-8859-1 while your String data contains characters outside the ISO-8859-1 charset. Better use the getBytes(charset) where in you can specify the charset yourself, such as UTF-8.

Answer 6

If the string itself is long, you definitely should avoid toString(), which makes another copy of the string. The most efficient way to write to stream should be something like this,

OutputStreamWriter writer = new OutputStreamWriter(
        new BufferedOutputStream(outputStream), "utf-8");

for (int i = 0; i < sb.length(); i++) {
    writer.write(sb.charAt(i));
}

Answer 7

Since java 8 you only need to do this:

Files.write(Paths.get("/path/to/file/file_name.extension"), stringBuilder.toString().getBytes());

You don't need any third party libraries to do that.

Answer 8

Based on https://stackoverflow.com/a/1677317/980442

I create this function that use OutputStreamWriter and the write(), this is memory optimized too, better than just use StringBuilder.toString().

public static void stringBuilderToOutputStream(
        StringBuilder sb, OutputStream out, String charsetName, int buffer)
        throws IOException {
    char[] chars = new char[buffer];
    try (OutputStreamWriter writer = new OutputStreamWriter(out, charsetName)) {
        for (int aPosStart = 0; aPosStart < sb.length(); aPosStart += buffer) {
            buffer = Math.min(buffer, sb.length() - aPosStart);
            sb.getChars(aPosStart, aPosStart + buffer, chars, 0);
            writer.write(chars, 0, buffer);
        }
    }
}

Answer 9

Benchmarks for most answers here + improved implementation: https://www.genuitec.com/dump-a-stringbuilder-to-file/

The final implementation is along the lines of

try {
    BufferedWriter bw = new BufferedWriter(
            new OutputStreamWriter(
                    new FileOutputStream(file, append), charset), BUFFER_SIZE);
    try {
        final int length = sb.length();
        final char[] chars = new char[BUFFER_SIZE];
        int idxEnd;
        for ( int idxStart=0; idxStart<length; idxStart=idxEnd ) {
            idxEnd = Math.min(idxStart + BUFFER_SIZE, length);
            sb.getChars(idxStart, idxEnd, chars, 0);
            bw.write(chars, 0, idxEnd - idxStart);
        }
        bw.flush();
    } finally {
        bw.close();
    }
} catch ( IOException ex ) {
    ex.printStackTrace();
}