Easy way to write contents of a Java InputStream to an OutputStream


I was surprised to find today that I couldn't track down any simple way to write the contents of an InputStream to an OutputStream in Java. Obviously, the byte buffer code isn't difficult to write, but I suspect I'm just missing something which would make my life easier (and the code clearer).

So, given an InputStream in and an OutputStream out, is there a simpler way to write the following?

byte[] buffer = new byte[1024];
int len = in.read(buffer);
while (len != -1) {
    out.write(buffer, 0, len);
    len = in.read(buffer);

This question is tagged with java io stream

~ Asked on 2008-09-04 03:46:16

23 Answers


Java 9

Since Java 9, InputStream provides a method called transferTo with the following signature:

public long transferTo(OutputStream out) throws IOException

As the documentation states, transferTo will:

Reads all bytes from this input stream and writes the bytes to the given output stream in the order that they are read. On return, this input stream will be at end of stream. This method does not close either stream.

This method may block indefinitely reading from the input stream, or writing to the output stream. The behavior for the case where the input and/or output stream is asynchronously closed, or the thread interrupted during the transfer, is highly input and output stream specific, and therefore not specified

So in order to write contents of a Java InputStream to an OutputStream, you can write:


~ Answered on 2016-09-11 21:44:31


As WMR mentioned, org.apache.commons.io.IOUtils from Apache has a method called copy(InputStream,OutputStream) which does exactly what you're looking for.

So, you have:

InputStream in;
OutputStream out;

...in your code.

Is there a reason you're avoiding IOUtils?

~ Answered on 2008-09-09 12:36:43


If you are using Java 7, Files (in the standard library) is the best approach:

/* You can get Path from file also: file.toPath() */
Files.copy(InputStream in, Path target)
Files.copy(Path source, OutputStream out)

Edit: Of course it's just useful when you create one of InputStream or OutputStream from file. Use file.toPath() to get path from file.

To write into an existing file (e.g. one created with File.createTempFile()), you'll need to pass the REPLACE_EXISTING copy option (otherwise FileAlreadyExistsException is thrown):

Files.copy(in, target, StandardCopyOption.REPLACE_EXISTING)

~ Answered on 2013-10-05 06:03:45


I think this will work, but make sure to test it... minor "improvement", but it might be a bit of a cost at readability.

byte[] buffer = new byte[1024];
int len;
while ((len = in.read(buffer)) != -1) {
    out.write(buffer, 0, len);

~ Answered on 2008-09-04 03:58:20


Using Guava's ByteStreams.copy():

ByteStreams.copy(inputStream, outputStream);

~ Answered on 2014-03-26 10:12:22


Simple Function

If you only need this for writing an InputStream to a File then you can use this simple function:

private void copyInputStreamToFile( InputStream in, File file ) {
    try {
        OutputStream out = new FileOutputStream(file);
        byte[] buf = new byte[1024];
        int len;
    } catch (Exception e) {

~ Answered on 2013-09-13 19:33:32


The JDK uses the same code so it seems like there is no "easier" way without clunky third party libraries (which probably don't do anything different anyway). The following is directly copied from java.nio.file.Files.java:

// buffer size used for reading and writing
private static final int BUFFER_SIZE = 8192;

  * Reads all bytes from an input stream and writes them to an output stream.
private static long copy(InputStream source, OutputStream sink) throws IOException {
    long nread = 0L;
    byte[] buf = new byte[BUFFER_SIZE];
    int n;
    while ((n = source.read(buf)) > 0) {
        sink.write(buf, 0, n);
        nread += n;
    return nread;

~ Answered on 2016-10-13 11:10:11


PipedInputStream and PipedOutputStream should only be used when you have multiple threads, as noted by the Javadoc.

Also, note that input streams and output streams do not wrap any thread interruptions with IOExceptions... So, you should consider incorporating an interruption policy to your code:

byte[] buffer = new byte[1024];
int len = in.read(buffer);
while (len != -1) {
    out.write(buffer, 0, len);
    len = in.read(buffer);
    if (Thread.interrupted()) {
        throw new InterruptedException();

This would be an useful addition if you expect to use this API for copying large volumes of data, or data from streams that get stuck for an intolerably long time.

~ Answered on 2009-02-12 10:37:28


For those who use Spring framework there is a useful StreamUtils class:

StreamUtils.copy(in, out);

The above does not close the streams. If you want the streams closed after the copy, use FileCopyUtils class instead:

FileCopyUtils.copy(in, out);

~ Answered on 2017-02-02 14:33:08


There's no way to do this a lot easier with JDK methods, but as Apocalisp has already noted, you're not the only one with this idea: You could use IOUtils from Jakarta Commons IO, it also has a lot of other useful things, that IMO should actually be part of the JDK...

~ Answered on 2008-09-04 10:50:10


Using Java7 and try-with-resources, comes with a simplified and readable version.

try(InputStream inputStream = new FileInputStream("C:\\mov.mp4");
    OutputStream outputStream = new FileOutputStream("D:\\mov.mp4")) {

    byte[] buffer = new byte[10*1024];

    for (int length; (length = inputStream.read(buffer)) != -1; ) {
        outputStream.write(buffer, 0, length);
} catch (FileNotFoundException exception) {
} catch (IOException ioException) {

~ Answered on 2015-08-21 08:52:19


Here comes how I'm doing with simplest for loop.

private void copy(final InputStream in, final OutputStream out)
    throws IOException {
    final byte[] b = new byte[8192];
    for (int r; (r = in.read(b)) != -1;) {
        out.write(b, 0, r);

~ Answered on 2016-08-22 01:17:50


Use Commons Net's Util class:

import org.apache.commons.net.io.Util;
Util.copyStream(in, out);

~ Answered on 2013-11-11 20:56:35


A IMHO more minimal snippet (that also more narrowly scopes the length variable):

byte[] buffer = new byte[2048];
for (int n = in.read(buffer); n >= 0; n = in.read(buffer))
    out.write(buffer, 0, n);

As a side note, I don't understand why more people don't use a for loop, instead opting for a while with an assign-and-test expression that is regarded by some as "poor" style.

~ Answered on 2015-12-10 00:29:32


I use BufferedInputStream and BufferedOutputStream to remove the buffering semantics from the code

try (OutputStream out = new BufferedOutputStream(...);
     InputStream in   = new BufferedInputStream(...))) {
  int ch;
  while ((ch = in.read()) != -1) {

~ Answered on 2019-01-20 02:04:10


This is my best shot!!

And do not use inputStream.transferTo(...) because is too generic. Your code performance will be better if you control your buffer memory.

public static void transfer(InputStream in, OutputStream out, int buffer) throws IOException {
    byte[] read = new byte[buffer]; // Your buffer size.
    while (0 < (buffer = in.read(read)))
        out.write(read, 0, buffer);

I use it with this (improvable) method when I know in advance the size of the stream.

public static void transfer(int size, InputStream in, OutputStream out) throws IOException {
    transfer(in, out,
            size > 0xFFFF ? 0xFFFF // 16bits 65,536
                    : size > 0xFFF ? 0xFFF// 12bits 4096
                            : size < 0xFF ? 0xFF // 8bits 256
                                    : size

~ Answered on 2019-07-11 00:13:01


I think it's better to use a large buffer, because most of the files are greater than 1024 bytes. Also it's a good practice to check the number of read bytes to be positive.

byte[] buffer = new byte[4096];
int n;
while ((n = in.read(buffer)) > 0) {
    out.write(buffer, 0, n);

~ Answered on 2013-09-13 14:24:37


PipedInputStream and PipedOutputStream may be of some use, as you can connect one to the other.

~ Answered on 2008-09-04 04:04:07


Another possible candidate are the Guava I/O utilities:


I thought I'd use these since Guava is already immensely useful in my project, rather than adding yet another library for one function.

~ Answered on 2012-12-11 05:42:34


Not very readable, but effective, has no dependencies and runs with any java version

byte[] buffer = new byte[1024];
for (int n; (n = inputStream.read(buffer)) != -1; outputStream.write(buffer, 0, n));

~ Answered on 2020-03-21 13:13:27


public static boolean copyFile(InputStream inputStream, OutputStream out) {
    byte buf[] = new byte[1024];
    int len;
    long startTime=System.currentTimeMillis();

    try {
        while ((len = inputStream.read(buf)) != -1) {
            out.write(buf, 0, len);

        long endTime=System.currentTimeMillis()-startTime;
        Log.v("","Time taken to transfer all bytes is : "+endTime);

    } catch (IOException e) {

        return false;
    return true;

~ Answered on 2015-04-02 18:48:24


Try Cactoos:

new LengthOf(new TeeInput(input, output)).value();

More details here: http://www.yegor256.com/2017/06/22/object-oriented-input-output-in-cactoos.html

~ Answered on 2017-09-21 16:06:09


you can use this method

public static void copyStream(InputStream is, OutputStream os)
     final int buffer_size=1024;
         byte[] bytes=new byte[buffer_size];
           int count=is.read(bytes, 0, buffer_size);
           os.write(bytes, 0, count);
     catch(Exception ex){}

~ Answered on 2014-03-06 11:23:20

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