How to convert outputStream to a byte array?

Question

How can I convert an OutputStream to a byte array? I have found that first I need to convert this OutputStream to a ByteArrayOutputStream. There is only write() method in this OutputStream class and I don't know what to do. Is there any idea?

Strictly speaking, you can't. An output stream is an output stream is an output stream. Once it exists, you could maybe wrap it in a PrintStream or something. — Ingo
– Ingo, Commented Apr 18, 2014 at 12:28

JohnK · Accepted Answer · 2022-06-15 12:11:08Z

112

Create a ByteArrayOutputStream.

Grab its content by calling toByteArray()

ByteArrayOutputStream baos = new ByteArrayOutputStream(); 
baos.writeTo(myOutputStream); 
baos.toByteArray();

Reference

edited Jun 15, 2022 at 12:11

JohnK

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answered Apr 18, 2014 at 12:25

washcloth

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3 Comments

Bruce Loth Over a year ago

For those clueless like me: ByteArrayOutputStream baos = new ByteArrayOutputStream(); baos.writeTo(myOutputStream); baos.toByteArray();

JackTheKnife Over a year ago

Side question. Are there any limitations to baos.toString() compared to baos.toByteArray()? I have used it first on some old projects but for some reason, I was not able to generate an SHA-1 hash digest from it but was working fine for the second one and still I have no clue why may be something in the reading file.

Liam Williams Over a year ago

Although this is a highly voted and accepted answer, it does not actually answer the question. The code in this answer shows how to write the contents of an empty ByteArrayOutputStream to another OutputStream myOutputStream via the writeTo method. If the idea was to demonstrate that ByteArrayOutputStream has a toByteArray method, then the intermediate baos.writeTo(myOutputStream) should instead be more like baos.write(new byte[] {1,2,3}), to demonstrate that those same bytes are then present in the toByteArray() call.

Community · Accepted Answer · 2017-05-23 12:25:24Z

If the OutputStream object supplied is not already a ByteArrayOutputStream, one can wrap it inside a delegate class that will "grab" the bytes supplied to the write() methods, e.g.

public class DrainableOutputStream extends FilterOutputStream {
  private final ByteArrayOutputStream buffer;
  public DrainableOutputStream(OutputStream out) {
    super(out);
    this.buffer = new ByteArrayOutputStream();
  }
  @Override
  public void write(byte b[]) throws IOException {
    this.buffer.write(b);
    super.write(b);
  }
  @Override
  public void write(byte b[], int off, int len) throws IOException {
    this.buffer.write(b, off, len);
    super.write(b, off, len);
  }
  @Override
  public void write(int b) throws IOException {
    this.buffer.write(b);    
    super.write(b);
  }
  public byte[] toByteArray() {
    return this.buffer.toByteArray();
  }
}

To reduce the overhead, the calls to super in the above class can be omitted - e.g., if only the "conversion" to a byte array is desired.

A more detailed discussion can be found in another StackOverflow question.

Community · Accepted Answer · 2017-05-23 12:32:28Z

4

You need to do 2 things

Using ByteArrayOutputStream write to it
Using toByteArray(), you will get the contents as byte[]

You could even extend it as mentioned here

edited May 23, 2017 at 12:32

CommunityBot

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answered Apr 18, 2014 at 12:25

John Eipe

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Comments

Dillon Ryan Redding · Accepted Answer · 2014-04-18 12:30:35Z

1

You could simply declare your output stream as a ByteArrayOutputStream then use ByteArrayOutputStream#toByteArray().

answered Apr 18, 2014 at 12:30

Dillon Ryan Redding

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Comments

zysaaa · Accepted Answer · 2021-11-30 19:16:30Z

If You try ByteArrayOutputStream bos=(ByteArrayOutputStream)outputStream then throw ClassCastException. I did it when I get OutputStream from HttpServletResponse and it is CoyoteOutputStream.(You can create file so dont create file temp).

You can use Java Reflection to convert OutputStream to byte[]:

 byte[] bytes = this.getBytes(outStream);

/**
 * Get byte from OutputStream
 *
 * @param outStream
 * @return
 */
@SneakyThrows
private byte[] getBytes(OutputStream outStream) {
    OutputBuffer outputBuffer = (OutputBuffer) this.getValueByName("ob", outStream);
    ByteBuffer byteBuffer = (ByteBuffer) this.getValueByName("bb", outputBuffer);
    return (byte[]) this.getValueByName("hb", byteBuffer);
}

/**
 * Get value from property
 *
 * @param name
 * @param value
 * @return
 */
@SneakyThrows
@SuppressWarnings("unchecked")
private Object getValueByName(String name, Object value) {
    List<Field> listFiled = new ArrayList<>();
    if (value.getClass().getSuperclass() != null) {
        listFiled.addAll(Arrays.asList(value.getClass().getSuperclass().getDeclaredFields()));
    }
    listFiled.addAll(Arrays.asList(value.getClass().getDeclaredFields()));
    Optional<Field> fieldOb = listFiled.stream()
        .filter(field -> StringUtils.equalsAnyIgnoreCase(name, field.getName()))
        .findFirst();
    if (fieldOb.isPresent()) {
        Field field = fieldOb.get();
        field.setAccessible(true);
        return field.get(value);
    }
    return StringUtils.EMPTY; // FIXME
}

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