RFR 8193832: Performance of InputStream.readAllBytes() could be improved (original) (raw)

Peter Levart peter.levart at gmail.com
Wed Dec 20 11:45:03 UTC 2017

Hi Brian,

On 12/20/2017 12:22 AM, Brian Burkhalter wrote:

On Dec 19, 2017, at 2:36 PM, Brian Burkhalter <brian.burkhalter at oracle.com> wrote:

You can also simplify the “for(;;) + break" into a do while loop:

do { int nread = 0; ... } while (n > 0); Good suggestion but I think that this needs to be "while (n >= 0)." Updated version here: http://cr.openjdk.java.net/~bpb/8193832/webrev.02/ Thanks, Brian

Looks good. There is one case that could be further optimized. When result.length <= DEFAULT_BUFFER_SIZE, the allocation of ArrayList could be avoided. Imagine a use case where lots of small files are read into byte[] arrays. For exmaple:

    public byte[] readAllBytes() throws IOException {         List<byte[]> bufs = null;         byte[] result = null;         byte[] buf = new byte[DEFAULT_BUFFER_SIZE];         int total = 0;         int n;         do {             int nread = 0;

            // read to EOF which may read more or less than buffer size             while ((n = read(buf, nread, buf.length - nread)) > 0) {                 nread += n;             }

            if (nread > 0) {                 if (MAX_BUFFER_SIZE - total < nread) {                     throw new OutOfMemoryError("Required array size too large");                 }                 total += nread;                 byte[] copy = (n < 0 && nread == DEFAULT_BUFFER_SIZE) ?                     buf : Arrays.copyOf(buf, nread);                 if (result == null) {                     result = copy;                 } else {                     bufs = new ArrayList<>(8);                     bufs.add(result);                     bufs.add(copy);                 }             }         } while (n >= 0); // if the last call to read returned -1, then break

        if (bufs == null) {             return result == null ? new byte[0] : result;         }

        result = new byte[total];         int offset = 0;         for (byte[] b : bufs) {             System.arraycopy(b, 0, result, offset, b.length);             offset += b.length;         }

        return result;     }

There is a possibility that JIT already avoids allocating ArrayList utilizing EA if all involved ArrayList methods inline, so this potential optimization should be tested 1st to see if it actually helps improve the "small file" case.

Regards, Peter

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