Why is ObjectOutputStream.writeObject faster than write bytes when writing a byte array?

Question

I did a small benchmark test and found that ObjectOutputStream.writeObject is faster than ObjectOutputStream.write(byte[] bytes) but I can't seem to find a possible explanation as under the hood, writeObject will call ObjectOutputStream.write(byte[] bytes) indirectly

Test code

public static void main(String[] args) throws Exception { byte[] bytes = new byte[10000]; for (int i = 0; i < 10000; ++i) { bytes[i] = (byte) (i % 256); } ByteArrayOutputStream out2 = new ByteArrayOutputStream(); try(ObjectOutputStream ostream2 = new ObjectOutputStream(out2)) { for (int i = 0; i < 10000; ++i) { ostream2.writeInt(bytes.length); ostream2.write(bytes, 0, bytes.length); } out2.reset(); long start = System.nanoTime(); for (int i = 0; i < 10000; ++i) { ostream2.writeInt(bytes.length); ostream2.write(bytes, 0, bytes.length); } long end = System.nanoTime(); System.out.println("write byte[] took: " + ((end - start) / 1000) + " micros"); } ByteArrayOutputStream out = new ByteArrayOutputStream(); try(ObjectOutputStream ostream = new ObjectOutputStream(out)) { for (int i = 0; i < 10000; ++i) { ostream.writeObject(bytes); } out.reset(); long start = System.nanoTime(); for (int i = 0; i < 10000; ++i) { ostream.writeObject(bytes); } long end = System.nanoTime(); System.out.println("writeObject took: " + ((end - start) / 1000) + " micros"); } }

Output

write byte[] took: 15445 micros

writeObject took: 3111 micros

user207421 · Accepted Answer · 2015-09-04 11:01:25Z

6

ObjectOutputStream.writeObject() conserves written objects. If you've already written that object, it only writes a handle to the same object, not the entire object.

answered Sep 4, 2015 at 11:01

user207421

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1 Comment

Adel Boutros Over a year ago

Indeed, you are correct. I replaced the code to have a new array everytime and the write byte array is now faster than writeObject. I totally forgot about the caching done by writeObject

Marko Topolnik · Accepted Answer · 2015-09-04 11:41:27Z

I wrote a slight modification of your code:

public class Test { public static final int REPS = 10000; public static void main(String argv[]) throws IOException { ByteArrayOutputStream out2 = new ByteArrayOutputStream(); try (ObjectOutputStream ostream2 = new ObjectOutputStream(out2)) { writeBytes(ostream2); out2.reset(); long start = System.nanoTime(); writeBytes(ostream2); long end = System.nanoTime(); System.out.println("write byte[] took: " + ((end - start) / 1000) + " micros"); } ByteArrayOutputStream out = new ByteArrayOutputStream(); try (ObjectOutputStream ostream = new ObjectOutputStream(out)) { writeObject(ostream); out.reset(); long start = System.nanoTime(); writeObject(ostream); long end = System.nanoTime(); System.out.println("writeObject took: " + ((end - start) / 1000) + " micros"); } } private static void writeObject(ObjectOutputStream ostream) throws IOException { for (int i = 0; i < REPS; ++i) { final byte[] bytes = bytes(); ostream.writeObject(bytes); } } private static void writeBytes(ObjectOutputStream ostream2) throws IOException { for (int i = 0; i < REPS; ++i) { final byte[] bytes = bytes(); ostream2.writeInt(bytes.length); ostream2.write(bytes, 0, bytes.length); } } static byte[] bytes() { byte[] bytes = new byte[REPS]; for (int i = 0; i < REPS; ++i) { bytes[i] = (byte) i; } return bytes; } }

Now the result is

write byte[] took: 51697 micros writeObject took: 57203 micros

Thank you for your answer. Indeed I totally forgot about the caching mechanism in writeObject

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