为什么在写字节数组时ObjectOutputStream.writeObject比写字节快？

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

Answer 1

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.

Answer 2

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