059bWhen Arguments Get out of Hand... (Follow-Up)

Author: Dr. Heinz M. KabutzDate: 2002-11-13Java Version: 1.4.1Category: Performance

Abstract: In this follow-up to our earlier newsletter on "When arguments get out of hand...", we look at the constant pool in Java classes and how this can be affected by how we compile our class.

Thanks to those of you who wrote back after my last newsletter. There are some issues I want to follow-up on.

To start off, the date on the newsletter was incorrect.

Then, the original topic of this newsletter was "Verrrrrry looooong Strings and other things". One of my subscribers from Poland suggested that the topic did not really represent the content as there is no real limit on how long ordinary Strings can be, except for the limit on char array length and physical memory. I renamed it to: "When arguments get out of hand..."

Talking about long constant Strings, many thanks to Frank Peters from Process Management Consulting in Cologne for pointing out the problem with long constant Strings to me at the beginning of June 2002. Apologies for forgetting to thank you in the original newsletter, our email conversation had slipped my conscious mind. Something that Frank pointed out to me in the original discussion was that if you have a unicode character, it might be converted to more than two bytes in certain circumstances, so the limit is actually less than 65535 in some cases.

Some readers also wrote to tell me that the problem with looong constant Strings used to appear with JSP pages generated by Dreamweaver.

Last, but definitely not least, thanks to Bjorn Carlin for sending me the secret to the 14 missing data members. Thank you also to John Bester from South Africa and Juan C. Valverde from ArtInSoft in Costa Rica for sending me their ideas.

The Missing 14 Fields

The limit on the fields is actually the limit in the constant pool where the names of the fields are stored. Bjorn told me that the 9th and 10th bytes of the class file tell you how many constant fields are in the class. He also sent me output from a little program he wrote that dumped the constant pool from a class.

Here is a piece of code that tells you the size of the constant pool for a classfile:

import java.io.*;

public class ConstantPoolSize {
  static void skip(InputStream in, int skip) throws IOException {
    while(skip > 0) {
      skip -= in.skip(skip);
    }
  }
    
  public static void main(String[] args) throws IOException {
    DataInputStream din = new DataInputStream(
      new FileInputStream(args[0]));
    skip(din, 8);
    int count = ((din.readByte() & 0xff) << 8) 
      | (din.readByte() & 0xff);
    System.out.println(count);
    din.close();
  }
}

I'll let you figure out the reason for skip() yourself ;-)

Run it against a small class such as:

public class Small {}

You will get different results depending with which debugging info options you compiled Small.java. The default compile will tell you that there are 13 entries in Small.class. If you add one field, you will have 15 entries, one for the name of the field, and one for the type of the field. For example, if you add an int a0;, you will have entries "I" and "a0".

You can compile BigClass with the -g:none option, then you can squeeze more fields in. In addition, if you call one field "I" and another "Code", you can get two more fields into your class. "I" is the type of int defined in the constants table. I don't know where "Code" comes from, but it works. The maximum number of fields I've therefore managed to squeeze in like this was 65526.

Now that's really overrevving Java ;-)

Heinz

P.S. Probably the worst mistake in the newsletter was that I said my rev counter was limited to 6000 revolutions per minute. That of course is nonsense. It is limited to 7000 revs. I checked today. When I wrote the newsletter, I did not feel like going to the garage to look. Besides, at 1:00am that might have made my neighbours unhappy ;-)

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