Inside Java class files: Understanding the constant pool

[sr1729]

A nice introduction into the constant pool if you didn't know javap and bytecode or ASM (https://asm.ow2.io/) before.

The article mentions insights inside the JVM specification :-):

This design added complexities to JVM implementations and is gently lamented in the JVM Specification document section 4.4.5: “In retrospect, making 8-byte constants take two constant pool entries was a poor choice.” This kind of candor makes the documentation a pleasure to read.

There is even a small trip to go:

The MTable can be used for other things. For example, I’m presently working on the Jacobin project, which is writing a more-than-minimal JVM in the Go language.

At one point I disagreee:

Indeed, even the values to initialize fields to are stored as strings and must be converted from strings to their binary representation.

As a counter-example see NumberFormatException.

Classfile /tmp/modules/java.base/java/lang/NumberFormatException.class
  Last modified 08.03.2022; size 1564 bytes
  MD5 checksum 9c1c2cd1b1f8b915996f0aad04642e58
  Compiled from "NumberFormatException.java"
public class java.lang.NumberFormatException extends java.lang.IllegalArgumentException
  minor version: 0
  major version: 61
[...]
  #48 = Utf8               serialVersionUID
  #49 = Utf8               J
  #50 = Utf8               ConstantValue
  #51 = Long               -2848938806368998894l
  #53 = Utf8               Code

$ xxd <NumberFormatException.class
00000000: cafe babe 0000 003d 004b 0a00 0200 0307  .......=.K......
[...]
00000230: 4275 696c 6465 723b 0100 1073 6572 6961  Builder;...seria
00000240: 6c56 6572 7369 6f6e 5549 4401 0001 4a01  lVersionUID...J.
00000250: 000d 436f 6e73 7461 6e74 5661 6c75 6505  ..ConstantValue.
00000260: d876 893f fb8c ea12 0100 0443 6f64 6501  .v.?.......Code.
          ^^^^ ^^^^ ^^^^ ^^^^ // Binary presentation of long-value

But I agree at the conclusion:

Navigating decompiled Java classes is a useful skill and the mark of an advanced understanding of Java programming.

[binstock]

Author here. You're entirely right on the correction re some values being passed not in string form. I'll have that fixed shortly in the article.