Re: JDB (how to generate debug code)

A Pietu Pohjalainen <>
24 Sep 2004 00:22:25 -0400

          From comp.compilers

Related articles
JDB (how to generate debug code) (2004-09-21)
Re: JDB (how to generate debug code) (A Pietu Pohjalainen) (2004-09-24)
| List of all articles for this month |

From: A Pietu Pohjalainen <>
Newsgroups: comp.compilers
Date: 24 Sep 2004 00:22:25 -0400
Organization: University of Helsinki
References: 04-09-128
Keywords: Java, debug
Posted-Date: 24 Sep 2004 00:22:25 EDT

napi <> wrote:
> Can anyone direct me to a doc/manual that describes how a compiler could
> generate debug code for the JDB (Java Debugger).

In the Java bytecode, generating debuggable versions of the class files
is done through the attribute mechanism: a classfile consists of some
administration data, such as magic identifier at the beginning
(0xCAFEBABE), minor and major versions of the class file format etc.
Then there are five tables, namely
- Constant pool
- Implemented interfaces
- Fields of the class
- Methods of the class
- Class level attributes

Fields-table and Methods-table can also contain attributes of their own,
I'll talk about them later.

Now, having sanely debuggable code takes part in (at least) three
places: the constant pool and Class-level attributes and attributes
of the Methods-table.

The class-level attributes are relevant to debuggability, as it contains
an attribute named 'SourceFile', which is used to recover the file that
this class was generated from. Most debuggers don't try to decompile the
bytecode to a higher-level form, but rely on the availability of the
source file.

Having your constants readable improves debuggability a lot. The
constant pool contains entries to the 'public' interface of the class:
class name, super class name, member variables, etc. An usual
obfuscation technique is to twist the public interface of the class to a
unreadable form. When debugging, it is advisable to have these in their
unobfuscated form.

Then, the Method table contains all the methods defined in the class. It
has two pre-defined attributes: Exceptions and Code. Exceptions contains
all the exception handlers in the method and Code contains the actual
bytecode of the method.

Now, Code contains yet another attributes: LineNumberTable and
LocalVariableTable. These are used to map bytecode ranges to lines in
the original source file and to give meaningful names to local variables
in the method, respectively.

A more thorough document can be found at almost any book that discusses
the Java VM; I used the one published by O'Reilly; 'Java Virtual Machine'
by Jon Meyer & Trow Downing, 1997. Other books on this topic (that I'm
aware of) include 'Inside the Java Virtual Machine' by Bill Venners and
'Programming for the Java(TM) Virtual Machine' by Joshua Engel. I
believe that any of these will do good enough.

Of course, the most accessible text is 'The Structure of the Java
Virtual Machine' by Tim Lindholm and Frank Yellin; It is also available
via web.

Pietu Pohjalainen

Post a followup to this message

Return to the comp.compilers page.
Search the comp.compilers archives again.