Determine whether .class file was compiled with debug info?

Question

How can I determine for any Java .class file if that was compiled with debug info or not?

How can I tell exactly what -g{source|lines|vars} option was used?

Answer 1

You must check the Code structure in the class file and look for LineNumberTable and LocalVariableTable attributes.

Tools like ASM or Apache Commons BCEL (Byte Code Engineering Library) will help: https://commons.apache.org/proper/commons-bcel/apidocs/index.html?org/apache/bcel/classfile/LineNumberTable.html

Answer 2

Building on the two answers above (Aaron Digulla and Pete Kirkham), you can inspect the jarfile or the classfile directly with javap -v. I find this a little simpler than using a classpath. Single quotes are needed with the ! for the jarfile case.

javap -v 'jar:file:///Absolute/Path/To/JarFile.jar!/package/name/using/slashes/ClassName.class' | grep "\(Compiled from\|LineNumberTable\|LocalVariableTable\)" | head

or (don't need the absolute path for checking a classfile)

javap -v Path/To/ClassName.class | grep "\(Compiled from\|LineNumberTable\|LocalVariableTable\)" | head

If results are unclear or the format changes, pipe to less instead of grep and take a look.

Answer 3

If you're on the command line, then javap -l will display LineNumberTable and LocalVariableTable if present:

peregrino:$ javac -d bin -g:none src/Relation.java 
peregrino:$ javap -classpath bin -l Relation 
public class Relation extends java.lang.Object{
public Relation();

peregrino:$ javac -d bin -g:lines src/Relation.java 
peregrino:$ javap -classpath bin -l Relation 
public class Relation extends java.lang.Object{
public Relation();
  LineNumberTable: 
   line 1: 0
   line 33: 4

peregrino:$ javac -d bin -g:vars src/Relation.java 
peregrino:$ javap -classpath bin -l Relation 
public class Relation extends java.lang.Object{
public Relation();

  LocalVariableTable: 
   Start  Length  Slot  Name   Signature
   0      5      0    this       LRelation;

javap -c will display the source file if present at the start of the decompilation:

peregrino:$ javac -d bin -g:none src/Relation.java 
peregrino:$ javap -classpath bin -l -c Relation | head
public class Relation extends java.lang.Object{
  ...

peregrino:$ javac -d bin -g:source src/Relation.java 
peregrino:$ javap -classpath bin -l -c Relation | head
Compiled from "Relation.java"
public class Relation extends java.lang.Object{
  ...

Programmatically, I'd look at ASM rather than writing yet another bytecode reader.